Triethylamine-induced Reactions of Methyl 2,3-Dibromo-2,3-diarylpropanoates in Methanol

Article abstract of DOI:10.1039/a706372e

The title compounds undergo elimination with methanolic triethylamine to afford the corresponding debrominated olefins, most of them through an E2 stereoconvergent process.

Full text of DOI:10.1039/a706372e

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124 J. CHEM. RESEARCH (S), 1998
J. Chem. Research (S),
1998, 124±125$
Triethylamine-induced Reactions of Methyl
2,3-Dibromo-2,3-diarylpropanoates in Methanol$
Mercedes A. Badajoz, Rosana S. Montani and
Mercedes C. Cabaleiro*
Instituto de Quimica Organica, Departamento de Quimica e Ing. Quimica,
Universidad Nacional del Sur, 8000 Bahia Blanca, Argentina
The title compounds undergo elimination with methanolic triethylamine to afford the corresponding debrominated
ole®ns, most of them through an E2 stereoconvergent process.
In a previous paper we described the methanolic methoxide-
and iodide-mediated reactions of methyl (R,S)- and (R,R)-
On this basis, the observation that these diastereoisomers
lead exclusively to their corresponding E ole®ns seems
to demand that the reactions should take place with syn
elimination. Probably, in spite of the favourable orbital
overlap accompanying anti elimination, the energetic
increase arising from interaction between the 3-aryl and the
CO2Me groups in transition state A would be more import-
ant than that due to a eclipsing geometry of the two aryl
groups (transition state B).%
2,3-dibromo-2,3-diarylpropanoates
1 and 2 respectively.
As a continuation of this study we now report the results of
the reaction of these compounds induced by triethylamine
(TEA) in methanol.
1
The H NMR spectra of the crude products of the elimin-
ation of the methyl (R,S)-2,3-dibromo-2,3-diarylpropanoates
(a, b, c, e, f and g) (Table 1) showed only signals of the
E-debrominated ole®ns.
The total selectivity of TEA, also found for I , towards
the debromination pathway might be attributed to
Pseudo-®rst-order kinetics were determined by using a
large excess of the base. According to the data recorded in
Table 1, electron-withdrawing groups such as those present
in compounds b, e, f and g, as well as an electron-releasing
group (compound c), increase the debromination rates only
slightly. This modest in¯uence may be taken as evidence
indicating a rate-controlling transition state with little devel-
opment of charge on either of the carbon atoms involved in
the elimination and is compatible with the kinetic response
of the iodide-induced debromination of the compounds in
the same solvent. These results, associated with the stereo-
chemical nature of the ole®nic products, may satisfactorily
be explained in terms of debromination proceeding by an
E2 mechanism.1
a
response of the 2-bromine to the e€ect of the adjacent
carbonyl group which favours C-20Br over C-30H bond
polarisation. However, the fact that the reactions induced
by MeO show a preference for dehydrobromination seems
to argue against the former explanation. With the available
evidence we do not see a justi®cation of this behaviour and
feel that more detailed investigation is required to uncover
the factors which dictate the discrepancy.
Comparison of the TEA-promoted debromination
reactions of the R,S substrates with those induced by I
indicates that they show closely similar characteristics,
both a€ording the product of trans-elimination (E).
However, for the reactions with the corresponding R,R iso-
mers the situation is quite the reverse. In this case, whereas
the iodide-promoted reaction leads predominantly to the
anti-debrominated ole®n (Z), the stereochemical outcome of
the reaction with TEA is the opposite (E). These results can
be reconciled with the hypothesis that the iodide ion nucleo-
philicity might assist the separation of the 3-bromine in a
transition state leading to trans elimination.1
As regards the corresponding diastereoisomeric substrates
(R,R; a, b, c, e, f and g), Table 1 indicates that the relative
magnitudes of the in¯uence of the aryl substituent on the
debromination rate are also typical of a concerted process.
Table 1 Rate coefficients of the debromination of (R,S)- and
(R,R)-ArCHBrCBrAr'CO2Me (0.006 mol dm 3) promoted by TEA
(4.5 mol dm 3) and TEA hydrochloride (2 mol dm 3) in methanol
at 30 8C
a
a
Ar
Ar'
kR,S
kR,R
a
b
c
d
e
f
Ph
Ph
Ph
Ph
Ph
0.85
1.58
1.03
ca. 75
2.51
1.19
0.27
0.60
0.29
ca. 75
0.52
0.47
p-ClC6H4
p-MeC6H4
p-MeOC6Hb4
p-NO2C6H4
Ph
In contrast to the former substrates, the introduction of a
p-methoxyaryl substituent on C-2 causes the compounds to
behave in a similar way to that exhibited by the reactions
promoted by I and MeO . Thus, the R,S and R,R d
isomers led exclusively to the corresponding dehydro-
brominated ole®n through a ®rst-order kinetic and relatively
rapid process, which is explained as a re¯ection of the
Ph
p-ClC6H4
p-NO2C6H4
p-MeOC6H4
g
h
Ph
Phb
1.26
>130
0.42
>130
aSecond-order coefficients in dm3 mol min
kinetics in min
.
bFirst-order
1
1
1
.
*To receive any correspondence (e-mail: mcabaler@criba.edu.ar).
$This is a Short Paper as de®ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1998, Issue 1]; there is there-
fore no corresponding material in J. Chem. Research (M).
%Consistent with this conclusion is the previously observed lower
rate of the iodide-promoted debromination of RR relative to that of
RS isomers.1

