Flash vacuum pyrolysis over magnesium. Part 1. Pyrolysis of benzylic, other aryl/alkyl and aliphatic halides

摘要

Flash vacuum pyrolysis over a bed of freshly sublimed magnesium on glass wool results in efficient coupling ofnbenzyl halides to give the corresponding bibenzyls. Where an ortho halogen substituent is present furtherndehalogenation gives some dihydroanthracene and anthracene. Efficient coupling is also observed fornhalomethylnaphthalenes and halodiphenylmethanes while chlorotriphenylmethane gives 4,4u0001-nbis(diphenylmethyl)biphenyl. By using α,αu0001-dihalo-o-xylenes, benzocyclobutenes are obtained in good yield, while thenisomeric α,αu0001-dihalo-p-xylenes give a range of high thermal stability polymers by polymerisation of the initiallynformed p-xylylenes. Other haloalkylbenzenes undergo largely dehydrohalogenation where this is possible, in somencases resulting in cyclisation. Deoxygenation is also observed with haloalkyl phenyl ketones to give phenylalkynes asnwell as other products. With simple alkyl halides there is efficient elimination of HCl or HBr to give alkenes. Fornaliphatic dihalides this also occurs to give dienes but there is also cyclisation to give cycloalkanes and dehalogenationnwith hydrogen atom transfer to give alkenes in some cases. For 5-bromopent-1-ene the products are those expectednfrom a radical pathway but for 6-bromohex-1-ene they are clearly not. For 2,2-dichloropropane and 1,1-ndichloropropane elimination of HCl occurs but for 1,1-dichlorobutane, -pentane and -hexane partial hydrolysisnfollowed by elimination of HCl gives E,E-, E,Z- and Z,Z- isomers of the dialk-1-enyl ethers and fully assigned 13CnNMR data are presented for these. With 6-chlorohex-1-yne and 7-chlorohept-1-yne there is cyclisation to givenmethylenecycloalkanes and -cycloalkynes. The behaviour of 1,2-dibromocyclohexane and 1,2-dichlorocyclooctanenunder these conditions is also examined. Various pieces of evidence are presented that suggest that these processes donnot involve generation of free gas-phase radicals but rather surface-adsorbed organometallic species.