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Catalytic Enantioselective House#Meinwald Rearrangement: Efficient Construction of All-Carbon Quaternary Stereocenters Dengke Ma, Chun-Bao Miao, and Jianwei Sun J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b07514 • Publication Date (Web): 20 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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Journal of the American Chemical Society

Catalytic Enantioselective House‒Meinwald Rearrangement: Efficient Construction of All-Carbon Quaternary Stereocenters Dengke Ma,† Chun-Bao Miao,‡ and Jianwei Sun†‡* †

Department of Chemistry and Shenzhen Research Institute, the Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, China; ‡ Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Changzhou, Jiangsu, China Supporting Information Placeholder ABSTRACT: A catalytic asymmetric House‒Meinwald rear-

Scheme 1. Asymmetric House‒Meinwald Rearrangement

rangement for the synthesis of both cyclic and acyclic ketones is disclosed. From readily accessible racemic tetrasubstituted epoxides, this approach provides efficient access to chiral ketones bearing α all-carbon quaternary stereocenters with high enantiocontrol. The observation of positive non-linear effects and nontrivial kinetic feature provided important insights into the mechanism.

Stereocontrolled construction of all-carbon quaternary stereocenters represents a longstanding challenge in organic synthesis.1 In particular, chiral ketones bearing an α-quaternary stereocenter are valuable synthetic intermediates.1,2 As a result, substantial efforts have been devoted to their synthesis, among which catalytic asymmetric α-arylation and α-alkylation of ketone enolates proved most prominent. However, some drawbacks still remain in this state-of-the-art. For example, to avoid regioselectivity issues, typically ketones with mono-enolizable α-positions or pre-defined enolates were used.1c,2 Moreover, the majority of current examples deal with cyclic ketones, and stereocontrol in these reactions benefits from the restricted conformation of tetrasubstituted enolates.1c,2 In addition, current strategies are incapable of generating α,α-diaryl quaternary stereocenters. In this context, herein we report a complementary approach by means of the House‒ Meinwald rearrangement for the efficient construction of both cyclic and acyclic ketones bearing an α,α-diaryl quaternary stereocenter. The House‒Meinwald rearrangement, first discovered in 1955, converts epoxides to aldehydes or ketones upon activation by acids, which provides efficient access to valuable α-substituted carbonyl compounds (Scheme 1a).3 However, despite its long history, this reaction has not been well-utilized for the synthesis of enantioenriched ketones/aldehydes. In 2006, Shi and co-workers reported an elegant chiral-transfer strategy from enantioenriched epoxides (Scheme 1b).4 In comparison, a catalyst-controlled approach would be more attractive.5 We envisioned that, upon activation by a chiral Brønsted acid, a racemic tetrasubstituted epoxide may undergo ring-opening to generate a carbocation bearing a vicinal hydroxy group, which may lead to semi-pinacol-type of rearrangement (Scheme 1c). The chiral counter anion is expected to induce chirality in the subsequent desymmetrizative alkyl shift, thus representing a catalytic asymmetric stereoconvergent variant.6 The tetrasubstituted epoxides can be easily obtained from cross-McMurry coupling followed by epoxidation.7

Our design requires the enantiodifferentiation of two sterically similar aryl groups on a carbocation intermediate, a known challenge in asymmetric catalysis.8 Epoxide 1a was used as the model substrate, in which a removable para-hydroxy group was used in one of the two aryl groups for electronic differentiation. Various chiral phosphoric acids were employed as potential catalysts.9 Gratifyingly, in the presence of 10 mol% of TRIP (A1), the reaction of 1a in DCM proceeded efficiently at 0 oC to provide α,αdiaryl cyclohexanone 2a as a single isomer, albeit with low enantioselectivity (Table 1, entry 1). Next, various chiral phosphoric acids were evaluated (see the SI for details). The results indicated that the spirocyclic bis(indane)-based acid C1 exhibited the best enantioselectivity (89% ee, entry 6). Then, different reaction parameters were tuned, including catalyst loading, reaction concentration, solvent, and additive. After considerable efforts, we were pleased to find that the reaction could give both excellent yield and enantioselectivity when run at 0.025 M concentration in mixed solvent PhCl/DCM (7:1) in the presence of 5 Å molecular sieves and only 1 mol% of catalyst (94% ee, full conversion, entry 8). The reaction was so efficient that it could also go to completion with 0.1 mol% of catalyst, although the enantioselectivity was slightly compromised (entry 9). It is worth noting that organocatalysis with such a high turn-over number is remarkable.10 With the optimized conditions established in entry 8 of Table 1, we next examined the reaction scope (Table 2). Various racemic tetrasubstituted epoxides were suitable for this rearrangement process, providing a diverse set of chiral cyclic ketones bearing α,α-diaryl quaternary stereocenters with high enantioselectivity. The product structure of 2k was confirmed by X-ray crystallography. It is notable that the presence of a hydroxy group in one of two aryl groups proved beneficial to override the influence of other functional groups, such as halogen, ester,

