The unusually slow hydrolysis rate of silyl methyl ketals in benzoquinone systems. The question of siloxy stabilization of an adjacent positive charge and stereoelectronic effects on ketal hydrolysis

Abstract

The kinetics of the acid-catalyzed monohydrolysis of six quinone bisketals were studied under pseudo-first-order conditions. The tert-butyldimethylsilyl methyl and the tert-butyldiphenylsilyl methyl ketals respectively hydrolyze about 400 times slower than does the corresponding dimethyl ketal and react nearly at the rate of the ethylene glycol ketal. An important consideration in understanding these results was the carbonium ion stabilizing ability of the siloxy substituent. Thus, and + values for the tert-butyldimethylsiloxy and the tert-butyldiphenylsiloxy groups were measured. These values indicated that siloxy groups are nearly as effective as a methoxy group in stabilizing a positive center provided a low-energy conformer is available for interaction of the oxygen nonbonding electron pair with the -center. If delocalization of the oxygen lone-pair electrons by the silicon is important, it does not manifest itself in the or + values. The data collected also illustrate the strong conformational dependence of siloxy groups interacting with -systems, and it is this effect that is thought to be the major contributor to the lower rate of hydrolysis observed for the silyl methyl ketal moieties in these quinone systems.

Enantioselective Synthesis of Spirocyclic Aminochroman Derivatives According to the CN(R,S) Strategy

“…Lactone 13 (7:3 epimeric mixture, Scheme ) was synthesized in two steps from (−)- 2 in 74% overall yield. The enolate of compound 13 (LDA, THF, −78 °C) was alkylated with 2-( tert -butyldimethylsiloxy)benzyl bromide ( 14 ) to furnish only the alkylated compound 15 in 44% yield. This compound exhibited (4 R ,9a R ) absolute configuration in accordance with a mechanism described in the literature 18 involving the fact that the electrophile came on the more stable enolate conformer bearing an equatorial phenyl group.…”

Palladium(II) and Gold(I) Complexes of a New O-Functionalized N-Heterocyclic Carbene Ligand: Synthesis, Structures, and Catalytic Application

“…Ag 2 O was purchased from SD-Fine Chemicals (India) and used without any further purification. (COD)PdCl 2 and (SMe 2 )AuCl were prepared according to the reported literature procedures, while 2-methoxybenzyl bromide was prepared by a procedure modified from that reported in the literature. 1 H and 13 C{ 1 H} NMR spectra were recorded in CDCl 3 on a Varian 400 MHz NMR spectrometer.…”

Time-Dependent Evolution of Adducts Formed between Deoxynucleosides and a Model Quinone Methide

“…1 H NMR (CDCl3): δ 0.23 (s, 6H), 0.99 (s, 9H), 2.06 (s, 3H), 5.10 (s, 2H), 6.81 (dd, J ) 8.0 and 1.1, 1H), 6.93 (td, J ) 8.0 and 1.1, 1H), 7.19 (td, J ) 8.0 and 1.5, 1H), 7.29 (dd, J ) 8.0 and 1.5, 1H). 13 (17). N-Bromosuccinimide (270 mg, 1.5 mmol) was added to a solution of 2-methyl-O-tert-butyldimethylsilylphenol (18) (300 mg, 1.4 mmol) in CCl4 (20 mL).…”