P.
Kovaříček‡
,
A. C.
Meister§
,
K.
Flídrová
,
R.
Cabot¶
,
K.
Kovaříčková||
and
J.-M.
Lehn
*
Institut de Science et d'Ingénierie Supramoléculaires, Université de Strasbourg, 8 allée Gaspard Monge, 67000 Strasbourg, France. E-mail: lehn@unistra.fr
First published on 10th February 2016
Competition among reagents in dynamic combinatorial libraries of increased complexity leads to reactional self-sorting (improved regioselectivity) in mixtures of aldehydes and oligoamines. High selectivity of a given library component is transferred to a different reacting component of low selectivity through a network of underlying equilibrating reactions which provide component exchange between all species. The selectivity of various carbonyl compounds in reactions with amines was also assessed towards the formation of defined sequences of residues along oligoamine chains. The approach was further exploited for defining selective dynamic protecting groups (DPGs), based on the reversible linkage between the substrate and the protecting group. They represent an intermediate approach between the conventional protecting groups and the protecting-group-free approach in organic synthesis. Removal of the protecting group is effected via dynamic exchange trapping by formation of a more stable product. The establishment of equilibrium eliminates the need for isolation and purification of the dynamically protected intermediate(s) and enables as well the selective sequential derivatisation of oligoamines. The DPG concept can be generalised to other reversible reactions and can thus represent a valuable alternative in the design of total synthesis of complex molecules.
Imines, resulting from the reversible condensation of a carbonyl component with an amine, are of particular interest for setting up DCLs, due to the ease of formation and exchange, usually under mild conditions. The heteroatomic imine bond is also satisfactorily orthogonal33–36 to many other reversible bonds and its dynamic nature can be “frozen” by reduction or other transformations.37 Simplification of the complex mixture of species in the library is achieved through adaptive sorting,38–41 for example in course of crystallisation,42–44 oxidation,45 distillation46,47 or coordination.48–52
Herewith we report the operation of a reactional self-sorting process in DCLs of imines of increasing complexity, driven by competition among the reagents in mixtures of aldehydes and oligoamines components and leading to improved selectivity in product formation and/or in site of reaction. The aldehydes in the studied DCLs are moderately selective in reactions with different amines to form various products such as imines, aminals or amino-lactones in the case of o-carboxybenzaldehyde.53,54 Competition between an increasing number of aldehydes for a given mixture of amines leads to increased selectivity of each of them. It leads to a simplification of the final composition resulting from the complexity of the DCL and its collapse into a system of reduced multiplicity (and lower entropy), amounting to a state that may be termed “simplexity” in a process of general type from chemical to biological systems.55,56 In fact, any recognition process, be it reactional or interactional (see Fig. 3 in ref. 57) results in the reduction of the complexity of a mixture, of its simplification, through the operation of competitive selection. On the supramolecular/interactional level it is expressed in the “instructed mixture paradigm”, whereby the behaviour of mixtures is driven by the instructions (molecular information) present in its members resulting in self-recognition (or self-sorting), a self-process,58 belonging to the general phenomenon of self-organization.9,59 The term “simplexity” does not carry the meaning that the system in itself is less complex, as there is in fact an increased complexity (increased number of reacting species) underlying and resulting in the simplification (increased selectivity) observed. We also propose a formal approach for the representation and comparison of DCLs (see below and ESI†) which has been used for evaluating the efficiency of selection in the self-sorting processes investigated here within the realm of dynamic covalent chemistry (DCC).1–5,28–30,57
