Am. Mcanoy et al., The loss of CO from the ortho, meta and para forms of deprotonated methyl benzoate in the gas phase, J CHEM S P2, (8), 2000, pp. 1665-1673
Citations number
39
Categorie Soggetti
Physical Chemistry/Chemical Physics
Journal title
JOURNAL OF THE CHEMICAL SOCIETY-PERKIN TRANSACTIONS 2
The ortho, meta and para anions of methyl benzoate may be made in the sourc
e of a mass spectrometer by the S(N)2(Si) reactions between HO- and methyl
(o-, m-, and p-trimethylsilyl)benzoate respectively. All three anions lose
CO upon collisional activation to form the ortho anion of anisole in the ra
tio ortho>>meta > para. The rearrangement process is charge directed throug
h the ortho anion. Theoretical calculations at the B3LYP/6-311++G(d,p)//HF/
6-31+G(d) level of theory indicate that the conversion of the meta and para
anions to the ortho anion prior to loss of CO involve 1,2-H transfer(s), r
ather than carbon scrambling of the methoxycarbonylphenyl anion. There are
two mechanisms which can account for this rearrangement, viz. (A) cyclisati
on of the ortho anion centre to the carbonyl group of the ester to give a c
yclic carbonyl system in which the incipient methoxide anion substitutes at
one of the two equivalent ring carbons of the three membered ring to yield
an intermediate which loses CO to give the ortho anion of anisole, and (B)
an elimination reaction to give an intermediate benzyne-methoxycarbonyl an
ion complex in which the MeOCO- species acts as a MeO- donor, which then ad
ds to benzyne to yield the ortho anion of anisole. Calculations at the B3LY
P/6-311++G(d,p)//HF/6-31+G(d) level of theory indicate that (i) the barrier
in the first step (the rate determining step) of process A is 87 kJ mol(-1
) less than that for the synchronous benzyne process B, and (ii) there are
more low frequency vibrations in the transition state for benzyne process B
than for the corresponding transition state for process A. Stepwise proces
s A has the lower barrier for the rate determining step, and the lower Arrh
enius factor: we cannot differentiate between these two mechanisms on avail
able evidence.