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J. CHEM. RESEARCH (S), 1998 125
Kinetic Procedures.ÐRates were measured at 3020.05 8C. The
reactions were initiated by adding a solution of the appropriate
substrate in methanol (50 cm3, 0.6 mmol) to a solution of triethyl-
amine (0.50 mol)±triethylamine hydrochloride (0.20 mol) bu€er in
the same solvent (50 cm3), which showed that the amine was the
only reactive basic species. Aliquots were withdrawn periodically,
quenched in water and extracted with dichloromethane. The solvent
was removed by evaporation and the residue analysed by 1H NMR
spectroscopy, following the disappearance of the 3-proton resonance
relative to the total methoxycarbonyl peak area. Second-order rate
values were obtained as usual from those of pseudo-®rst-order ones.
The kinetics of the impure (R,R)-b and -c compounds were also
monitored by 1H NMR spectroscopy in a similar way, except that
the area corresponding to the methoxycarbonyl protons due to
the R,S isomer was subtracted from the total methoxycarbonyl
resonance intensity.
known stability of p-methoxybenzyl cations bearing an
a-carbonyl substituent.2
In the case of the R,S and R,R h substrates, ®rst-order
rate coecients of nearly the same value were observed
(Table 1), together with stereoconvergent debromination
leading to the E ole®n. This is an expected consequence of
the in¯uence of the p-methoxy substituent on the 3-phenyl
ring to enhance the leaving-group tendency of the 3-
bromine, leading to a 3-carbonium ion which undergoes
rotation before ejection of the 2-bromine to give the thermo-
dynamically more stable ole®n.}
Experimental
Triethylamine3 and methanol4 were puri®ed according to reported
procedures.
Product analysis.ÐThe reactions were quenched after eight half-
lives, worked up in the usual way and analysed by 1H NMR
spectroscopy.
Methyl 2,3-dibromo-2,2-diarylpropanoates were obtained as
described.1 With the exception of (R,R)-b and (R,R)-c, all the dia-
stereoisomers could be separated by repeated silica gel chromatog-
raphy using light petroleum (bp 60±80 8C)±diethyl ether (49:1) as
eluant. The substrates (R,R)-b and -c were obtained as mixtures
containing 11% and 30% of their R,S isomers respectively. The
mps of the solid compounds were as follows. a: (R,S) 104±106 8C,
(R,R) 98±100 8C; b: (R,S) 65±68 8C; d: (R,R) 102±105 8C; e: (R,S)
153±156 8C, (R,R) 129±132 8C; f: (R,S) 72±74 8C; g: (R,S) 103±
104 8C; h: (R,S) 130±133 8C.
We acknowledge the ®nancial assistance of the Secretaria
de Ciencia y Tecnologia (UNS) and FONCYT (CONICET).
Received, 1st September 1997; Accepted, 25th November 1997
Paper E/7/06372E
References
1 M. A. Badajoz, R. S. Montani and M. C. Cabaleiro, J. Chem.
Soc., Perkin Trans. 2, 1996, 1717, and references cited therein.
2 J. J. Blumenstein, V. C. Ukachukwu, R. S. Mohan and D. L.
Whalen, J. Org. Chem., 1993, 58, 944.
3 N. Barlow, D. R. Marshall and C. J. M. Stirling, J. Chem. Soc.,
Perkin Trans. 2, 1977, 1920.
}A di€erent result was obtained for the reaction of these substrates
with MeO and I , since only solvolytic products were then formed.
We tentatively assign the lack of solvolysis to the presence of
the large excess of triethylamine which probably interacts with the
carbonium centre precluding attack by the solvent.
4 R. A. More O'Ferrall, F. Larkin and P. Walsh, J. Chem. Soc.,
Perkin Trans. 2, 1982, 1573.