At first, different aldehydes were reacted with different amines in order to test their differences in reactivity and the nature of the product formed. Salicylaldehyde (SALAL), pyridine-2-carboxaldehyde (PYRAL) and 2-carboxy-benzaldehyde (CAXAL) were selected as prototypical aldehydes. On the other hand, isopentylamine (IPA), N,N′-dimethyl-1,3-diaminopropane (Me2PDA) and piperidine (PIP) were selected as their amine counterparts. As solvent a mixture of d6-DMSO with 1% of D2O was used to guarantee solubility of all partners, in which the water addition maintains constant water content during the aldehyde–amine condensation. On the basis of previous studies, differences in reaction outcome was expected for the various aldehyde–amine pairs,53 anticipating that: (a) SALAL would have a preference for primary amines to form imines,53 (b) PYRAL would react with diamines to provide a cyclic aminal53 and (c) CAXAL would react with secondary amines to afford amino-lactones through trapping of the intermediary iminium by the neighbouring carboxylate group.54 These anticipated selectivities towards formation of different condensation products were verified in separate experiments: SALAL reacted with IPA to provide the expected imine, PYRAL gave the six-membered cyclic aminal with Me2PDA, and CAXAL indeed reacted with PIP to form the amino-lactone. These reactions proceeded with quantitative conversion and the resulting preferentially formed products may be considered as “matching pairs”, effecting reactional recognition, like supramolecular systems effect interactional recognition (Fig. 1).4,57
Checking first the combinations opposite to the “matching pairs” (see above) showed that both aldehydes reacted with both amines: when CAXAL was mixed with 1 eq. of IPA, equilibrium was reached almost instantaneously giving a dynamic mixture of the imine and lactone formed with IPA (details in ESI, Section 3.2†); on the other hand, SALAL reacted with PIP in approximately 30% conversion to the aminal formed from two amine molecules and one aldehyde. In subsequent experiments, the propensity towards the formation of the “matching pair” product was tested in presence of a competing amine, i.e. each aldehyde was reacted with an equimolar mixture of the two amines. SALAL showed a very high preference for imine formation reacting solely with IPA and leaving PIP unreacted in the solution. However, the conversion of the aldehyde in this case was not complete, approximately 10% remaining unreacted. In contrast, CAXAL under the same reaction conditions provided two products: the lactone on the PIP ring was formed in about 39% yield and the dynamic imine–lactone product from IPA was present in about 61% yield (details in the ESI, Section 3.2†). Finally, the full [2 × 2] library was considered: SALAL and CAXAL were mixed with both IPA and PIP in equimolar amounts and the resulting product mixture was examined using NMR spectroscopy. Equilibrium was reached after 5 hours and the mixture contained only the two “matching pair” species, the imine SALAL–IPA and the lactone CAXAL–PIP, both in quantitative conversion.
This remarkable simplicity achieved in the comparatively complex mixture, contrasts with the ability of both aldehydes to react with both amines, and vice versa each amine with the two aldehydes. It demonstrates the occurrence of a selectivity enhancement of a poor selector by competition with a strong selector. Specifically, a species reacting non-selectively with several compounds is brought to become selective towards a given product, when put in presence of a competing species of high selectivity which monopolises one of the components. The process amounts to a competitive selectivity enhancement. The formation of the matching pair products can also be seen as resulting from agonist amplification9,57 by competition via component exchange through an underlying network of equilibrating reactions (Fig. 2).
Stoichiometry is of course a crucial parameter providing the enhanced selection of the inherent self-sorting process. In the present case, the components react in 1:1 ratio to form the preferred product. Therefore, the initial mixture must contain the components in this ratio so that the condensation of the matching pair in quantitative conversion depletes all of the starting materials, which in turn do not interfere with the formation of the second matching pair in the complex system. This is clearly demonstrated by the fact that whereas the aforementioned mixture SALAL–CAXAL–IPA–PIP can give, in principle, 14 products (accounting for 2 imines, 4 homo- and 2 heteroaminals, 4 hemiaminals and 2 lactones), only two products are observed in the mixture containing all four species in equimolar amounts.
As the final concentration of species can play a role in the selection process,60 we repeated the library experiments at several concentrations: 4, 10, 20, 60 and 100 mM of the components. The equilibrium composition of the library in this range was constant showing no concentration dependency.
