Nucleophilic rearrangement

Another feature of systems that are subject to B-strain is their reluctance to form strained substitution products. The cationic intermediates usually escape to elimination products in preference to capture by a nucleophile. Rearrangements are also common. 2-Methyl-2-adamantyl p-nitrobenzoate gives 82% methyleneadamantane by elimination and 18% 2-methyl-2-adamantanol by substitution in aqueous acetone. Elimination accounts for 95% of the product from 2-neopentyl-2-adaman l p-nitrobenzoate. The major product (83%) from 2-r-butyl-2-adamantyl p-nitrobenzoate is the rearranged alkene 5. 

After the migration has taken place, the atom at the migration origin (A) must necessarily have an open sextet. In the third step, this atom acquires an octet. In the case of carbocations, the most common third steps are combinations with a nucleophile (rearrangement with substitution) and loss of H (rearrangement with elimination). 

Schematically, the two major types of reactions undergone by epoxides are rearrangements and addition of nucleophiles. Rearrangements can lead to toxic intermediates, precursors, or metabolites, whereas nucleophilic additions can lead to alkylation of biomacromolecules, i.e., formation of covalent adducts.

Harada, T. Mukaiyama, T. Trityl antimonate-catalyzed sequential reactions of epoxides with silylated nucleophiles. Rearrangement of epoxides and C-C or C-O bond forming nucleophilic reaction onto the intermediate carbonyl compounds. Bull. Chem. Soc. Jpn. 1993, 66, 882-891. 

The rearrangement of cyclopropylcarbene to cyclobutene was studied theoretically using B3LYP at the 6-311G(d) level.58 The results show that the rearrangement of cyclopropylcarbene to cyclobutene is an electrophilic rearrangement, very different from the traditional mechanism of nucleophilic rearrangement for carbocations. 

The carbanion generated from the dinitrile 24 causes a Smiles-type nucleophilic rearrangement to produce new anions 25 and 26. The tandem intramolecular cyclization of the latter affords dihydrothiophene 27. Thieno[2,3-3][l,6]naphthyridine 28 is derived from 27 in four steps (Scheme 6) <1995H(41)1307>. 

Nucleophilic rearrangements formally occur in three steps. These steps may be illustrated by the rearrangement of a carbonium ion. A typical example is provided by the neopentyl system. It will be recalled that the SN2 mechanism at such a centre is slow, because of the steric hindrance caused by the /-butyl group in the (5-position. Furthermore, normal elimination is impossible, because there is no (5-hydrogen. 

The Sn-C bond can be cleaved with hydrogen peroxide44-63 or peroxyacids.64 The reactions take place with retention of configuration, and presumably involve a nucleophilic rearrangement, for example equation 5-50.44 Ozonolysis has been used on a few occasions (e.g. equation 5-51),65 and also oxygen in the presence of a transition metal catalyst.66... 

The first example of a nucleophilic rearrangement leading to the formation of the Si—C—Si structure is shown in the following equation (63) ... 

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