1,3-Bishomocubane: a kinetic rock, a thermodynamic powerhouse and a compelling chiral synthetic scaffold

Abstract

Over the last several decades, saturated polycyclic cage compounds have remained a point of interest for organic chemists because of their unique characteristics and reactivity. For the first time, a detailed analysis of the synthesis, properties and transformations of 1,3-bishomocubanes, which fall under the rare category of chiral cage compounds, is provided in this article. This review which also includes the authors’ work in this area over the last decade is expected to serve as a valuable resource for chemists interested in the fascinating chemistry and properties of polycyclic cage compounds.

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Introduction

The chemistry of saturated polycyclic cage systems, namely adamantane, snoutane, triprismane, tetrahedrane, pentaprismane, cubane, homocubane, bishomocubane, trishomocubane, dodecahedrane, golcondane etc. has continuously emerged over time. Along with the appealing symmetry in the structures, these compounds also possess distinctive properties.¹ The angle strain, torsional strain, negative heat of combustion and abnormal positive heat of formation² make this class of compounds special. The complex procedures and lower yields make the synthesis of cage compounds a challenging task, but their wide range of applications motivate scientists to investigate these compounds in detail. The cage compounds have been utilized as high-energy materials,³ propellants and explosives,⁴ ligands,^5,6 in natural product synthesis^7,8 and in medicinal chemistry.^9–11

Adamantane was the first polycyclic cage compound to be synthesized in the laboratory. It was first synthesized by Prelog and Seiwerth in 1941¹² and became popular once a viable synthesis was reported by Schleyer in 1957.¹³ Today, the adamantane chemistry is well developed and enjoys a variety of applications. Adamantane is used in medicinal chemistry,¹⁴ fuels¹⁵ and smart displays.¹⁶

The first practical synthesis of cubane was reported by Eaton and Cole in 1964,¹⁷ which was later modified by Chapman.¹⁸ At present, a number of cubane derivatives have been synthesized and they are being utilized for a variety of purposes in areas as diverse as synthetic, medicinal and materials chemistry.¹⁹ Considering the difficult synthesis and low yields in the case of cubane-based energy materials, the scientific community started exploring the scope of synthesis of homocubane, bishomocubane and trishomocubane derivatives which can show similar properties. Homocubanes can be obtained by ring contraction of bishomocubanes²⁰ or by [2+2] cycloaddition reactions of suitable tricyclic compounds.²¹ The chemistry of homocubane, bishomocubane and trishomocubane has been reviewed by Marchand in 1989.²

1,3-Bishomocubane (BHC)

1,3-Bishomocubane 1 is a unique motif containing eight methine (CH) units and two methylene (CH₂) carbons arranged perfectly in a C2 symmetric manner (Fig. 1). According to the revised von Baeyer nomenclature system,²² 1,3-bishomocubane can be numbered as shown in Fig. 1 and naming was done according to IUPAC system as pentacyclo[5.3.0.0.^2,50.^3,90^4,8]decane. 1,3-Bishomocubane 1 is an interesting candidate from the stereochemical perspective because, out of the five possible bishomocubane isomers, 1,3-bishomocubane 1 is the only one that is chiral. Usually, it is difficult to imagine the C2 axis of symmetry by considering the molecule in a 3D view. However, Schlegel projection provides a more convenient way to visualize the C2 axis in the BHC molecule 1.²³ The derivatives of 1,3-bishomocubane 1 may or may not be C2 symmetric, depending on the nature and position of the substituents.

Fig. 1 Nomenclature of 1,3-bishomocubane.

The bishomocubane molecule 1 captures more attention due to its ring strain. According to Allinger force field calculations, the increase in strain energy accompanied by photochemical ring closure of dicyclopentadiene (DCPD) 2 to bishomocubane 1 moiety is Δ_strain = 52.28 kcal mol⁻¹ (Fig. 2(a)).²⁴ In 1993, Dilling continued the study of bishomocubane chemistry towards molecular mechanics calculations.²⁵ To understand the reactivities and properties, calculations have been performed on several BHC derivatives. Enthalpies of formation and strain energies of BHC hydrocarbons were calculated by MM2 method (Fig. 2(b)). Compared to the strain energies of cubane²⁵ and homocubane,²⁵ all five isomers of bishomocubane (1–1d) exhibit lower strain energies due to small deviations from the normal tetrahedral bond angles. Among all the bishomocubanes, the strain energy mainly relies on fused 4-membered rings, which are more highly strained than 5-membered rings.

Fig. 2 Theoretical study of 1,3-bishomocubane in comparison with other bishomocubanes.

