Open Access Article
Arianna
Brandolese
ab,
Mark D.
Greenhalgh
ac,
Titouan
Desrues
d,
Xueyang
Liu
d,
Shen
Qu
a,
Cyril
Bressy
*d and
Andrew D.
Smith
*a
aEaStCHEM, School of Chemistry, University of St Andrews, St Andrews, Fife KY16 9ST, UK. E-mail: ads10@st-andrews.ac.uk
bDepartment of Chemical and Pharmaceutical Sciences, University of Ferrara, Via L. Borsari, 46, 44121 Ferrara, Italy
cDepartment of Chemistry, University of Warwick, Coventry, CV4 7AL, UK
dAix Marseille Univ, CNRS, Centrale Marseille, iSm2 Marseille, France. E-mail: cyril.bressy@univ-amu.fr
First published on 8th March 2021
The sequential acylative kinetic resolution (KR) of C2-symmetric (±)-1,2-syn and (±)-1,3-anti-diols using a packed bed microreactor loaded with the polystyrene-supported isothiourea, HyperBTM, is demonstrated in flow. The sequential KRs of C2-symmetric (±)-1,2-syn and (±)-1,3-anti-diols exploits Horeau amplification, with each composed of two successive KR processes, with each substrate class significantly differing in the relative rate constants for each KR process. Optimisation of the continuous flow set-up for both C2-symmetric (±)-1,2-syn and (±)-1,3-anti-diol substrate classes allowed isolation of reaction products in both high enantiopurity and yield. In addition to the successful KR of C2-symmetric (±)-1,2-syn and (±)-1,3-anti-diols, the application of this process to the more conceptually-complex scenario involving the sequential KR of C1-symmetric (±)-1,3-anti-diols was demonstrated, which involves eight independent rate constants.
A representative example of this powerful amplification process is the acylative double catalytic kinetic resolution (DoCKR) of C2-symmetric (±)-1,2-syn and (±)-1,3-anti-diols (Fig. 1).5–8 Due to the bis-alcohol functionality within the substrates, two KR processes can be in operation and are represented by the rate constants k1 and k2 for the first KR, and k3 and k4 for the second KR. If k1 > k2 and k3 > k4, the enantiomer of monoester preferentially generated in the 1st KR is also preferentially acylated in the 2nd KR (so called additive Horeau amplification). In principle, this allows the generation of both the diester (from two successive KRs) and diol in high enantiopurity and with opposite absolute configuration (shown in the blue and red boxes in Fig. 1). The enantiopurity and absolute configuration of the monoester product will be highly dependent upon reaction conversion, with its configuration matching that of the diester at low conversion, and the diol at high conversion. By necessity, there will be an inflection point between these two extremes where the monoester is racemic. In addition, the product distribution and hence relative yields of diester, diol and monoester obtained in such a sequential KR process will be dependent upon the relative magnitude of the combined rate constants for the 1st KR process relative to the 2nd KR process. If both KR processes are reasonably selective, then this can be simplified as the relationship between the largest rate constant for each KR step (i.e. in the example in Fig. 1, k1vs. k3).
