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I. Introduction

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    23957

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    A. Definitions

    Stereoselectivity is “the pre­ferential for­ma­tion of one stereoisomer over another in a chem­ical reaction”.1 This selectivity can be divided into diaster­eo­selectivity and enantioselectivity. “En­antio­selectivity in a reaction is either the preferential formation of one enantiomer of the product over the other or the preferential reaction of one enantiomer of the (usually racemic) starting material over the other...Diastereoselectivity is the preferential formation in a reaction of one diastereoisomer of the product over others.”2 Although diastereo­selectivity almost always refers to product formation, it also can apply to preferential con­sump­tion of one diastereomer.3

    B. Factors Affecting Stereoselectivity

    Stereoselectivity in radical reactions is determined by a combination of factors that includes steric, stereoelectronic, confor­mational, tor­sional, and configurational effects as well as reaction tem­perature.4 Each of these effects can be linked to a particular aspect of structure. Steric effects are ­the repul­s­ive interactions that develop between closely ap­proach­ing spe­cies (e.g., a neutral molecule and a free radical) or between two groups within the same structure. Stereo­elec­tronic effects are geometry-dependent, orbital inter­ac­tions that favor formation or consumption of one stereo­isomer over another. Con­for­mational effects are differences in stereo­selec­tiv­ity due to dif­fer­ences in the population of various conformers. Tor­sional effects are the destabil­izing inter­actions that develop as elec­trons in bonds on adjacent atoms move closer to each other. Fin­ally, con­fig­urational effects in radical reactions are differ­ences in stereoselectivity due to pyram­idal radi­cals that under­go reac­tion faster than inversion of configuration.

    Although stereoselectivity in a reaction often results from a com­bin­ation of the effects just described, it is possible to identify three important situations where a particular effect appears to be dominant. First, in addition and abstraction reactions, where the radical center is not adjacent to a ring oxygen atom, the greater role of steric effects causes reaction to occur along the least-hindered pathway. When a radical is centered on an atom adjacent to a ring oxygen atom (as occurs in pyran­os-1-yl and furanos-1-yl radicals) orbital interactions become the factor most frequently de­ter­mining stereo­selectivity. Finally, in reactions that form new five- and six-membered rings, ster­eo­selectivity usually is deter­mined by maximizing stability of a chair-like or boat-like trans­ition state.

    This page titled I. Introduction is shared under a All Rights Reserved (used with permission) license and was authored, remixed, and/or curated by Roger W. Binkley and Edith R. Binkley.