Cationic polymerization and insertion chemistry in the reactions of vinyl ethers with (α-Diimine)PdMe+ species

Article abstract of DOI:10.1021/ja100491y

The reactions of (α-diimine)PdMe⁺ species (1, α-diimine = (2,6-ⁱPr₂-C₆H₃)N - CMeCMei - N(2,6-ⁱPr₂-C₆H₃)) with vinyl ethers CH₂ - CHOR (2a-g: R = ^tBu (a), Et (b), SiMe₃ (c), SiMe₂Ph (d), SiMePh₂ (e), SiPh ₃ (f), Ph (g); 2a-g: R = ^tBu (a), Et (b), SiMe₃ (c), SiMe₂Ph (d), SiMePh₂ (e), SiPh₃ (f), Ph (g)) were investigated. Two pathways were observed. First, 1 initiates the cationic polymerization of 2a-c with concomitant decomposition of 1 to Pd ⁰. This reaction proceeds by formation of (α-diimine) PdRAε(CH₂i - CHOR)⁺ Iε complexes (RAε = Me or CH₂CHMeOR from insertion), in which the vinyl ether Ci - C bond is polarized with carbocation character at the substituted carbon (C_int). Electrophilic attack of C _int on monomer initiates polymerization. Second, 1 reacts with stoichiometric quantities of 2a-g by formation of (α-diimine)PdMe(CH ₂i - CHOR)⁺ (3a-g), insertion to form (α-diimine)Pd(CH₂CHMeOR)⁺ (4a-g), reversible isomerization to (α-diimine)Pd(CMe₂OR)⁺ (5a-g), β-OR elimination of 4a-g to generate (α-diimine)Pd(OR)(CH ₂i - CHMe)⁺ (not observed), and allylic C-H activation to yield (α-diimine)Pd(I³-C₃H ₅)⁺ (6) and ROH. Binding strengths vary in the order 2a > 2b ～ 2c > 2d ～ 2g > 2e > 2f. Strongly electron-donating OR groups increase the binding strength, while steric crowding has the opposite effect. The insertion rates vary in the order 3a < 3b < 3c < 3d < 3e < 3f < 3g; this trend is determined primarily by the relative ground-state energies of 3a-g. The β-OR elimination rates vary in the order O^tBu < OSiR₃ < OPh. For 2d-g, the insertion chemistry out-competes cationic polymerization even at high vinyl ether concentrations. β-OR elimination of 4/5 mixtures is faster for SbF ₆^- salts than B(C₆F₅) ₄^- salts. The implications of these results for olefin/vinyl ether copolymerization are discussed.

Full text of DOI:10.1021/ja100491y

Published on Web 03/16/2010
Cationic Polymerization and Insertion Chemistry in the
Reactions of Vinyl Ethers with (r-Diimine)PdMe+ Species
Changle Chen, Shuji Luo, and Richard F. Jordan*
Department of Chemistry, The UniVersity of Chicago, 5735 South Ellis AVenue,
Chicago, Illinois 60637
Received January 19, 2010; E-mail: rfjordan@uchicago.edu
Abstract: The reactions of (R-diimine)PdMe+ species (1, R-diimine ) (2,6-iPr2-C6H3)NdCMeCMedN(2,6-
iPr2-C6H3)) with vinyl ethers CH2dCHOR (2a-g: R ) tBu (a), Et (b), SiMe3 (c), SiMe2Ph (d), SiMePh2 (e),
t
SiPh3 (f), Ph (g); 2a-g: R ) Bu (a), Et (b), SiMe3 (c), SiMe2Ph (d), SiMePh2 (e), SiPh3 (f), Ph (g)) were
investigated. Two pathways were observed. First, 1 initiates the cationic polymerization of 2a-c with
concomitant decomposition of
1
to Pd0. This reaction proceeds by formation of (R-
diimine)PdR′(CH2dCHOR)+ π complexes (R′ ) Me or CH2CHMeOR from insertion), in which the vinyl
ether CdC bond is polarized with carbocation character at the substituted carbon (Cint). Electrophilic attack
of Cint on monomer initiates polymerization. Second, 1 reacts with stoichiometric quantities of 2a-g by
formation of (R-diimine)PdMe(CH2dCHOR)+ (3a-g), insertion to form (R-diimine)Pd(CH2CHMeOR)+ (4a-g),
reversible isomerization to (R-diimine)Pd(CMe2OR)+ (5a-g), ꢀ-OR elimination of 4a-g to generate (R-
diimine)Pd(OR)(CH2dCHMe)+ (not observed), and allylic C-H activation to yield (R-diimine)Pd(η3-C3H5)+
(6) and ROH. Binding strengths vary in the order 2a > 2b ∼ 2c > 2d ∼ 2g > 2e > 2f. Strongly electron-
donating OR groups increase the binding strength, while steric crowding has the opposite effect. The insertion
rates vary in the order 3a < 3b < 3c < 3d < 3e < 3f < 3g; this trend is determined primarily by the relative
ground-state energies of 3a-g. The ꢀ-OR elimination rates vary in the order OtBu < OSiR3 < OPh. For
2d-g, the insertion chemistry out-competes cationic polymerization even at high vinyl ether concentrations.
-
-
ꢀ-OR elimination of 4/5 mixtures is faster for SbF6 salts than B(C6F5)4 salts. The implications of these
results for olefin/vinyl ether copolymerization are discussed.
Introduction
identified, including (i) coordination of the CH2dCHX monomer
to the LnMR+ active species through the X group rather than
The development of catalysts that are capable of polymerizing
or copolymerizing functionalized vinyl monomers (CH2dCHX)
by insertion mechanisms would enable the synthesis of new
polyolefins with enhanced properties.1 The discovery by Brookhart
and co-workers that (R-diimine)PdR+ catalysts copolymerize
ethylene and acrylate monomers to highly branched copolymers
was a seminal development in this field.2 More recently, several
groups have shown that (ortho-phosphino-arenesulfonate)PdR
catalysts copolymerize ethylene with acrylates, vinyl ethers,
vinyl fluoride, acrylonitrile, vinyl acetate, and other comonomers
to linear copolymers.3 However, in other cases, it has been found
that CH2dCHX monomers deactivate olefin polymerization
catalysts. Several common deactivation processes have been
the CdC bond to form unreactive LnMR(κ-X-XCHdCH2)+
adducts;4 (ii) formation of LnMCH(X)CH2R species that are
resistant to subsequent insertion reactions due to X chelation,
which blocks the site required for monomer coordination,5 and
the electronic influence of the X substituent;6 and (iii) ꢀ-X
elimination of LnMCH2CHXR or LnMCHRCH2X species to
form unreactive LnMX species.7 Moreover, metal catalysts can
(3) (a) Drent, E.; van Dijk, R.; van Ginkel, R.; van Oort, B.; Pugh, R. I.
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9
10.1021/ja100491y  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 5273–5284 5273

A R T I C L E S
Chen et al.
Scheme 1
initiate undesired radical or ionic homopolymerization of
CH2dCHX monomers.8,9
Vinyl ethers (CH2dCHOR) are attractive candidates for
insertion copolymerization with olefins because their steric and
electronic properties can be tuned by varying the R group,
possibly enabling the deactivation reactions noted above to be
avoided. Several potential problems can be anticipated for vinyl
ethers in insertion polymerization. First, vinyl ethers are highly
susceptible to cationic polymerization,10 and electrophilic olefin
polymerization catalysts are good cationic initiators.11,12 For
example, [(P∼N)PdMe(CH3CN)][BF4] (P∼N ) o-C6H4(PPh2)-
(NdCHAr), Ar ) C6H5, 4-F-C6H5) species initiate cationic
polymerization of ethyl vinyl ether to generate poly(ethyl vinyl
ether) with a molecular weight of ca. 8000.11b Second, vinyl
ethers can coordinate to metals not only through the CdC bond,
as in [NEt4][PtCl3(CH2dCHOEt)]13 and PdCl2[NMe2CH2-
CMe2(CHdCHOMe)],14,15 but also through the OR group, as
Scheme 2
t
t
in(C5H4Me)2Zr(OBu)(η1-O-EtOCHdCH2)+andRuH(CO)(PBu2Me)2-
(η1-O-EtOCHdCH2)+.15c,16 Third, the insertion barriers for
LnMR′(CH2dCHOR) species are predicted to be high. For
example, the insertion barrier of (HNdCHCHd
NH)PdMe(CH2dCHOMe)+ was calculated by DFT to be ca. 6
kcal/mol higher than that for (HNdCHCHdNH)Pd-
Me(CH2dCH2)+.17 Nevertheless, insertions of vinyl ethers into
metal hydrides, such as RuH(CO)(PtBu2Me)2(η2-CH2Cl2)+,
(tBu3SiO)3TaH2, and Os3H2(CO)10, have been reported.16,18,19
Also, CH2dCHOEt inserts into the Pd-acetyl bond of
[(P∼N)Pd(COCH3)(CH3CN)][BF4] to generate [(P∼N)Pd-
CH(OEt)CH2COCH3][BF4].11b Finally, LnMCH2CH(OR)R′ spe-
cies formed by 1,2 insertion of LnMR′(CH2dCHOR) species
may undergo ꢀ-OR elimination, which would terminate chain
growth. Wolczanski showed that (tBu3SiO)3TaH2 undergoes 1,2
insertion of CH2dCHOR (R ) alkyl, Ph) to generate
(tBu3SiO)3TaH(CH2CH2OR) species, which undergo ꢀ-OR
elimination. However, the ꢀ-OR elimination rate decreases as
the size of R increases, and (tBu3SiO)3TaH(CH2CH2OtBu) is
stable.18
(6) (a) Foley, S. R.; Shen, H.; Qadeer, U. A.; Jordan, R. F. Organometallics
2004, 23, 600. (b) Philipp, D. M.; Muller, R. P.; Goddard, W. A.;
Storer, J.; McAdon, M.; Mullins, M. J. Am. Chem. Soc. 2002, 124,
10198. (c) Szabo, M. J.; Jordan, R. F.; Michalak, A.; Piers, W. E.;
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Mitoraj, M.; Michalak, A. J. Mol. Modeling 2005, 11, 341.
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Rheingold, A. L. J. Am. Chem. Soc. 2002, 124, 12080.
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125, 796.
(9) (a) Schaper, F.; Foley, S. R.; Jordan, R. F. J. Am. Chem. Soc. 2004,
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Organomet. Chem. 2005, 690, 4349.