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| Fig. 1 Horeau amplification in the sequential double acylative KR of C2-symmetric (±)-diols, where k1 > k2 and k3 > k4. | ||
Despite the potential to harness additive Horeau amplification in these processes, the majority of examples of catalytic acylative KR of diols are selective for formation of the monoester product,5,6 and hence do not benefit from amplification of product enantiopurity imparted by the 2nd KR process. To date, only work from our research groups has demonstrated this concept, through the use of isothiourea catalysis in the sequential KR of (±)-1,3-anti and (±)-1,2-syn diols (Scheme 1).7,8 The sequential KR of (±)-1,3-anti-diol 2a was achieved using (2R,3S)-HyperBTM 1 as catalyst (1 mol%) and propionic anhydride as acylating agent (1.05 equiv.), to give (R,R)-diol 2a (41% yield, >99
:
1 er), (R,R)-monoester 3a (7% yield, 66
:
34 er) and (S,S)-diester 4a (41% yield, 98
:
2 er). This process was shown to exhibit Horeau amplification, where both KR steps displayed the same sense of enantiodiscrimination.7 Based on the product distribution, it can be estimated that the relative rates of each KR process are close to parity (i.e. k1 ≈ k3). The sequential KR of (±)-1,2-syn-diol 5a was also shown to satisfy the conditions of Horeau amplification; however the relative rates of the two KR steps were significantly different, with the 1st KR being ∼8 times that of the 2nd KR.8 The significantly smaller rate constant for the second acylation can presumably be attributed to the increased steric hindrance associated with the α-ester substituent introduced following the first acylation event. This scenario makes it impossible to isolate diol and diester in both high yield and enantiopurity, and necessitates the sequential KR being driven to higher conversion to access highly enantiomerically enriched material. As such, treatment of (±)-1,2-syn-diol 5a with (2S,3R)-HyperBTM (ent)-1 (1 mol%) and isobutyric anhydride (1.5 equiv.) in CHCl3 gave (R,R)-diol 5a (8% yield, >99
:
1 er), (R,R)-monoester 6a (31% yield, >99
:
1 er) and (S,S)-diester 7a (50% yield, 97
:
3 er).8 In this case, the diol and monoester can be combined to give the (R,R)-enantiomer in 39% yield and >99
:
1 er. These two sequential KR processes, although related, demonstrate the complexity of these systems, and how modulation of reaction conversion can be used to maximise both the yield and enantiopurity of the various reaction products.
One approach to improve the sustainability of organocatalytic transformations is the immobilisation of the organocatalyst on a heterogeneous polymer support.9,10 In order to off-set the additional time and expense associated with synthesis, the supported catalyst must be readily recovered and recycled multiple times without loss in activity. The use of a packed bed reactor and application in a continuous flow set-up is a particularly-attractive technological solution that significantly simplifies catalyst recovery.9e–h The use of polymer-supported isothioureas as recyclable enantioselective catalysts was first demonstrated by Pericàs in formal cycloaddition processes.11 Further collaborative work between Pericàs and Smith has established the use of polymer-supported isothioureas for the KR of secondary alcohols, tertiary alcohols and BINOL derivatives in batch and flow.12 Building on these precedents, this manuscript investigates the possibility of performing the sequential KR of C2-symmetric (±)-1,2-syn- and (±)-1,3-anti-diols, as well as C1-symmetric (±)-1,3-anti-diols, in flow13 using a polymer-supported variant of HyperBTM.
:
6 er, along with the (S,S)-enantiomer of both diol 5a and monoester 6a in >99
:
1 er and 98
:
2 er, respectively, and a combined yield of 38% (entry 5).
