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

Chapter 1: Introduction
combine more slowly with the aromatic radical anion. ESR studies show the presence
of radical species, although these studies were inconclusive since they were unable to
show that the radical anion was an intermediate in the dearomatisation rather than
another trace species, or the reduced nitrobenzene.
1.3.1.b Nitrile
Cyano groups are highly susceptible to 1,2-addition reactions with organolithiums, and
the lithiation of benzonitriles normally requires the use of a non-nucleophilic base. 14
However, in an extension of his dearomatisation of cyanonaphthalenes, 15 Ortiz found
that at very low temperatures, lithiated phosphine boranes (20) underwent
dearomatising addition in a 1,6-fashion.
CN
CN
PPh 2 BH 3
Li 20
CN
H 2 O
PPh 2 BH 3
21
65%
THF, HMPA (6 eq)
−90 °C 12 hr
PPh 2 BH 3
RX
PPh 2 BH 3
R
CN
22
R = Me, 97% 1:1.2 dr
= allyl, 97% 1:1.5 dr
= Bn, 77% 97:1 dr
16
Scheme 1.7 – dearomatisation of benzonitrile
The above summary shows that alkylation of the dearomatised intermediate proceeds
in much better yield than simple protonation; indeed most of the remaining mass
balance accompanying 21 (32%) were products of 1,2-addition. Ortiz does not
comment on these results which apparently show the electrophilic addition affecting
the prior nucleophilic addition, which could indicate a reversible reaction. The
dearomatisation was highly regioselective with no 1,4-addition observed;
dearomatisation was also achieved for 3-chloro (79%) and 3-methyl (36%)
benzonitriles to give 1,3 dienes similar to 21.
21