The structures of several dimethyl ketals of 1,3-bishomocubane were assigned with the help of NMR studies by Price and Griffin in 1964.²⁶ In 1975, Dilling examined the proton NMR spectra of various 1,3-bishomocubanes and concluded that for any isomeric pair, the methylene proton which is anti to the cage skeleton is deshielded as compared to the syn proton (Fig. 3).²⁷

Fig. 3 NMR study of 1,3-bishomocubane.

Synthesis

1,3-Bishomocubane chemistry started when Schenck et al. performed the [2+2] photocyclization of cyclopentadiene dimer 2 in the presence of UV light to get 1,3-bishomocubane 1 in 1963 (Scheme 1),²⁸ and this created a significant milestone in the field of bishomocubane chemistry. When the parent hydrocarbon (DCPD) 2 was irradiated under UV light in the presence of sensitizer acetone, along with bishomocubane, numerous other products were formed (3, 4 and 5). Without a sensitizer, using cyclohexane as solvent, an unidentified trace amount of brown oil was observed. Later on, numerous derivatives of endo-dicyclopentadiene were converted to bishomocubane derivatives via [2+2] photochemical cycloaddition.²⁹

Scheme 1 Synthesis of 1,3-bishomocubane.

The irradiation of dicyclopentadienone 6 in the crystalline state results in a polymer only (Scheme 2).²⁹ This process initiates more likely via n–π* transition and favours the formation of BHC in the presence of solvent and pyrex filter. The same reaction was also performed by irradiation through quartz. The polymer formation might be caused by the intermolecular attack of the excited state I.

Scheme 2 Synthesis of 1,3-bishomocubanone.

In 1973, a new pathway was developed by Klunder and Zwanenburg to access 1,3-bishomocubane derivatives 9 and 10via Wagner–Meerwein rearrangement of homocubane 8 (Scheme 3).³⁰ This method is a single-step procedure with 67% yield of the major isomer. Even though this method offers a reasonable yield in a single step, it still can’t be viewed as a primary approach to synthesize 1,3-bishomocubane because access to the homocubane 8 itself is challenging. Apart from this, 1,3-bishomocubane or its derivatives were observed as minor products in several other cases.

Scheme 3 Synthesis of 1,3-bishomocubane derivatives.

Until 2009, synthesizing unsubstituted C2 symmetric bishomocubane 1 was cumbersome because direct irradiation of cyclopentadiene dimer 2 led to a mixture of products due to possible molecular interconversions.³¹ Fokin et al. successfully addressed this challenge by carrying out the photolysis of monoethylene ketal of cyclopentanone dimer 11 and converted it into the unsubstituted 1,3-bishomocubane 1 in subsequent five steps with an overall 45% yield (Scheme 4).³²

Scheme 4 Modified method for the synthesis of 1,3-bishomocubane.

The substituted 1,3-bishomocubanes can also be prepared via [2+2] photocycloaddition pathway which was reviewed by Dilling in 1966.²⁹ Osawa et al. studied the factors affecting [2+2] photochemical ring closure in 1977.³³ The reactions were found to be under the influence of three criteria: (1) distance between two double bonds (r), (2) orientation of the double bond (θ), and (3) strain energy (Δ_strain).

Indeed, synthesizing bishomocubane derivatives is always a challenging task. Among all, the synthesis of 1,3-bishomocubane diester is a fascinating challenge in bishomocubane chemistry. The history behind synthesizing precursor (Thiele's ester) for the diester molecule makes it more interesting.

Thiele's ester

In 1900, Thiele reported the carbonation of dicyclopentadiene 2 to get potassium cyclopentadiene carboxylate.³⁴ Next year, Thiele also reported the heterodimers of dicyclopentadiene carboxylic acid 17, which arose from the Diels–Alder reaction between the three possible regioisomers of cyclopentadiene carboxylic acid 16 (Scheme 5).³⁴ Although, 72 Diels–Alder adducts are theoretically possible, Thiele separated the major isomer 18 by repeated crystallization, because only one isomer predominates in practice. Thiele also carried out the esterification to get the diester. Later on, the acid and the ester were known as Thiele's acid 18 and Thiele's ester 19, respectively. The preparation of Thiele's acid 18 by this method was demanding as the yields were low and repeated recrystallization was required to get the compound 18 in pure form. Almost after a century, the Marchand group, in 1998, improved the procedure by preparing the sodium cyclopentadienide instead of potassium cyclopentadienide and obtained the product, primarily consisting of Thiele's acid 18, on a 55 g scale in 80% yield.³⁵ They also noticed that it was impossible to eliminate the regioisomers completely by recrystallization. In 2004, Marchand et al. reported that the esterification and recrystallization could give Thiele's ester 19 in pure form.³⁶

Scheme 5 Synthesis of Thiele's ester.