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Iva’n, B. Designed Polymers by Carbocationic Macromolecular
Engineering: Theory and Practice; Hanser: Munich, 1992. (c) Maty-
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M. In ComprehensiVe Polymer Science Pergamon; Allen, G., Bev-
ington, J., Eds.; Oxford: U.K., 1989; Vol. 3, pp 673-696. Vinyl ethers
are poor substrates for radical polymerization. See: (f) Matsumoto,
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4, 277. (g) Kamachi, M.; Tanaka, K.; Kuwae, Y. J. Polym. Sci., Part
A: Polym. Chem. 1986, 24, 925.
Recently, we reported that (R-diimine)PdMe+ species (R-
diimine ) (2,6-iPr2-C6H3)NdCMeCMedN(2,6-iPr2-C6H3))
copolymerize 1-hexene and CH2dCHOSiPh3 to OSiPh3-
substituted polyhexene (Scheme 1).20 Alkyl vinyl ethers such
as CH2dCHOtBu are not suitable comonomers in this system
due to competing cationic polymerization and Pd0 formation.
Phenyl vinyl ether is also unsuitable because (R-diimine)
Pd(CH2CHMeOPh)+ species generated by CH2dCHOPh insertion
undergo rapid ꢀ-OPh elimination, ultimately forming (R-di-
imine)Pd(η3-C3H5)+, which is catalytically inactive, and PhOH.
Further studies showed that (R-diimine)PdMe+ species undergo
up to three sequential insertions of CH2dCHOSiPh3, ultimately
forming Pd allyl products (Scheme 2).21 The product distribution
is dependent on the anion.
(11) (a) Baird, M. C. Chem. ReV. 2000, 100, 1471. (b) Chen, C. L.; Chen,
Y. C.; Liu, Y. H.; Peng, S. M.; Liu, S. T. Organometallics 2002, 21,
5382. (c) Chen, Y. C.; Reddy, K. R.; Liu, S. T. J. Organomet. Chem.
2002, 656, 199. (d) Albietz, P. J., Jr.; Yang, K.; Lachicotte, R. J.;
Eisenberg, R. Organometallics 2000, 19, 3543. (e) Albietz, P. J., Jr.;
Yang, K.; Eisenberg, R. Organometallics 1999, 18, 2747. (f) Shaffer,
T. D.; Ashbaugh, J. R. J. Polym. Sci., Part A: Polym. Chem. 1997,
35, 329. (g) Kawaguchi, T.; Sanda, F. O.; Masuda, T. J. Polym. Sci.,
Part A: Polym. Chem. 2002, 40, 3938.
(12) For vinyl ether polymerizations mediated by combinations of metal
complexes and silanes, see: (a) Nagashima, H.; Itonaga, C.; Yasuhara,
J.; Motoyama, Y.; Matsubara, K. Organometallics 2004, 23, 5779.
(b) Crivello, J. V.; Fan, M. J. Polym. Sci., Part A: Polym. Chem. 1991,
29, 1853.
In this paper, we describe a comprehensive study of the
reactions of (R-diimine)PdMe+ species with a set of vinyl ethers
with varying steric and electronic properties, CH2dCHOR
(13) Elder, R. C.; Pesa, F. Acta Crystallogr. 1978, B34, 268.
(14) McCrindle, R.; Ferguson, G.; Khan, M. A.; McAalees, A. J.; Ruhl,
B. L. J. Chem. Soc., Dalton. Trans. 1981, 4, 986.
t
(2a-g: R ) Bu (a), Et (b), SiMe3 (c), SiMe2Ph (d), SiMePh2
(e), SiPh3 (f), Ph (g)). We first describe studies of the reaction
of (R-diimine)PdMe+ with excess 2a-g to probe the potential
(15) (a) Chang, T. C. T.; Foxman, B. M.; Rosenblum, M.; Stockman, C.
J. Am. Chem. Soc. 1981, 103, 7361. (b) Watson, L. A.; Franzman, B.;
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118, 11134. (e) Cotton, F. A.; Francis, J. N.; Frenz, B. A.; Tsutsui,
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B.; Svensson, M. Organometallics 2001, 20, 2813.
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(19) Boyar, E.; Deeming, A. J.; Arce, A. J.; De Sanctis, Y. J. Organomet.
Chem. 1984, 276, C45.
(20) Luo, S.; Jordan, R. F. J. Am. Chem. Soc. 2006, 128, 12072.
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12892.
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Eisenstein, O.; Caulton, K. G. Organometallics 2000, 19, 2281.
9
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Cationic Polymerization and Insertion Chemistry
A R T I C L E S
cationic polymerization reactivity of these substrates. We then
discuss the reactions of (R-diimine)PdMe+ with 1 equiv of
2a-g, under conditions where the concentration of vinyl ether
is too low to support cationic polymerization, in order to probe
the coordination, insertion, chain-walking, and ꢀ-OR elimination
reactivity. These studies provide new insights to how to
circumvent potential chemical obstacles to the insertion copo-
lymerization of vinyl ethers and olefins.
polymerization of 2a. The NMR spectra of the
-[CH2CH(OtBu)]n- polymer are essentially identical to those
of -[CH2CH(OtBu)]n- generated by cationic initiators such as
[Ph3C][B(C6F5)4] or [Li(Et2O)2.8][B(C6F5)4] under the same
conditions. These -[CH2CH(OtBu)]n- polymers are atactic
(mm/mr/rr ) 1:3:1) and contain methyl (-CH(OtBu)CH3),
aldehyde (-CH2C(dO)H), and acetal (-CH(OtBu)2) end groups
and internal -CHdCH- units (ca. 3 mol %).23 These features
are characteristic of a cationic polymerization process.10,11,23,24
Interestingly, rapid formation of Pd0 was observed during the
polymerization of 2a. The reaction of 1[B(C6F5)4] with excess
2b at 20 °C is similar.
Results
Generation of (r-Diimine)PdMe+ (1). The cationic species
(R-diimine)PdMe+ (1) was generated by the methods shown in
eq 1 and eq 2. The reaction of (R-diimine)PdMeCl and Ag[SbF6]
in Et2O generates [(R-diimine)PdMe(OEt2)][SbF6] (1[SbF6]),
which was isolated as a yellow solid. 1[SbF6] is stable in CD2Cl2
at -60 °C for at least 12 h but decomposes slowly at 20 °C.2d,22
The Et2O ligand is readily displaced by vinyl ethers to generate
[(R-diimine)PdMe(CH2dCHOR)][SbF6] (3[SbF6]) vinyl ether
adducts.
The reaction of 1[B(C6F5)4] with
a
large excess
CH2dCHOSiMe3 (2c, 50 equiv, 1.2 M) in CD2Cl2 at 20 °C
results in inefficient cationic polymerization to form
-[CH2CH(OSiMe3)]n- (7% conversion, 20 h). Pd0 formation
was also observed early in this reaction. However, when less
than 10 equiv (0.25 M) of 2c was used, no cationic polymer-
ization was detected. In contrast, the reaction of 1[B(C6F5)4]
with excess 2d-g (ca. 80 equiv, 2.0 M) in CD2Cl2 at 20 °C
does not generate polymer or Pd0.25 The reactions of 1[SbF6]
with excess 2a-g gave very similar results.
These results imply that the cationic polymerization and Pd0
formation processes are closely related, and that both processes
can be circumvented by using arylsilyl or aryl vinyl ethers.
Initiation of Cationic Polymerization of 2a-c. Two likely
initiators for the cationic polymerization in eq 3 are the (R-
diimine)PdMe+ cation itself or H+ generated from adventitious
water (e.g., via formation of (R-diimine)PdMe(H2O)+). Bulky
2,6-disubstituted pyridines have been used as proton traps to
quench the H+-initiated cationic polymerization of vinyl ethers
and other monomers.11f,24b,26 Similarly, pyridine was shown to
quench the cationic polymerization of vinyl ethers initiated by
(P∼N)PdMe(H2O)+ species.11b However, control experiments
show that 2,6-tBu2-pyridine (DTBP) has only a minimal effect
on the rate and yield of the 2a polymerization in eq 3. This
Alternatively, (R-diimine)PdMe+ was generated in situ by
the reaction of (R-diimine)PdMeCl and [Li(Et2O)2.8][B(C6F5)4]
in CD2Cl2 at 20 °C (eq 2). In the absence of Lewis bases, this
reaction produces the dinuclear species [{(R-diimine)PdMe}2(µ-
Cl)][B(C6F5)4], which is stable at 20 °C at least for 24 h, along
with an equimolar amount of unreacted [Li(Et2O)2.8][B(C6F5)4].4a
In the presence of Lewis bases (L) that are sufficiently strong
to displace (R-diimine)PdMeCl from [{(R-diimine)PdMe}2(µ-
Cl)]+, (R-diimine)PdMe(L)+ species are formed. This in situ
generated cationic system will be referred to as 1[B(C6F5)4]
below.
result suggests that (R-diimine)PdR′+ (R′
) Me or
CH2CH(OR)Me formed by insertion, vide infra) species initiate
this reaction.27 A reasonable mechanism for the 1-mediated
polymerization of 2a-c that accounts for these observations is
(23) The-CHdCH-units are formed by elimination of HOR from the
-[CH2CH(OR)]n-chain. See: Hashimoto, T.; Kanai, T.; Kodaira, T.
J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 675.
(24) (a) Wang, Q.; Baird, M. C. Macromolecules 1995, 28, 8021. (b) Ke´ki,
S.; Nagy, M.; Dea´k, G.; Zsuga, M. J. Phys. Chem. B 2001, 105, 9896.
(25) When the in situ generated 1[B(C6F5)4] contains excess LiB(C6F5)4,
2d is slowly cationically polymerized. However, Pd0 formation was
not observed.
Cationic Polymerization of Vinyl Ethers by 1[B(C6F5)4]
and 1[SbF6]. We first studied the reactions of 1[B(C6F5)4] with
excess CH2dCHOR (2a-g) to probe trends in cationic polym-
erization reactivity. As shown in eq 3, the reaction of 1[B(C6F5)4]
with excess 2a at 20 °C results in rapid (5 min) and quantitative
(26) (a) De, P.; Faust, R. Macromolecules 2006, 39, 7527. (b) Hadjikyri-
acou, S.; Acar, M.; Faust, R. Macromolecules 2004, 37, 7543. (c)
Bennevault, W.; Peruch, F.; Defieux, A. Macromol. Chem. Phys. 1996,
197, 2603. (d) Cho, C. G.; Feit, B. A.; Webster, O. W. Macromolecules
1992, 25, 2081.
