Electrophilic addition X-Y reagents

Electrophilic addition of the halogens and related X—Y reagents to alkenes and alkynes has been a standard procedure since the beginning of modem organic chemistry.1 Anti electrophilic bromination of such simple compounds as cyclohexene and ( )- and (Z)-2-butene, and variants of this reaction when water or methanol are solvents (formation of halohydrin or their methyl ethers, respectively), are frequently employed as prototype examples of stereospecific reactions in elementary courses in organic chemistry. A simple test for unsaturation involves addition of a dilute solution of bromine in CCU to the... 

E. Block, A. L. Schwan, Electrophilic Addition of X-Y Reagents to Alkenes and Alkynes, in Comprehensive Organic Synthesis (B. M. Trost, I. Fleming, Eds.), Vol. 4, 329, Pergamon Press, Oxford, 1991. 

We considered that nucleophilic addition is promoted when electron transfer from the reagent to the substrate is facilitated.41 Consequently, we proposed that X, Y and Z be classified according to their electrophilicity, as measured by the energy of their antibonding orbitals o cx, o CY and o cz. Thus, the best electron acceptor is considered to be the bulkiest group. This approach uses only one parameter and eliminates the tricky problem of defining effective bulk. See, however, the discussion on pp. 168-169. 

It seems clear that cluster-substituted carbonium ions are easily generated, are very stable thermally, and are of sufficient electrophilic reactivity to be useful in the synthesis of many new functional cluster compounds. Before we consider further their exceptional stability and the mode of their stabilization, it is of interest to mention some chemical consequences of the high stability of a positive charge generated a to the apical carbon atom of the (OC)9Co3C cluster. For instance, one might expect the addition of the positive part of an ionic or polar reagent, X+Y-, to the... 

Usually y.B-unsaturated acids (260) are employed in these lactonizations. The initial step involves an electrophilic attack at the double bond, either by a proton or a reagent X—Y (264). An activated species (261) is thus generated, suitable for the nucleophilic addition of the carboxy group, which in principle may close to a six- or five-membered lactone ring (263 or 262 equation 92). Normally, (262) is preferred on the basis of vector approach analysis. 

In addition to consideration of whether electrophiles interact with Pd or C atoms in Pd—C bonds, c-bond metathesis (interaction of reagent X—Y across a Pd—C bond) may be appropriate for some systems. 

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