Later, in 2012, Desouter-Lecomte's group presented a preliminary simulation towards the control of Cope rearrangement of the most stable isomer of Thiele's ester 19 by laser control technique (Scheme 6(a)).³⁷ For that, they carried out an experimental investigation for the dimerization of methylcyclopentadienyl carboxylate which forms the most stable isomer, Thiele's ester 19. The authors stipulated that making the metal salt can control the volatility of monomeric cyclopentadiene ester 16. They isolated the sodium salt of carboxylated cyclopentadiene 22 in 74% yield by reacting cyclopentadiene 21 with sodium hydride and dimethyl carbonate (Scheme 6(b)).³⁷ The addition of a saturated solution of NH₄Cl in DCM to a highly reactive methylcyclopentadienyl carboxylate 22 leads to rapid dimerization and affords a mixture of 19a, 19 and 19b in 20 : 63 : 17 ratio. This technique is useful to get the desired isomer without any rearranged byproducts.

Scheme 6 (a) Cope rearranged product of Thiele's ester; (b) control of cope rearrangement of Thiele's ester by laser control technique.

This method was modified by Wulff et al. in 2015 to get cyclopentadiene metal salt 22 in 94% yield. They also further optimized the preparation of Thiele's ester 19 to get the major isomer in 64% yield (Scheme 7).³⁸

Scheme 7 Modified method for the synthesis of Thiele's ester.

Despite the method of synthesizing Thiele's acid and ester was known since 1900, the precise position of carboxylate groups remained uncertain. Subsequently, Peters concluded with the help of UV spectroscopic study that both the carboxylate groups were in conjugation with the double bonds,³⁹ which was contrary to the earlier conclusion drawn by Alder group.⁴⁰ Later, Finnegan and McNees used spectral data to confirm the earlier result from the Peters group.⁴¹ Even with many investigations, the precise location of carboxyl groups was ambiguous. In 1968, Dunn and Donohue presented evidence for four possible alternative isomers 19, 19c, 19d and 19e (Scheme 8(a))⁴² and the correct structure was established by preparing 5-membered cyclic anhydride 25 (Scheme 8(b)). This result was in contrast with a report from Bouboulis,⁴³ which proposed the structure 19e for the diacid. Later, in 1990, Marchand and coworkers provided a detailed structural analysis of Thiele's ester 19 by analyzing 1D and 2D NMR data,⁴⁴ which was in agreement with the structure 19 proposed by Dunn and Donohue (Scheme 8(b)).

Scheme 8 (a) Conjugated isomers of Thiele's ester; (b) confirmation of the structure of Thiele's ester.

In 1959, Peters reported the major isomer of Thiele's dimer in which both of the carboxyl groups were conjugated with the double bond, whereas in the minor isomer, only one of the carboxyl groups was conjugated and this data was solely on the basis of UV spectra.³⁹ This claim was reinvestigated by Marchand's group in 1998. In this effort, two of the minor isomers, 26 and 27, were also isolated and characterized (Fig. 4).³⁵ Also, it was suggested that the minor isomer reported by Peters was indeed 27 but not 26. Further, to prove the structure of the minor isomer 27 unequivocally, Marchand and coworkers recorded X-ray structure of its [2+2] cycloadduct 28.³⁵

Fig. 4 Some minor isomers of C2-symmetric 1,3-bishomocubane diester.

This year, our group reported a new derivative of BHC, i.e. 1,3-BHC-keto alcohol 31 (Scheme 9).⁴⁵ This was synthesized from commercially available dicyclopentadiene 2. The synthetic steps involve the allylic oxidation of dicyclopentadiene (DCPD) 2 by SeO₂ followed by PCC oxidation and MBH (Morita–Baylis–Hillman) reaction to give MBH-DCPD alcohol 30.⁴⁶ This on photocyclization provided BHC-keto alcohol 31.

Scheme 9 Synthesis of BHC-keto alcohol.

Stereochemical aspects

Generally, the polycarbocyclic cage compounds possess highly symmetrical skeletons but still could be chiral if only axis of symmetry is present. The 1,3-bishomocubane falls into the rare category of C2 symmetric chiral polycarbocyclic cage compounds. Because of the distinct stereochemical properties, the synthetic transformations of 1,3-bishomocubane are found to be quite selective in nature.

The synthesis of optically active 1,3-bishomocubane from a racemic monoprotected dimer of cyclopentadienone 11 was demonstrated by Nakazaki and Naemura (Scheme 10).⁴⁷ The compound 11 was resolved using (+)-2-(1-aminoethyl) naphthalene as the resolving agent and later on converted to optically active (−)-1,3-bishomocubane (−)-1 (1S, 2S, 3S, 4S, 5R, 7S, 8S, 9R) and the absolute configuration was assigned on the basis of CD (circular dichroism) spectra. The very next year, Nakazaki et al. studied the chiroptical properties of (−)-1, which states that the structure of (−)-1,3-bishomocubane deviates very little from the O_h symmetric cubane.⁴⁸

Scheme 10 Synthesis of optically active 1,3-bishomocubane.

