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resulted in the formation of the propene-coordinated species,
4
, suggesting 1,2-al-
kene insertion followed by
-bromo elimination. Propene is gradually lost from 4
and is trapped by unreacted
2
to form
7
. The cationic Pd(II)-halide species arising
from
4
by propene loss converts into the chloro-bridged dimer
6
(via
5
). The struc-
ture of
6
as a dicationic complex with two aluminum tetrachloride counter-anions
was established by an X-ray crystal structure determination. The identity of the ha-
lide ligand in
4
has not been established but the formation of 6 opens up the pos-
sibility of an aluminum-assisted
-bromo abstraction pathway shown in Scheme
9.1. Once formed,
7
undergoes 1,2-insertion of propene to form the known
-
agostic Pd(II)
-tert-
butyl compound
8
[14].
The migratory insertion rates of bound vinyl bromide and propene in
3
and
7
,
respectively, were directly measured by monitoring the disappearance of the corre-
sponding Pd-CH
3
resonance. For propene our value was in close agreement with
that reported by Brookhart [15]. For vinyl bromide, an Arrhenius plot was con-
structed from rate measurements done between -74 and -37
C. Our values to-
gether with those of Brookhart [9, 15] are reported in Tab. 9.2.
A Hammett plot of the relative insertion rates of substituted alkenes versus
p
[20] yielded a straight line with a
positive
(+3.41) (Fig. 9.5).
Of note is that the line encompasses values obtained by both Brookhart [9, 15]
and us. Theoretical calculations have also led to a lower insertion barrier for acry-
late compared to ethene [16]. This can be contrasted with a
negative
value of
ob-
tained by Bercaw for alkene insertion in Cp*NbH(alkene) [17a]. Additionally, the
second-order rate constants obtained by Wolczanski for alkene insertion into the
Ta(V)-H bond follow the trend H
OR >> halide (F, Cl, Br) [12]. The decrease in
the rate of insertion with increasing electron withdrawing effect of the substituent
on the alkene in the case of early transition metal compounds has been attributed
to the development of a positive charge on the carbon bearing the substituent
either during alkene coordination or the subsequent insertion step [12, 17]. In
Wolczanski's case, it is not been possible to separate the effect of the substituent
on binding versus insertion, and the trend for the actual insertion step remains
an open question. We ascribe the increase in insertion rate for the palladium-
methyl complex to a ground-state effect. An alkene with an electron-withdrawing
substituent coordinates less strongly to the electrophilic metal (i.e.
-donation is
more important than
-back-donation) [18]. Thus, a weaker metal-alkene bond has
to be broken for the insertion to proceed (i.e., the destabilization of the alkene
Tab. 9.2
Kinetic data for insertion of alkenes into palladium(II)-methyl bond
a)
.
10
3
k[s
-1
]
(
H
[kcal mol
-1
]
(
S
[cal K
-1
mo
-1
]
Alkene
Propene
0.54
Ethene [15]
1.9
14.2±0.1
-11.2±0.8
Vinyl bromide
22.0
11.9±0.1
-16.8±0.1
Methyl acrylate [9]
55.0
12.1±1.4
-14.1±7.0
a)
Measured or extrapolated to 236.5 K.
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