Alkane desaturation by concerted double hydrogen atom transfer to benzyne

Benzynes are capable of concerted removal of two vicinal hydrogen atoms from a hydrocarbon, a discovery enabled by the thermal generation of reactive benzyne intermediates through the hexadehydro-Diels–Alder cycloisomerization reaction of triyne substrates. A new route for C–H activation This paper reports a new route for the dehydrogenation of an alkane to an alkene — a reaction of long-standing interest to synthetic chemists. The authors show that benzynes are capable of concerted, bimolecular removal of two vicinal hydrogen atoms from a hydrocarbon. The discovery of this chemical transformation was enabled by the thermal generation of reactive benzyne intermediates through the hexadehydro-Diels–Alder cycloisomerization reaction of triyne substrates. This appears to be the first report of a single-step, bimolecular reaction in which two hydrogen atoms are simultaneously transferred from a saturated alkane. The removal of two vicinal hydrogen atoms from an alkane to produce an alkene is a challenge for synthetic chemists 1 , 2 . In nature, desaturases and acetylenases are adept at achieving this essential oxidative functionalization reaction, for example during the biosynthesis of unsaturated fatty acids 3 , eicosanoids, gibberellins 4 and carotenoids 5 . Alkane-to-alkene conversion almost always involves one or more chemical intermediates in a multistep reaction pathway; these may be either isolable species (such as alcohols or alkyl halides) or reactive intermediates (such as carbocations, alkyl radicals, or σ-alkyl-metal species). Here we report a desaturation reaction of simple, unactivated alkanes that is mechanistically unique. We show that benzynes are capable of the concerted removal of two vicinal hydrogen atoms from a hydrocarbon. The discovery of this exothermic, net redox process was enabled by the simple thermal generation of reactive benzyne intermediates through the hexadehydro-Diels–Alder cycloisomerization reaction of triyne substrates 6 . We are not aware of any single-step, bimolecular reaction in which two hydrogen atoms are simultaneously transferred from a saturated alkane. Computational studies indicate a preferred geometry with eclipsed vicinal C–H bonds in the alkane donor.