Preparation and synthetic applications of Ã-alkoxystannanes as precursors to Ã-alkoxyorganolithiums

Abstract

The preparation of a-alkyl and a,a-dialkyl a alkoxystannanes is described, and their synthetic utility as precursors of a-alkoxyorganolithium reagents is demonstrated.

N,N-Diethyl-. N,N-diisopropyl- and N phenylcarbamate protected a-hydroxytrimethylstannanes are prepared (34-83% yields). These compounds are shown to undergo tin-lithium exchange to provide stable a- alkoxyorganolithium species that trap aliphatic and aromatic aldehydes to provide protected 1,2-diols in good yields (53-93%). In contrast, prepared MOM-protected derivatives (75-78% yield) do not undergo tin-lithium exchange as cleanly as their tributyl-analogues do. Removal of the N,N-dialkyl-, and N-phenylcarbamate protecting groups is accomplished with alane and Li.AllL, respectively. An enantiomerically enriched (>97% ee) N,N-diethylcarbamate protected a-alkoxyorganolithium is trapped using benzaldehyde with complete retention of configuration (HPLC).

The addition of N.N-diethylcarbamate protected a-alkoxyorganolithiums to benzaldehyde provides the protected 1.2-diols in approximately 1: 1 ratios. Organomagnesium species prepared from a-alkoxyorganolithiums using MgBr2•OEt2 give high selectivities (87: 13), while other organometallic species incorporating metals such as Al, B, and Zn show no reactivity.

The addition of organomagnesium reagents to acylstannanes provides one route to a,a.-dialkyl a.-alkoxystannanes. Other organometallic reagents resulted in complex reaction mixtures. The enantioselective addition of simple alkyl organometallic reagents to ( l-tributylstannyl)propan-1-one and ( I -tributylstannyl)ethan-1-one provides a,a.-dialkyl a-alkoxystannanes with low levels of enantioselectivity (<56% ee) and poor reproducibility.

Finally, the chromatographic separation of diastereomeric carbamates prepared from (S)-a-naphthylethylamine is a useful method of obtaining a,a-dialkyl cx-alkoxystannanes in high levels of diastereomeric purity (91-97% de). Cleavage of the (S)-a-naphthylethylcarbamate group (alane) and protection of the enantiomerically enriched a-hydroxystannane as the N,N-diethylcarbamate provides access to configurationally stable a,a-dialkyl a-alkoxyorganolithium reagents. Trapping with benzaldehyde is shown to proceed with complete retention of configuration.