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Ar
BPin
R
R'
R
R'
Ar-I
57
γ
-selective
coupling
56
Pd(dba)
2
PPh
3
, Ag
2
O,
dioxane, 100 °C
R = Alkyl or Aryl,
R' = Alkyl
Ar
BPin
R
Ar
R
Ar
58
59
α
-selective
coupling
R = Alkyl
Scheme 11.5
Suzuki-Miyaura coupling of unsymmetrical allylboronic esters.
conditions required the use of significantly lower amounts of PPh
3
,
although this still proved to be the ligand of choice. Thus, using only 2
equiv. of phosphine to Pd, moderate to good yields were obtained for the
desired coupling products with high to perfect enantioselectivities
(Table 11.1).
For simple aliphatic substrates, g : a ratios from 78 : 22 to 95 : 5 were
obtained, but consistently higher regioselectivities were observed for
systems containing an aryl group in the allylic position (Table 11.1, Entries
1-3; Table 11.2, Entry 1). Interestingly, if this aryl substituent is on the
olefin, the reactions are uniformly a-selective (Table 11.1, Entries 4-6).
In order to examine this reaction in more detail and to assess the potential
involvement of p-allyl-Pd intermediates in the reaction, deuterated substrate
71 was prepared, in which 90% of the deuterium label is at the vinylic
position and the allyl unit is substituted with two identical methyl groups
[eqn (11.17)].
55
If a fully equilibrated p-allyl-Pd species is implicated, a 50 : 50
mixture of 72 and 73 would be expected. In the event, an 85 : 15 ratio of
72 : 73 was observed, with the major product resulting from g-selective
coupling.
55
(11
:
17)
This implies that the reaction proceeds via a facile reductive elimination
following g-transmetallated intermediate II. Based on the deuterium label
distribution in the product, less than 5% of intermediate II is isomerized to
IV via p-allyl intermediate III (Scheme 11.6).
55
Although the related deuterated substrate flanked by two phenyl groups
could be prepared, it underwent facile borotropic shifts that resulted in
scrambling of the deuterium and rendered this same analysis impossible.
However, analyzing the product distribution from substrates 74 and 75
(Scheme 11.7) provides some interesting insight. As expected, (E)-74
undergoes a highly g-selective coupling giving the expected product (76)ina
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