Vani
Singhania
a,
Margery
Cortes-Clerget
a,
Jade
Dussart-Gautheret
a,
Bhornrawin
Akkachairin
ab,
Julie
Yu
a,
Nnamdi
Akporji
a,
Fabrice
Gallou
c and
Bruce H.
Lipshutz
*a
aDepartment of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106, USA. E-mail: lipshutz@chem.ucsb.edu
bProgram on Chemical Biology, Chulabhorn Graduate Institute, Center of Excellence on Environmental Health and Toxicology (EHT), Ministry of Education, 54 Kamphaeng Phet 6, Laksi, Bangkok 10210, Thailand
cNovartis Pharma AG, CH-4057 Basel, Switzerland
First published on 27th December 2021
Esterification in an aqueous micellar medium is catalyzed by a commercially available lipase in the absence of any co-factors. The presence of only 2 wt% designer surfactant, TPGS-750-M, assists in a 100% selective enzymatic process in which only primary alcohols participate (in a 1:1 ratio with carboxylic acid). An unexpected finding is also disclosed where the simple additive, PhCF3 (1 equiv. vs. substrate), appears to significantly extend the scope of usable acid/alcohol combinations. Taken together, several chemo- and bio-catalyzed 1-pot, multi-step reactions can now be performed in water.
Given the importance of the ester linkage in so many industrial products, it seems remarkable that such a functional group has yet to be constructed via a straightforward biocatalytic-based esterification; that such green technology indicative of the options available to synthetic chemists in water, in the absence of any organic solvent, and in particular, dipolar aprotic choices including DMSO,15 has yet to be described. We now report that use of an economical, commercially available lipase can, indeed, catalyze esterification in water as the reaction medium, and do so using a 1:1 stoichiometric ratio of acid and alcohol. Moreover, these take place in the absence of any co-factor. The key to successful levels of conversion is the presence of the designer surfactant TPGS-750-M (DL-α-tocopherol methoxypolyethylene glycol succinate) in the reaction medium.16 Even further levels of conversion can be anticipated by the presence of a simple additive (1 equiv.), as disclosed herein. Applications of this new esterification, together with chemocatalysis, leads to unprecedented tandem, 1-pot processes illustrative of the potential for organic synthesis to be conducted in sequential fashion, all done in water.
Using these optimized conditions (i.e., 1:1 acid:alcohol, 30 °C), several esterification reactions were examined, catalyzed by lipase derived from Rhizomucor miehei (Scheme 1; commercially available; see the ESI†). Given an enzyme's typical preference for certain structural features associated with reaction partners, as witnessed with carboxylic acids, somewhat greater flexibility was observed in terms of the alcohol that participated in the esterification process. While products 3–16 reflect these enzymatic requirements, they are solely representative of the possibilities for lipases, perhaps in general, to effect esterification in water. Unexpectedly, while the presence of 2 wt% TPGS-750-M (i.e., 20 mg mL−1 of water) in the buffered aqueous medium proved beneficial in most cases, increasing the amount to either 4 or 6 wt%, unlike that observed with KRED,18 led to no further benefit. While these esterifications in water seem unexpected, the key to success may be the presence of the micelles. Hence, the water-insoluble product esters likely locate within their inner cores, which feature a purely hydrophobic pocket. Thus, opportunities for competitive hydrolysis by water are negated due to this “reservoir” effect.18
In attempts to extend these lipase-catalyzed esterification reactions to a broader range of heteroaromatic ring-containing partners (i.e., beyond those in 15 and 16; Scheme 1), educt 3-(2-thiophenyl)propionic acid (17; Fig. 1), the analog of 3-phenylpropionic acid (Scheme 1, R1 = Ph) was examined, along with phenethyl alcohol (18), which had successfully led to product 7. In the event, and notwithstanding the thiophene's identical distance from the reactive carboxyl site, esterification was completely shut down (Fig. 1; 0% yield). Unexpectedly, therefore, when acid 17 was admixed with either the phenyl- or p-bromophenylpropionic acid (see 20 in Scheme 2) and the same alcohol (18), the major ester formed contained the thiophene subunit, product 19! This finding initiated a search for an additive that, ultimately, enabled esterification of 17 to 19 in 72% that was otherwise completely inhibited (notwithstanding the presence of the surfactant). This phenomenon is unlikely to be a simple solvent effect, since common solvents like methylene chloride (no conversion), hexanes (25% yield), cyclohexane (29% yield) and toluene (50% yield) were comparatively ineffective.19 Also as shown in Fig. 1, among the additives investigated (A1–A6), trifluoromethylbenzene (PhCF3; A6) was selected given its effectiveness, commercial availability, non-chlorinated status, and attractive economics.
