Key words asymmetric catalysis - organocatalysis - enol silanes
Silicon-mediated organic synthesis has become an important subject that is gaining more attention in recent years.[1 ] In this context, silicon–hydrogen exchange reactions interconvert silylated (C−SiR3 or O−SiR3 ) and hydrogenated (C−H or O−H) compounds, exhibiting promising potential in asymmetric synthesis.[2 ] Catalyzed by chiral acids, such transformations can be used to access highly valuable enantiopure enol silanes. For example, we have recently shown that symmetric ketones can be desymmetrized using strong and confined imidodiphosphorimidate (IDPi) catalysts (Scheme [1a ]). Alternatively, racemic enol silanes can be formally hydrolyzed using the exact same catalyst, enabling access to the opposite enol silane enantiomer (Scheme [1b ]). We have also been successful at expanding the scope of such hydrolytic-type kinetic resolutions to racemic enol silanes derived from 2-substituted ketones. However, we have never applied our approach to a deprotosilylative kinetic resolution of α-branched ketones. We expected this reaction design to be somewhat challenging since Yamamoto and co-workers have previously shown that Lewis acid assisted Brønsted acids, in the absence of a stoichiometric silyl group acceptor, readily catalyze the isomerization of the kinetic enol silane to its corresponding thermodynamic, achiral isomer.[3 ] This reactivity has also been observed by Takasu and co-workers who have used Tf2 NH as catalyst for the same isomerization.[4 ]
Scheme 1 Asymmetric catalytic silicon–hydrogen exchange reactions. Applied in (a) a desymmetrization of achiral ketones, (b) a protodesilylative kinetic resolution of racemic enol silanes, and (c) a deprotosilylative kinetic resolution of racemic, 2-substituted ketones (this work).
Remarkably, we have now found that conditions can be developed that allow for a highly enantioselective kinetic resolution to occur in deprotosilylative reactions of 2-substituted cyclic ketones with tert -butyldimethyl(2-methylallyl)silane (Scheme [1c ]). Our method provides an alternative entry to enantioenriched enol silanes, which are of high value in several fundamental applications.[5 ]
[6 ]
[7 ]
Our studies commenced with the identification of a proper acid catalyst for the asymmetric silicon–hydrogen exchange reaction of 2-phenylcyclohexan-1-one (1a ) and methallylsilane 2 .[8 ] As reported in our previous work, while moderately acidic Brønsted acids, such as chiral phosphoric acids (CPA)[9 ] imidodiphosphates (IDP),[10 ] and disulfonimides (DSI)[11 ] were ineffective, the desired enol silane products were obtained under the catalysis of the much more acidic IDPi catalysts (see the Supporting Information, Table S1).[12 ]
[13 ] The thermodynamically favored enol silane 4a was observed to be the major product (3a :4a = 1:2) when the reaction was performed at 25 °C in toluene-d
8 using IDPi 5a (Scheme [2 ]). Replacement of the Tf substituent with a C6 F5 SO2 group gave catalyst 5b , which led to an even higher 3a :4a ratio of 1:5. We also investigated different aryl substituents at the 3,3′-positions of the binaphthyl backbone and, to our delight, the kinetically favored product 3a could indeed be obtained as the major regioisomer when catalyst IDPi 5c , bearing a 3-Ph-C6 H4 substituent, was employed. Further endeavors focused on modifying the inner core of the catalyst. For example, IDPi 5d possessing a C6 F5 SO2 group enabled formation of enol silane 3a with good regioselectivity and a promising enantioselectivity of 88:12. With our newly developed catalyst 5e bearing a 2-C10 F7 inner core substituent, the e.r. of the desired enol silane 3a could be further improved to 93:7 with a conversion of roughly 50%. Ultimately, beneficial effects on both regioselectivity and enantioselectivity were observed by decreasing the temperature to 0 °C, furnishing 3a in 56:1 r.r. and 96:4 e.r.. Gratifyingly, ketone 1a can be recovered in 94:6 e.r. with high selectivity (s ).
