At trigonal carbon

All the mechanisms so far discussed take place at a saturated carbon atom. Nucleophilic substitution is also important at trigonal carbons, especially when the carbon is double bonded to an oxygen, a sulfur, or a nitrogen. Nucleophilic substitution at vinylic carbons is considered in the next section at aromatic carbons in Chapter 13. 

As we have seen (Section 4, p. 191) the range of effective molarities associated with ring-closure reactions is very much greater than that characteristic of intramolecular general acid-base catalysis the main classification is therefore in terms of mechanism. By far the largest section (I, Tables A-D) gives EM s for intramolecular nucleophilic reactions. These can be concerted displacements (mostly at tetrahedral carbon), stepwise displacements (mostly addition-elimination reactions at trigonal carbon), or additions, and they have been classified in terms of the nucleophilic and electrophilic centres. 

In later sections, heterolytic reactions at trigonal carbon centres, and at phosphorus, and reactions of radicals will be treated. 

A.H.M. Renfrew et al.. Stepwise versus Concerted Mechanisms at Trigonal Carbon Transfer of the 1,3,5-Triazinyl Group Between Aryl Oxide Ions in Aqueous Solution, J. Am. Chem. Soc., 1995,117, 5484. 

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 the presence of RUCI3 and oxidant t-BuOOH, 2 was converted to triketone 120 in a good yield (Scheme 39) [139]. Consistent with the preferred exo attack at trigonal carbons of sumanene, reaction of 120 with methylmagnesium bromide selectively gave the product 121 as a single isomer, where all three methyl groups are at the exo positions. 

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