Selectivities that deviate from SN1 stereoelectronic models in the nucleophilic substitutions of tetrahydropyran acetals were investigated. When weak nucleophiles were employed, stereoselectivities conformed to known SN1 stereoelectronic models. In contrast, stereoselectivities in the substitutions of acetals with strong nucleophiles depended on reaction conditions. Erosions in selectivities were observed when strong nucleophiles were employed in the absence of coordinating counterions. These erosions in selectivities are attributed to rates of nucleophilic additions to oxocarbenium ion intermediates that approach the diffusion limit. When triflate counterions were present, however, SN2-like pathways became accessible with strong nucleophiles. In most cases examined, the major stereoisomers formed from reactions that proceeded through SN2-like pathways were opposite to the major stereoisomers formed from the analogous reactions that proceeded through SN1 pathways.

This work details an in-depth evaluation of an unprecedented mechanism for the hydrosilylation of carbonyl compounds catalyzed by (PPh3)2Re(O)2I. The proposed mechanism involves addition of a silane Si-H bond across one of the rhenium-oxo bonds to form siloxyrhenium hydride intermediate 2 that reacts with a carbonyl substrate to generate siloxyrhenium alkoxide 4, which, in turn, affords the silyl ether product. Compelling evidence for the operation of this pathway includes the following: (a) isolation and structural characterization by X-ray diffraction of siloxyrhenium hydride intermediate 2, (b) demonstration of the catalytic competence of intermediate 2 in the hydrosilylation reaction, (c) 1H and 31P{1H} NMR and ESI-MS evidence for single-turnover conversion of 2 into 1, (d) observation of intermediate 2 in the working catalyst system, and (e) kinetic analysis of the catalytic hydrosilylation of carbonyl compounds by 1.

The effect of nucleophile strength on diastereoselectivity in the nucleophilic substitution of cyclic acetals was explored. Stereoselectivity remained constant and high as nucleophilicity increased until a threshold value was reached. Beyond this point, however, selection of Lewis acid determined whether stereochemical inversion or erosion was observed.The development of stereocontrolled glycosylation reactions is complicated by the fact that these processes may proceed via S N 1-like 1-3 or S N 2-like 4-10 mechanisms. Changes in the glycosyl donor, 11,12 nucleophile, 13,14 activator, 15 and solvent 16 can alter selectivity unpredictably. This report documents the relationship between nucleophile strength and stereoselectivity for the substitution reactions of cyclic acetals; we describe dramatic changes in stereoselectivity and provide mechanistic rationales for these findings. This study provides insight applicable to the development of new stereoselective glycosylation reactions.Acetal 1 was treated with a panel of nucleophiles having known nucleophilicity parameters (N) 17 in the presence of Me 3 SiOTf (Table 1). A nucleophile's N value is a direct measure of reactivity: it correlates logarithmically with its rate of reaction with carbocationic electrophiles. 17 Reactions with π-nucleophiles spanning more than four orders of magnitude of nucleophilicity led to selective formation of 1,4-trans products (entries 1 and 2). A roughly one hundred-fold further increase in N, however, associated with application of silylketene acetal nucleophiles 9-11, resulted in reversal of diastereoselectivity: 1,4-cis products were formed selectively (entries 3-5). This dichotomy in stereochemical outcomes suggests a change in reaction mechanism. 18We have reported previously an electrostatic model to explain the trans selectivities observed in the reactions of acetal 1 with weak nuceophiles (e.g., 7 and 8). 19,20 These reactions occur by S N 1-type mechanisms involving oxocarbenium ion intermediate I (Scheme 1). 21 Axial attack on the electrostatically preferred axial conformer I ax affords trans products via a chairlike transition state. This model, however, does not account for the cis selectivities observed when strong nucleophiles 9-11 react with 1. It is unlikely that the 1,4-cis ester products cis-(4-6) arise from disfavored equatorial conformer I eq because increased nucleophile strength should not alter the conformational equilibrium of the oxocarbenium ion. Moreover, the selectivities of reactions of I ax and I eq should be independent of nucleophile reactivity unless reaction rates approach the diffusion limit. 22 kwoerpel@uci.edu. The stereochemical inversion observed in Me 3 SiOTf-activated reactions of electrophile 1 with silylketene acetals 9-11 can be explained by S N 2-like substitutions 23-27 of triflate-trapped contact ion-pairs 28 II via transition state III (Scheme 2). Transition state III is consistent with the electrostatic model. As the triflate group departs from the axial orientation, the trans...

A family of bis(oxazoline) complexes of coordinatively unsaturated monomeric rhodium(II) (2a,b, 3a,b) are described. These complexes serve as catalysts for cyclopropanation of olefins by ethyl diazoacetate, giving excellent yields (66-94%). Enantioselectivities for the cis product isomers are good (61-84%). The reaction shows an unusual preference for formation of the cis isomers. Catalytic aziridination of N-aryl imines with ethyl diazoacetate is also described.