Chemistry Reference
In-Depth Information
complexes is more ordered and chirality transfer from an appropriate an-
cillary can be expected. Indeed, a series of papers by Yu and co-workers
described this approach, using protected amino acids as chiral ancillaries
and rather sophisticated substrates. The most interesting feature of this
process was an extremely strong influence of alkali metal counterions on
both material and optical yields,
372
showing a strong dependence of the
outcome on the structure of intermediate complexes in which the substrate
and chiral ancillary are bonded in a proper configuration through chelation
of counterions (Scheme 9.127).
373
The same protected amino acids were shown by Cui and co-workers to be
fairly effective for induction of planar chirality in the oxidative alkenylation
of dimethylaminomethylferrocene (Scheme 9.128).
374
A promising development of directed palladation methodology uses
temporarily installed directing groups that are subsequently removed from
the products. Thus, a 2-pyridylsulfoxide was reported by Carretero and co-
workers as a general and removable directing group in alkenylations per-
formed in the presence of K
2
S
2
O
8
or PhI(OAc)
2
as terminal oxidants.
375,376
The nitrogen atom of the pyridine ring and not the sulfoxide oxygen lend the
directing assistance, as e.g. phenylsulfoxide residues failed to show the ef-
fect. This methodology was successfully used in the synthesis of resveratrol
(Scheme 9.129).
A similar 2-pyridylsulfonyl substituent at the pyrrole or indole nitrogen
was established by Carretero and co-workers to serve as an excellent dir-
ecting substituent. Although other acyls and sulfonyls are known also to
direct alkenylation at a 2-site, the use of the 2-pyridylsulfonyl group was
reported to afford a more reliable alkenylation protocol, useful for a broad
range of indoles, pyrroles and olefins. The method uses Cu(II) as terminal
oxidant and in addition to monoalkenylation also affords bisalkenylation
(Scheme 9.130).
377
Ph
COONa
COOH
Pd(O Ac)
2
(5 mo l%)
Boc-Ile-OH, BQ, KHCO
3
t-AmOH, 90°C, 48 h
*
COOBu
+
73% (ee 9 7%)
Scheme 9.127 Enantioselective alkenylation of a racemic 1,1-diphenylpropionate.
Pd(O Ac)
2
(5 mo l%)
Boc-Phe-O H, K
2
CO
3
DMF, air , 60°C, 5 h
NMe
2
NMe
2
Z
Fe
+
Fe
Z
Z=COOR, Ar,Cy,CONMe
2,
P(O)(OEt)
2
65-98% (ee 91-99% )
Scheme 9.128 Enantioselective
directed
oxidative
alkenylation
of
dimethylaminomethylferrocene.
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