Silicon-induced diastereoselectivity of the catalyzed phosphorylation of 1-(perfluoroalkyl)-ω (trialkylsilyl)alkan-1-ols

Abstract

The stereochemistry of catalytic phosphorylation of 1 (perfluoroalkyl) ω (trialkyl silyl)alkan 1 ols with pentavalent phosphorus acid monochlorides (phosphonochloridates, methyl(phenyl)phosphinic chloride, and phosphorochloridates) was studied. The effects of the structures of alcohols and phosphorylating agents on the degree of the reaction dia stereoselectivity were investigated. Methylphosphonochloridates were found to react most stereoselectively; the diastereoselectivity of the phosphorylation is independent of the donor or acceptor character of substituents at the phosphorus atom, being determined by their volumes. In the series of silylalkanols, the diastereoselectivity of the reaction the higher the closer the Si atom to the reactive site of the molecule, the larger the volume of the perfluoroalkyl substituent, and the more pronounced the electron withdrawing properties of the substituents at the Si atom. A reaction mechanism is proposed that rationalizes the stereoselectivity of the reaction.In many cases, introduction of a silicon atom into a certain position of an organic substrate molecule changes the directions of various reactions and their rates or selec tivities. This is explained by the ability of silicon to stabi lize free radicals and positive and negative charges and to interact with both n and π donors. 1 The effect of the silicon atom on stereochemistry of reactions of organophosphorus compounds has been re ported only for a limited number of examples. For in stance, the high stereoselectivity of the phosphorylation of α silyloxy aldehydes 2,3 and their N trimethylsilyl imides 4 with diethyl trimethylsilyl phosphite is explained by the formation of a cyclic or bicyclic transition state as a result of intra and intermolecular coordination of the silicon atom with the O or N atom, respectively. In addi tion, stereospecific addition of silyl phosphites to oxo and iminofuranoses has been described 5 . However, the latter reactions are stereospecific only because of steric factors.Previously, 6 we showed that the phosphorylation of 1 perfluorobutyl 2 trimethylsilylethanol (1a) with aryl methylphosphonochloridates in the presence of catalytic amounts of metallic Mg proceeded fairly diastereoselec tively.The goal of the present work was to scrutinize the stereochemistry of the catalytic phosphorylation of racemic 1 (perfluoroalkyl) ω (trialkylsilyl)alkan 1 ols R 1 R 2 2 Si(CH 2 ) n CH(R F )OH (1-3) with racemic phospho rus acid monochlorides R 3 R 4 P(O)Cl (4a-i, 5, 6a,b) (Scheme 1) and to elucidate the effect of the structure of the phosphorylating agent on the ratio of the resulting diastereomers. The role of such factors as the distance between the silicon atom and the reactive site, the volume of the perfluorinated substituent R F , and the donating or withdrawing character of substituents at the Si atom in the alcohol to be phosphorylated was also studied.Equimolar mixtures of alcohols 1-3 and acid chlo rides 4-6 were heated at 140-160 °C in the presence of 2.5 mol. % Mg until evolution of HCl...