Unsaturated system electrophilic addition

P. B. de la Mare and R. Bolton, Electrophilic Additions to Unsaturated Systems, 2nd ed., Elsevier, New York, 1982. 

Such reactions involve the addition of organometallic reagent to unsaturated systems like alkynes, alkenes, allenes, or related structures in order to create a new C-C bond and C-M bond at the same time. The C-M bond can then be functionalized by an electrophile to generate a wide range of products. 

De la Mare, P.B.D. Bolton, R. Electrophilic Additions to Unsaturated Systems. Elsevier Scient. Publ. Co. Amsterdam, Holland, 1982 p 247. 

Therefore, although it is generally accepted that electrophilic mercuration of di- and polycyclic systems containing a double bond takes place in accordance with the exo-syn addition rule, at least a part of the reported results shows that the strain of the unsaturated system is insufficient to be the only determining factor for syn addition of mercury salts. 

During the coverage period of this chapter, reviews have appeared on the following topics reactions of electrophiles with polyfluorinated alkenes, the mechanisms of intramolecular hydroacylation and hydrosilylation, Prins reaction (reviewed and redefined), synthesis of esters of /3-amino acids by Michael addition of amines and metal amides to esters of a,/3-unsaturated carboxylic acids," the 1,4-addition of benzotriazole-stabilized carbanions to Michael acceptors, control of asymmetry in Michael additions via the use of nucleophiles bearing chiral centres, a-unsaturated systems with the chirality at the y-position, and the presence of chiral ligands or other chiral mediators, syntheses of carbo- and hetero-cyclic compounds via Michael addition of enolates and activated phenols, respectively, to o ,jS-unsaturated nitriles, and transition metal catalysis of the Michael addition of 1,3-dicarbonyl compounds. ... 

In general, the mechanisms of nucleophilic additions to double bonds have not been as much studied or systemized as those of electrophilic addition. Reactions 7.51 and 7.52 are examples of the very useful Michael condensation, in which a carbanion adds to an a,/ -unsaturated carbonyl or nitrile compound. The usefulness of these reactions arises from the fact that the number of ways of building longer carbon chains from smaller ones is limited. 

The presence of two electrophilic reaction centers in the molecule of o -unsaturated carbonyls is responsible for their ability to participate in the synthesis of heterocycles. Such compounds can react as ambident electrophiles owing to delocalization of electron density in a C=C-C=0 system. The addition of nucleophiles to these molecules can proceed in one of two main directions—via attack of the carbonyl group (1,2-addition) or involving the / -carbon (1,4-addition). 

Our interest in this chapter is in silver-catalyzed cycloisomerization reactions. Therefore, we shall present different silver-catalyzed cycloisomerization reactions as a function of the nucleophilic and electrophilic moiety. Cycloisomerization reactions including the classical ene-yne cycloisomerization (with X = CHR, Scheme 5.1), and the related heterocyclization reactions with heteroatoms embedded in unsaturated systems (X = NR, O Scheme 5.1) belong to the same reaction family. In addition, the alkynyl part can be exchanged for an allene unit. Internal or external nucleophiles (Nu) can then stabilize, through cascade reactions, the positive charge created.24... 

At 0°C, with HBr 4-bromo-4,5-dihydro-2-(2-pyrrolyl)furanium bromides 284 are seen in the NMR spectra, resulting from the addition of HBr to the protonated furan moiety (Scheme 63) <1998MI30>. Thus, it is demonstrated that under certain conditions pyrrole compounds can undergo electrophilic addition like nonaromatic unsaturated systems. 

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