(27) (a) This result may also suggest that (R-diimine)PdMe(OH2)+ is too
crowded to be deprotonated by DTBP. It was not possible to test the
influence of pyridine on the 1-initiated cationic polymerization since
pyridine displaces the R-diimine ligand. (b) DTBP significantly retards
the polymerization of 2a by [Li(Et2O)2.8][B(C6F5)4].
(22) Johnson, L. K.; Kilian, C. M.; Arthur, S. D.; Feldman, J.; Mccord,
E. F.; Mclain, S. J.; Kreutzer, K. A.; Bennett, M. A.; Coughlin, E. B.
PCT Int. Appl. WO 9623010, 1996.
9
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A R T I C L E S
Chen et al.
Scheme 3
Scheme 4
Table 1. 1H and 13C NMR Chemical Shifts for Free CH2dCHOtBu
(2a) and 3a[SbF6] (CD2Cl2, -60° C)
CH2dCHOtBu
chemical shift (δ)
coord
shown in Scheme 3. Coordination of the vinyl ether to a (R-
a
resonance
free
∆
diimine)PdR′+
species
generates
an
(R-diimine)-
PdR′(CH2dCHOR)+ π complex (A), in which the vinyl ether
CdC bond is polarized with carbocation character at the
substituted carbon (Cint), due to the electrophilic character of
the cationic Pd center.15a,c Electrophilic attack of Cint of A on
monomer then initiates cationic polymerization. Pd0 formation
can occur by reductive elimination of growing species B or more
likely by ꢀ-H elimination of B followed by R′-H reductive
elimination.28,29
Hint
Htrans
Hcis
C(CH3)3
CH2
CH
6.47
4.22
3.93
1.21
7.09
-0.62
0.95
0.97
-0.14
36.0
-1.1
-7.2
3.27
2.96
1.35
90.4
54.4
147.1
76.4
148.2
83.6
C(CH3)3
a ∆ ) δfree - δcoord
.
Reaction of 1[SbF6] or 1[B(C6F5)4] with 1 Equiv of
CH2dCHOR (2a-g). Scheme 3 suggests that it might be
possible to probe for vinyl ether insertion chemistry by
generating (R-diimine)PdMe+ species in the presence of sto-
ichiometric quantities of vinyl ether, such that excess substrate
is not present to propagate cationic polymerization. Indeed, as
shown in the Scheme 4, reaction of 1[SbF6] or 1[B(C6F5)4] with
1 equiv of 2a-g proceeds by initial formation of (R-
diimine)PdMe(CH2dCHOR)+ (3a-g),30 followed by 1,2 inser-
tion to produce (R-diimine)Pd(CH2CHMeOR)+ (4a-g) and
reversible isomerization to form (R-diimine)Pd(CMe2OR)+
(5a-g). The 4a-g/5a-g mixtures react further at 20 °C by
ꢀ-OR elimination of 4a-g to generate (R-diimine)Pd-
(OR)(CH2dCHMe)+ (not observed), followed by allylic C-H
activation to yield (R-diimine)Pd(η3-C3H5)+ (6) and ROH as
the ultimate products. These reactions are discussed in detail
in the following sections.
was detected in the presence of excess vinyl ether over the
temperature range of -60 to 20 °C, and no EXSY cross peaks
between the free and coordinated vinyl ether resonances were
observed at -60 or -20 °C. These results show that the
exchange between the free and coordinated vinyl ether is slow.
Key NMR data for the tert-butyl vinyl ether complex
3a[SbF6] are listed in Table 1. The CH2dCHOtBu 1H resonance
is shifted downfield and the CH2dCHOtBu resonances are
shifted upfield from the corresponding resonances of free
CH2dCHOtBu. Furthermore, the CH2dCHOtBu 13C resonance
is shifted significantly upfield, while the OCMe3 resonance is
shifted only slightly upon coordination. These data are charac-
teristic for CdC-bound π complexes, such as CpFe(CO)2-
(CH2dCHOEt)+ and Cp′2Zr(OtBu)(η2-H2CdCHOtBu)+.15a,c In
contrast, for O-coordinated CH2dCHOR adducts, such as
(C5H4Me)2Zr(OtBu)(η1-O-EtOCHdCH2)+andRuH(CO)(PtBu2Me)2-
(η1-O-EtOCHdCH2)+, the Htrans and Hcis 1H NMR resonances
are shifted downfield, and the CH2dCHOEt and
CH2dCHOCH2Me 13C NMR resonances are shifted significantly
upfield from those of free vinyl ether.15c,16 The NMR spectra
of 3b-g[SbF6] are similar to those of 3a[SbF6], showing that
these species are also CdC π complexes.
Generation of [(r-Diimine)PdMe(CH2dCHOR)][SbF6]
(3a-g[SbF6]). The reaction of 1[SbF6] and 2a-g in CD2Cl2 at
-60 °C generates the corresponding adducts [(R-
diimine)PdMe(CH2dCHOR)][SbF6] (3a-g[SbF6]) in >90%
yield.31 No broadening of the H NMR resonances of 3a-g
1
The in situ generated (R-diimine)PdMe+ system 1[B(C6F5)4]
is less useful for the observation of 3[B(C6F5)4] complexes
because displacement of (R-diimine)PdMeCl from the interme-
diate [{(R-diimine)PdMe}2(µ-Cl)]+ cation (eq 2) by vinyl ethers
is slow. 1[B(C6F5)4] does not react with 2a in CD2Cl2 at -60
°C, indicating that under these conditions the vinyl ether does
not break the chloride bridge. However, when the [{(R-
diimine)PdMe}2(µ-Cl)]+/[Li(Et2O)2.8][B(C6F5)4]/2a mixture is
warmed to 20 °C for 10 min, [(R-diimine)PdMe(CH2d
CHOtBu)][B(C6F5)4] (3a[B(C6F5)4]) is formed in 78% yield. The
(28) The end groups in species C and D were not detected by NMR,
possibly because their NMR resonances overlap with, and are much
weaker than, those of other end groups generated by chain transfer,
which is fast (ca. 46-[CH2CH(OtBu)]n-chains are produced per Pd).
The end group D may be incorporated into the polymer.
(29) (R-Diimine)PdR2 complexes (R ) nPr, nBu, iBu) are moderately stable
at room temperature. These species slowly decompose (days) to
alkanes, alkenes, and unidentified Pd(0) species. See ref 2b.
(30) If the anion is not specified for complexes 3, 4, and 5, the statement
-
-
is true for both the SbF6 and B(C6F5)4 salts.
(31) The major impurity is [{(R-diimine)PdMe}2(µ-Cl)][SbF6].
9
5276 J. AM. CHEM. SOC. VOL. 132, NO. 14, 2010

Cationic Polymerization and Insertion Chemistry
A R T I C L E S
Table 2. Reactivity of CH2dCHOR with 1[SbF6]a
CH2dCHOR (2), R )
tBu (2a)
Et (2b)
SiMe3 (2c)
SiMe2Ph (2d)
SiMePh2 (2e)
SiPh3 (2f)
Ph (2g)
K2/ethylene (-60 °C)b
0.88(2)
5.35d
0.22(1)
1.66d
-0.2(1)
0.84(5)
0.12(1)
1
0
K2/2c (-20 °C)c
0.34(4)
0.5(1)
3.2(2)
>33f
0.19(2)
0.8(1)
0.08(1)
0.30(2)
0.6(1)
∆G2/2c (kcal/mol, -20 °C)e
kinsert,3 (10-4 s-1, 0 °C)
kinsert,3 (10-4 s-1, 20 °C)
-0.8(1)
0.33(2)
6.3(4)
21.5(1)
0.21
0.33
15.0(9)
20(1)
1.3(1)
8.1(5)
1.6
(1)
5.2(3)
15.0(9)
∼27
>33f
20.7(1)
>19i
>19i
111(7)
>33f
>33f
>33f
∆Gq
(kcal/mol, 0 °C)g
21.0(1)
2.2
20.3(1)
20.0(1)
19.8(1)
19.4(1)
unknownj
unknownj
>5000l
insert,3
K5/4 (0 °C)h
>19i
>19i
>19i
K5/4 (20 °C)h
2.7
1200(70)
4440(260)
>19i
>19i
>19i
kꢀ-OR,obs (10-6 s-1, 20 °C)k
kꢀ-OR (10-6 s-1, 20 °C)m
156(9)
246(15)
378(23)
>2220n
>3120n
>4920n
>7560n
>5000n
b
c
a The uncertainties are based on replicate runs. K2/ethylene ) [3][CH2dCH2][(R-diimine)PdMe(CH2dCH2)+]-1[2]-1 at equilibrium. K2/2c
)
[3][2c][3c]-1[2]-1 at equilibrium. d ∆S of equilibrium is assumed to be negligible, so ∆G does not change over temperature.33 K2a,b/ethylene(-20 °C) )
exp{(213/253)ln[K2a,b/ethylene(-60 °C)]}; K2a,b/2c ) K2a,b/ethylene/K2c/ethylene
.
e ∆G2/2c ) -RTlnK2/2c
.
f More than 95% of 3 undergoes insertion within 15 min
g
h
at 20 °C. The insertion barrier for 3a-g at 0 °C, ∆Gq
) -RTln(kinsert,3h/kBT). K5/4 ) [5]/[4] at equilibrium. i Complexes 4c-f were not detected
insert,3
by NMR. j 4g and 5g were not observed due to fast ꢀ-OR elimination of 4g. k The observed first-order rate constant for consumption of the total of 4
and 5. l More than 95% of 3g is converted to 6 and phenol within 10 min at 20 °C. m The first-order rate constant for ꢀ-OR elimination of 4, kꢀ-OR
kꢀ-OR,obs (K5/4 + 1). n Lower limit assuming K5/4 > 20.
)
remaining species in solution are [{(R-diimine)PdMe}2(µ-Cl)]+
(8%) and the insertion products 4a[B(C6F5)4] (10%) and
5a[B(C6F5)4] (4%, Scheme 4). Complex 3b[B(C6F5)4] is formed
in 27% yield by the reaction of 1[B(C6F5)4] and 2b in CD2Cl2 at
20 °C for 10 min. The remaining species in solution are [{(R-
diimine)PdMe}2(µ-Cl)]+ (8%), the insertion products 4b[B(C6F5)4]
(18%) and 5b[B(C6F5)4] (35%), and allyl complex 6 (12%).
