A Route to Carbasugar Analogues - Jonathan Clayden - The ...

2.4 – Mechanistic discussion
Whilst no mechanistic studies were published, Meyers proposed a polar mechanism for
the dearomatising addition to 2-naphyloxazolines 38 giving azaenolate 132 (Scheme
2.24). Meyers states that the reactions gradually formed red solutions over the course
or an hour, which decolourised upon quench. 31 The reaction also tolerated a large
range of organolithiums.
Ph OMe Ph OMe Ph OMe
O
N
Li
O
NLi
n-Bu
MeI
O
N
n-Bu
n-Bu
38 132
97%
d.r. 47:3
Scheme 2.24 – polar reaction mechanism proposed by Meyers
In contrast, the colour of our dearomatisation reaction varies with substrate, regardless
of whether addition occurs or not. The reaction is complete within minutes, and deep
colouration observed immediately indicating that trace amounts of intermediate form
rapidly. Such a rapid dearomatisation, without warming above –78 °C is quite
remarkable; for example the kinetically more favourable intramolecular dearomatising
cyclisations of Clayden require warming to ambient temperature. 88
Miyano and Bertoli propose that the dearomatising addition of organometallic reagents
to BHA benzoates and dinitrobenzenes respectively proceed through a SET pathway
(section 1.3.1). Whilst these claims were made with different degrees of experimental
support, they lend credence to the possibility of an ET process. Two approaches have
been undertaken to study the mechanism of reaction; EPR spectroscopy and radical
trapping experiments, and are described below.
2.4.2.a Electron paramagnetic resonance (EPR)
The defining characteristic of a radical is its unpaired electron, which it paramagnetic.
Just as NMR spectroscopy measures the energy difference of nuclear spin states in an
applied magnetic field, so does EPR spectroscopy measure the difference of spin states
of the unpaired electron. This energy, denoted g, is of similar magnitude to microwave
radiation, and can be characteristic of the type of compound, but has to be measured
78