In 2017, Thiele's acid 18 was resolved after more than a century since its inception. Wulff et al. performed chiral resolution of Thiele's acid 18 by preparing the diastereomeric salt with brucine (Scheme 11).⁴⁹ The preparation of enantiopure Thiele's ester (−)-19 and (+)-19 was also reported using the resolved Thiele's acid 18. Eventually, this method provides another way to furnish enantiopure (−)-pentacyclo[5.3.0.0.^2,50.^3,90^4,8]decane 1.

Scheme 11 Resolution of Thiele's acid.

Rearrangements

The high rigidity and significant amount of ring strain of polycyclic cage molecules, including bishomocubanes, make them especially susceptible to unusual rearrangements. The rearrangement of 1,3-bishomocubane to 1,4-bishomocubyl moiety by 1,2-alkyl migration in the presence of PCl₅ was reported. Afterwards, in 1966, Dauben and Whalen reported a rearrangement of the 1,1-bishomocubyl (pentacyclodecyl cation) system via solvolysis.²⁰ In light of the intriguing carbocation rearrangement of strained polycyclic systems, Dilling and Reineke presented an acetolysis of 1,3-bishomocubyl tosylates 34 and 36 which occurred via bridged carbocation intermediate I (Scheme 12).⁵⁰ Later, the evidence for the intermediacy of bridged carbocation I was confirmed by the Dilling group in 1969.⁵¹ It was found that acetolysis of syn-10-(tosyloxy)-1,3-bishomocubane 34 gave the product 35 without any rearrangement (Scheme 12(a)). However, anti-10-(tosyloxy)-1,3-bishomocubane 36 afforded a mixture of rearranged product 38 as major (85%) and unreacted anti-alcohol 37 (15%) with overall 75% yield (Scheme 12(b)).^50,51 In these rearrangements, the migration of the C–C bond antiperiplanar to the leaving group results in different products.

Scheme 12 Solvolysis of syn- and anti-10-(tosyloxy)-1,3-bishomocubane.

In 1971, Klunder and Zwanenburg reported homoketonization of a homocubane system involving the base-catalyzed transformation of a homocubane bridgehead alcohol and its acetate to a half-cage ketone.⁵² Afterwards, the same group extended their work to the bishomocubane system in 1973.⁵³ Homoketonization of 1,3-bishomocubane bridgehead alcohols 39a and 39b took place on treatment with NaOMe in EtOH under reflux conditions (Scheme 13). This process proceeded by the cleavage of the C2–C5 bond exclusively in the direction of the least hindered ketone. The authors have also reported the stereochemistry of cage opening of alcohols to ketones as endo by performing the reaction with EtOD. This was supported by force field calculations by Osawa and coworkers.⁵⁴

Scheme 13 Homoketonization of 1,3-bishomocubane derivatives.

The exceptional cationic rearrangement of 1,3-bishomocubanone 7 to brendane derivatives 41 was presented by the Mehta group in 1976, which was facilitated by the Schmidt reaction (Scheme 14).⁵⁵ Nucleophilic addition of azide ion to a protonated carbonyl of 7 generates the key intermediate I. Regiospecific bond migration in I and C3–C4 bond migration in the intermediate II leads to a stabilized cyclopropylcarbinyl cation III. Mesylate ion opens the cyclopropane ring from the exo face to form a brendane derivative 41.

Scheme 14 Synthesis of brendane derivative from 1,3-bishomocubane.

Ring expansion reactions of cyclic ketones with diazomethanes are well known. However, the reaction of 6-membered rings is faster than 5-membered rings with CH₂N₂ and accordingly it is difficult to synthesise 6-membered cyclic ketones from 5-membered cyclic ketones because in such case rather than cyclohexanone, cycloheptanone forms predominantly.⁵⁶ In 1975, Yonemitsu and coworkers reported ring expansion reactions of cage systems with diazomethane.⁵⁷ In 1977, the Yonemitsu group observed the formation of dimerized product 44 or 45 along with ring expansion when bishomocubane was treated with an excess of CH₂N₂ in ether for 16 h (Scheme 15).⁵⁸ In 1983, Hirao, Yonemitsu and coworkers reported the conditions for the formation of a mixture of homologous ketone and dimer and also exclusively the ketone.⁵⁹ As reported by the Hirao group, along with ring expanded homoketones, 42 and 43, which are formed in 37% yield, aldol-type novel dimeric hydroxy ketones 44 and 45 are formed in 32% yield. But in the presence of methanol, the reaction yields ketone 42 predominantly in 70% yield.⁵⁹

Scheme 15 Reaction of 1,3-bishomocubane with diazomethane.