Complexes 3c-g[B(C6F5)4] are not observed in the reaction of
1[B(C6F5)4] and 2c-g at 20 °C due to fast insertion.
exo isomers) of 3i (∆Gq
determined from the coalescence of the Pd-Me resonances.
) 10.3 kcal/mol, -50 °C) was
rotation
Competitive Binding of Ethylene and Vinyl Ethers to 1. The
relative binding strengths of vinyl ethers 2a-c to 1[SbF6] were
determined by competition experiments with ethylene (eq 5).
The reactions of 1[SbF6] with excess ethylene and 2a-c were
monitored by 1H NMR at -60 °C until the reaction quotient Q
) [3][CH2dCH2][(R-diimine)PdMe(CH2dCH2)+]-1[2]-1 reached
a constant value (K2/ethylene), indicating that the equilibrium in
eq 5 had been reached. 2d-g bind to Pd much more weakly
than ethylene does, so their binding affinities cannot be measured
accurately by eq 5. The binding affinities of 2d-g were
determined by competition experiments with 2c (eq 6). These
experiments were conducted at -20 °C in order to accelerate
the approach to equilibrium. Equilibrium constants for eq 5 and
Two rotamers are possible for
a
(R-diimine)
PdMe(CH2dCHOR)+ species, which differ in the orientation
of the vinyl ether (eq 4). The 1H NMR spectra of 3a-g contain
one set of sharp R-diimine, Pd-Me, and vinyl ether resonances
over the temperature range of -70 to 20 °C (0 °C for 3g;
insertion is rapid above this temperature), suggesting that either
rotation around the Pd-vinyl ether bond is very fast or, more
likely, one rotamer is highly favored. NOSEY spectra of 3c-f
contain Pd-Me/CHMe2, Hcis/CHMe2, and Hint/CHMe2′ cross
peaks but no cross peaks between the Pd-Me and Hint or Hcis
resonances. These results are consistent with a structure in which
the CdC bond is oriented perpendicular to the N-N-Pd-Me
square plane and the OR group points toward the Pd-Me group
(endo rotamer).32
6
are listed in Table 2. The reactions of [(R-
diimine)PdMe(CH2dCH2)][B(C6F5)4] (generated in situ)2d with
excess ethylene and 2a-c gave very similar equilibrium
-
constants (Table 3), showing that the counteranion (SbF6 vs
B(C6F5)4-) does not strongly influence the relative binding
ability of these substrates to 1.
To further understand the structures and dynamics of
these complexes, two less crowded examples, [{(2,6-
i
P r - C
H
) N d C A n C A n d N ( 2 , 6 -
2
6
3
iPr2-C6H3)}PdMe(CH2dCHOSiPh3)][SbF6] (3h, An,An )
acenaphthyl) and [{(4-Me-C6H5)NdCMeCMedN(4-Me-
C6H5)}PdMe(CH2dCHOSiPh3)][SbF6] (3i) were prepared. 3h
is similar to 3c-f and exists as the endo isomer. However, at
-70 °C in CD2Cl2, 3i exists as a 1/1 mixture of endo and exo
rotamers, which were identified by NOSEY correlations. The
barrier for olefin rotation (i.e., interconversion of the endo and
Insertion of [(r-Diimine)PdMe(CH2dCHOtBu)][B(C6F5)4]
(3a[B(C6F5)4]). The tBu vinyl ether adduct 3a[B(C6F5)4] reacts to
form [(R-diimine)Pd{CH2CHMe(OtBu)}][B(C6F5)4] (4a[B(C6F5)4],
66%) and [(R-diimine)Pd{CMe2(OtBu)}][B(C6F5)4] (5a[B(C6F5)4],
25%) in 2 h at 20 °C (Scheme 4). The first-order rate constant for
the consumption of 3a[B(C6F5)4] was measured by 1H NMR at 0
(33) (a) This assumption was made by Brookhart and by Guan in studies
of competitive binding of ethylene and methyl acrylate to (R-
diimine)PdR+ species. (b) Mecking, S.; Johnson, L. K.; Wang, L.;
Brookhart, M. J. Am. Chem. Soc. 1998, 120, 888. (c) Popeney, C. S.;
Guan, Z. B. J. Am. Chem. Soc. 2009, 131, 12384.
(32) (a) Two rotamers of [{(2,6-Me2-C6H3)NdCAnCAndN(2,6-Me2-
C6H3)}PdMe(η2-CH2dCHO2CCH3)}+ were observed. See ref 6. (b)
(Me2bipy)PdMe(η2-CH2dCHCl)+ (Me2bipy ) 4,4′-Me2-2,2′-bipyri-
dine) prefers the exo structure. See ref 7a.
9
J. AM. CHEM. SOC. VOL. 132, NO. 14, 2010 5277

A R T I C L E S
Chen et al.
Table 3. Reactivity of CH2dCHOR with 1[B(C6F5)4]a
CH2dCHOR (2), R )
tBu (2a)
Et (2b)
SiMe3 (2c)
SiMe2Ph (2d)
SiMePh2 (2e)
SiPh3 (2f)
<0.01
Ph (2g)
K2/ethylene (-60 °C)b
kinsert,3 (10-4 s-1, 0 °C)
kinsert,3 (10-4 s-1, 20 °C)
K5/4 (0 °C)d
1.2(7)
0.33(2)
7.1(4)
0.27
0.39
2.3(1)
3.2(2)
0.21(1)
0.80(5)
0.17(1)
0.04(1)
∼20
>33c
>19e
>33c
>19e
>33c
>19e
>33c
>19e
>33c
2.5
2.8
>19e
K5/4 (20 °C)d
>19e
>19e
>19e
>19e
unknownf
kꢀ-OR,obs (10-6 s-1, 20 °C)g
kꢀ-OR (10-6 s-1, 20 °C)i
912(55)
3460(200)
32(2)
51(3)
134(8)
107(6)
>5000h
>640j
>1020j
>2680j
>2140j
>5000j
b
a The uncertainties are based on replicate runs. K2/ethylene ) [3][CH2dCH2][(R-diimine)PdMe(CH2dCH2)+]-1[2]-1 at equilibrium. c More than 95% of
d
3 undergoes insertion within 15 min at 20 °C. K5/4 ) [5]/[4] at equilibrium. e Complexes 4c-f were not detected by NMR. f 4g and 5g were not
observed due to fast ꢀ-OR elimination of 4g. g The observed first-order rate constant for consumption of the total of 4 and 5. h More than 95% of 3g is
converted to 6 and phenol within 10 min at 20 °C. i The first-order rate constant for ꢀ-OR elimination of 4, kꢀ-OR ) kꢀ-OR,obs(K5/4 + 1). j Lower limit
assuming K5/4 > 20.
Scheme 5
and 20 °C (Table 3). For the alkyl ligand of 4a[B(C6F5)4], the 1H
and COSY NMR spectra contain a multiplet at δ 4.86 (CH), a
doublet of doublets at δ 0.40 and a triplet at δ 0.93 (CH2), and a
doublet at δ 1.15 (CH3). For the alkyl ligand of 5a[B(C6F5)4], the
1H NMR spectrum comprises a singlet at δ 0.64. NMR data show
that both 4a[B(C6F5)4] and 5a[B(C6F5)4] contain unsymmetrical
R-diimine ligands, indicating that these species are four-coordinate
rather than three-coordinate complexes. To probe for O-chelation
140 min at -40 °C). The adduct [5a-MeCN][B(C6F5)4] was not
in 4a and 5a, the reaction with CH3CN was investigated. As shown
detected. Therefore, at this temperature, 5a[B(C6F5)4] slowly
in Scheme 4, the 4a[B(C6F5)4]/5a[B(C6F5)4] mixture reacts with
rearranges to 4a[B(C6F5)4], which is then rapidly trapped by
MeCN in CD2Cl2 at -40 °C to produce a single product,
MeCN (Scheme 4).
(R-diimine)Pd{CH2 CHMe(Ot Bu)}(NCMe)+
([4a-
The interconversion of 4a[B(C6F5)4] and 5a[B(C6F5)4] occurs
by a normal chain-walking mechanism, that is, ꢀ-H elimination
to generate (R-diimine)PdH(CH2dCMe(OtBu))+ (not observed)
followed by re-insertion (Scheme 5). DFT studies show that
the energy difference between 4a[B(C6F5)4] and 5a[B(C6F5)4]
is small (E4a - E5a ) 0.2 ( 1.0 kcal/mol), which is consistent
with the fact that both isomers are observed.
MeCN][B(C6F5)4]), quantitatively. The PdCH2CH(OtBu)Me (δ
67.2) and PdCH2CH(OCMe3)Me (δ 72.8) 13C NMR resonances
i
of [4a-MeCN][B(C6F5)4] appear in the normal range for Pr and
tBu ethers.34 In contrast, the PdCH2CH(OtBu)Me (δ 88.3) and
PdCH2CH(OCMe3)Me (δ 88.9) 13C resonances of 4a[B(C6F5)4]
and the PdCMe2(OtBu) (δ 82.8) and PdCMe2(OCMe3) (δ 90.3)
resonances of 5a[B(C6F5)4] are shifted far downfield from this
range, indicating the presence of O-chelation in these species.35-38
Conversion of 4a[B(C6F5)4] and 5a[B(C6F5)4] to
(r-Diimine)Pd(η3-C3H5)[B(C6F5)4] (6[B(C6F5)4]). The 4a[B(C6F5)4]/
5a[B(C6F5)4] mixture reacts to produce (R-diimine)Pd(η3-
Interconversion of 4a[B(C6F5)4] and 5a[B(C6F5)4]. The
5a[B(C6F5)4]/4a[B(C6F5)4] ratio remains constant (0.39) during
the formation of these species from 3a[B(C6F5)4] at 20 °C and
their subsequent conversion to 6 at 20 °C (Scheme 4). These
results imply that 4a[B(C6F5)4] and 5a[B(C6F5)4] interconvert
rapidly on the laboratory time scale but slowly on the NMR
chemical shift time scale at this temperature. NMR monitoring
studies reveal that, during the reaction of 4a[B(C6F5)4]/
5a[B(C6F5)4] with MeCN (1.2 equiv), 4a[B(C6F5)4] is converted
to [4a-MeCN][B(C6F5)4] rapidly (within 5 min at -60 °C), while
5a[B(C6F5)4] is converted to [4a-MeCN][B(C6F5)4] slowly (ca.
t
C3H5)[B(C6F5)4] (6[B(C6F5)4]) and BuOH quantitatively over
the course of 15 days at 20 °C in CD2Cl2 solution (Scheme 4).
