Alkynes other reactions associated with

F. Other Reactions Associated with Alkyne Hydrosilylation. 1732... 

Due to the low reactivity of alkyl and arylorganozinc reagents towards alkenes and alkynes, it appears clear that the carbozincation chemistry for this class of reagents is intimately associated with transition metal catalysts. Some of the metal-catalyzed/promoted reactions do indeed produce organozinc reagents as the final organometallic species that can further react with an appropriate electrophile, whereas other processes lead to highly functionalized products by an entirely different pathway. 

The (n,n excited state of a ketone has electrophilic character, similar to that associated with alkoxy radicals, and it is not surprising that these excited states readily attack carbon-carbon multiple bonds. The overall reaction that normally ensues is a cycloaddition, giving a four-membered oxygen heterocycle—an oxetane from an alkene addend (4.62), or an oxete from an alkyne addend (4.63). Some oxetanes are of interest in their own right, but many are useful intermediates in the synthesis of other compounds. 

In addition to the preceding advances primarily associated with heterocyclic substrates, similar additions were observed with simple ALformyl substrates (Scheme 3-100). Moreover, terminal alkynes were effective substrates in additions to styrenes and 1,3-dienes. Based on this combination of recent developments, the nickel-catalyzed fiinctionalization of a variety of relatively unreactive C-H bonds has now been demonstrated. Given the range of substrates that participates, many other classes of reactions that involve nickel-catalyzed C-H fimctionalization are to be expected. The special role of the combined effect of nickel and Lewis acid cocatalysis has emerged as an important influence in this growing body of catalytic processes. 

To account for the rates of reaction and product distributions observed in these and other studies, Fahey and co-workers suggested that two competitive processes are involved. As shown in equations 9.68 through 9.70, both processes begin with the formation of an alkyne-HCl complex or its kinetic equivalent, an association of the alkyne and HCl in a solvent cage. If the developing carbocation is sufficiently stable, the complex can proceed to a vinyl cation-chloride ion pair, leading to product formation by an AdE2 mechanism (equation 9.69). Alternatively, the complex can react with added chloride ion to form product by an AdnS mechanism (equation 9.70). In the absence of added chloride ion, another molecule of HCl may serve the same... 

Alkenes and alkynes are capable of normal substitution reactions like other nucleophiles, but this is rarely an associative process with 18-electron systems because alkenes and alkynes are poor nucleophiles. Such systems require prior dissociation of a ligand to allow coordination of the alkene, which can then form strong complexes because of the 7C back bonding from the metal to the Jt orbital of the C—C multiple bond. The bonding n electrons have relatively low nucleophilicity. However, in coordinatively unsaturated systems, prior dissociation is not a problem. The expected initial reactions are shown in Scheme 5.21. 

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