The Stereochemistry of Substitution at Trigonal Carbon

Both nucleophilic and electrophilic attack on trigonal carbon can take place by two pathways (Fig. 5.8)— direct attack on one of the n bonds attached to the double bond (path a), or by attack on the n bond (path b), with the formation of an ionic intermediate, followed by the loss, respectively, of a nucleofugal group X or an electrofugal group M. There are also formally unimolecular pathways, SN1 and SE1, with ionisation followed by capture by the nucleophile or electrophile, but the former is neither favourable nor common,502 and the latter unknown. 

The stereochemistry of the direct attack can be expected to resemble the corresponding reactions at a saturated carbon (Sections 5.1.1.1 and 5.1.1.2)—inversion for nucleophilic substitution, and retention, or perhaps occasionally inversion, for electrophilic substitution. In practice, SN2 reactions at trigonal carbon are rare,503 and their stereochemistry, where inversion is known,504 barely established. Electrophilic attack, like the reactions of vinyllithium reagents with protons, aldehydes and carbon dioxide, takes place invariably 

In practice, retention of configuration is commonly observed, as in the stereospecific reactions of the geometric isomers 5.101 — 5.102,506 showing that the 60° rotation, to give the intermediate 5.97, is understandably more frequent than the 120° rotation. 

2 Electrophilic Substitution by Addition-Elimination. Electrophilic attack has an exactly parallel series of events, which is best known from the electrophilic substitution of vinylsilanes. The electrophile attacks from above (or below) the n bond in the vinylsilane 5.103, moving towards the creation of an intermediate carbocation 5.104. Rotation about the cr bond can take place either clockwise by 60° to give the 

The exception to this pattern is bromodesilylation, and a few similar reactions, in which the electrophilic attack 5.113 is followed by nucleophilic opening of the bridged intermediate 5.114.507 Rotation 5.115, followed by anti elimination of the silyl group and bromide ion 5.116, give the product of inversion of configuration 5.117. 

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