The determination of the stereochemical features of reactions involving organometallic species is a long-standing issue, which is still to be appropriately addressed. The study of the stereochemistry of products after a reaction of an organometallic species with a suitable electrophile has been the most commonly used strategy, [1] and often the only possible approach.[2] However the term ªsuitable electrophileº is intrinsically imprecise. The reaction of an organometallic reagent with an electrophile is always a possible source of interference when the goal is to study the organometallic compound itself. The degree of retention/inversion/ loss of stereochemical integrity is highly specific for each organometallic ± electrophile pair and is therefore not available in many cases. Different electrophiles can give rise to a different distribution of products with partial (or complete) retention (or inversion) of their configurations.[3] Even though retention of configuration is the most common reaction pathway, the exact behavior of each particular system is, at the present time, simply unpredictable. [4,5] Among the different organometallic reagents utilized in organic chemistry, organozinc compounds occupy an intermediate position in terms of the electronegativity of the metal and hence reactivity. As a general trend for organometallic reagents devoid of functionality, the more covalently bonded species are those which display high energy barriers to inversion of the carbon center (or topoisomerization barriers), [6] and vice versa. Thus, activation energies for inversion in primary organometallic compounds of the type R n M increase in the order M Li < Mg < Zn, and the inversion of Al and Hg compounds is slow on the NMR time scale. [7a,b] In general, NMR spectrometers are designed to operate between 200 8C and À 100 8C or less, which allows the study of a wide range of systems with activation energies ranging from approximately 5 to 25 kcal mol À1