The library was also assessed for the effect of pH. To this end, the pre-equilibrated library was examined on addition up to 4 equivalents of acid (MeSO3H) or up to 2 eq. of base (t-BuOK). Addition of 1 eq. of acid leads to almost complete hydrolysis of the aminals of both SALAL and PYRAL, while on addition of the second equivalent of MeSO3H the imine hydrolysis reached 60%. Continued titration by acid led to complete hydrolysis of the imines and extensive hydrolysis of the CAXAL derived lactone, finally giving only the hydrolysed products, after addition of 4 equivalents of acid. In contrast, upon titration by base, disappearance of the lactone signals was observed. Interestingly, this lactone depletion was not accompanied by the emergence of the aldehyde peak of CAXAL, but rather of a new imine of CAXAL with IPA and liberation of SALAL was observed. This example clearly displays the operation of the underlying network of interconnected equilibrating reactions which leads to the complex adaptation of the dynamic library to a stimulus.
The selectivity enhancement displayed here for [n × n] dynamic libraries represents a process of competitive selection, whereby simplification results from competition within the set of equilibrating constituents undergoing component exchange. This behaviour relates also to the process of co-evolution in a dynamic library, leading to the synergistic expression of given constituents.9,61
We have developed a formal method for quantification of selection in dynamic combinatorial libraries and especially for (self)-sorting processes which involve coupled reactions. It is based on a matrix representation of the combinatorial library including the particular case of the selection induced by increased complexity of the mixture (see ESI, Section 2,† for detailed description).
When SALAL was reacted with IPA (20 mM, buffered d6-DMSO, see ESI, Section 3,† for details) the formation of the imine followed second order kinetics with the rate constant of 0.01 M−1 s−1 and 90% overall conversion to the imine (note that in the reaction without a buffer the conversion was quantitative). When the experiment was repeated in presence of 1 eq. of CAXAL, significantly slower rate (k = 0.001 M−1 s−1) and only 77% conversion of SALAL was observed (Chart 1), showing that competition of the two aldehydes for one amine affects both the equilibrium (conversion) and the kinetics (rate of formation). Finally, when the experiment was performed in presence of 1 eq. of both CAXAL and PIP, forming the corresponding lactone, the original SALAL conversion of 90% was restored and the rate of formation was even accelerated (k = 0.03 M−1 s−1) due probably to the catalytic effect of the secondary amine.64 Thus, as expected, on the way towards the simplexity state, the evolution of the fractions of the different library constituents depends on the kinetics until equilibrium has been reached.
In the simplest case of N-benzylethylenediamine (BnEDA), the chain has one primary and one secondary nitrogen, expected to represent the reactivity of both IPA and PIP respectively. When an equimolar mixture of SALAL and CAXAL was reacted with 1 eq. BnEDA the dominant species in the solution (70%) was the expected imine–lactone: the imine of SALAL formed on the primary nitrogen and the lactone of CAXAL closed on the secondary nitrogen (Fig. 5, see ESI, Section 3.5,† for details). This predominant formation of a single product is remarkable given the fact that the mixture can in principle generate a large number of other species.
With extended chains with three nitrogen sites, such as diethylenetriamine (en2N3) or bis(3-aminopropyl)amine (pr2N3), selective formation of aminals with PYRAL can be assessed. Closure of the aminal ring bridges two nitrogen sites, terminal and central, while the third one, the other primary amine, is available for imine formation. Thus, when either of the triamines was reacted with 2 eq. of PYRAL, the imine–aminal was the major product formed. In contrast, when the triamines were reacted with 2 eq. of SALAL, the terminal bis-imine was the only product. Remarkably, reaction of the two aldehydes mixed in 1:1 ratio with the two triamines, resulted in selective formation of the expected products, the SALAL-imine and PYRAL-aminal, with respectively 77% and 84% conversion for en2N3 and pr2N3 (Fig. 6).
The non-symmetric triamines en-prN3 and spermidine were also examined in the reaction with the two aldehydes. When en-prN3 was reacted with 1 eq. of PYRAL and 1 eq. of SALAL the desired imine–aminal species with five membered aminal ring formed in 85% conversion, with trace amounts of six-membered isomer. When spermidine was reacted under the same conditions, only one size of the aminal ring can be formed as seven-membered rings are much more difficult to form compared to the six-membered ones, and indeed the NMR spectrum revealed that the six-membered PYRAL aminal bearing the SALAL-imine on the C4-arm was formed in overall conversion of 71% (Fig. 7a, see ESI, Section 3.5,† for details).
