Flash vacuum pyrolysis over magnesium. Part 1. Pyrolysis of benzylic, other aryl/alkyl and aliphatic halides
作者：R. Alan Aitken、Philip K. G. Hodgson、John J. Morrison、Adebayo O. Oyewale
DOI：10.1039/b108663d
日期：2002.1.23
Flash vacuum pyrolysis over a bed of freshly sublimed magnesium on glass wool results in efficient coupling of benzyl halides to give the corresponding bibenzyls. Where an ortho halogen substituent is present further dehalogenation gives some dihydroanthracene and anthracene. Efficient coupling is also observed for halomethylnaphthalenes and halodiphenylmethanes while chlorotriphenylmethane gives 4,4′-bis(diphenylmethyl)biphenyl. By using α,α′-dihalo-o-xylenes, benzocyclobutenes are obtained in good yield, while the isomeric α,α′-dihalo-p-xylenes give a range of high thermal stability polymers by polymerisation of the initially formed p-xylylenes. Other haloalkylbenzenes undergo largely dehydrohalogenation where this is possible, in some cases resulting in cyclisation. Deoxygenation is also observed with haloalkyl phenyl ketones to give phenylalkynes as well as other products. With simple alkyl halides
there is efficient elimination of HCl or HBr to give alkenes. For aliphatic dihalides this also occurs to give dienes but there is also cyclisation to give cycloalkanes and dehalogenation with hydrogen atom transfer to give alkenes in some cases. For 5-bromopent-1-ene the products are those expected from a radical pathway but for 6-bromohex-1-ene they are clearly not. For 2,2-dichloropropane and 1,1-dichloropropane elimination of HCl occurs but for 1,1-dichlorobutane, -pentane and -hexane partial hydrolysis followed by elimination of HCl gives E,E-, E,Z- and Z,Z- isomers of the dialk-1-enyl ethers and fully assigned 13C NMR data are presented for these. With 6-chlorohex-1-yne and 7-chlorohept-1-yne there is cyclisation to give methylenecycloalkanes and -cycloalkynes. The behaviour of 1,2-dibromocyclohexane and 1,2-dichlorocyclooctane under these conditions is also examined. Various pieces of evidence are presented that suggest that these processes do
not involve generation of free gas-phase radicals but rather surface-adsorbed organometallic species.
在玻璃棉上覆盖一层新升华的镁,进行闪式真空热解,能有效促使苄基卤化物耦合生成相应的联苄。当有邻位卤素取代基存在时,进一步脱卤生成部分二氢蒽和蒽。卤甲基萘和二苯基甲烷也能高效耦合,而三苯基氯甲烷则生成4,4′-双(二苯甲基)联苯。用α,α′-二卤代邻二甲苯可以获得较高产率的苯并环丁烯,而异构的α,α′-二卤代对二甲苯,通过形成的对二甲苯的聚合,可以得到一系列高热稳定性的聚合物。其他卤代烃苯大体上会脱卤化氢,某些情况下能产生环化反应。同样可以观察到,苯基卤代烷烃脱去羰基生成苯乙炔以及其他产物。简单的烷基卤化物则高效地脱去HCl或HBr生成烯烃。脂肪族二卤化物也会发生这一反应生成二烯,但不发生环化反应生成环烷烃,或在某些情况下发生氢原子转移的脱卤反应生成烯烃。5-溴戊-1-烯的产物符合自由基途径的预期,但6-溴己-1-烯并不符合。2,2-二氯丙烷和1,1-二氯丙烷能脱去HCl,但1,1-二氯丁烷、戊烷和己烷则能部分水解,随后脱去HCl,生成E,E-, E,Z-和Z,Z-异构体二烷-1-烯基醚,并且得到了这些物质的13C NMR全归属数据。6-氯己-1-炔和7-氯庚-1-炔能发生环化反应生成亚甲基环烷烃和环炔烃。本文还考察了1,2-二溴环己烷和1,2-二氯环辛烷在上述条件下的行为。本文给出了众多种证据,表明这些反应过程不涉及气相自由基的形成,而是表面吸附的金属有机物种。
