The microscopic steps responsible for the perfectly
alternating copolymerization of ethylene and CO
catalyzed by 1,10-phenanthroline (phen) based palladium complexes
have been studied. Palladium carbonyl alkyl,
carbonyl acyl, ethylene alkyl, and ethylene acyl complexes
[(phen)Pd(R)(L)+Ar‘4B-
(Ar‘ =
3,5-(CF3)2C6H3; R, L
=
CH3, CO (2); CH3,
C2H4 (3);
CH2CH3, C2H4
(7); C(O)CH3, CO (8);
C(O)CH3, C2H4 (13);
CH2CH2C(O)CH3,
C2H4
(15);
CH2CH2C(O)CH3, CO
(16);
C(O)CH2CH2C(O)CH3,
C2H4 (17);
C(O)CH2CH2C(O)CH3,
CO (18)); and the β-
and γ-keto chelate complexes
(phen)PdCH2CH2C(O)CH3
+
(14) and
(phen)PdC(O)CH2CH2C(O)CH3
+
(19)] have
been prepared. An X-ray structure of the carbonyl acyl complex
(phen)Pd(C(O)CH3)(CO)+Ar‘4B-·CH2Cl2
(8·CH2Cl2) has been obtained. The migratory insertion
reactions of 2, 3, 7, 13,
16, and 17 have been studied by low-temperature NMR techniques. The barriers for insertion increase in
the following order:
ΔG
⧧
R
→
CO
≈ 15 kcal/mol
(−66 °C) <
ΔG
⧧
Ac
→
C
2
H
4
≈ 17 kcal/mol (ca. −45 °C) <
ΔG
⧧
R
→
C
2
H
4
≈ 19 kcal/mol (−25 °C). The relative binding
affinities of ethylene and CO to Pd methyl, acyl, and chelate complexes
have been determined by combining stepwise
measurements of the binding affinities of ligands with intermediate
strength to (phen)Pd(CH3)(L)+ (CO
> MeSPh
> CH3CN ≈ C2H4 >
C6H5CN ≫ OEt2) with relative
equilibrium constants for ethylene/CO binding between acyl
and alkyl complexes. The copolymerization mechanism has been
determined from the kinetic and thermodynamic
data. The catalyst resting state is a carbonyl acyl complex which
is in equilibrium (K
5(25 °C) = (7.1 ±
3.5) × 10-4)
with a less stable ethylene acyl intermediate which undergoes β-acyl
migratory insertion to generate a Pd alkyl
species followed by rapid reaction with 2 equiv of CO to reform the
resting state. This model is tested by comparing
calculated and experimental turnover frequencies.
The cationic Pd(II) complexes,
[(phen)Pd(CH3)(L)]+[BAr‘4]-
phen = 1,10−phenanthroline; L = Et2O,
Me3SiC⋮CSiMe3; Ar‘ =
3,5-(CF3)2C6H3)
catalyze the hydrosilation and dehydrogenative silation of
olefins.
Hydrosilation of ethylene, tert-butylethylene,
1-hexene, and cyclohexene by HSiR3 (R =
CH2CH3, C6H5)
occurs in
the presence of 1 mol %
[(phen)Pd(CH3)(L)]+[BAr‘4]-.
The reaction of tert-butylethylene with
HSi(i-Pr)3 in the
presence of
[(phen)Pd(CH3)(L)]+[BAr‘4]-
yields neohexane and
t-BuCHCHSi(i-Pr)3.
Low-temperature NMR
experiments revealed that the catalyst resting state for the silations
of ethylene and alkyl-substituted olefins is [(phen)Pd(SiR3)(η2-H2CCHR‘)]+[BAr‘4]-.
Evidence for rapid, reversible silyl migration at −70 °C was
observed by 1H
NMR spectroscopy. Deuterium labeling studies show that the
intermediate Pd(II) alkyl complexes can isomerize
via a series of β-hydride eliminations followed by reinsertions of
olefin prior to reaction with DSiEt3.
Styrene
undergoes both hydrosilation and dehydrogenative silation in the
presence of
[(phen)Pd(CH3)(L)]+[BAr‘4]-
or [(phen)Pd(η3-CH(CH3)C6H5)]+[BAr‘4]-
yielding ethylbenzene,
R3SiCH2CH2C6H5
and trans-R3SiCHCHPh (R =
CH2CH3, CH(CH3)2).
1H NMR spectroscopy revealed that the π-benzyl
complexes
[(phen)Pd(η3-CH(CH2SiR3)C6H5)]+[BAr‘4]- and
[(phen)Pd(η3-CH(CH3)C6H5)]+[BAr‘4]-
are the catalyst resting states for the silation reactions of
styrene.
The β-methyl migratory insertion chemistry of a series of
cis-coordinated styrene methyl complexes of
palladium
[(phen)Pd(CH3)(p-X-C6H4CHCH2)+Ar‘4B-
(2X) (phen = 1,10-phenanthroline; X =
CF3, Cl, H, CH3,
OCH3; Ar‘ =
3,5-(CF3)2C6H3)] has
been investigated. Complexes 2X are prepared in
situ from the addition of
p-X-styrene to CD2Cl2 solutions
of
(phen)Pd(CH3)(OEt2)+Ar‘4B-
(1). The X-ray structure of 1 has been
determined
[P21/c; a =
16.460(4) Å, b = 18.911(3) Å, c =
17.374(3) Å; β = 117.996(14)°; V =
4775.2(15) Å3; Z = 4] at
−78
°C. The rearrangement of 2X, via β-CH3
migratory insertion then arene coordination, to yield
(phen)Pd(η3-CH(CH2CH3)(C6H4-p-X)+Ar‘4B-
(3X) has been studied by 1H NMR spectroscopy.
The rearrangement is accelerated by
electron-withdrawing groups; a Hammett analysis at −29.2 °C reveals
that log k values are best fit by σp
parameters:
ρp = 1.1 ± 0.1, r = 0.992. The
anti-isomer of 3H has been structurally
characterized: P21/n; a =
13.950(4) Å, b
= 15.582(5) Å, c = 24.004(8) Å; β =
101.961(24)°; V = 5105(3) Å3;
Z = 4. Complex 3H reacts further
with
styrene to form
(phen)Pd(η3-CH(CH3)(C6H5)+Ar‘4B-
(4) and (E)-β-methylstyrene; kinetic and
isotopic labeling
experiments have been employed to determine the mechanism of this
benzyl exchange reaction. The relative binding
affinities of the p-X-styrenes of 2X have been
determined for the equilibria
p-X-C6H4CHCH2
+ 2H ⇌ C6H5CHCH2 + 2X at −66 °C. The
electron-rich styrenes bind tightest to Pd; a Hammett plot of these
equilibrium
constants yields ρp = −2.2 ± 0.1 and r
= 0.999. The kinetic and thermodynamic data indicate that both
the ground
and transition-states are stabilized by electron-rich styrenes and, of
the two, the substituent effects are greatest in the
ground-states. Since the relative rates are determined by the
relative differences in energy of the ground and
transition
states, the migratory insertion reaction of 2X is
accelerated by styrenes bearing electron-withdrawing
substituents.
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