In a further extension to four amino sites, the reaction of triethylenetetramine (en3N4) with 2 equivalents of SALAL gave a complex mixture of products: four different imine signals (three sharp and one broad) and three aminal peaks were observed in the NMR spectrum, corresponding to all possible combinations of imine–aminal structures formed in an essentially statistical fashion. However, when 1 equivalent of PYRAL was added to the mixture, preferential formation of its aminal on the middle secondary amino groups enhanced the formation of the imines of SALAL at the extremities leading to an equilibrium conversion of about 60% (Fig. 7c).
These experiments clearly demonstrate that despite the ambiguous reactivity of both the amine sites and the aldehydes (imine–aminal-lactone equilibria), the system of higher complexity involving all components led to a pronounced competition-enhanced selectivity towards a preferred species, as compared to less complex mixtures. These results provide a remarkable illustration of selectivity amplification, i.e. simplification induced by an increase in complexity.
Amine protection with carbonyl compounds enables C-alkylations,73–76 O-alkylations77,78 or CC double bond dihydroxylation.79 As protecting aldehydes, salicylaldehyde80–83 and formaldehyde84–86 have been used for selective imine or aminal formation respectively followed by the typical acidic hydrolysis work up.73–76 Deprotection can be effected by transimination on application of hydrazides87 or hydroxylamine derivatives.82,88
The dynamic nature of the reversible imine bond forming reaction gives the opportunity for facile protecting group removal in conditions which do not affect acylated amines or similar derivatives. Imine hydrolysis under acidic conditions can be replaced by much milder thermodynamically driven imine exchange. To this end, addition of a hydrazine, a hydrazide or an alkoxyamine leads to cleavage of the imine to give a hydrazone, an acylhydrazone or an oxime (respectively) of the carbonyl partner in quantitative conversion with liberation of the free primary amino group. The double implementation of the features of DCC gives the advantage to perform the three-step reaction sequence, protection–derivatisation–deprotection, in a “one pot” fashion without the need for isolation and purification of the intermediates (Fig. 8). A number of such processes have been explored (Fig. 8; Table 1). To be successful, this selective DPG approach requires of course high imine formation, high transimination on deprotection, as well as stability of the imine in the conditions used for the derivatization. Comprehensive method optimisation, synthetic details and full characterisation of all products are provided in the ESI, Section 4.† In the following text, the reported yields represent isolated amount of pure products.
Ref. | Electrophile 1 | Electrophile 2 | Solvent | Deprotecting agent | Yield |
---|---|---|---|---|---|
A1 | Cbz-GlyONp | — | CH3CN | BnONH3Cl | 82% |
A2 | pNO2PhCOCl | — | CH3CN | BnONH3Cl | 69% |
A3 | Ph2NCOCl | — | CH3CN | PhCONHNH2 | 70% |
A4 | PhNCO | — | CH3CN | BnONH3Cl | 86% |
B1 | pNO2PhCOCl | Cbz-GlyONp | CH3CN | PhCONHNH2 | 78% |
B2 | Cbz-GlyONp | Boc-LeuOSu | CH3CN | PhCONHNH2 | 83% |
B3 | Cbz-GlyONp | Boc-AlaOSu | CH3CN | PhCONHNH2 | 82% |
B4 | Ph2NCOCl | PhCOCl | EtOH | BnONH3Cl | 75% |
B5 | CbzCl | PhNCO | CH3CN | PhCONHNH2 | 71% |
B6 | PhNCO | CbzCl | EtOH | BnONH3Cl | 82% |
In the reaction of MeDAP with the p-nitrophenyl activated ester of protected glycine (Cbz-GlyONp) using dynamic protection by 1 eq. SALAL, the N-Cbz-glycyl substituent was introduced on the secondary amine, giving isolated yield of 82% for the full three-step one-pot reaction sequence. Importantly, when the reaction was performed without the addition of SALAL, it afforded 85% isolated yield of a product consisting of a mixture of regioisomers due to unsufficient difference in reactivity of the two nitrogen atoms. The regioisomers were present in 2:1 ratio in favour of the product of acylation on the secondary amine. The product of double acylation of MeDAP was isolated as well in about 6% yield. We explored the versatility of the protocol by varying the reagents, the solvent and also the nature of the deprotecting agent. The results are summarized in Table 1 (entries A1–A4). Good isolated yields in the range of 69–86% of the desired products were obtained after the complete three-step reaction sequence.