The mechanism illustrated that the highly electrophilic carbonyl carbon of bishomocubanone 7 reacts rapidly with diazomethane to give betaine intermediate I, which is stabilized either by ring closing reaction to give epoxide 47 or by Wagner–Meerwein type rearrangement to form homoketone 43 (Scheme 16).⁵⁹ The highly reactive starting material ketone 7 present in the reaction medium reacts with diazo intermediate, to give a dimer 46via a dimeric betaine II. The formation of epoxide 47 from betaine I is unfavorable owing to the angle strain. In 1980, Zwanenburg et al. utilized the strategy of ring expansion reactions with diazomethane to prepare basketanes from homocubanones.⁶⁰ Krow reviewed this chemistry as a part of his article on one carbon ring expansion strategy in 1987.⁶¹

Scheme 16 Mechanism for the reaction of 1,3-bishomocubane with diazomethane.

A well-known reaction for the synthesis of lactones from cyclic ketones is the Baeyer–Villiger reaction. In 1976, Mehta, Pandey and Ho reported a regiospecific Baeyer–Villiger oxidation of polycyclic ketones with ceric ion.⁶² Treatment of 1,3-bishomocubanone 7 with ceric ammonium sulfate or ceric ammonium nitrate in acetonitrile furnished a single product 48 (Scheme 17). However, when ketone 7 was treated with m-chloroperbenzoic acid, a mixture of lactones 48 and 49 was formed in 5 : 1 ratio.

Scheme 17 Baeyer–Villiger reaction of 1,3-bishomocubane.

In the same year, Yonemitsu and coworkers reported the formation of rearranged five-membered lactone 51 along with six-membered lactone 48 and 49 when 1,3-bishomocubanone 7 was treated with m-CPBA in chloroform (Scheme 18).⁶³ The mechanism suggests the oxidation of BHC and the formation of carbocation intermediates I and II by the heterolytic cleavage of 50 rather than migration. In 1978, the same group gave evidence for the carbocation mechanism of rearranged product 51 by examining a few factors, such as the solvent effect, kinetic treatment, and methyl substituent effects.⁶⁴

Scheme 18 Rearrangement of 1,3-bishomocubanone.

In 1977, Luh carried out Ag(I) catalyzed reaction on pentacyclo [5.3.0.0.^2,50.^3,90^4,8]-decan-6,10-dione 52 (Scheme 19).⁶⁵ The compound 52 on treatment with Lewis acid AgClO₄ afforded a retro [2+2] adduct cyclopentadienone dimer 53 in quantitative yield. The metallo-cage complex I is formed by cleavage of most strained C3–C4 σ bond. The diene was obtained by the migration of kinetically or thermodynamically favored C8–C9 bond over migration of C2–C5 bond.

Scheme 19 Ag(I) catalyzed reaction of 1,3-bishomocubane derivative.

In 1980, the group of Zwanenburg explored the base-induced homoketonization of 4-acetoxypentacyclo[5.3.0.0.^2,50.^3,90^4,8] decan-6-one 54 and its ketal 56 (Scheme 20).⁶⁶ They cogently expressed that the effect of β-substitution overshadows thermodynamic control in ring-opening reaction. Later, in 1985, the same group exploited compound 56 by preparing a few polycyclic compounds via eliminative ring fission.⁶⁷ The cause of product formation was discussed with the help of MM2 studies later in the same year.⁶⁸

Scheme 20 Homoketonization of 1,3-bishomocubane derivatives.

In 1984, Zwanenburg group attempted an attractive and direct approach for the synthesis of 5-methoxyhomocubane carboxylic acid by a Favorskii ring contraction (Scheme 21(d)).⁶⁹ But the cage ketone 58 undergoes unforeseen cage opening reactions due to electronic participation of bridgehead methoxy group. In 1986, they provided an efficient route to control the cage opening reactions in 1,3-bishomocubane and homocubane systems.⁷⁰ As discussed, BHC with bridgehead methoxy group resulted in cage opening product. The attempt to increase the leaving group tendency by the addition of AgNO₃ under basic conditions leads to the formation of 59 (Scheme 21(a)).⁶⁹ Treatment of BHC with HCl in toluene led to an instantaneous rearrangement to tricyclodecenedione 60 (Scheme 21(b)).⁶⁹ An efficient route provided for Favorskii ring contraction starts from 4,5-dibromo-1,3-bishomocubanone 61 (Scheme 21(c)).⁷⁰

Scheme 21 Favorskii ring contraction of 1,3-bishomocubane derivatives.

