Siloxy transfer

The bis-trimethylsilyl ethers of stereoisomeric 5,7-undecanediol 78 (as well as the free diols) give upon chemical ionization (protonation with QH9) MH+ ions, which show two major processes (reaction 41). One corresponds to a Grob-type fragmentation 05-cleavage) of the ion 79 (formed by loss of Me3SiOH from protonated 78) to yield the silylated aldehyde 80. The second major reaction path involves loss of CH4 from MH+, followed by cyclization of the intermediate 81 (siloxy transfer) to 82 and ft-cleavage (loss of 1-hexene) to generate 8467. This reaction sequence, in which a centrally located structural unit rather than a peripheral one is eliminated, is rarely observed under chemical ionization and deserves further study. 

Mattay et al. examined the regioselective and stereoselective cyclization of unsaturated silyl enol ethers by photoinduced electron transfer using DCA and DCN as sensitizers. Thereby the regiochemistry (6-endo versus 5-exo) of the cyclization could be controlled because in the absence of a nucleophile, like an alcohol, the cyclization of the siloxy radical cation is dominant, whereas the presence of a nucleophile favors the reaction pathway via the corresponding a-keto radical. The resulting stereoselective cis ring juncture is due to a favored reactive chair like conformer with the substituents pseudoaxial arranged (Scheme 27) [36,37]. 

CP/MAS NMR study of CH4 activation on [(=SiO)2Ta(H)J shows the formation even at 150 °C of methyUdene and methyUdyne species by an a-H elimination process on several sites that should correspond to the tris-hydride on other sites a methyl group is transferred to the surface, leading to the formation of (=Si-Me) and of [(=SiO)3Ta]. Correlation with EXAFS suggests that the tris-hydride should exist on surface sites (=20%) quite distant from siloxy bridges whereas methyl transfer to the surface should happen on the specific sites (=80%) close to the siloxy bridges. The latter, which are formally 10 electrons species, exhibit a moderate to weak activity in methane C-H activation. To the best of our knowledge, this is the first observation of methyl group transfer on a surface (Scheme 2.18). 

A procedure for alkylation of C=0 double bonds in the presence of (metal-free) organocatalysts and non-metallic nucleophiles has been reported by the Iseki group for trifluoromethylation of aldehydes and ketones [185]. On the basis of a previous study of the Olah group [186, 187] which showed the suitability of non-chiral phase-transfer catalysts for trifluoromethylation of carbonyl compounds, Iseki et al. investigated the use of N-benzylcinchonium fluoride, 182, as a chiral catalyst. The reaction has been investigated with several aldehydes and aromatic ketones. Trifluoromethyltrimethylsilane, 181, was used as nucleophile. The reaction was, typically, performed at —78 °C with a catalytic amount (10-20 mol%) of 182, followed by subsequent hydrolysis of the siloxy compound and formation of the desired alcohols of type 183 (Scheme 6.82). 

The formation of an a-siloxy Rh(III) intermediate resulting from initial transfer of the silyl moiety to the ketone oxygen (as opposed to a Rh(III) alkoxide) has been postulated based on spin-trapping experiments and EPR studies. Ojima,... 

Unsaturated esters can be formed from 1-siloxy-l-alkoxycyclopropanes in the reaction with a quinone, such as 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) or tetrachloro-l,4-ben-zoquinone, where ring-opened C —C and C —O bonded adducts are likely to be involved as intermediates formed via a single electron transfer mechanism. - Application to C — C bond... 

Similarly, p3ridine addition to aldehydes is also promoted by TMSOTf in a three-component reaction to form [l-(trimethyl-siloxy)alkyl]p3Tidinium salts (eq 62), which may act as group transfer reagents or as precursors for the analogous phosphonium... 

Scheme 8 anri-Diastereo- and enantioselective siloxy-crotylation in the transfer hydrogenative coupling of primary alcohols with alkynes via hydride-shift enabled ji-aUyl formation. Yields are of material isolated by flash chromatography on silica gel. Diastereoselectivity was determined through NMR analysis of crude... 

Chirality transfer from an a-silylalcohol to a-carbamoyloxy- and a-siloxyallyl-carbanions has been investigated using a Brook rearrangement-mediated 5 2 protonation in y-carbamoyloxy- and y-siloxy-ct-silylallyl alcohols. A reaction mechanism that proceeds along one of two pathways that involves a concerted protonation of a silicate intermediate and a concerted lithiation of the intermediate followed by protonation with retention or by protonation after racemization has been proposed. 

Einaga, H., Nojima, M., and Abe, M., Photooxygenation (electron transfer) of sdyl enol ethers derived from 1-indanone silyl group and medium effects on the competitive formation of 3-siloxy-1,2-dioxetane and a-peroxy ketone. Main Group Metal Chem., 22, 539, 1999. 

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