Alkynes, photoinduced alkylation

Photoinduced Alkylation of Alkenes and Alkynes by Alkanes, Alcohols and Alkyl Halides... 

From a synthetic point of view, bond forming steps are the most important reactions of radical ions [202]. Several principle possibilities have been described in Section 8.1 and are summarized in Scheme 52. Many carbo- and heterocyclic ring systems can be constructed by (inter- and intramolecular) radical addition to alkenes, alkynes, or arenes. Coupling of carbonyl radical anions leads to pinacols either intra-or inter-molecular which can be further modified to give 1,2-diols, acyloins or alkenes. Radical combination reactions with alkyl radicals afford the opportunity to synthesize macrocyclic rings. These radical ion-radical pairs can be generated most efficiently by inter- or intramolecular photoinduced electron transfer. 

IV-Aryl and iV-alkyl substituted imidazole-fused enediynes 785 undergo thermally promoted Bergman cycloaroma tization to the benzimidazoles 786 at 145 °C (Scheme 194). Kinetic data indicate that iV-aryl substitution enhances the rate by up to sevenfold <2004TL3621>. 4,5-Bis-(alkyn-l-yl)imidazoles also undergo photoinduced Bergman cycloaromatization <2005TL1373>. 

The facile photosensitized oxidation of tetraalkylstannanes (R4 n) and related group-14 compounds has been widely exploited by Mella and co-workers to form carbon-centered radicals. Photoinduced electron transfer from tetraalkylstannanes to a sensitizer, such as aromatic nitriles and esters (including tetramethyl pyromellitate, TMPM), affords a radical cation [R4 n] that can fragment to form an alkyl radical (R) together with the R3Sn+ cation. The alkyl radicals can then react with electron-poor alkenes, alkynes, aromatics, or the radical anion formed from the photosensitizer to form new carbon-carbon bonds. Careful choice of the photosensitizer can ensure that the radical anion selectively reduces the radical adduct (derived from addition to a double bond) rather than the first-formed alkyl radical (Scheme 5). 

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