Compound 6[B(C6F5)4] was prepared independently and fully
characterized.
The most likely mechanism for the conversion of 4a[B(C6F5)4]/
5a[B(C6F5)4] to 6[B(C6F5)4] is ꢀ-OR elimination of 4a[B(C6F5)4]
to generate (R-diimine)Pd(OR)(CH2dCHMe)+ (not observed),
followed by allylic C-H activation (Scheme 4). Similar C-H
activation reactions to form allyl species were reported by
Bercaw for R-diimine Pt and Pd hydroxide complexes.39 In
related work, Hosokawa showed that in situ generated ClP-
d(OH)(propene) undergoes allylic C-H activation to form {(π-
allyl)PdCl}2 and H2O.40 Also, the reaction of Pd(II) chloride
salts with R-olefins to generate (π-allyl)Pd(II) complexes is well-
known.41
(34) 13C NMR (CDCl3): Me2CHOCMe3, δ 63.4; Me2CHO CMe3, δ
72.7.
(35) The possible coordination of the excess CH2dCHOtBu or Et2O present
in solution was ruled out because NMR data show that these species
are free. After removal of volatiles from the mixture, the NMR
resonances for 4a and 5a are unchanged, confirming that excess
CH2dCHOtBu and Et2O do not bind to these species. ꢀ-H agostic
interactions were not detected in 4a[B(C6F5)4] and 5a[B(C6F5)4] by
ultra-low-temperature NMR (CDCl2F solution,-130 °C). Under similar
conditions (CDCl2F,-120 °C), the 1H NMR resonance for the agostic
H in (R-diimine)Pd{CH(CH2-µ-H)CH3}+ (δ-7.85) was observed. See
ref 2d.
As noted above, the 5a[B(C6F5)4]/4a[B(C6F5)4] ratio remains
constant during the conversion of 4a[B(C6F5)4]/5a[B(C6F5)4] to
6[B(C6F5)4], indicating that the 4a[B(C6F5)4]/5a[B(C6F5)4] ex-
change is faster than the ꢀ-OtBu elimination of 4a[B(C6F5)4].
(36) For comparison, the O(CH2CH3)2 13C NMR resonance for (R-
diimine)PdMe(OEt2)+ appears at δ 71.5, while the corresponding
resonance for free O(CH2CH3)2 appears at δ 65.7. See ref 2e.
(39) (a) Driver, T. G.; Williams, T. J.; Labinger, J. A.; Bercaw, J. E.
Organometallics 2007, 26, 294. (b) Bercaw, J. E.; Hazari, N.; Labinger,
J. A.; Oblad, P. F. Angew. Chem., Int. Ed. 2008, 47, 9941. (c) Williams,
T. J.; Caffyn, A. J. M.; Hazari, N.; Oblad, P. F.; Labinger, J. A.;
Bercaw, J. E. J. Am. Chem. Soc. 2008, 130, 2418.
(37) O-Chelation
in
the
analogous
species
Ru(η2-
CH2CH2OCH3)(CO)(PtBu2Me)2+ was established by X-ray diffraction.
The RuCH2CH2OCH3 and RuCH2CH2OCH3 13C NMR resonances
occur at δ 89.6 and 60.6 ppm, respectively. See ref 16.
(38) DFT calculations confirmed that 4 and 5 have O-chelated structures.
See the Supporting Information.
(40) Hosokawa, T.; Tsuji, T.; Mizumoto, Y.; Murahashi, S. I. J. Organomet.
Chem. 1999, 574, 99.
(41) (a) Morellir, D.; Ugo, R.; Conti, F.; Donati, M. Chem. Commun. 1967,
16, 801. (b) Ketley, A. D.; Braatz, J. Chem. Commun. 1968, 3, 169.
9
5278 J. AM. CHEM. SOC. VOL. 132, NO. 14, 2010

Cationic Polymerization and Insertion Chemistry
A R T I C L E S
Scheme 6
complexes at 0 °C follows the order 3f > 3e > 3d > 3c. However,
the formation of 3c-f[B(C6F5)4] from the reaction of 1[B(C6F5)4]
with 2c-f is slower than the subsequent insertion reactions, so
accurate kinsert,3 values could not be obtained in these cases.
The direct insertion products 4c-f were not detected by
NMR, which implies that K5c-f/4c-f > 19. DFT studies show that
both 4c and 5c have O-chelated structures and that 5c is 3.6 (
1 kcal/mol more stable than 4c, consistent with the fact that
only 5c is observed. 5c,f[B(C6F5)4] react with MeCN to afford
[4c,f-MeCN][B(C6F5)4] in >90% yield, which confirms that
4c,f[B(C6F5)4] and 5c,f[B(C6F5)4] readily interconvert.
Complexes 5c-f react to generate 6 (100%) and ROH via
rearrangement to 4c-f followed by ꢀ-OR elimination and allylic
C-H activation. In the case of 5c,d, the ROH products react
further to yield R2O and H2O. The observed ꢀ-OR elimination
rate constant (kꢀ-OR,obs) follows the order f > e > d > c. As
observed for 2a,b, the observed ꢀ-OR elimination rate constant
of 4c-f/5c-f[SbF6] (kꢀ-OR,obs) is greater than that of 4c-f/
5c-f[B(C6F5)4].
Therefore, the conversion of 4a[B(C6F5)4]/5a[B(C6F5)4] to
6[B(C6F5)4] obeys pre-equilibrium kinetics. The observed first-
order rate constant kꢀ-O Bu,obs for consumption of the total of
t
4a[B(C6F5)4] and 5a[B(C6F5)4] and the formation of 6[B(C6F5)4]
1
was measured by H NMR. The first-order rate constant for
ꢀ-OtBu elimination (kꢀ-O Bu) of 4a[B(C6F5)4] is given by eq 7.42
t
Values for these rate constants are listed in Table 3.
NMR monitoring studies of the reaction of phenyl vinyl ether
-
-
kꢀ-OtBu ) kꢀ-OtBu,obs(K5a/4a + 1)
(7)
complex 3g (B(C6F5)4 and SbF6 salts) at 0-20 °C reveal
smooth quantitative conversion to 6 and phenol. Neither the
1,2 insertion product 4g nor its chain-walk isomer 5g was
detected, which indicates that ꢀ-OPh elimination of 4g is fast.44
Other r-Diimine Ligands. The insertion rate constant of
sterically open 3h (kinsert ) 2.0 × 10-4 s-1 at 0 °C) is 4 times
smaller than that of 3f (8.1 × 10-4 s-1 at 0 °C). This difference
is consistent with the trend observed in analogous ethylene
insertion reactions.2d,45 The ꢀ-OSiPh3 elimination rate of
Model Allylic C-H Activation Reaction. Attempts to prepare
discrete (R-diimine)Pd(OR)+ species to probe if they react with
propene by allylic C-H activation as proposed in Scheme 4
were unsuccessful. However, the model compound
[{(tmeda)Pd(OPh)}n][B(C6F5)4]n (7, tmeda ) N,N,N′,N′-tetram-
ethyl ethylenediamine) was generated in situ by the reaction of
43
(tmeda)Pd(OPh)2 with [HNMe2Ph][B(C6F5)4]. The base-free
i
i
species 7 is believed to be a labile oligomer in solution (see
Experimental Section). Complex 7 reacts with MeCN to generate
the monomeric species [(tmeda)Pd(OPh)(NCMe)][B(C6F5)4] (7-
MeCN). Both 7 and 7-MeCN react with propylene to produce
[(tmeda)Pd(η3-C3H5)][B(C6F5)4] and phenol at 20 °C (Scheme 6).
The propylene complex [(tmeda)Pd(OPh)(CH2dCHMe)][B(C6F5)4]
was not detected by NMR monitoring of these reactions, which
suggests that the allylic C-H activation step is facile.
[{(2,6-Pr2-C6H3)NdCAnCAndN(2,6-Pr2-C6H3)}PdCMe2(OSiPh3)]-
[SbF6] (5h) (kꢀ-OSiPh ,obs ) 1.37× 10-4 s-1 at 20 °C) is also slower
3
that that of 5f (kꢀ-OSiPh ,obs ) 3.78 × 10-4 s-1 at 20 °C). These
3
results show that decreasing the steric crowding around the Pd
center decreases the rate of both the CH2dCHOSiPh3 insertion
and ꢀ-OSiPh3 elimination reactions.
Discussion
Reaction of 1[SbF6] with 2a. The reaction of 1[SbF6] with
2a is similar to that of 1[B(C6F5)4] with 2a, and the NMR data
for 5a[SbF6]/4a[SbF6] are very similar to those for 5a[B(C6F5)4]/
4a[B(C6F5)4]. The insertion rate constant (kinsert,3a) and equilib-
rium constant (K5a/4a) are also very similar for both cases (Table
2). However, the ꢀ-OR elimination rate constant of 4a[SbF6]
(kꢀ-OR) is 6.5 times greater than that of 4a[B(C6F5)4] (Tables 2
and 3).
Reactions of 1[B(C6F5)4] and 1[SbF6] with 2b-g. The key
features of these reactions that are different from the reactions
of 2a are summarized in this section, and key equilibrium and
rate constants are given in Tables 2 and 3. Ethyl vinyl ether
(2b) binds less strongly than 1[B(C6F5)4] but inserts more rapidly
compared to 2a. Interestingly, while K5b/4b is greater than K5a/
4a, ꢀ-OEt elimination of 4b is 103 faster than ꢀ-OtBu elimination
of 4a. Similar results are observed for the reaction of 1[SbF6]
with 2b. The insertion rate constant (kinsert,3b) and equilibrium
constant (K5b/4b) are very similar for both the [B(C6F5)4]- and
[SbF6]- anions, while the ꢀ-OR elimination rate constant of
4b[SbF6] (kꢀ-OR) is 1.3 times greater than that of 4b[B(C6F5)4].
The reactions of 1[SbF6] with 2c-g are similar to those with
2a. The insertion rate constant kinsert,3 of the silyl vinyl ether
The results described above provide insights into the cationic
polymerization and insertion processes observed in the reactions
of vinyl ethers with (R-diimine)PdMe+ (1). First, the efficiency
of 1-initiated cationic polymerization varies in the order
CH2dCHOtBu (2a) > CH2dCHOEt (2b) > CH2dCHOSiMe3
(2c), and this process is not observed for CH2dCHOSiMe2Ph
(2d), CH2dCHOSiMePh2 (2e), CH2dCHOSiPh3 (2f), or
CH2dCHOPh (2g). The 1-initiated cationic polymerization of
2a,b and the corresponding Pd0 formation preclude the use of
these vinyl ethers as comonomers in olefin polymerization
(Scheme 1).20 The 1-initiated cationic polymerization of 2c is
fast enough to compete with 1-initiated 1-hexene/2c insertion
copolymerization and renders this process inefficient.