The operation of DCC under thermodynamic control has been exploited here for the protection and deprotection steps. The establishment of equilibrium offers the possibility to perform sequences of reactions in systems of increasing complexity. Thus, the equilibrium mixtures in the previous experiments before isolation of products consist of the oxime or acylhydrazone of SALAL together with a MeDAP derivative displaying a free primary amino group (complete conversion in deprotection). We have therefore investigated the possibility to perform a controlled sequential derivatisation in a “one pot” fashion (Fig. 8), in which the imine formation by SALAL directs the sequence of derivatization to give products bearing the residue introduced first on the secondary amine group and the second one on the primary amine site. The reaction was repeated for various combinations of electrophiles, in two solvents and with two deprotecting agents giving yields of the desired products in the range of 71–83% (92–95% per step, Table 1 entries B1–B6). In contrast, when the reaction was performed in absence of the SALAL-imine protecting group using N,N-diphenylcarbamoyl chloride and benzoyl chloride as electrophiles, a mixture of all four possible products was obtained in yields from 16 to 34% (details in the ESI, Section 4.1.1.9†).
When PYRAL was mixed with MeDAP, efficient formation of the aminal within 2 hours was confirmed by NMR. The equilibrated solution was then treated with phenyl isocyanate forming the urea moiety and the protecting group was removed by the exchange reaction with O-benzylhydroxylamine which converts the aminal of PYRAL completely to the corresponding oxime. The resulting product C1 was isolated in 74% yield, accompanied with 2% of the opposite regioisomer (Table 2). It is thus possible to perform a double derivatisation of the diamine in a “one pot” fashion by addition of a second electrophile after deprotection. To this end, we have used phenyl isocyanate as the first electrophile and the activated ester of N-protected phenylalanine or benzyl chloroformate in the second acylation (isolated yields 76 and 74%, respectively), demonstrating the potential of the approach implementing dynamic protecting groups and mild deprotection procedures compared to conventional protecting groups used in peptide synthesis.
The reaction employing PYRAL protection via aminal formation was studied with several different electrophiles used in the first acylation, but the isolated product always consisted of the diamine derivatised on the secondary amine (due to aminal–imine equilibrium, see above). Even after extensive optimisation of reaction conditions (details in the ESI, Section 4.1.3†) and replacement of PYRAL with formaldehyde, reported in the literature as aminal forming reagent,84,85 the terminal regioisomer was only obtained with phenyl isocyanate. Literature reports describe the use of aminal forming aldehyde to drive the selectivity of the Michael addition of acrylonitrile to polyamines.85,90 In this vein, MeDAP was reacted first with 1 eq. of PYRAL (or SALAL) and then 1 eq. of acrylonitrile was added. Formation of the product of Michael addition (Fig. 10a) was followed by NMR (at r.t. in d4-methanol) revealing that the reaction time in presence of PYRAL was much longer (35% conversion after 24 hours) than in the case of SALAL protected version (quantitative in 24 hours). Moreover, while SALAL drove the reaction with full selectivity for the addition on the secondary nitrogen, the reaction mixture with PYRAL consisted of both regioisomers in ratio 1:2.5 in favour of the same product as with SALAL (due to the aminal–imine dynamic interconversion). When the acrylonitrile Michael addition was repeated without any protecting aldehyde, again a mixture of products was obtained, but in this case favouring the addition on the primary amine. This finding indicates kinetic resolution between primary and secondary amines in the acrylonitrile Michael addition, which was supported by the reaction of 1,3-diaminopropane with 2 eq. of acrylonitrile providing a quantitative yield of the N,N′-bis(2-cyanoethyl)-1,3-diaminopropane (Fig. 10b, details in the ESI, Section 4.4†).91 In conclusion, the PYRAL protection is a suitable approach in the case of highly reactive isocyanate species, presumably because it reacts faster than is the rate of intramolecular aminal–imine interconversion. In the case of acyl chlorides this equilibrium, although strongly shifted towards the aminal, leads to non-selective derivatisation of both amino groups. Acrylonitrile, reacts preferentially with primary amines and SALAL protection is needed to achieve the selective reaction on the secondary nitrogen.
