Counterattack reagents

The intermediate A -cyanoammonium bromide has been trapped, and its structure confirmed by chemical, analytical, and spectral data. The BrCN in this reaction has been called a counterattack reagent, that is, a reagent that accomplishes, in one flask, two transformations designed to give the product. ... 

Counterattack reagents in reactions of heterocycles 89T1233. Electrochemistry of heterocycles 87MI10. 

For a review of counterattack reagents, see Hwu, J.R. Gilbert, B.A. Tetrahedron 1989, 45, 1233. For a monograph that discusses most of the reactions in this section, see Stowell, J.C. Carbanions in Organic Synthesis, Wiley, NY, 1979. For a review, see Noyori, R., in Alper, H. Transition Metal Organometallics in Organic Synthesis, Vol. 1, Academic Press, NY, 1976, pp. 83-187. 

When a halogen atom is employed to direct orr/i( -metallation, the Cr(CO)3 unit then allows substitution to follow, provided that an appropriate electrophile is employed (Scheme 10.58). In the case of butyrolactone, ring opening by the lithium reagent 10.225 releases an alkoxide that counterattacks onto the arene, leading to formation of lactone 10.227. 

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