52897-15-1Relevant articles and documents
Making Mercury-Ptotosensitized Dehydrodimerization into an Organic Synthetic Method: Vapor Pressure Selectivity and the Behavior of Functionalized Substrates
Brown, Stephen H.,Crabtree, Robert H.
, p. 2935 - 2946 (2007/10/02)
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 3o>2o>1o 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. kdis/krec is estimated for a number of radicals studied.The very hindered 3o 1,4-dimethylcyclohex-1-yl radical is notable in having a kdis/krec as high as 7.1.
