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.
Type
Thesis, PhD Doctor of Philosopy
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