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retention of configuration. Thus, as shown in eqn (11.29), when PCy
3
is used
as a ligand for Ni, the reaction proceeds with retention, whereas with SIMes it
proceeds with inversion of configuration. n-Butanol proved to be a key addi-
tive in the reaction in order to achieve high levels of stereospecificity. Of the
derivatives employed, the carbamate shown in 119 gives the best combination
of high yield and enantiospecificity, with other protecting groups for the
alcohol giving lower yields or selectivities. Interestingly, however, when the
pivalate ester 121 is employed instead, the requirement for an extended
aromatic ring on the starting electrophile is lifted, dramatically expanding the
scope of this important transformation [eqn (11.30)].
(11
:
29)
120
119
(11
:
30)
122
121
In related work appearing soon after the Jarvo study, Watson and
co-workers also demonstrated that the pivalate group could be effectively
employed in the stereospecific Suzuki-Miyaura coupling reaction shown in
eqn (11.31).
88
Again, nickel catalysis was employed but in this case an added
ligand was not required. Optimal conditions included the use of arylborox-
ines, NaOMe as a base at 70 1C in toluene with 5% Ni(COD)
2
as catalyst.
Under these conditions, a variety of diarylethanes were produced in 86-99%
ee. Interestingly, as under the conditions used by Jarvo and co-workers, the
presence of a p-system with extended conjugation was typically required, but
one example was given a in which a biphenyl substituent was employed.
Again, the reaction was found to proceed with inversion of configuration.
(11
:
31)
123
12
4
In addition to the use of C-O bonds as nascent electrophiles, Watson and
co-workers also described the cleavage of C-N bonds, activated as ammo-
nium groups.
89
As shown in eqn (11.32), the Suzuki-Miyaura coupling of
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