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Table 1. Evaluation of Reaction Conditionsa

Table 2. Scope for the Synthesis of Cyclic Ketonesa

a

Reaction condition: 1a (0.05 mmol), catalyst (10 mol%), 5Å MS (20 mg), and solvent (1 mL), 0 oC. b Clean conversion to the desired product was observed by 1HNMR analysis of the crude mixture using CH2Br2 as internal standard. Ee value was determined by HPLC with a chiral stationary phase. c Run with mixed solvent PhCl/DCM (7:1), c = 0.025 M. d Run with 1 mol% of catalyst, 12 h. e Run with 0.1 mol% of catalyst, 23 h. triflate, alkyne, aldehyde, and alkoxy groups. Chiral cyclooctanone 2q could also be formed. Finally, the high efficiency of this process was also demonstrated by gram-scale preparation of the corresponding products 2a and 2c. Next, we were interested in probing the capability of this protocol for acyclic ketones. Due to the more flexible conformation, such stereocontrol is expected to be more challenging.1a Indeed, epoxide 3a bearing two ethyl groups was found to have much lower reactivity than the cyclic counterpart 1a. A higher catalyst loading (10 mol%) and higher temperature (rt) as well as a longer reaction time (51 h) were needed (eq 1). While the desired ketone 4a was successfully obtained, unfortunately, the enantioselectivity was disappointingly low (39% ee). Nevertheless, we re-screened the catalysts and other conditions (see the SI for more details), which indicated that the BINOL-derived acid A3 could catalyze the same transformation in DCE with excellent enantioselectivity (92% ee, eq 1). We next examined the generality for the synthesis of other chiral acyclic ketones (Table 3). Indeed, this protocol proved efficient for the synthesis methyl and ethyl ketones (4a-f) with high enantioselectivity. However, further examination indicated that the same catalyst could not be simply extended for the ketones with longer alkyl chains. For example, with A3 as catalyst, the epoxide bearing two n-butyl groups led to 4g with only 36% ee, indicating that these acyclic cases were very sensitive to the alkyl chain length and further corroborating the challenge in generating acyclic quaternary stereocenters. After further optimization, we found that the chiral phosphoric acid A5 exhibited superior performance (89% ee for 4g). The cases with alkyl chloride and azide functioanilities were also highly enantioselective with this new protocol (97% ee for 4i-j). It is worth noting that these ketones may not be easily obtained by asymmetric α-arylation or alkylation reactions since

a Reaction conditions: 1 (0.4 mmol), (R)-C1 (1 mol%), 5Å MS (160 mg), PhCl/DCM (7:1, 16 mL), 0 oC. b Run with 10 mol% of (R)-C. c 0 oC → rt.

both α-positions are enolizable and the desired reactive α-position is sterically much more demanding than the undesired but more accessible α′ position, let alone the challenging enantiocontrol.

Table 3. Scope for the Synthesis of Acyclic Ketonesa

a Reaction scale: 3 (0.4 mmol), catalyst (10 mol%), 5Å MS (160 mg), DCE (8 mL).

A possible mechanism is depicted in Scheme 2. Upon acid activation, the epoxide undergoes regioselective opening to form benzylic cation I, which is paired with the chiral counter anion. This anion may also have a hydrogen-bonding interaction with the hydroxy group. This type of chiral ion pair may also have a pseudo resonance structure I' in the form of para-quinone methide (pQM).11 Although the chiral information in I is much closer to the reactive center, remote stereocontrol on p-QM is also feasible.11


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Subsequent semi-pinacol-type alkyl shift proceeds with enantiocontrol by either chiral counter anion (in I) or hydrogen bonding (in I').12 In the latter case (I'), the hydroxy group loses contact with the chiral catalyst component, which is hard to explain the observed high enantiocontrol. While water has been previously proposed to relay hydrogen bonding in this remote control, 13 the use of molecular sieves in our reaction excludes this possibility. Thus, we proposed a transition state involving multiple catalyst molecules to relay hydrogen bonding (e.g., II), thus forming a restricted conformation to benefit stereocontrol. We observed a positive non-linear relationship between the catalyst and product ee’s in the reaction of epoxide 3a (Figure 1). The chirality amplification may suggest that a higher-order catalyst aggregate is operative in the key transition state, consistent with II.14 Moreover, a kinetic study of this reaction indicated that the reaction exhibits an order of 1.6 in catalyst. While both I and II might simultaneously function, a clear interpretation of the kinetic result would need further studies. In contrast, in the case of the cyclic epoxide 1a, a linear relationship between the catalyst and product ee’s was observed (see the SI for details), suggesting that the stereodetermining transition state might be different from that with the acyclic cases. Finally, no kinetic resolution of substrate 3a was observed at partial conversion.