Finally, the sequential derivatisation shown above for diamines was also investigated with the biologically relevant triamine spermidine. In this case, the reaction sequence started with protection by 1 eq. of PYRAL forming the aminal and thus leaving only one primary amino group available for the subsequent reaction with the activated ester of an amino acid (Cbz-GlyONp). The protecting group was removed by exchange reaction with benzyloxyamine and without isolation, the subsequent protection by 1 eq. of SALAL was introduced. The selective imine formation at the other terminal NH2 group of the starting polyamine left the central secondary amine free, thus allowing for selective derivatisation in the middle of the chain by benzyl chloroformate. Deprotection was again performed by imine exchange with benzyloxyamine and the last remaining nitrogen atom was thereafter derivatized using p-nitrobenzoyl chloride (Fig. 12). This 7-step, one-pot reaction sequence performed at r.t. under ambient atmosphere afforded after a single purification step the desired triply derivatized product in 31% yield (85% calc. per step), demonstrating the potential of dynamic selective protecting groups in organic synthesis.
The presented strategy of selective DPGs showed remarkable versatility when SALAL was used as the protecting agent. On the other hand, inverted sequence of derivatisation by employing PYRAL was limited to cases in which the acylating reagent reacts faster than the rate of equilibration of the protecting group between several species. These two approaches are thus complementary and when combined, provide a powerful tool to perform selective and/or sequential functionalization of polyamines.
The selective DPG concept has been exploited towards the regioselective derivatization of various amines with different reagents ranging from isocyanates to activated esters of amino acids. The establishment of equilibrium in the reaction of the substrate with the protecting group as well as in the deprotection step eliminates the need for isolation and purification of intermediates and allows to run reactions in “one-pot” fashion. Furthermore, the thermodynamic control over the reversible condensations opens the possibility to perform sequences or networks of reactions in systems of increasing complexity, as it was shown in the case of selective sequential derivatisation of polyamines. Exploitation of other reversibly formed species, such as acetals or boroxines, can provide useful alternatives in total synthesis of complex products. Altogether the present work represents an extension of the thermodynamically driven DCC, implementing dynamic organic reactivity, into the traditionally kinetically governed realm of organic synthesis and provides ground for further exploration of its potential.
Finally, in a broad perspective, the data presented provide a remarkable illustration of simplification within a given constitutional dynamic library, induced by an increase in complexity, which leads to enhanced competition within the set of equilibrating constituents undergoing component exchange via a network of interconnected reactions engaged in agonistic and antagonistic relationships with feedback loops. In general terms, higher complexity results in simplification through competition.
Footnotes |
† Electronic supplementary information (ESI) available: Experimental details, NMR spectra, synthetic protocols and mathematical model for formal dynamic combinatorial library description. See DOI: 10.1039/c5sc04924e |
‡ Current address: Department of Low-dimensional Systems, J. Heyrovsky Institute of Physical Chemistry, Dolejškova 2155/3, 18223 Prague 8, Czech Republic. |
§ Current address: Merck KGaA, Frankfurter Straße 250, 64293 Darmstadt, Germany. |
¶ Current address: Department of Chemistry, University of Cambridge, Lensfield Road, CB2 1EW Cambridge, United Kingdom. |
|| Current address: Institute of Chemical Technology Prague, Department of Research and Development, Technická 5, 160 00 Prague, Czech Republic. |
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