Scheme 2 Impact of PhCF3 on inverted substrate pair (i.e., vs. reaction pair in Fig. 1). |
The unpredictable yet very intriguing impact of the additive on the inverted combination of reaction partners (i.e., phenylpropanoic acid, 20 and 2-thiophenemethanol, 21) was also studied. In the event, similar results were found where, in the absence of additive, no conversion to ester 22 was observed. However, when PhCF3 (1 equiv.) was in the aqueous reaction medium, a remarkable 50% yield was obtained (Scheme 2).
Further studies were carried out on the effect induced by this additive (PhCF3) on the lipase derived from Rhizomucor miehei,20 focusing on several control reactions (Table 2). The highest yield was observed in the presence of one equivalent of additive (entry 3); using half this amount dramatically lowered the extent of conversion (entry 2). Using PhCF3 in excess (entry 4) led to a decrease in yield to 57%. Neither the micellar medium alone (entry 1) nor just the aqueous buffer (entry 5) is capable of mediating the intended reaction, a strong indication that acid 17 by itself, as observed previously (vide supra), is not an acceptable reaction partner for this lipase-mediated esterification.
Entry | Solventa | Additiveb (equiv.) | Yieldc (%) |
---|---|---|---|
a 1 mL of solvent used for 0.5 M global concentration. b One equiv. of this additive represents ca. 6% of the total reaction volume. c Isolated yield of 19. | |||
1 | 2 wt% TPGS-750-M/0.01 M buffer | — | 0 |
2 | 2 wt% TPGS-750-M/0.01 M buffer | PhCF3 (0.5) | 14 |
3 | 2 wt% TPGS-750-M/0.01 M buffer | PhCF 3 (1) | 72 |
4 | 2 wt% TPGS-750-M/0.01 M buffer | PhCF3 (2) | 57 |
5 | 0.01 M buffer (no surfactant) | PhCF3 (1) | 0 |
The remarkable impact of the combination of an additive A1–A6, together with the buffer and surfactant in the pot, was further investigated on other substrates 19, and 23–28 (Fig. 2). Thus, in addition to the initial discovery involving esterification to product 19, the isolated yield using PhCF3 of esters 23 and 27 increased from 0 to 36% and from 0 to 50%, respectively. And while more modest enhancements in yields were observed for products 24–26 and 28, the overall net positive trend using PhCF3 is clear.
Fig. 2 Impact of various additives (see Fig. 1) on lipase-catalysed esterification reactions. |
The manner in which PhCF3 influences enzymatic esterification may be due to a positive allosteric regulation, altering the enzymatic cavity. Typically, however, this phenomenon relies on far more functionalized molecules;21 indeed, the overall dramatic effect mediated by such a simple additive as PhCF3 appears to be unprecedented, perhaps suggesting that enzymatic modification leading to greater substrate tolerance may not require the types of modulators in current use. Another explanation may be the direct alteration of the entrance to the enzymatic pocket, thereby adding a variable element of “promiscuity” to the level of acceptance associated with its “natural” structural features.22 Yet another explanation is that this additive may be providing a hydrophobic layer that alters the extent of lid opening at the enzymatic site.23 While a more definitive analysis as to which role is operative awaits further scrutiny, these observations may be indicative of future discoveries perhaps applicable to other enzymatic arrays, thereby augmenting the already huge potential of bio-catalytic processes in organic synthesis.
