Making Mercury-Ptotosensitized Dehydrodimerization into an Organic Synthetic Method: Vapor Pressure Selectivity and the Behavior of Functionalized Substrates

Article abstract of DOI:10.1021/ja00190a031

Mercury-photosensitized dehydrodimerization in the vapor phase can be made synthetically useful by taking advantage of a simple reflux apparatus (Figure 1), in which the products promptly condense and are protected from further conversion.This vapor pressure selectivity gives high chemical selectivity even at high conversion and on a multigram scale.Mercury absorbs 254-nm light to give the ³P₁ excited state (Hg*), which homolyses a C-H bond of the substrate with a 3^o>2^o>1^o selectivity.Quantitative prediction of product mixtures in alkane dimerization and in alkane-alkane cross-dimerizations is discussed.Radical disproportionation gives alkene, but this intermediate is recycled back into the radical pool via H atom attack, which is beneficial both for yield and selectivity.The method is very efficient at constructing C-C bonds between highly substituted carbon atoms, yet the method fails if a dimer has four sets of obligatory 1,3-syn methyl-methyl steric repulsions, as in the unknown 2,3,4,4,5,5,6,7-octamethyloctane.We have extended the range of substrates susceptible to the reaction, for example to higher alcohols, ethers, silanes, partially fluorinated alcohols, and partially fluorinated ethers.We see selectivity for dimers involving C-H bonds α to O or N and for S-H over C-H.An important advantage of our experimental conditions in the case of alcohols is that the aldehyde or ketone disproportionation product (which is not subject to H^. attack) is swept out of the system by the stream of H₂ also produced, so it does not remain and inhibit the rate and lower the selectivity. k_dis/k_rec is estimated for a number of radicals studied.The very hindered 3^o 1,4-dimethylcyclohex-1-yl radical is notable in having a k_dis/k_rec as high as 7.1.