The trend in the efficiency of 1-initiated cationic polymeri-
zation rate, 2a > 2b > 2g, is consistent with the classical trend
for cationic polymerization by other initiators.10-12,46 The key
(44) 2,6-Dimethylphenyl vinyl ether, 2-tert-butylphenyl vinyl ether, pen-
tafluorophenyl vinyl ether, and 4-nitrophenyl vinyl ether react similarly
to 2g.
(45) Consistent with this trend, the insertion of 3i (kinsert,3i ) 1.76 ×
10-4 s-1 at 0 °C) is slower than that of 3f or 3h. However, in this
case, the insertion product is not stable and multiple species and
Pd0 are formed.
(42) Wright, M. R. An Introduction to Chemical Kinetics; John Wiley &
Sons: New York, 2004; p 346.
(43) Alsters, P. L.; Baesjou, P. J.; Janssen, M. D.; Kooijman, H.; Roetman,
A. S.; Spek, A. L.; Koten, G. V. Organometallics 1992, 11, 4124.
(46) (a) Aoki, S.; Stille, J. K. Macromolecules 1970, 3, 473. (b) Plesch,
P. H.; Shamliam, S. H. Eur. Polym. J. 1990, 26, 1113. (c) Shildknecht,
C. E.; Zoss, A. O.; Grosser, F. Ind. Eng. Chem. 1949, 41, 2891. (d)
Matsumoto, H. J.; Okamura, S. Kobunshi Kugaku 1968, 26, 702.
9
J. AM. CHEM. SOC. VOL. 132, NO. 14, 2010 5279

A R T I C L E S
Chen et al.
diimine)PdMe(CH2dCHOR)+ adduct (kinsert,3). The kinsert,3a,b
values are relatively small, so cationic polymerization dominates
for 2a,b. However, the kinsert,3d-g values are more than 4 times
larger than kinsert,3b, and thus insertion chemistry predominates
for 2d-g. The value for kinsert,3c is only 2 times larger than
Scheme 7
kinsert,3b, and both insertion and cationic polymerization proceed
for 2c; the latter process is favored at higher 2c concentrations
because it is first-order in 2c, while the insertion of 3c is zero-
order in 2c.
It should be noted that, in some cases, cationic polymerization
of silyl vinyl ethers is terminated or prevented by desilylation
of the growing siloxy carbenium ion by nucleophilic attack at
the SiR3 group to yield oligomers or polymers with
-CH2C(dO)R end groups. For example, Rimmer showed that
the reaction of CH2dCHOiBu and CH2dCPh(OSiMe3) with a
HCl·CH2dCHOiBu/TiCl4 initiator system yields oligomers of
general structure H(CH2CHOiBu)nCH2C(dO)Ph.51 This reaction
proceeds by cationic polymerization of CH2dCHOiBu, occasional
addition of the growing alkoxy carbenium ion to
CH2dCPh(OSiMe3) to yield H(CH2CHOiBu)nCH2CPh(OSiMe3)+,
and reaction with Cl- to yield H(CH2CHOiBu)nCH2C(dO)Ph and
Me3SiCl. Desilylations of this type can occur when Cl- or F- are
present.52 Several lines of evidence establish that the absence of
cationic polymerization of 2d-f by 1 is not due to such desilylation
reactions. First, essentially quantitative yields of the Pd allyl
products derived from multiple insertion, but no aldehyde products,
are observed in the reaction of 1 with excess CH2dCHSiPh3 (2f,
Scheme 2). Second, the relatively stable, poorly nucleophilic anions
[SbF6]- or [B(C6F5)4]- are used as counterions for 1. Third, as
noted above, Ag[SbF6], [Li(OEt2)2.8][B(C6F5)4], [Ph3C][B(C6F5)4],
and [H(OEt2)2][B(C6F5)4] all initiate cationic polymerization of
2c-f under the conditions studied here.53
parameter that influences the reactivity of these substrates is
the electron-donating ability of the OR group, which activates
the CdC bond for electrophilic attack and stabilizes the growing
alkoxycarbenium ion.47 It is surprising that 2c-f are not
cationically polymerized by 1 since silyl vinyl ethers including
2c,d are readily polymerized by Lewis acids such as EtAlCl2,
SnCl4, TiCl4, and BF3.48 Moreover, kinetic studies of the
reactions of Ar2CH+ cations with vinyl ethers show that silyl
vinyl ethers have similar or higher nucleophilicity compared to
analogous alkyl vinyl ethers.49 Also, we showed that Ag[SbF6],
[Li(OEt2)2.8][B(C6F5)4], [Ph3C][B(C6F5)4], and [H(OEt2)2]-
[B(C6F5)4] initiate cationic polymerization of 2c-g under the
conditions studied here. The low efficiency (2c) or absence
(2d-f) of cationic polymerization of 2c-f by 1 is due to
competing insertion chemistry, which consumes 1. As noted
above, 1[B(C6F5)4] undergoes up to three sequential insertions
of 2f, ultimately forming Pd allyl products (Scheme 2).21
Analogous multiple insertion reactions occur for 2c-f. Similarly,
it is surprising that 2g is not cationically polymerized by 1 since
other cationic initiators such as [4-ClC6H4CO][SbF6] and SnCl4
readily polymerize this monomer.50 In this case, fast insertion
and ꢀ-OPh elimination out-compete cationic polymerization and
produce (R-diimine)Pd(η3-C3H5) (6) and PhOH (Scheme 1). The
Pd allyl products of these reactions are not active for cationic
polymerization.
Liu et al. reported that [(1-PPh2-2-NdCHAr-C6H4)-
PdMe(H2O)][BF4] (Ar ) 4-FC6H4, C6H5) initiates fast cationic
polymerization of alkyl vinyl ethers but not of CH2dCHOSiMe3.11c
Our results suggest that insertion chemistry similar to that observed
for (R-diimine)PdMe+ may out-compete cationic polymerization
in the latter case.
Under conditions where the cationic polymerization is
circumvented, that is, by using low concentrations of 2a-c or
by using 2d-g, the vinyl ether undergoes the CdC π coordina-
tion, insertion, chain-walking, ꢀ-OR elimination, and allylic
C-H activation process shown in Scheme 4. The relative
binding strengths of CH2dCHOR vary in the order
CH2dCHOtBu (2a) > CH2dCHOEt (2b) ∼ CH2dCHOSiMe3
(2c) > CH2dCHOSiMe2Ph (2d) ∼ CH2dCHOPh (2g) >
CH2dCHOSiMePh2 (2e) > CH2dCHOSiPh3 (2f). The binding
strength trend reflects a combination of electronic and steric
effects. The σ-donation component dominates the Pd-olefin
bonding in (R-diimine)PdR(olefin)+ species due to the poor
The competition between cationic polymerization, which
dominates for 2a,b, and insertion chemistry, which dominates
for 2d-g, is controlled by the relative rates of these processes
(Scheme 7). As noted above, literature data suggest that the
inherent reactivity of alkyl and silyl vinyl ethers toward cationic
polymerization is similar, which in turn suggests that the key
factor that influences the competition between polymerization
and insertion is the insertion rate constant of the (R-
(47) (a) Yuki, H.; Hatada, K.; Nagata, K.; Emura, T. Polym. J. 1970, 1,
269. (b) Hatada, K.; Nagata, K.; Yuki, H. Bull. Chem. Soc. Jpn. 1970,
43, 3195. (c) Hatada, K.; Nagata, K.; Hasegawa, T.; Yuki, H.
Makromol. Chem. 1977, 178, 2413. (d) Hatada, K.; Kitayama, T.;
Nishiura, T.; Shibuya, W. J. Polym. Sci., Part A 2002, 40, 2134.
(48) (a) Murahashi, S.; Nozakura, S.; Sumi, M. J. Polym. Sci., Part B 1965,
3, 245. (b) Nozakura, S.; Ishihara, S.; Inaba, Y.; Matsumura, K.;
Murahashi, S. J. Polym. Sci. 1973, 11, 1053. (c) Solaro, R.; Chiellini,
E. Gazz. Chim. Ital. 1976, 106, 1037. (d) Palos, I.; Cadenas-Pliego,
G.; Knjazhanski, S. Y.; Jimenez-Regalado, E. J.; Casas, E. G. D.;
Ponce-Ibarra, V. H. Polym. Degrad. Stab. 2005, 90, 264.
(51) (a) Lang, W. H.; Sarker, P. K.; Rimmer, S. Macromol. Chem. Phys.
2004, 205, 1011. (b) Sarker, P. K.; Rimmer, S. Macromol. Chem. Phys.
2005, 206, 1280.
(52) (a) Sawamoto, M.; Okamoto, C.; Higashimura, T. Macromolecules
1987, 20, 2693. (b) Cramail, H.; Deffieux, A. J. Phys. Org. Chem.
1995, 8, 293. (c) Kuwajima, I.; Nakamura, E.; Shimizu, M. J. Am.
Chem. Soc. 1982, 104, 1025. (d) Huffman, J. W.; Potnis, S. M.; Satish,
A. V. J. Org. Chem. 1985, 50, 4266.
(49) (a) Bartl, J.; Steenken, S.; Mayr, H. J. Am. Chem. Soc. 1991, 113,
7710. (b) Burfeindt, J.; Patz, M.; Muller, M.; Mayr, H. J. Am. Chem.
Soc. 1998, 120, 3629.
(53) It is highly unlikely that the LiCl that is generated as a byproduct in
in situ activation of (R-diimine)PdMeCl with [Li(OEt2)2.8][B(C6F5)4]
inhibits or prevents cationic polymerization of 2c-g because LiCl is
not soluble in CD2Cl2, and the discrete catalyst 1[SbF6] does not initiate
cationic polymerization of 2c-g.
(50) (a) Plesch, P. H.; Shamlian, S. H. Eur. Polym. J. 1990, 26, 1113. (b)
Heublein, G.; Wondraczek, R. Acta Polym. 1980, 31, 558. (c)
Yamamoto, K.; Higashimura, T. Polymer 1975, 16, 815.