| Entry | (±)-5a [M] | (i-PrCO)2O [M] | i-Pr2Net [M] | Product ratio (5a : 6a : 7a)b |
5a er (S,S) : (R,R)c |
6a er (S,S) : (R,R)c |
7a er (R,R) : (S,S)c |
|---|---|---|---|---|---|---|---|
| a Conditions: (±)-5a (0.5 mmol), r.t., flow rate 0.1 mL min−1. b Determined by 1H NMR spectroscopic analysis of the crude reaction product mixture. c Determined by CSP-HPLC analysis. d Diol and anhydride in one syringe, i-Pr2NEt in the other. e 2.0 mmol scale. f Isolated yield. | |||||||
| 1 | 0.2 | 0.3 | 0.32 | 17 : 39 : 44 |
99 : 1 |
85 : 15 |
95 : 5 |
| 2 | 0.1 | 0.15 | 0.16 | 10 : 44 : 46 |
>99 : 1 |
75 : 25 |
94 : 6 |
| 3 | 0.2 | 0.4 | 0.44 | 1 : 40 : 59 |
— | >99 : 1 |
75 : 25 |
| 4 | 0.2 | 0.35 | 0.35 | 7 : 41 : 52 |
99 : 1 |
97 : 3 |
90 : 10 |
| 5 , | 0.2 | 0.35 | 0.35 |
7 : 40 : 53
|
>99 : 1 (3%)
|
98 : 2 (35%)
|
94 : 6 (48%)
|
Using the conditions optimised for diol 5a, the generality of this sequential KR process in flow was investigated using a selection of electronically- and sterically-differentiated (±)-1,2-diols (Scheme 2). (±)-1,2-Diarylethane-1,2-diol derivatives 5b–5d were all resolved with excellent selectivity with results comparable with those obtained with the homogeneous catalyst.8 The sequential KR of p-methoxy-substituted diol 5d provided products with higher enantioenrichment than substrates 5b and 5c bearing electron-withdrawing p-chloro or p-trifluoromethyl substituents. These results are consistent with the KR of simple benzylic alcohols using isothiourea catalysts, where the presence of an electron-rich aromatic substituent provides enhanced stabilisation of the acylation transition state through π–cation interactions.17 The method was also extended to naphthyl-substituted diol 5e, however low solubility of 5e in CHCl3 required the use of a different solvent system (1
:
1 THF
:
CHCl3). Under these conditions, diol 5e and diester 7e were recovered with high enantioenrichment (97
:
3 er and 94
:
6 er, respectively); while monoester 6e was obtained with only moderate enantioenrichment (70
:
30 er). This can be attributed to slightly lower conversion being observed, presumably reflecting the reduced swelling of the catalyst resin in 1
:
1 THF
:
CHCl3 and consequently leading to a lower residence time. Finally, the KR of (±)-1,2-diols 5f and 5g bearing alternative π-donor systems (alkene and alkyne) was also performed. Only moderate selectivity, coupled with higher conversions were obtained in both cases, consistent with the sequential KR of these substrates using the homogenous catalyst.8 These results are also consistent with the observation that the isothiourea-catalysed KR of simple allylic and propargylic alcohols is generally less selective than the KR of benzylic alcohols.17
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Scheme 2 Reaction scope for C2-symmetric (±)-syn-1,2-diols. a Ratio determined by 1H NMR spectroscopic analysis of the crude reaction product. b Solvent 1 : 1 THF : CHCl3. | ||
:
1 THF
:
CHCl3 at room temperature provided good solubility. To test the suitability of this solvent system for KR selectivity, control experiments were conducted using the homogenous catalyst, HyperBTM 1, in both CH2Cl2 and 1
:
1 THF
:
CHCl3 for the KR of 1,3-diol 2a at room temperature.16 Comparable conversion and product enantioenrichment was obtained and therefore process optimisation in flow was conducted using this mixed solvent system. A brief optimisation was conducted using 1,3-diol 2a, with the use of fewer equivalents of anhydride, lower reagent concentration and a higher flow rate, relative to the KR of 1,2-diols, found to be optimal.16
Using the optimised conditions, the sequential KR of (±)-anti-1,3-diols was explored (Scheme 3). In all cases, the (±)-anti-1,3-diols substrates were synthesised in >95
:
5 dr, with only trace amounts of the syn-diastereomer present. In keeping with the KR of these substrates using the homogenous catalyst,7 the diol and diester products were obtained in consistently good yield and enantioenrichment, whilst the monoester was isolated as a minor component with low enantioenrichment in each case. Also in keeping with the previous work,7 the monoester was generally isolated as a mixture of anti- and syn-diastereomers, whilst both diol and diester were obtained as essentially single diastereomers. This phenomenon results in the sequential KR enhancing the diastereopurity of the products, in addition to the anticipated separation of enantiomers, and can presumably be attributed to a small rate constant for the 2nd acylation for the syn-diastereomer.
