Silyl transfer

Silyl migrations readily occur in silylated ylides to give the ylides of optimum stability. Thus, deprotonation of the salts (21) and (23) gave the ylides (22) and (24), respectively. Intermolecular silyl transfers, from one ylide (or the corresponding phosphonium salt) to another, also lead to maximum stabilization. Silyl transfer does not occur in the product (26) from methylenetrimethylphosphorane and the chlorodisilane (25), pre-... 

Scheme 6.25. Zr-catalyzed addition of silyl ketene acetals to aldehydes requiring added iPrOH, which is proposed to facilitate silyl transfer and release of the active catalyst after each C-C bond formation.

The mechanism proposed for this transformation is outlined in Scheme 24 (235). The slow step of this reaction is silyl transfer from the copper alkoxide 353. This step may occur through the intermediacy of an external silicon source (intermolecular) or by internal transfer of the silyl group (intramolecular). To probe this issue, these workers conducted a double-crossover experiment involving two distinct nucleophiles with different silyl groups, 342a and 359, and examined the products prior to desilylation. The results show conclusively that silicon transfer has a significant intermolecular component, and is somewhat sensitive to the solvent, Eq. 199. 

Mukaiyama Michael reactions of alkylidene malonates and enolsilanes have also been examined (244). The stoichiometric reaction between enolsilane (342a) and alkylidene malonate (383) proceeds in high selectivity however, catalyst turnover is not observed under these conditions. The addition of HFIP effectively promotes catalyst turnover, presumably by protonation and silyl transfer from the putative copper malonyl enolate generated in this reaction. The reaction proved general for bulky P-substituents (aryl, branched alkyl), Eq. 209. 

Attempted formation of the 4-silyl-substituted nucleophilic carbene (111) by deprotonation of the corresponding triazolium salt with KH led to the triazole (112), the product of apparent [1,2]-Si migration.A crossover experiment indicated that silyl transfer is intermolecular. 

The silatropic ene pathway, that is, direct silyl transfer from an silyl enol ether to an aldehyde, may be involved as a possible mechanism in the Mukaiyama aldol-type reaction. Indeed, ab initio calculations show that the silatropic ene pathway involving the cyclic (boat and chair) transition states for the BH3-promoted aldol reaction of the trihydrosilyl enol ether derived from acetaldehyde with formaldehyde is favored [60], Recently, we have reported the possible intervention of a silatropic ene pathway in the catalytic asymmetric aldol-type reaction of silyl enol ethers of thioesters [61 ]. Chlorine- and amine-containing products thus obtained are useful intermediates for the synthesis of carnitine and GABOB (Scheme 8C.26) [62],... 

Loss of one C — H bond proton or silyl transfer, the cleavage of original C — H bond. 

The silyl enol ether does not enjoy the advantage of the h drogen-honded structure so it pixfcrs the T-alkene for steric reasons. It might form only in this shape from the E-enoiaie hut it might also equilibrate hy silyl transfer. 

To test this hypothesis, several silylene/silyl transfer experiments were carried out with the compounds 1 and 2. Here we report the cross experiments with alkyl(chloro)germanes, transfer experiments with chloro- and diphosphanes, and trapping reactions with three phosphaalkenes. 

The mechanism of the Mukaiyama aldol reaction largely depends on the reaction conditions, substrates, and Lewis acids. Linder the classical conditions, where TiCl4 is used in equimolar quantities, it was shown that the Lewis acid activates the aldehyde component by coordination followed by rapid carbon-carbon bond formation. Silyl transfer may occur in an intra- or intermolecular fashion. The stereochemical outcome of the reaction is generally explained by the open transition state model, and it is based on steric- and dipolar effects. " For Z-enol silanes, transition states A, D, and F are close in energy. When substituent R is small and R is large, transition state A is the most favored and it leads to the formation of the anf/-diastereomer. In contrast, when R is bulky and R is small, transition state D is favored giving the syn-diastereomer as the major product. When the aldehyde is capable of chelation, the reaction yields the syn product, presumably via transition state h. ... 

Dioxaphospholens.— In the presence of tertiary amines the phosphate (73) isomerizes to the silyloxyphosphorane (74). The order of efficiency of the base (imidazole > pyridine > EtjN) suggests that it is functioning as a nucleophile, and silyl transfer probably occurs in the adduct (75). 

As in the uncatalyzed reactions with enamines (vide supra), there is potentially more than one point where stereochemical differentiation can occur (Scheme 59). Selectivity can occur if the initial addition of the enol ether to the Lewis acid complex of the a,/J-unsaturated acceptor (step A) is the product-determining step. Reversion of the initial adduct 59.1 to the neutral starting acceptor and the silyl enol ether is possible, at least in some cases. If the Michael-retro-Michael manifold is rapid, then selectivity in the product generation would be determined by the relative rates of the decomposition of the diastereomers of the dipolar intermediate (59.1). For example, preferential loss of the silyl cation (or rm-butyl cation for tert-butyl esters step B) from one of the isomers could lead to selectivity in product construction. Alternatively, intramolecular transfer of the silyl cation from the donor to the acceptor (step D) could be preferred for one of the diastereomeric intermediates. If the Michael-retro-Michael addition pathway is rapid and an alternative mechanism (silyl transfer) is product-determining, then the stereochemistry of the adducts formed should show little dependence on the configuration of the starting materials employed, as is observed. 

Instead of an anionic nucleophile, the protonated neutral form can be used in combination with N,0-bis(trimethylsilyl)acetamide (BSA see Scheme 26) [65, 98, 99,100]. The reaction is initiated by catalytic amounts of acetate ions. Silyl transfer from BSA to the acetate generates the anion of N-(trimethylsilyl)aceta-mide which then deprotonates dimethyl malonate. In the subsequent allyhc substitution, one equivalent of acetate is generated which, as before, reacts with BSA. The use of BSA has the advantage that only catalytic amounts of a base are present in the reaction mixture and that the neutral protonated nucleophhe can... 

Spectroscopic data has been obtained that is consistent with the formation of four-membered ring adducts 61/62 as the kinetic products of the reaction. The step leading to these cyclic products has been shown to be reversible prolonged exposure of 61/62 to the reaction conditions led to the conversion of these metastable oxetanes to 63, the thermodynamic product of the reaction. The investigators have speculated that the formation of oxetane adducts in this study is a consequence of a slow silyl transfer step 60—>63. Thus, these observations highlight the fine balance that can exist between the various reaction pathways available to the adduct of the C-C bond-forming step (cf 60). 

Reaction of (50) with alkyl halides gives exclusive a-alkylation. With aldehydes and ketones, a-addi-tion again takes place to give (52) via intramolecular silyl transfer with concomitant loss of lithium cyanide (c/. 25 Scheme 30). Treatment of (52) with p-Ts0H H20 gives the cyclopentenone annelation product. The allylic cyanohydrin anion (53) also gives a-adducts upon treatment with aldehydes and ketones at -78 °C, whereas reaction with electrophiles at 0 C affords -y-adducts (c/. 25). 

Silyl transfer. l,l-Di(r-butyl)silacyclopropanes readily submit the di-r-butylsilyl residue to a-keto esters to form 4-aIkoxy-l,3,2-dioxasiloles. In the case of an aUyloxy ester the situation is set up for the Ireland-Claisen rearrangement. ... 

The silyl transfer to imines delivers silaaziridines that can enter cross-coupling with alkynes to afford alylic amines. ... 

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