Regiospecific synthesis of trimethylsilyl enol ethers via silatropic rearrangements

Abstract

1619(CH,)2CCH,Br -(CH,),C=CH, + B r a (1) (CH,),CCH,Br + O2 -, (CH3),C(OO*)CH2Br (2) propagation reactions. The sensitivity of a n autoxidation to Br. is illustrated by the observation that the oxidizability of 1 is increased by a factor of -100 by the addition of 0.04 A4 HBr.The rate of autoxidation of 1 in the presence of (CH3)3COOH (0.5 to 2 M ) does not depend on the partial pressure of oxygen indicating that reaction 1 is not important in this system. This is not unexpected because a t such large hydroperoxide concentrations any bromine atoms that are formed should undergo chain transfer with the hydroperoxide to regenerate peroxy radicals. (3)This was confirmed by the observation that catalysis of the oxidation of 1 by HBr was eliminated by 2 M (CH3)3COOH (vide Table I).The only rate controlling propagation reaction in this system is, therefore E r -+ (CH,),COOH -HBr + (CH3)3COO* kn' (CH,),COO* + (CH,),CHCH,X -(CH,),COOH + (CH3),CCH2X (4) and the propagation rate constant that is measured is k,'.Measurements of homopropagation rate constants a t 30' for organic substrates with unactivated C-H's (e.g., isobutane 3) and organic substrates with a-bromo and a-chloro substituents (e.g., isopropyl bromide 4 and isopropyl chloride 5 ) is not possible because kinetic chain lengths are too short." A propagation rate constant of 0.003 M-I sec-' for the autoxidation of 3 has, however, been estimated" while rate constants for reaction of 4 and 5 with (CH3)3COO. have been calculated to be -0.0006 A4-I sec-I.l1The reactivity of compounds 1 to 5 to (CH3)3COO. decreases in the order l > 2 -3 > 4 -5 whereas on purely electronic grounds the reactivity order should be 3 > 1 -2 > 4 -5. Thus a CH2Br group activates an adjacent C-H bond relative to a CH3 group because anchimeric assistance from the P-bromine overshadows the deactivating effect expected from electron withdrawal by the bromine. The similarity between the rate constants for 3 and 2 implies that the deactivating effect of a P-chloro substituent is almost exactly balanced by the accelerating effect from anchimeric assistance. W e have previously concludedl * that rate constants for reaction of organic compounds with (CH3)3COO. a t 30' correlate with the strength of the weakest bond in the molecule (D[R-HI) by the empirical equation log ( k p ' / M -i sec-') = 15.4 -0 .2D[R-H]with D[R-HI in kilocalories per mole. As values of k,' for 1 and 2 would be expected to fit this relationship 1 must have a tertiary C-H bond that is -7 kcal mol-' weaker than 4 and -4 kcal mol-' weaker than 3, while the tertiary C-H's in 2 and 3 have approximately the same strength.The estimated stabilizing effect of a CHzBr substituent relative to a CH3 substituent of 4 kcal mol-] is in reasonable agreement with the estimate of 2.5 kcal mol-' obtained by Cain and Solly12 from a kinetic study of the thermolysis of 1 -bromomethyl-and 1 -methyl-4-chlorobicylo[2.-2.01hexanes and the estimate of 3 kcal mol-' made by Skell and SheaI3 from a study of the brominations of l-bromobutane and propane.W e b...

Use of cyclopropanes and their derivatives in organic synthesis

“…yielded hydroazulene190 in 70% yield.99 Thermolysis of 191 at 515 °C gave 194 in 57% yield.100 Pyrolysis of bicyclopentane 195 gave a high yield of the three isomeric esters 196, 197, and 198 in a ratio of 1h i b i t e d .…”

Silyl enol ethers and silyl ketene acetals

“…A conceptually different approach 26 relies on silatropic rearrangement 27 of trimethylsilyl /?-ketoesters and related species (Scheme 17.13). The requisite silyl /?-ketoesters are readily prepared 28 by acylation of the appropriate carboxylic acid dianion followed by silylation.…”

“…As was stated earlier, quenching of the equilibrating mixture of anions (17) and (18) with trialkylsilyl chlorides results in O-silylation. Competitive attack at oxygen, to give esters (26), can be minimized by increasing the steric bulk of the silyloxy silicon substituents. Competitive attack at oxygen, to give esters (26), can be minimized by increasing the steric bulk of the silyloxy silicon substituents.…”

“…-SIR, R 3 COCl OSIR: (24) (26) α-Ketosilanes (acylsilanes) 46 , /?-ketosilanes 47 , and ayff-epoxysilanes 48 all undergo more or less clean thermal rearrangement to silyl enol ethers (Scheme 17.27), although none of these routes is likely to compete directly with the more standard methods so far discussed (see also Chapter 5). …”

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“…36%) (Scheme 10, Table 4). Reproducibly, 9% of the hydrolysis product Nallylbenzamide (25a) were formed, maybe by a thermal sigmatropic rearrangement process [30] followed by hydrolysis during workup, along with other by-products that were not isolated. Different hydrolysis and workup conditions did not increase the yield of 29a.…”