Contributions by the surfactant and additive could be individually evaluated based on the “reservoir effect” first discovered in enzymatic ketone reductions (KREDs).18 This phenomenon was also observed in the lipase-catalyzed esterification between 3-phenylpropanoic acid 20 and (4-chlorophenyl)methanol 29 leading to ester 9 (Fig. 3). Thus, while esterification in only buffer did not exceed 5% after a 24 hour period (reflecting commonly observed enzymatic inhibition), the presence of only 2 wt% TPGS-750-M increased the level of conversion to 65%. The same reaction with added PhCF3 (A6, 1 equiv.) further raised the conversion to 73%. Noteworthy, however, was the dramatic increase in reaction rate, reaching 70% after only two hours.
The same lipase (Rhizomucor miehei) was also found to selectively catalyze esterification with primary alcohols to the complete exclusion of secondary alcohols. Hence, while valeric acid (1) was converted in the presence of 1-octanol (32) to product 33 (86%), identical treatment with 2-octanol (30) afforded none of the corresponding ester 31 (Scheme 3). This specificity prevails even when the alcohol functional groups are present within the same molecule. Thus, irrespective of the connectivity of the alcohol on a sp2 carbon (phenol, 35) or an sp3 carbon (secondary alcohol, 37), the primary alcohol is esterified exclusively. The same selective outcome favoring esterification of a primary alcohol is observed in the presence of PhCF3 (1 equiv.).
With water being the common denominator associated with chemo- and bio-catalysis, virtually unlimited opportunities are available for combining each in 1-pot sequences. The benefits to be realized from such a synthetic strategy include not only minimizing workups and thereby, waste creation, but also time invested (“time economy”),24 and processing (“pot economy”).25 Importantly, while maintaining global concentrations that typically range between 0.25 and 1 M,18 the sequence of reactions involving chemo- or bio-catalysis can be varied (vide infra).
For example, a sequence involving bio- followed by chemo-catalysis is shown in Scheme 4. Lipase-catalyzed esterification of 39 with 40 leads to product 41. Without isolation, and given the presence of the aryl bromide being amenable to a ppm Pd-catalyzed Suzuki–Miyaura coupling26 involving arylboronic acid 42, final product 43 is isolated in 82% overall yield. It is important to note the lack of competing hydrolysis of the intermediate ester after the increase of pH upon addition of base, presumably reflecting the preferred location of newly formed ester 41 within the lipophilic inner micellar cores, rather than in the basic aqueous medium.
These tandem, mixed bio- and chemo-catalysis sequences are not limited to two steps. As shown in Scheme 5, catalytic hydrogenation of cinnamic acid 44 in water in the presence of the Pd/C27 led to intermediate acid 20. Subsequent lipase-catalyzed esterification with alcohol 45 gave gem-dibromocyclopropane 46, which was then reduced by nickel nanoparticles28 to give product 47 in 65% overall isolated yield, all done, sequentially, in water.
Further extending the options for this approach to chemoenzymatic catalysis and, importantly, substantiating enzymatic compatibility, another 3-step sequence was carried out as illustrated in Scheme 6. In this case, an initial, traditional Pd-catalyzed Sonogashira coupling was performed29 specifically as a test of enzymatic compatibility with such high, indeed unsustainable amounts of both Pd and Cu.30 Thus, and notwithstanding these levels of metal present in the aqueous medium, initial product alcohol 50 was successfully subjected (after adjustment to pH = 2) to lipase-catalyzed esterification.31 The 1-pot reaction mixture containing ester 51 was subsequently adjusted to pH 7, after which addition of an alcohol dehydrogenase (ADH-101)18 provided secondary nonracemic alcohol 52 in 59% overall isolated yield (ee >99%).
An even more complex carboxylic acid containing an indole (53) was also amenable to a tandem, 1-pot process (Scheme 7). An initial biocatalytic esterification to 28, which benefited from the presence of PhCF3 (vide supra; Fig. 2), was followed (without isolation) by two consecutive chemocatalysis steps: a Pd-catalyzed Suzuki–Miyaura vinylation to 55, and thence olefin reduction27 to arrive at ester 56. It is worthy of note that the first, selective enzymatic step takes place without protecting group chemistry (of the indole), in further alignment with the 12 Principles of Green Chemistry.32,33
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1sc05660c |
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