Making mercury-photosensitized dehydrodimerization into an organic synthetic method. Vapor-pressure selectivity and the behavior of functionalized substrates

• Published in: Journal of the American Chemical Society Vol. 111; no. 8; pp. 2935 - 2946
• Main Authors: Brown, Stephen H, Crabtree, Robert H
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 01.04.1989
• Subjects: 400201 - Chemical & Physicochemical Properties; 400500 - Photochemistry; ALCOHOLS; ALKANES; CHEMICAL PREPARATION; CHEMICAL REACTION KINETICS; CHEMICAL REACTIONS; CHEMICAL REACTORS; Chemistry; DATA; DESIGN; DIMERIZATION; DIMERS; EFFICIENCY; ELEMENTS; ETHERS; Exact sciences and technology; EXPERIMENTAL DATA; FLUIDS; GASES; General and physical chemistry; HYDRIDES; HYDROCARBONS; HYDROGEN COMPOUNDS; HYDROXY COMPOUNDS; INFORMATION; INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; KINETICS; MERCURY; METALS; NUMERICAL DATA; ORGANIC COMPOUNDS; ORGANIC OXYGEN COMPOUNDS; ORGANIC SILICON COMPOUNDS; Photochemistry; PHOTOSENSITIVITY; Physical chemistry of induced reactions (with radiations, particles and ultrasonics); PHYSICAL PROPERTIES; POLYMERIZATION; QUANTUM EFFICIENCY; REACTION KINETICS; SENSITIVITY; SILANES; SILICON COMPOUNDS; SYNTHESIS; THERMODYNAMIC PROPERTIES; VAPOR PRESSURE; VAPORS; Hydrocarbon; Saturated compound; Reaction mechanism; Kinetics; Quantum yield; Dimerization; Mercury; Photosensitizer; Monocyclic compound
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja00190a031

Mercury-photosensitized dehydrodimerization in the vapor phase can be make synthetically useful by taking advantage of a simple reflux apparatus, 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 {sup 3}P{sub 1} excited state (Hg*), which homolyses a C-H bond of the substrate with a 3{degree} > 2{degree} > 1{degree} 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 2,3,4,4,5,6,7-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 bond {alpha} 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{sup {sm bullet}} attack) is swept out of the system by the stream of H{sub 2} also produced, so it does not remain and inhibit the rate and lower the selectivity. k{sub dis}/k{sub rec} is estimated for a number of radicals studied. The very hindered 3{degree} 1,4-dimethylcyclohex-1-yl radical is notable in having a k{sub dis}/k{sub rec} as high as 7.1.