Initially, the sequential KR process was applied to C2-symmetric 1,3-diols 2a–d, bearing a π-system adjacent to the reactive carbinol centres (Scheme 3). Allylic diol 2a was resolved with both diol and diester obtained with excellent enantioenrichment. This result provides some contrast to the KR of 1,2-diols, where the resolution of allylic and propargylic diols was significantly less selective. The KR of benzylic 1,3-diol 2b was achieved with even higher selectivities, again consistent with the usual observation that benzylic alcohols are resolved more selectively than allylic alcohols.17 Variation of the electronic nature of the aromatic substituents was next probed, with the introduction of either electron-donating or electron-withdrawing substituents (2c and 2d) providing diol and diester products with excellent enantioenrichment.
To probe this scenario, two C1-symmetric (±)-anti-1,3-diols, 2e and 2f, were applied in the sequential KR process with excellent selectivity. Diols 2e and 2f, and their corresponding diesters, 4e and 4f, were recovered in good yield and with high enantioenrichment. The corresponding monoesters 3e′ and 3e′′ and 3f′ and 3f′′ were all isolated with low er and with lower dr than the starting anti-1,3-diols. Of particular note, the two constitutional isomers of monoester obtained from each reaction (for example e′ and e′′) were determined to have opposite configuration: whilst monoester e′/f′ was enriched in the same enantiomeric series as the (R,R)-diol, the monoester e′′/f′′ was enriched in the same enantiomeric series as the (S,S)-diester. At low conversion both constitutional isomers of monoester would be enriched in the (S,S)-enantiomer, whilst at high conversion both constitutional isomers of monoester would be enriched in the (R,R)-enantiomer. The intriguing results displayed in Scheme 5 can therefore be explained as a product of the 8 independent rate constants outlined above and the reaction conversion, which is controlled by the concentration of reagents, catalyst bed loading and flow-rate. Overall these results conveniently highlight the kinetic complexity of this system, and embody many of the principles popularised by Horeau.
Finally, to provide further insight into these processes, and contrast the sequential KR of (±)-syn-1,2- and (±)-anti-1,3-diols, selectivity values (s)14 for the individual KR steps were calculated for each process (Scheme 6). The s value for the first KR step was evaluated in each case by using the conversion and er of the recovered diol. This calculation assumes the diol only participates in the first step, and that its er is unaffected by the KR of the monoester. Due to the high conversion required for the sequential KR of (±)-syn-1,2-diols, the s value calculated for the 1st KR would be very inaccurate and therefore the sequential KR was repeated using 1 equiv. of anhydride.16 The s value for the 2nd KR is more conveniently assessed by performing a simple KR of a racemic sample of monoester. For the sequential KR of (±)-syn-1,2-diols, the s values for the 1st and 2nd KR steps were calculated as 13 and 38, respectively, demonstrating the 2nd KR step is significantly more selective than the 1st KR. This trend is in keeping with the sequential KR process using the homogenous catalyst,8,12a albeit the magnitude of the s values in flow are slightly lower than observed in batch. For the KR of (±)-anti-1,3-diols, the opposite relationship between the two KR steps was observed, with the s values for the 1st and 2nd KR steps calculated as 12 and 7, respectively, demonstrating the 1st KR step is more selective. This subtle difference between the KR of 1,2- and 1,3-diols, in addition to the difference in product distribution obtained due to different overall rate constants for diol and monoester acylation, highlights how appreciation of Horeau's pioneering work and insight is essential for understanding these sequential KR processes. It is also notable that although none of the individual s values are particularly high (e.g. >50), the products are still obtained in highly enantioenriched form due to the operation of the additive Horeau amplification.
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| Scheme 6 Assessment and comparison of s values for individual KR steps for the sequential KR of both (±)-1,2- and (±)-1,3-diols. | ||
Footnote |
| † Electronic supplementary information (ESI) available: Characterisation and HPLC analyses. See DOI: 10.1039/d1ob00304f |
| This journal is © The Royal Society of Chemistry 2021 |