Scission, alkynes

Radical-mediated silyldesulfonylation of various vinyl and (a-fluoro)vinyl sulfones 21 with (TMSlsSiH (Reaction 25) provide access to vinyl and (a-fluoro)vinyl silanes 22. These reactions presumably occur via a radical addition of (TMSlsSi radical followed by /)-scission with the ejection of PhS02 radical. Hydrogen abstraction from (TMSlsSiH by PhS02 radical completes the cycle of these chain reactions. Such silyldesulfonylation provides a flexible alternative to the hydrosilylation of alkynes with (TMSlsSiH (see below). On oxidative treatment with hydrogen peroxide in basic aqueous solution, compound 22 undergoes Pd-catalyzed cross-couplings with aryl halides. 

Silylstannanes undergo regiospecific addition to alkenes and alkynes with scission of the Sn—Si bond, as illustrated in reactions 15 and 16272. 

Alkyne Coordination, Dimerization, and Scission on a Tungsten—Triiridium Cluster... 

Alkynes, coordination, dimerization, and scission on tungsten-triiridium cluster core, 127-134 Alloys... 

Alkyne scission reactions also occur and some are of exceptional interest and importance. Schrock, et al.166 discovered the following remarkable reaction ... 

In saturated solvents, Co2(CO)e(RC2H) complexes react with nor-bornadiene to give (7t-C5H5)Co(CO)2 (141). The cyclopentadienyl ligand arises from a reverse Diels-Alder scission of norbornadiene. In aromatic solvents, Co4(CO)g(arene) complexes are formed the fate of the alkyne was not determined. Co2(CO)g(PhC2Ph) reacts with norbornadiene to give Co2(CO)4(norbornadiene)2 (142). 

Alkyne scission also occurs when a rhodium or iridium trimetallic cluster is heated under vacuum (449). 

Perhaps the most remarkable illustration of the ability of metals to activate alkynes comes from reactions in which complete scission of the carbon-carbon triple bond occurs. On the stoichiometric level these include examples in which carbyne complexes are produced from alkyne completes as in the melt-thermolysis of CpCo(PPh3)(RCsCR) [112] or from reactions of alkynes with unsaturated metal species (Scheme 4-34) [113]. The remarkable alkyne metathesis reaction (Scheme 4-35), which involves overall cleavage and regeneration of two o-and four rt-bonds, is conceptually related. A variety of functionalized alkynes can be tolerated as metathesis substrates [114] and especially effective catalysts for these reactions are Mo(VI)-and W(VI)-carbyne complexes. Metallacyclobutadienes 64, formed by the reaction of the alkyne with a metal-carbyne complex, appear to be central intermediates in these reactions and the equilibrium between metallacycle and alkyne/metal-carbyne is observable in some cases [115]. 

Approximately 20 examples of alkyne scission have now been observed over the past 20 years or so and are detailed in Tables 1-3 below, classified according to the type of reaction involved. The reactions have been divided, rather arbitrarily in some cases, into three categories ... 

Table 1. Alkyne scission in reactions of alkynes with complexes.

Alkyne Scission on Meta Cluster Frameworks Table 2. Alkyne scission by thermolysis of metal-alkyne complexes. 223... 

Historically speaking, the most studied reactions are those involving the formation of [M3( 3-CR)2Cp3] clusters where M = Co, Rh, Ir. This can be achieved from mononuclear units such as CpM(CO)2 or from preformed alkyne clusters M3(CO)(/ 3-C2R2)Cp3 in each case the scission step occurs at a trinuclear metal... 

Table 3. Alkyne scission by reaction of alkyne complexes with cluster-building reagents.

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