Reactions and reactivity of allylic and benzylic carbamates.
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Date
1995
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Abstract
The purpose of this investigation was to study the effect of the carbamate group on the
reactions and reactivity of substituted 1,1-diphenylmethane and a,ro-diphenyl allylic
compounds. A series of carbamates were prepared and reacted with a variety of
electrophiles and nucleophiles.
l-(O-N,N-dimethylcarbamoyloxy)-l,l-diphenylmethane (i) reacted with a variety of
electrophiles to afford a-substituted carbamate products (H).
Reactions of allylic carbamates with electrophiles proceed with substitution at the carbon
atom a or y to the carbamate. 1-(O-N,N-diethylcarbamoyloxy)-1,3-diphenyl-2-propene
(iii) only reacted with methyl iodide to afford the y-substituted product (iv). Reactions of
1-(O-N,N-diethylcarbamoyloxy)-l ,5-diphenyl-2,4-pentadiene (v) with electrophiles were
all unsuccessful.
Nucleophilic substitution reactions were performed with carbamates (i), (iii) and (v). No
success was achieved in the reactions of (i) with nucleophiles. Carbamates (iii) and (v)
reacted with a few oxygen nucleophiles to afford allylic ethers with simultaneous
elimination of the carbamate group.
Several properties ofterminal-diphenyl carbamates have been revealed:
(a) Benzylic carbamate (i) reacted successfully with a variety of electrophiles.
Nucleophilic substitution is not favoured with the benzylic carbamate. This indicates that
SN2 elimination of the carbamate does not occur in this molecule.
(b) Allylic carbamate (iii) only reacts with methyl iodide indicating that the bulk of the
diethyl substituents on the carbamate group, the bulk of the incoming electrophile and the
size of the phenyl groups are fundamental to the success of reaction. Methylation occurred
only at the y-position.
(c) Conjugated allylic carbamate (v) did not react with any electrophiles, suggesting
that, in addition to the steric factors, the stability offered in the retention of conjugation in
the molecule prevented the formation of the electrophilic substitution products.
(d) Unsymmetrical allylic carbamates (iii) and (v), in which the carbamate occupies a
benzylic position, are more stable than symmetrical allylic carbamates (vi) and (vii) which
decompose to the corresponding alcohol.
(e) Nucleophilic substitution of (iii) and (v) occurred with SN2' elimination of the
carbamate group, the reaction proceeding in a way which faciliates the formation of the
most conjugated product possible.
Nucleophilic substitution of (iii) with the phenoxide anion resulted in the allyl aryl ether
(viii). Several attempts were made to promote the Claisen rearrangement to afford the
allylic phenol (ix) but without success.
Exploitation of the migrational ability and the leaving group ability of the carbamate
moiety were extended to the synthesis of /13-flavene (xi) from allylic alcohol (x). Several
attempts were made to synthesise (x) via the Aldol condensation and Grignard reactions.
The synthesis of (xi) was hindered by the failure to produce allylic alcohol (x).
During the course of these investigations an unexpected decomposition reaction was
discovered. The purification by distillation of allylic carbamate (xii) afforded allylic amine
(xiii) in quantitative yield. The driving force for the reaction is the intramolecular SN2'
elimination of the carbamate with the simultaneous loss of carbon dioxide. This reaction
may have scope in the synthesis of allylic amines.
Description
Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1995.
Keywords
Carbamates., Insecticides., Theses--Chemistry.