Scheme 2. Proposed Mechanism

The chiral ketone products could serve as precursors to a diverse array of chiral molecules containing all-carbon quaternary stereocenters (Scheme 3). For example, cyclohexanone 2a could be converted to oxime 5. The para-hydroxy group could be efficiently triflated (6a and 6c) and then easily converted to other functional groups (7‒9) or removed (10). The congested ketone functional group in 10 could also be converted to unsaturated enone 11, exocyclic olefin 12, and enol triflate 13 as well as internal olefin 14. Furthermore, a cross-aldol reaction between 10 and p-bromobenzaldehyde led to enone 15 efficiently. Finally, the cyclohexanone motif could condense with 2-aminobenzaldehyde to form tetrahydroacridine 16 with a quaternary stereocenter in the 4-position.15 It is noteworthy that in all these transformations, no loss of the high enantiopurity was observed.

Scheme 3. Product Transformations

Initial rate (mol L-1 min-1)

100 90 80 70 60 50 40 30 20 10 0

ee of 4a (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Journal of the American Chemical Society

0

20

40

60

ee of (S)-A3 (%)

80

100

0.00045 0.0004 0.00035 0.0003 0.00025 0.0002 0.00015 0.0001 0.00005 0

y = 1.02x1.61 R² = 0.99

0

0.002

0.004

0.006

0.008

Concentration of (S)-A3 (mol/L)

Figure 1. Left part: non-linear effects with 3a, (S)-A3, and 5Å MS, rt, DCE; Right part: kinetic study with 3a and (S)-A3, rt, CD2Cl2. To probe the role of the para-hydroxy group, we used the methylated analog 1a' for a control reaction. Under the standard conditions, only 39% ee was observed, implying the above p-QM intermediate plays an important role for stereocontrol (eq 2). However, 2a' could be obtained with good enantioselectivity with catalyst D. Similar results were observed with acyclic substrate 3a'. The previously optimized conditions for 3a resulted in only 28% ee, but the enantioselectivity could be improved to 72% ee with catalyst E (eq 3). In these cases, the ion pair (I') would be the key form for enantiocontrol.11j

(a) HONH2•HCl, NaHCO3, MeOH, reflux; (b) DMAP, Et3N, Tf2O, DCM, 0 oC ~ rt; (c) PdCl2, PPh3, Cs2CO3, vinyltrifluoroborate, THF/H2O (9:1), 80 oC; (d) Pd2(dba)3, Xantphos, DIPEA, thiophenol, dioxane, reflux; (e) Pd(dppf)Cl2, KOAc, bis(pinacolato)diboron, dioxane, reflux; (f) Pd(OAc)2, PPh3, Et3N, HCOOH, DMF, 65 oC; (g) IBX, DMSO, 85 oC; (h) MePPh3Br, tBuOK, THF, 60 oC; (i) KHMDS, Tf NPh, THF, ‒78 oC ~ rt; (j) 2 Pd(OAc)2, PPh3, Et3N, HCOOH, DMF, 65 oC; (k) KOH, 4bromobenzaldehyde, EtOH/H2O (5:1), rt; (l) RuCl2(PPh3)3, tBuOK, 2-aminobenzaldehyde, dioxane, 80 oC. In summary, we have developed a catalytic enantioconvergent House‒Meinwald rearrangement for the synthesis of both cyclic and acyclic ketones bearing all-carbon quaternary stereocenters.



By proper design of the tetrasubstituted epoxides and suitable choice of the catalysts, this reaction proceeded with broad scope, excellent efficiency and functional group compatibility as well as high regio- and stereocontrol under mild conditions (typically without cryogenic conditions). Low catalyst loading was also demonstrated. This reaction also represents an attractive complement to the existing strategies (e.g., α-arylation of ketones) for the synthesis of chiral ketones bearing α-quaternary stereocenters. The observation of positive non-linear effects and the nontrivial kinetic feature also provided important insights into the mechanism. The chiral products are useful precursors to a wide range of chiral molecules containing all-carbon quaternary stereocenters.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental procedures and compound characterization (PDF) X-ray data (CIF)

AUTHOR INFORMATION Corresponding Author [email protected].

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT Financial support was provided by Hong Kong RGC (16302617, 16302318, 16311616) and Shenzhen Science and Technology Innovation Committee (JCYJ20170818113708560, JCYJ20160229205441091).

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