9
5280 J. AM. CHEM. SOC. VOL. 132, NO. 14, 2010

Cationic Polymerization and Insertion Chemistry
A R T I C L E S
the difference in the insertion barriers of 3a-g is the ground-
state energy of the vinyl ether adduct. As noted above, strongly
electron-donating OR groups increase the binding strength and
hence lower the ground-state energy and increase the insertion
barrier, while steric crowding has the opposite effect.2d Com-
plexes 3a-g all insert more slowly than (R-diimine)PdMe(eth-
ylene)+.
The 1,2 insertion products (R-diimine)Pd{CH2CH(OR)Me}+
(4a-f) rapidly and reversibly isomerize to the chain-walk
isomers (R-diimine)Pd{CMe2(OR)}+ (5a-f) via ꢀ-H elimina-
tion/reinsertion. The equilibrium constants K5/4 increase in the
order K5a/4a < K5b/4b , K5c-f/4c-f (only 5 is observed), showing
that as the R group changes from alkyl to silyl the preference
for 5 versus 4 increases. This trend may reflect the trend in
electron-donating ability of the OR group OtBu > OEt >
OSiR3.56 Previous studies have shown that LnM-R bonds are
strengthened by the presence of electron-withdrawing substi-
tutents on CR of the R group, which can stabilize the δ- charge
on CR resulting from the inherent polarization of the M-C
bond.57 Electron-donating groups are expected to exert the
opposite effect, so strongly electron-donating -OR groups
exhibit small K5/4 values. Steric factors may also influence K5/
4; however, this issue is difficult to assess since the relative steric
crowding in 4 versus 5 will be sensitive to the identity of the
OR group.
Complexes 4a-g undergo ꢀ-OR elimination. The observed
ꢀ-OR elimination rate constants for the 4/5 mixtures (kꢀ-OR,obs
)
vary in the order OtBu < OSiR3 < OPh. This trend parallels the
expected order of leaving group ability -OtBu < -OSiR3 <
-OPh.58 ꢀ-OR elimination is also more facile for small OR
groups; for example, ꢀ-OEt elimination is much faster than
ꢀ-OtBu elimination.59 The fast ꢀ-OAr elimination of 4g and
analogous (R-diimine)PdCH2CHOAr+ species44 precludes the
use of aryl vinyl ethers as comonomers in 1-catalyzed olefin
polymerization (Scheme 1).
Figure 1. Energy diagram for insertion of 3a-g[SbF6]. Relative ground-
state energies (versus 3c) are based on K2/ethylene and K2/2c values in Table
2. Transition-state energies are based on kinsert,3 data in Table 2. ∆Gq for 3j
is based on the kinsert,ethylene value in ref 2d.
-
The data in Tables 2 and 3 show that the anion (SbF6 vs
-
B(C6F5)4 ) has only a small influence on the vinyl ether binding
back-bonding ability of the cationic Pd(II) center.54 The trend
in binding strengths, 2a > 2c > 2g, parallels the trend in the
DFT-calculated HOMO energies 2a (-5.76 eV) > 2c (-5.86
eV) > 2g (-5.95 eV).17,55 The DFT-calculated HOMO energies
of 2c and 2b (-5.89 eV) are similar, but 2c is more sterically
bulky and hence binds more weakly compared to 2b. The trend
in binding strength 2c > 2d > 2e > 2f is expected because these
substrates become poorer donors and more sterically crowded
as methyl groups are replaced by phenyl groups.
The vinyl ether adducts 3a-g[SbF6] undergo insertion with
exclusively 1,2 regioselectivity. The insertion rate constants
(kinsert,3, Table 2) vary in the order 3a < 3b < 3c < 3d < 3e < 3f
< 3g. An energy diagram for the insertion of 3a-g and (R-
diimine)PdMe(ethylene)+ based on ground-state energies (versus
3c) derived from competitive binding studies (K2/ethylene and K2/
2c in Table 2) and insertion barriers determined from insertion
rate constants (kinsert,3, Table 2; kinsert,ethylene ) 900 × 10-4 s-1 at
0 °C2d) is shown in Figure 1. The major factor contributing to
strength (K2a-c/ethylene), the vinyl ether insertion rates (kinsert,3),
or subsequent chain-walking (K5/4). However, the kꢀ-OR,obs values
for 4a-f[SbF6]/5a-f[SbF6] are 1.3 to 6.5 times greater than
(56) The electron-donating ability trend OtBu > OEt > OSiR3 parallels the
order of pKa values of the conjugate acids HOR: HOtBu (17) > HOEt
(15.9) > HOSiMe3 (12.7) > HOSiPh3 (10.8).
(57) (a) Axe, F. U.; Marynick, D. S. J. Am. Chem. Soc. 1988, 110, 3728.
(b) Cotton, J. D.; Markwell, R. D. J. Organomet. Chem. 1990, 388,
123. (c) Cotton, J. D.; Crisp, G. T.; Daly, V. A. Inorg. Chim. Acta
1981, 47, 165. (d) Cross, R. J.; Gemmill, J. J. Chem. Soc., Dalton
Trans. 1981, 2317. (e) Cawse, J. N.; Fiato, R. A.; Pruett, R. L. J.
Organomet. Chem. 1979, 172, 405. (f) Anderson, G. K.; Cross, R. J.
Acc. Chem. Res. 1984, 17, 67. (g) Kuhlmann, E. J.; Alexander, J. J.
Coord. Chem. ReV. 1980, 33, 195. (h) Calderazzo, F.; Noack, K.
Coord. Chem. ReV. 1966, 1, 118. (i) Koga, N.; Morokuma, K. J. Am.
Chem. Soc. 1986, 108, 6136. (j) Galamb, V.; Pa´lyi, G.; Boese, R.;
Schmid, G. Organometallics 1987, 6, 861. (k) Clot, E.; Besora, M.;
Maseras, F.; Me´gret, C.; Eisenstein, O.; Oelckers, B.; Perutz, R. N.
Chem. Commun. 2003, 490. (l) Clot, E.; Me´gret, C.; Eisenstein, O.;
Perutz, R. N. J. Am. Chem. Soc. 2009, 131, 7817. (m) Evans, M. E.;
Burke, C. L.; Yaibuathes, S.; Clot, E.; Eisenstein, O.; Jones, W. D.
J. Am. Chem. Soc. 2009, 131, 13464.
(54) (a) Burns, C. T.; Jordan, R. F. Organometallics 2007, 26, 6726. (b)
Burns, C. T.; Jordan, R. F. Organometallics 2007, 26, 6737. (c)
Willner, H.; Aubke, F. Organometallics 2003, 22, 3612. (d) Lupinetti,
A. J.; Strauss, S. H.; Frenking, G. Prog. Inorg. Chem. 2001, 49, 1. (e)
Rix, F. C.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 1137. (f)
Rix, F. C.; Brookhart, M.; White, P. S. J. Am. Chem. Soc. 1996, 118,
4746.
(58) As noted by Caulton, since ꢀ-OR elimination implies migration of
the RO- anion, OPh should be more prone to migrate than OEt. (a)
Ozerov, O. V.; Watson, L. A.; Pink, M.; Caulton, K. G. J. Am. Chem.
Soc. 2004, 126, 6363. (b) The order of leaving ability of OR groups
is inverse to the order of pKa values of their conjugate acids HO R:
HOtBu > HOEt > HOSiR3 > HOPh (9.8).
(59) (a) The fact that -O Et is a better leaving group than -OtBu also
contributes. (b) The ꢀ-OR elimination of (tBu3SiO)3TaH(CH2CH2OR)
ˇ
(55) (a) Kaeˇer, P.; Tobicˇik, J.; Kuzma, M.; Cerveny´, L. J. Mol. Catal. A
t
2003, 195, 235.
is much slower for R ) Bu than R ) Et. See ref 18.
9
J. AM. CHEM. SOC. VOL. 132, NO. 14, 2010 5281

A R T I C L E S
Chen et al.
2,6-Di-tert-butylpyridine was degassed by three freeze-pump-thaw
cycles and distilled from CaH2 under reduced pressure. (cod)PdCl2
(cod ) 1, 5-cyclooctadiene),61 (cod)PdMeCl,62(R-diimine)PdMeCl
(R-diimine ) (2,6-(iPr)2C6H3)NdCMeCMedN(2,6-(iPr)2C6H3)),22
(R-diimine)PdMe(OEt2)][SbF6],22,63 [(η3-C3H5)Pd(µ-Cl)]2,64 and
(tmeda)Pd(OPh)243 were prepared by literature procedures. KOPh
was synthesized by the reaction of KN(SiMe3)2 and phenol in THF
and purified by washing with hexanes. [Li(Et2O)n][B(C6F5)4] and
[HNMe2Ph][B(C6F5)4] were obtained from Boulder Scientific. The
Et2O content in [Li(Et2O)n][B(C6F5)4] was determined by 1H NMR
with C6Me6 as internal standard (n ) 2.8). CH2dCHOtBu,
CH2dCHOEt, and CH2dCHOSiMe3 were obtained from Aldrich,
degassed by three freeze-pump-thaw cycles, and dried over CaH2.
CH2dCHOSiMe2Ph,65 CH2dCHOSiMePh2,65 CH2dCHOSiPh3,65
and CH2dCHOPh66 were prepared using literature procedures. All
other chemicals were purchased from Aldrich and used without
further purification.
those of 4a-f[B(C6F5)4]/5a-f[B(C6F5)4]. As is evident from
Scheme 4, this effect could be due to changes in kꢀ-OR or K5/4.
However, the K5/4 values that can be measured (K5a/4a and K5b/
4b) are very similar for the two anions, suggesting that the
difference in kꢀ-OR,obs values is due to the kꢀ-OR term. This is
likely to be true for the other vinyl ethers 2c-g, as well. One
possible explanation for the anion effect is that SbF6-, which
-
is more strongly coordinating than B(C6F5)4 , may bind weakly
to 4, increasing the effective steric crowding and accelerating
ꢀ-OR elimination.60 This is consistent with the observation that
sterically bulky R-diimine ligands promote ꢀ-OR elimination
-
-
rate of the SbF6 salts compared to B(C6F5)4 salts of (R-
diimine)PdCH2CHMeOR+ and explains the anion effect on the
product distribution in the multiple insertion reactions in Scheme
2.21
NMR spectra were recorded on Bruker DMX-500 or DRX-400
spectrometers in Teflon-valved tubes at 20 °C unless specified
otherwise. 1H and 13C chemical shifts are reported relative to SiMe4
and were determined by reference to the residual solvent signals.
Coupling constants are reported in hertz. For H2CdCHX substrates,
Conclusions
The Brookhart olefin polymerization catalyst (R-diimine)P-
dMe+ (1) reacts with vinyl ethers by two general pathways.
