Marta Marin-Luna

Thermal cyclization of phenylallenes that contain ortho-1,3-dioxolan-2-yl groups: new cascade reactions initiated by 1,5-hydride shifts of acetalic H atoms.

A series of 2-(1,3-dioxolan-2-yl)phenylallenes that contained a range of substituents (alkyl, aryl, phosphinyl, alkoxycarbonyl, sulfonyl) at the cumulenic C3 position were prepared by using a diverse range of synthetic strategies and converted into their respective 1-(2-hydroxy)-ethoxy-2-substituted naphthalenes by smooth thermal activation in toluene solution. Electron-withdrawing groups at the C3 position accelerated these tandem processes, which consisted of 1) an initial hydride-like [1,5]-H shift of the acetalic H atom onto the central cumulene carbon atom; 2) a subsequent 6π-electrocyclic ring-closure of the resulting reactive ortho-xylylenes; and 3) a final aromatization step with concomitant ring-opening of the 1,3-dioxolane fragment. If the 1,3-dioxolane ring of the starting allenes was replaced by a dimethoxymethyl group, the reactions led to mixtures of two disubstituted naphthalenes, which were formed by the migration of either the acetalic H atom or the methoxy group, with the latter migration occurring to a lesser extent. Two of the final 1,2-disubstituted naphthalenes were converted into their corresponding naphtho-fused dioxaphosphepine or dioxepinone through an intramolecular transesterification reaction. A DFT computational study accounted for the beneficial influence of the 1,3-dioxolane fragment on the carbon atom from which the H-shift took place and also of the electron-withdrawing substituents on the allene terminus. Remarkably, in the processes that contained a sulfonyl substituent, the conrotatory 6π-electrocyclization step was of lower activation energy than the alternative disrotatory mode.

Substituent Effects in the Silylation of Secondary Alcohols: A Mechanistic Study

Relative rates for the silylation of C4-substituted 1-(naphthalen-1-yl)ethanol substrates with tert-butyldimethylsilyl chloride (TBSCl) in CDCl3 catalyzed by 9-azajulolidine (TCAP) have been measured. Hammett plot analysis of the resulting selectivity data yields two intersecting linear correlations. A small positive slope of ρ=+0.09 is observed for donor-substituted alcohols, while silylation rates for acceptor-substituted alcohols correlate best with a slightly larger negative slope (ρ=-0.48). The reaction of methanol with TBSCl catalyzed by 4-(N,N-dimethylamino)pyridine (DMAP) has been studied at several different theoretical levels in chloroform solution. Silyl-group transfer between silylated DMAP and methanol occur over an exceedingly flat surface with barely defined minima and transition states. Reaction pathway calculations for the Lewis base and general base catalyzed mechanisms for reaction of TBSCl with C4-substituted 1-(naphthalen-1-yl)ethanol compounds predict a close competition of both pathways.