In 1985, the photo-induced isomerization of 1,3-bishomocubane radical cations in γ irradiated freon matrices was studied with the help of EPR and electronic spectra by Shida et al.⁷¹ At the same time, Andrews et al. followed photochemical rearrangements of 1,3-bishomocubane radical cations in solid argon with the help of absorption spectra.⁷²

Marchand and coworkers reported an unprecedented rearrangement of 1,3-bishomocubyl ring system in 1987.⁷³ When methyl-3,10-dinitropentacyclo[5.3.0.0.^2,50.^3,90^4,8] decane-1-carboxylate 64 was treated with base and K₃Fe(CN)₆, the methyl-3,9-dinitro-exo-10-methoxypentacyclo[5.3.0.0.^2,50.^3,90^4,8]decane-8-carboxylate 65 was formed (Scheme 22). During this rearrangement, 1,3-bishomocubyl anion radical was proposed as an intermediate.

Scheme 22 Rearrangement of 1,3-bishomocubane derivative.

Ueda et al. in 1993, reported a base promoted rearrangement of 1,5-dibromopentacyclo [5.3.0.0.^2,50.^3,90^4,8] decane-6,10-dione 66.^74,75 The expected Favorskii type contraction was not observed, instead, a ring cleaved product 67 was formed (Scheme 23), the reason may be ring strain and stability of bromocarbanion intermediate. Later in the same year mechanistic evidence was presented for this base promoted rearrangement by the Ueda group. Intermediate I was formed when 66 reacted with hydroxide ion. This converts to bromocarbanion intermediate via Haller–Bauer type reaction. The reaction of II with H⁺(or D⁺) gives III, which reacts with OH⁻ to form intermediate Vvia intermediate IV. This indicated that rather than Favorskii rearrangement, ring cleavage and succeeding elimination of bromine atom occurred. Here basicity plays a major role in order to bring about Haller–Bauer type reaction and to complete the cycle of intermediates capable to convert rearranged product.

Scheme 23 Haller–Bauer type reaction of 1,3-bishomocubane derivative.

By the end of the 20th century, many cationic, anionic, or radical reactions were known, but the reaction of 1,3-bishomocubane involving a carbene intermediate was still unreported. Our group published the first carbene trapping experiment on the 1,3-bishomocubane system 7 in 2012 (Scheme 24).⁷⁶ The pentacyclo[5.3.0.0.^2,50.^3,90^4,8]decane-6-one 7 was treated with CBr₄/PPh₃ followed by ⁿBuLi to get 1,3-bishomocubyl carbene intermediate I which was successfully trapped using norbornene as a 1 : 1 mixture of isomers of 69. The kinetic and thermodynamic stability of the intermediate was predicted using DFT calculations which suggested higher stability for carbene intermediate I over cycloalkyne intermediate II in the ground state. An effort was also made to synthesize C2 symmetric bis-vinylidenecarbene, which remained unsuccessful.

Scheme 24 Rearrangement of 1,3-bishomocubane via carbene intermediate.

In 2019, an extended pinacolone rearrangement on 1,3-bishomocubane dimethylcarboxylate 11 was reported by Wulff et al. (Scheme 25(a)).^77a The rearrangement took place across an sp³–sp³ σ-bond and was reported to be regio- and stereospecific (Scheme 25(a) and (b)).

Scheme 25 Pinacolone rearrangement of bishomocubane-dimethylcarboxylate.

Both stepwise and concerted mechanisms were presented for the above transformation. Protonation of either of the alcohols leads to intermediate II, then fragmentation of C2–C5 bond of cyclobutane generates an olefin 72 (via intermediate III) or 73 or their regioisomers (not shown). The 1,2-migration of benzene ring in the final step is irreversible and dictates the stereochemical outcome of the overall process by generating a new stereogenic centre. The mechanism was briefly investigated by computational data. According to DFT calculations, the low energy conformer has one of the two phenyl rings on C11 antiperiplanar to C2–C5 bond and C12–O bond is approximately synperiplanar. Reaction via these conformations leads exclusively to product 72.

This year, the same group gave a further explanation for the mechanism of extended pinacolone rearrangement of BHC-diester 11 (Scheme 25(b)).^77b During the reaction, the formation of the side product 74 was observed along with the rearranged product 72, which was isolated and characterized. This gave the evidence that product 72 formed via a stepwise mechanism involving carbocation intermediate III.

Miscellaneous transformations

The rearrangement of BHC moiety was explored extensively with significant data, and there are diverse transformations that can be found in the literature. In 1974, Yonemitsu and coworkers reported the synthesis of bisnorditwistane 78 by the reduction of BHC 7 in acetic acid for 10 h in 74% yield (Scheme 26).⁷⁸ In 1980, the same group gave a detailed explanation for the hydrogenation of cyclobutanes in strained cage compounds (BHC).⁷⁹

Scheme 26 Synthesis of bisnorditwistane from 1,3-bishomocubane.

