Sterically hindered organometallic reagents

The strongly basic organometallic reagents inevitably attack at the less substituted, less sterically hindered carbon of the oxirane, as does any strong nucleophile (Fig. 10.36). 

Nucleophilic Substitution. Eabom reported that fluorosi-lanes are more reactive toward organometallic reagents than the corresponding chlorosilanes, and therefore more suited for preparation of sterically hindered organosilanes. Two examples of this enhanced reactivity are shown for the Tsi anion (eq 1) and a dianion (eq 2). The use of less-hindered reagents in stoichiometric amounts still allow for the sequential introduction of nucleophiles (eq 3).l ... 

Entries 1-4 in Table 3 illustrate the tendency for a Cram selective process in additions to aldehydes of type (4 equation 1). In contrast, when (4) is treated with the aluminum additive (1) prior to exposure to organometallics, the nucleophilic addition results in an anti-Cram product. The resulting facial selectivity may be most easily rationalized by considering transition state structure (6), which defmes the anti-Cram face of the aldehyde to be less hindered by virtue of precoordination of the aluminum reagent (1) to the less sterically demanding Cram face. For example, comparison of entries 2, 6 and 9 to the corresponding entries 5, 8 and 10 in Table 3 illustrates the dranuitic effect that the aluminum additive (1) has on the facial selectivity of the reaction. This approach to anti-Cram selectivity, however, does suffer... 

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