Synthetic applications of flash vacuum pyrolysis over magnesium

Abstract

The thermal reaction over activated magnesium of some 120 organic substrates has been investigated under flash vacuum pyrolysis(FVP) conditions. The activation was achieved by freshly resublimed Grignard grade magnesium onto glass wool at 700°C under vacuum. Magnesium prepared in this form showed an effective dehalogenating activity on a wide range of organic halides. FVP of simple aliphatic halides like 2-chloro-2,3-dimethylbutane and 2,3-dichloro-2,3-dimethylbutane, resulted in dehydrohalogenation to give the corresponding monoenes and dienes. The dehydrobromination of neopentyl bromide over magnesium gave a mixture of 2-methyl-2-butene and 2-methyl-l-butene, a product mixture that cannot be accounted for by either radical or carbene chemistry. A similar process was observed with terminal dihalides to give dienes, however with increased chain length a dehalogenation process involving hydrogen transfer predominates to give monoenes. The debromination of 1,3-dibromopropane over magnesium led to cyclopropane. Dehydrohalogenation was similarly observed with cyclic dihalides, haloalkenes and haloalkynes and for substrates of adequate chain length, dehalogenation of the haloalkenes or haloalkynes was followed by cyclisation on to the unsaturated end of the molecule to give cyclic products. Germinal dihalides underwent dehalogenative homocoupling to give symmetrical dienes except for 1,1-dichloropropane and 2,2-dichloropropane where simple dehydrochlorination was observed to give the chloroalkenes. FVP of benzylic- and benzylidene- halides over magnesium led to dehalogenative homocoupling to give bibenzyls and stilbenes respectively. Various substituted bibenzyls and stilbenes were prepared in moderate to high yields. The amount of magnesium surface available for reaction and a comparative study of the thermal reaction of zinc, calcium and magnesium with benzyl chloride was carried out. On pyrolysis of o-halobenzyl halides and o-halobenzylidene halides in the presence of excess magnesium, the loss of both side chain and ring halogens led to coupling and cyclisation to 9,10-dihydrophenanthrene and phenanthrene respectively. The ease of ring dehalogenation decreases from iodine to bromine and chlorine, with fluorine virtually unreactive. Attempts to prepare symmetrical disubstituted phenanthrenes from substituted o-halobenzylidene chlorides met with only limited success. Under the pyrolytic conditions, benzotrichloride was converted to α, α'-dichlorostilbene, α, α, α', α'-tetrachlorobibenzyl and diphenylacetylene, the yield of the last two products being dependent on the reaction conditions. The thermal dehalogenation of α, α'-dihalo-o-xylenes over magnesium gave benzocyclobutene. The same process afforded fluorinated benzocyclobutenes and benzodicyclobutene in good yields. The debromination of α, α, α', α'-tetrabromo-m-xylene over magnesium led to pyrene. For haloalkylbenzenes, the basic reaction on pyrolysis over magnesium was dehydrohalogenation to arylalkenes, but this was sometimes accompanied by bond isomerisation and rearrangement. In the case of 1-chloro-4-phenylbutane, an additional process of cyclisation on to the phenyl ring to give tetrahydronaphthalene was observed. The thermal reaction of 3-chloropropiophenone and 2-bromoacetophenone over magnesium led not only to dehalogenation products but also to deoxygenated products. 2-Bromoethyl phenyl ether mainly underwent dehalogenation and fragmentation to phenol. The dehalogenation of chlorobenzene and 1,2-dihalobenzenes was also achieved on pyrolysis over magnesium. Although the main product obtained from the latter was triphenylene, the involvement of benzyne as an intermediate is in doubt. The dehalogenation of long chain acid chloride over magnesium, Was accompanied by fragmentation to give a mixture of alkenes and alkanes. The magnesium-induced 1,6-dehalogenation of α, α'-halogenated p -xylenes in the gas phase led to p-xylylenes, which polymerised. α, α'-Dichloro-p-xylene gave poly(p-xylylene) and poly(a-chloro-p-xylyene); while α, α, α', α'-tetrachloro-p-xylene and α, α, α', α'-tetrabromo-p-xylene gave poly(α, α'-dichloro-p-xylylene) and poly(α, α'-dibromo-p-xylyene) respectively. Also prepared were poly(α, α,α',a'-tetrachloro-p-xylylene) and poly(α, α, α', α'-tetrafluoro-p- xylylene) from the corresponding hexahalo-p-xylene. Thermal stability measurements and the solid state 13C NMR, including molecular weight estimation were carried out on the polymers. Some of the processes observed with aryl compounds on pyrolysis Over magnesium were extented to halogenated thiophene compounds, especially for the preparation of 1,2-dithienylethenes and derivatives. Attempts to prepare benzodithiophenes from halo-dichloromethyl- thiophenes gave halogenated products and isomeric benzodithiophenes. The isomeric benzodithiophenes obtained and the likely route to them was investigated. Attempts to generated 2,3-thiophyne by dehalogenation of mono- and di-halothiophe.nes were inconclusive. The thermal reaction of a series of epoxides in the presence or absence of magnesium was investigated. The main process in the absence of magnesium, was rearrangement to carbonyl compounds, while in the presence of magnesium, deoxygenation and dehydration reactions were observed in addition to the thermal rearrangement.