In 1978, Blum and Zlotogorski isolated metallocyclic intermediate 83 while carrying out Ir(I) catalyzed conversion of 1,3-bishomocubane 1 to cyclopentadiene dimer 2 (Scheme 27).⁸⁰ Subsequently, in 1984, Zlotogorski et al. reported a difference in reactivity of 1,3-bishomocubane 1 with different Rh(I) catalysts.⁸¹ [Rh(norbornadiene)Cl]₂ gave the same product as that obtained while using Ir(I), while the use of [Rh(CO)₂Cl]₂ resulted in carbonyl insertion product 82. The unusual ring expansion and decomposition of BHC 1 could be seen in the presence of metal complex at higher temperature.

Scheme 27 Formation of metallocyclic intermediate of 1,3-bishomocubane.

The photothermal metathesis of BHC 84 was demonstrated by Mukai et al. in 1976 (Scheme 28).⁸² It was observed that 1,3-bishomocubane derivative 84 underwent a decarbonylation under both thermal and photochemical conditions. The mechanism proceeds with decarbonylation via a radical pathway.

Scheme 28 Photothermal metathesis of 1,3-bishomocubane derivative.

The gas-phase thermolysis of 1,3-bishomocubane derivative 87 astonishingly ended up giving benzyne 92 (Scheme 29). In 1986, Brown et al.⁸³ subjected 1,3-bishomocubane derivative 87 to flash vacuum pyrolysis and benzyne thus formed was trapped in an argon matrix. The benzyne 92 was characterized with the help of IR spectroscopy.

Scheme 29 Gas-phase thermolysis of 1,3-bishomocubane derivative.

In 1986, Roth et al. performed photolysis of 1,3-bishomocubane 1 using photoexcited chloranil 94 and the reaction was followed using chemically induced dynamic nuclear polarisation (CIDNP) technique (Scheme 30).⁸⁴ In this attempt, 1,3-bishomocubane radical cation 95 was generated and studied. The analysis showed that having spin and charge in one strategic bond allows maximum release of ring strain.

Scheme 30 Photolysis of 1,3-bishomocubane.

The highly strained pyramidalized alkene intermediate of bishomocubane was trapped successively by Marchand group in 1999.⁸⁵ BHC-dicarboxylic acid 96 was converted to diiodo bishomocubane 97 and reaction with MeLi in dry THF at −78 °C generated cage alkene intermediate I by the elimination of I₂ (Scheme 31). This intermediate was trapped stereoselectively by 1,3-diphenylisobenzofuran 98 (DPIBF) and also with 9-methoxyanthracene 101 to afford the corresponding [4+2] cycloaddition products 100 and 102, respectively.

Scheme 31 Generation and trapping of pyramidalized alkene intermediate of 1,3-bishomocubane.

1,3-Bishomocubanes as energetic compounds

Mechanical modelling suggested a good amount of strain energy in 1,3-bishomocubane systems.²⁵ Marchand et al. and our group exploited this property to make high-energy compounds using 1,3-bishomocubane system. In 1984, Marchand and Suri prepared 3,5,5-trinitropentacyclo [5.3.0.0.^2,60.^3,100^4,8]decane 109 (Scheme 32).⁸⁶ The starting material was prepared using the method developed by Herz et al. in 1975.⁸⁷

Scheme 32 Synthesis of trinitro-bishomocubane.

The bis(nitratomethyl)-1,3-bishomocubane (DNMBHC) 111 was synthesized and investigated as an energetic material by our group in 2013 (Scheme 33).⁸⁸ In this study, DNMBHC 111 was found to be a suitable high-energy, high-density material. The high heat of combustion, positive heat of formation along with high detonation pressure and satisfactory detonation velocity make it a worthy fuel or fuel additive.

Scheme 33 Synthesis of dinitratomethylene-bishomocubane.

The very next year three nitro substituted derivatives of bishomocubane were prepared by our group (Scheme 34).⁸⁹ The newly prepared nitromethyl-1,3-bishomocubanes 112, 113 (NMBHC), nitromethylene-1,3-bishomocubane 114 (NMyBHC) and bisnitromethyl-1,3-bishomocubane 115 (DNTMBHC) were investigated by semi-empirical and ab initio methods, TGA and IR spectroscopy. The studies showed that NMBHC 112, 113 and NMyBHC 114 could be the prime candidates to replace rocket propellant-1 (RP-1) as a fuel, whereas DNTMBHC 115 can be an appropriate additive for RP-1.

Scheme 34 Synthesis of nitratomethylene-bishomocubane.

