Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 441

Y Y PdAr Y Pd

ArPd +

Ar

R () n R () n R () n

ArPdX

Y Ar Y PdX Y

or

PdX Ar Ar

R ( n R () n R () n

Y Ar Y

Ar

R ( n R () n

Scheme 9.139 Chelation control over regioselectivity.

PhB (OH) 2
Pd(OAc) 2 (5 mol%)
AgOAc, CuF 2, KHF 2
Me 2 CO, 8 5°C, 5 h

Et Et

OAc Ph OAc

Scheme 9.140 Retention of double-bond position in the deborylative arylation of
allylic acetates.

O

Pd(IPr)(OTs) 2 (6 mol% )
Cu(OTf) 2, O 2, DMA
35°C, 2 4 h

HO

B Ph

+

() 7

O

HO Ph

() 7

89%

Scheme 9.141 Retention of double-bond position in alkenes not capable of intra-
molecular chelation.

Probably, however, the regioselectivity is governed by other factors,
besides involving intramolecular chelation in intermediates. A very inter-
esting system was reported by Werner and Sigman. 402 The complex
Pd(I i Pr)(OTs) 2 403 was shown to serve in the systemwith Cu(OTf) 2 (20 mol%) as
an intermediate reoxidizer and O 2 as terminal reoxidizer in DMA at 40 1C for
the arylation of a wide range of functionally o-substituted terminal olefins by
ethylene glycol esters of arylboronic acids to yield the arylation products with
high regioselectivity (conservation of double-bond position) and stereo-
selectivity (E-isomers were the major products), disregarding the nature of
functionality, which excludes intramolecular chelation as the main factor to
account for the high regioselectivity of PdH elimination (Scheme 9.141).
Even more interesting is the complete absence of racemization in a
proximal chiral centre, which would necessarily have occurred if double-
bond migration took place. This finding permits thermodynamic control
with equilibrium bond shifts favouring the most stable conjugated position
to be completely ruled out (Scheme 9.142).