Scheme 2 Reaction development. Reactions were conducted with rac -1a (0.05 mmol), methallyl-TBS agent 2 (2.0 equiv.), and catalyst 5a –5e (1.0 mol%) in toluene (0.1 M) at indicated temperature. a Conversions were determined by GC analysis, calibrating with 1,3,5-trimethoxybenzene as internal standard. b The regioisomeric ratio (r.r. = 3a :4a ) was determined by 1 H NMR analysis. c The enantiomeric ratio (e.r.) was determined by HPLC analysis. d
s = ln[(1 − conv.)(1 – ee
1a
)] / ln[(1 − conv.)(1 + ee
1a
)]. e Reactions in toluene-d
8 monitored by 1 H NMR.
Under these optimized reaction conditions, we next explored the substrate scope of the silicon–hydrogen exchange reaction with several racemic 2-substituted ketones. In most cases, the reactions proceeded cleanly and the desired kinetic enol silane regioisomers were obtained in high selectivities along with the recovered ketones. As summarized in Scheme [3 ], product 3a can be obtained in 49.5% yield and 96:4 e.r. with ketone 1a recovered in 47.6% yield and 94:6 e.r. on a 0.1 mmol scale. Substrates with strong electron-donating groups (Me, OMe) and a strong electron-withdrawing group (F) at the para position of the phenyl ring were well tolerated under the reaction conditions, affording the corresponding enol silane products 3b –3d in 49–50.5% yields with 93:7–95:5 e.r., and ketones 1b –1d in 47.5–49% yields with 92:8–95:5 e.r., respectively. It is noteworthy that the catalytic system is very well compatible with the silicon–hydrogen exchange reaction of a 7-membered ketone, furnishing the enol silane product 3e in 45.4% yield with 98:2 e.r. and the recovered ketone 1e in 42.3% yield with 96.5:3.5 e.r.. In this case, a remarkably high selectivity of 211 was obtained.
Scheme 3 Substrate scope of the enol silane synthesis from 2-substituted cyclic ketones 1 and methallyl-TBS agent 2 . Reactions were conducted with rac -1 (0.1 mmol), methallyl-TBS agent 2 (2.0 equiv.), and catalyst 5e (1.0 mol%) in toluene (0.1 M) at 0 °C. a Conv. = (ee
1
) / (ee
1
+ ee
3
). b All yields were determined by crude 1 H NMR analysis with CH2 Br2 as internal standard. c The regioisomeric ratio (r.r.) was determined by 1 H NMR analysis. d The enantiomeric ratio (e.r.) was determined by HPLC analysis. e
s = ln[(1 − conv.)(1 – ee
1
)] / ln[(1 − conv.)(1 + ee
1
)]. f With 5 mol% catalyst 5e .
Toward a deeper understanding of the reaction, two comparison experiments were carried out using two enantiomerically pure substrates (Scheme [4 ]). Only 6% conversion was observed after 56 hours at 0 °C from the reaction of ketone (S )-1a , which furnished enol silane (S )-3a and ketone (S )-1a without loss of enantiopurity but with moderate regioselectivity (3a :4a = 5.3:1) (eq. 1). In stark contrast, the reaction of ketone (R )-1a proceeded much faster, providing enol silane (R )-3a as the only regioisomer and ketone (R )-1a was recovered with 99:1 e.r. (eq. 2). Interestingly, when the reaction of rac -1a was performed at room temperature, the e.r. of enol silane 3a gradually decreased, and ketone 1a remained nearly racemic throughout the reaction (see the Supporting Information, Figure S11). These control experiments indicate that racemization of the ketone hardly occurs at 0 °C, but takes place at room temperature, suggesting potential for a dynamic kinetic resolution upon identification of a suitable acid catalyst.
Scheme 4 Asymmetric catalytic silicon–hydrogen exchange reactions with enantiopure ketones 1a (0.1 mmol), methallyl-TBS agent 2 (2.0 equiv.), and catalyst 5e (1.0 mol%) in toluene (0.1 M) at 0 °C. (1) Reactivity comparison reaction of ketone (S )-1a with 2 , and (2) the reaction of ketone (R )-1a with 2 .
We have developed access to enantiopure enol silanes from 2-substituted ketones, via silicon–hydrogen exchange reaction using a strongly acidic and confined IDPi catalyst. The newly established catalytic system complements our previously reported methods. We are currently exploring this remarkably general approach to obtain a variety of functionalized molecules and toward developing catalysts that can realize a dynamic kinetic asymmetric silicon–hydrogen exchange reaction.
