Silyl groups, vinylic

This /J-silyl phosphorane is a synthon for a vinylmetal reagent, Vinylation occurs with concomitant migration of the silyl group. 

An important contribution to silylium ion chemistry has been made by the group of Muller, who very recently published a series of papers describing the synthesis of intramolecularly stabilized silylium ions as well as silyl-substituted vinyl cations and arenium ions by the classical hydride transfer reactions with PhjC TPEPB in benzene. Thus, the transient 7-silanorbornadien-7-ylium ion 8 was stabilized and isolated in the form of its nitrile complex [8(N=C-CD3)]+ TPFPB (Scheme 2.15), whereas the free 8 was unstable and possibly rearranged at room temperature into the highly reactive [PhSi /tetraphenylnaphthalene] complex. ... 

The reaction of silylborane with 1-halo-l-lithio-l-alkenes yields 1-boryl-l-silyl-l-alkenes via borate formation followed by 1,2-migration of silyl group (Equation (90)).76,240 The mechanism seems to be closely related to that proposed for the silaboration of isocyanide (Figure 2). Vinyl-substituted carbenoids, l-chloro-l-lithio-2-alkenes, react with silylpinacolborane to give l-boryl-l-silyl-2-alkanes in good yield (Equation (91)).241 This methodology is applied to the synthesis of l-boryl-l-silyl-2-cyclobutene.2 2 Similar reactions are carried out with other carbenoid... 

Addition of the (l-silylalkyne)titanium complex to carbonyl compounds and imines occurs at the (3-position to the silyl group, as shown in Fig. 9.2. However, the reaction with sBuOH takes place exclusively at the carbon—titanium bond a to the silyl group to give the (P-silylalkenyl)titanium species, as in Eq. 9.5 (values in square brackets denote the regioselectivity) [24], where the vinyl—titanium bond is visualized by the outcome of the iodi-nolysis. The overall reaction can therefore be regarded as the hydrotitanation of silylace-... 

The generation of a-ferrocenyl-P-silyl substituted vinyl cations of type 28 does not require superacidic conditions, they can be generated by protonation of l-ferrocenyl-2-trialkylsilyl alkynes with trifluoroacetic acid at room temperature. The SiR3-groups with larger alkyl substituents increase the lifetime of this type of carbocations. 

Vinyl cations of type 28 with a-aryl or a-alkyl substituents and two P-silyl groups and with an anion of very low nucleophilicity can be generated at room temperature in non-coordinating solvents from 30 by a Si-H to C-H hydride transfer reaction. For 29 (R = t-butyl), an X-ray structure determination has been reported (43, 52, 53). 

Not only the ring size but also the number of stabilising silyl groups in the -position is essential for the stability of the vinyl cations. Thus, reaction of alkyne 16 with tityl cation gave both stereoisomers of aikenylsilane 18 as the only products in 80-85% isolated yield (Scheme 3). This result suggests, that the generated / -silyl-substituted vinyl cation intermediate 17 did not persist under the applied reaction conditions but underwent a second hydride transfer with the formation of compound 18. 

Reaction of linear conjugated dienes with la at —10 °C in hydrocarbon solvent in the presence of McsSiCl/AlCls affords stereospecific tra i-l-silyl-3-vinyl-cyclopentanes, indicating a [3 + 2] cycloaddition of the allyl group of la with a carbon arbon double bond of the diene [Eq. (9)]. In the [3 + 2] annulation reaction, of greater significance is the tram conformation of the trimethylsilyl group and vinyl groups. 

The Claisen rearrangement of allyl vinyl ethers is a classic method for the stereoselective synthesis of y,J-unsaturated esters. The allylic C-H activation is an alternative way of generating the same products [135]. Reactions with silyl-substituted cyclohexenes 197 demonstrate how the diastereoselectivity in the formation of 198 improves (40% to 88% de) for the C-H insertion reactions as the size of the silyl group increases (TMS to TBDPS) (Tab. 14.14). Indeed, in cases where there is good size differentiation between the two substituents at a methylene site, high diastereo- and enantioselectivity is possible in the C-H activation. 

A variation of GTP, referred to as aldol GTP, involves polymerization of a silyl vinyl ether initiated by an aldehyde [Sogah and Webster, 1986 Webster, 1987]. Both initiation and propagation involve nucleophilic addition of the vinyl ether to the aldehyde carbonyl group with transfer of the trialkyl silyl group from vinyl ether to the carbonyl oxygen (Eq. 5-82). The reaction has similar characteristics as GTP. The product is a silated poly(vinyl alcohol) (PVA), which can be hydrolyzed by acid to PVA (Eq. 5-83). 

Enyne 7a having a silyloxy group on the alkyne gives cyclic compound 8a having a vinyl silyloxy moiety, which is converted into methyl ketone 9a by desilylation. In a similar manner, enyne 7b affords bicyclic methyl ketone 9b in 68% yield after deprotection of the silyl group. However, ynoate 7c and yne-phosphonate 7d do not give cyclized compounds. Ene-ynoate 12, which is obtained by treatment of enol ether 11 with BuLi affords cyclic enol ether 13 in good to moderate yields " (Scheme 4). 

Highly substituted 2,3-dihydrofurans 44 (Scheme 1.3.18) would make particularly interesting starting materials for the asymmetric synthesis of tetrahydrofu-rans, structural motifs which can be found in many important natural products, including polyether antibiotics, lignans, and nucleosides [30]. Not only the activated double bond but also the vinylic silyl group of 44 should allow useful synthetic transformations. 

The Nazarov reaction [196] is a conrotatory electrocyclization involving four electrons over a five-carbon span. Usually, a more highly substituted cyclopentenone is obtained. However, contrathermodynamic products may be generated by placing a silyl group at the p-position of a bare vinyl moiety in the cross-conjugated dienone [197]. The acceptor facilitates and controls the regiochemistry of the cyclization process. 

In certain reactions, introduction of a bulkier silyl group in substrates may decrease the stereoselectivity. For example, photolysis of a mixture of benzaldehyde and silyl vinyl ethers 31 affords a mixture of diastereomeric [2 + 2] cycloadducts 32 and 33... 

In contrast, /1-silyl-substitution is predicted by the calculations to be far more stabilizing than /1-alkyl substitution. Thus, the isodesmic equation 3 predicts for 9 a stabilization by the /1-silyl group of 38 kcalmol-1, while the /1-methyl substitution in 10 gives only a stabilization of 28 kcalmol-1 [MP3/6-31G(d)//3-21G(d)]5. The stabilization by the silyl substituent is markedly orientation-dependent. Thus, the perpendicular conformation of the /1-silylethyl cation 9p is higher in energy by 29.6 kcalmol-1 compared with the bisected conformation 9 [MP3/6-31G(d)//3-21G(d)]5. The open /1-silyl-substituted vinyl cation 11 is lower in energy by 28.6 kcalmol-1 and 20.5 kcalmol-1 [MP3/6-31G(d)//3-21G(d)] compared with the vinyl cation (equation 4, R = H) and the 1-propenyl cation (equation 4, R = Me), respectively5. 

Kresge and Tobin80 investigated the /1-silicon effect on the hydrolysis of vinyl ethers (equation 29) and found a rate acceleration on the hydrolysis of 175 compared with 176, and hence a stabilizing effect of the /1-silyl group on the intermediate -ethoxy carbocation 177 compared with 178. The acceleration is small the rate factor (175) (176) of 129 is equivalent to a free energy of activation difference AAG of 2.9 kcalmol-1,... 

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