To explore the scope of using polycyclic cage-derived compounds in propellant systems, our group carried out extensive quantum mechanical studies in 2017.⁴ A total of 28 cage compounds were screened for their thermodynamic, propulsive and detonation properties. The nitrogen and oxygen-containing bishomocubane derivatives could be the candidates of choice for liquid propellants while using oxygen as the oxidizer. In the case of monopropellants, several cage compounds showed promising properties on comparing with isopropylnitrate (IPN). The application of cage compounds as solid propellants is subject to their ability to polymerize, and none of the cage compounds have explosive properties.

In 2019, homocubanedimer (HCD) 116 and bis(nitratomethyl)-1,3-bishomocubane (DNMBHC) 115 were compared with RP-1 and their blends for the utility of rocket propulsion by Kumbhakarna et al. (Fig. 5).⁹⁰ The DNMBHC 115 and BHCD 117 pyrolyzed with severe micro explosions during droplet combustion studies. Along with the desired properties, DNMBHC 115 and BHCD 117 being completely miscible with RP-1 makes them prime candidates to be used as additives in rocket fuel. In the subsequent year, BHCD 117, DNMBHC 115, and diazido-dimethyl-bishomocubane (DADMBHC) 119 were tested against the RP-1 surrogate fuel by the same group (Scheme 35).⁹¹ The analysis using color-ratio pyrometry (CRP) indicated DNMBHC 115 to be the most propitious as a liquid propellant.

Fig. 5 Structure of homocubane and bishomocubane dimer.

Scheme 35 Introduction of triazole and tetrazole to bishomocubane molecule.

In 2015, our group reported the novel high-nitrogen bishomocubanes, namely diazidodimethyl bishomocubane (DADMBHC) 119, ditetrazolobishomocubane (DTetzBHC) 121, and diphenyltriazolo-dimethylbishomocubane(DPTrizDMBHC) 120 (Scheme 35).⁹² The analysis was performed using NASA-CEA, TGA-IR and ab initio methods. DADMBHC 119 was found to be an ideal propellant specifically for volume-limited applications. The HTPB can be replaced by using DADMBHC 119 or DTetzBHC 121 as a fuel binder. DADMHBC 119 can be seen as a replacement for RP-1, whereas DTetzBHC 121 can be an additive to RP-1. The detonation properties suggested low potential as explosives for these compounds.

Recently, in 2024, our group again came up with several highly energetic novel hydroxymethyl-bishomocubanone derivatives. The ketoalcohol 31 was synthesized according to Scheme 9, and from that precursor, around twelve BHC derivatives were synthesized and characterized (Scheme 36).⁴⁵ As per the calculated ballistic properties, it was found that the synthesized compounds are superior to conventional fuels RP-1 and HTPB and among all, dinitroazide 126, dinitronitrate 125 and dinitrate 127 were found as excellent candidates for volume-limited applications.

Scheme 36 Synthesis of nitro and azide derivatives of BHC.

Conclusions and future outlook

Besides the uniqueness of structure, properties, and reactivity, polycyclic cage compounds possess various applications. At the same time, the methods to synthesize cage compounds such as cubane, homocubane, and bishomocubane are often complex and laborious. Therefore, chemists have made dedicated efforts to improve synthetic strategies, as described already. From the previous works and modified methods, it is possible to synthesize cage compounds in a kilogram scale nowadays. Numerous studies have been conducted on the bishomocubane moiety, due to the strain and rigidness associated with its skeleton. For the same reasons, reactions involving unusual rearrangements captivate considerable attention. Even though BHCs are highly rigid, at higher temperature or in the presence of light, BHC undergoes decomposition due to the strain of the molecule.

So far, cage compounds have not been used as ligands in natural product synthesis and as high-energy density materials. Our group has carried out detailed studies on the energetic properties of 1,3-bishomocubane and remains actively involved in further exploration of the potential of bishomocubane. As Eaton described cubane, the bishomocubane skeleton can also be seen as a kinetic rock but a thermodynamic powerhouse. The skeleton possesses untapped potential in the domain of asymmetric synthesis as well. In the future, the enantiopure derivatives of 1,3-bishomocubane (BHC) may find applications as catalysts, co-catalysts, or ligands in asymmetric synthesis. BHC and its derivatives can have many potential applications in propellent chemistry and in material and polymer chemistry owing to their unique physical and thermal properties. Since cubane has already been investigated as a benzene bioisostere, we envision the BHC molecule as a promising alternative and suggest the possibility of investigating and reporting more of its applications in future.

Data availability statement

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

INNN Thanks CSIR, ISRO and DRDO India for financial support at different times for research on cage compounds. AD thanks the Ministry of Education, Government of India for a Prime Minister's Research Fellowship (PMRF) and GLT thanks CSIR India for a Senior Research Fellowship. This article is dedicated to Professor Alan P. Marchand in honor of his pioneering contributions to polycyclic cage chemistry.

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Footnote

† Present address: Department of Chemistry, M. V. M. Science and Home Science College, Rajkot, Gujarat 360 005, India.

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