First, 1 initiates the cationic polymerization of vinyl ethers 2a-c.
This pathway results in the decomposition of 1 to Pd0. Second,
1 reacts with stoichiometric quantities of 2a-g by π complex
formation, insertion, chain-walking, ꢀ-OR elimination, and
allylic C-H activation to form (R-diimine)Pd(η3-C3H5)+ (6) and
ROH. For silyl vinyl ethers 2d-f, the ꢀ-OR elimination is
sufficiently slow that up to three sequential vinyl ether insertions
can occur prior to ꢀ-OR elimination.
H
cis is the H that is cis to Hint and Htrans is the H that is trans to Hint.
13C NMR resonances were assigned with the assistance of DEPT-
-
135 experiments. The NMR spectra of B(C6F5)4 salts contain
signals for the free B(C6F5)4 anion. 13C{1H} NMR (CD2Cl2): δ
-
148.5 (d, J ) 242), 137.0 (d, J ) 247), 135.6 (d, J ) 244), 123.1
(br, Cipso). 13C{1H} NMR (CD2Cl2, -60 °C): δ 147.4 (d, J ) 238),
137.7 (d, J ) 244), 135.8 (d, J ) 236), 123.2 (br, Cipso). 19F NMR
(CD2Cl2): δ -132.1 (br s, 8F, Fortho), -161.3 (t, J ) 20, 4F, Fpara),
-165.2 (t, J ) 18, 8F, Fmeta). 19F NMR (CD2Cl2, -70 °C): δ -132.5
(br s, 8F, Fortho), -161.7 (t, J ) 20, 4F, Fpara), -164.9 (t, J ) 18,
8F, Fmeta). 11B NMR (CD2Cl2): δ -16.1 (br s). 11B NMR (CD2Cl2,
-70 °C): δ -15.8 (br s).
Electrospray mass spectra (ESI-MS) were recorded on freshly
prepared samples (ca. 1 mg/mL in CH2Cl2) using an Agilent 1100
LC-MSD spectrometer incorporating a quadrupole mass filter with
an m/z range of 0-3000. Typical instrumental parameters included
the following: drying gas temperature 350 °C, nebulizer pressure
35 psi, drying gas flow 12.0 L/min, and fragmentor voltage 0, 70,
or 100 V. In all cases where assignments are given, the observed
isotope patterns closely matched calculated isotope patterns. The
listed m/z value corresponds to the most intense peak in the isotope
pattern.
The cationic vinyl ether polymerization and associated Pd0
formation and the ꢀ-OR elimination to form Pd allyl species
that are unreactive for olefin insertion are catalyst deactivation
processes that must be avoided in order to achieve 1-catalyzed
olefin/vinyl ether copolymerization. The copolymerization of
1-hexene with CH2dCHOSiPh3 (2f) by 1 is possible because
insertion is much faster than cationic polymerization and ꢀ-OR
elimination is relatively slow for this vinyl ether. However, the
more electron-rich vinyl ethers 2a-c are not suitable comono-
mers for 1 because for these substrates cationic polymerization
out-competes insertion, and aryl vinyl ethers are unsuitable
because ꢀ-OAr elimination is fast. The ability to strongly
influence the reactivity of vinyl ethers with metal catalysts by
modification of the vinyl ether structure, combined with the
ability to tune catalyst behavior by modification of the ancillary
ligands and anions, may enable broad use of vinyl ethers as
comonomers in insertion polymerization of olefins.
Polymerization of CH2dCHOtBu (2a) by 1[B(C6F5)4]. An
NMR tube was charged with (R-diimine)PdMeCl (13.6 mg, 0.0242
mmol), [Li(Et2O)2.8][B(C6F5)4] (22.0 mg, 0.0246 mmol), and 2,6-
di-tert-butylpyridine (9.2 mg, 0.048 mmol). CD2Cl2 (0.4 mL) and
2a (0.68 mmol) were added by vacuum transfer at -196 °C. The
tube was warmed to 20 °C, shaken vigorously, and monitored
periodically by NMR. 1H NMR spectra showed that
-[CH2CH(OtBu)]n- had formed within 5 min, and 2a was
quantitatively converted to polymer after 20 h. Key NMR data for
Experimental Section
Methods and Materials. All manipulations were performed
using drybox or Schlenk techniques under purified nitrogen or on
a high-vacuum line unless indicated otherwise. Nitrogen was
purified by passage through activated molecular sieves and Q-5
oxygen scavenger. Dichloromethane was dried over CaH2, stored
over P2O5, and freshly vacuum transferred prior to use. Tetrahy-
drofuran was distilled from sodium/benzophenone. Pentane, hex-
anes, toluene, and benzene were either distilled from sodium/
benzophenone or purified by passage through activated alumina
and BASF R3-11 oxygen removal catalyst. CD2Cl2 and CDCl3 were
degassed by three freeze-pump-thaw cycles and dried over P2O5.
1
-[CH2CH(OtBu)]n-. H NMR (CDCl3): δ 9.70 (m, -CH2CH-
(OtBu)CH2C(dO)H), 5.53 (br, -CH2CH(OtBu)CHdCHCH2-),
5.40 (br, -CH2CH(OtBu)CHdCHCH2-), 4.90 (br m,
-CH2CH(OtBu)2), 4.00 (br, -CH2CH(OtBu)CHdCHCH2-), 3.65
(CH3CH(OtBu)-), 3.59 (br, -[CH2CH(OtBu)]n-), 2.35
(-CH2CH(OtBu)CHdCHCHH′-), 1.63 (br, -[CH2CH(OtBu)]n-),
1.60 (-CH2CH(OtBu)CHdCHCH2-), 1.45 (-CH2CH(OtBu)2),
(61) Drew, D.; Doyle, J. R. Inorg. Synth. 1990, 28, 348.
(62) Ru¨lke, R. E. Inorg. Chem. 1993, 32, 5769.
(63) McCord, E. F.; McLain, S. J.; Nelson, L. T. J.; Arthur, S. D.; Coughlin,
E. B.; Ittel, S. D.; Johnson, L. K.; Tempel, D.; Killian, C. M.;
Brookhart, M. Macromolecules 2001, 34, 362.
(64) Tatsuno, Y.; Yoshida, T.; Otsuka, S. Inorg. Synth. 1990, 28, 342.
(65) Schaumann, E.; Tries, F. Synthesis 2002, 2, 191.
(66) Okimoto, Y.; Sakaguchi, S.; Ishii, Y. J. Am. Chem. Soc. 2002, 124,
1590.
(60) (a) Macchioni, A.; Bellachioma, G.; Cardaci, G.; Travaglia, M.;
Zuccaccia, C.; Milani, B.; Corso, G.; Zangrando, E.; Mestroni, G.;
Carfagna, C.; Formica, M. Organometallics 1999, 18, 3061. (b)
Zuccaccia, C.; Macchioni, A.; Orabona, I.; Ruffo, F. Organometallics
1999, 18, 4367. (c) Macchioni, A.; Magistrato, A.; Orabona, I.; Ruffo,
F.; Rothlisberger, U.; Zuccaccia, C. New J. Chem. 2003, 27, 455. (d)
Macchioni, A. Eur. J. Inorg. Chem. 2003, 195.
9
5282 J. AM. CHEM. SOC. VOL. 132, NO. 14, 2010

Cationic Polymerization and Insertion Chemistry
A R T I C L E S
1.36 (CH3CH2CH(OtBu)-), 1.19 (br, -[CH2CH(OtBu)]n-), 1.10
(CH3CH(OtBu)-), 0.78 (CH3CH2CH(OtBu)-). 1H-1H COSY
correlations (CDCl3): δ/δ 5.53 (br, -CH2CH(OtBu)CHdCHCH2-)/
2.35 (-CH2CH(OtBu)CHdCHCHH′-), 5.40 (br, -CH2CH-
(OtBu)CHdCHCH2-)/4.00 (br, -CH2CH(OtBu)CHdCHCH2-),
4.90 (br m, -CH2CH(OtBu)2)/1.45 (-CH2CH(OtBu)2), 3.65 (CH3CH-
(OtBu)-)/1.10 (CH3CH(OtBu)-), 3.59 (br, -[CH2CH(OtBu)]n-)/
1.63 (br, -[CH2CH(OtBu)]n-), 1.36 (CH3CH2CH(OtBu)-)/0.78
(CH3CH2CH(OtBu)-). 13C{1H} NMR (CDCl3): δ 73.2 (br,
-CH2CH(OCMe3)-), 67.6 (br, mm -CH2CH(OtBu)-), 67.2 (br,
mr -CH2CH(OtBu)-), 66.5 (br, rr -CH2CH(OtBu)-), 45.6 (br,
-CH2CH(OtBu)-), 29.6 (br, -CH2CH(OCMe3)-); mm/mr/rr )
1:3:1.
Generation of [(r-Diimine)PdMe(CH2dCHOtBu)][SbF6]
(3a[SbF6]). An NMR tube was charged with 1[SbF6] (20.0 mg,
0.0237 mmol), and CD2Cl2 (0.4 mL) and 2a (0.024 mmol) were
added by vacuum transfer at -196 °C. The tube was warmed to
-78 °C, shaken to dissolve and thoroughly mix the components,
and placed in an NMR probe that had been precooled to -60 °C.
NMR spectra at -60 °C showed that 3a[SbF6] (90%) had formed.
1H NMR (CD2Cl2, -60 °C): δ 7.37-7.28 (m, 6H), 7.09 (dd, J )
13, 4, 1H, Hint), 3.27 (d, J ) 13, 1H, Htrans), 2.96 (d, J ) 4, 1H,
Hcis), 2.92 (m, 1H, CHMe2), 2.87 (m, 2H, CHMe2), 2.72 (m, 1H,
CHMe2), 2.30 (s, 3H, NdCMe), 2.23 (s, 3H, NdCMe), 1.41 (d, J
) 7, 3H, CHMe2), 1.35 (s, 9H, OCMe3), 1.32 (d, J ) 7, 3H,
CHMe2), 1.26 (d, J ) 7, 3H, CHMe2), 1.24 (d, J ) 7, 3H, CHMe2),
1.16 (d, J ) 7, 3H, CHMe2), 1.15 (d, J ) 7, 3H, CHMe2), 1.14 (d,
J ) 7, 3H, CHMe2), 1.08 (d, J ) 7, 3H, CHMe2), 0.14 (s, 3H,
PdMe). 13C{1H} NMR (CD2Cl2, -60 °C): δ 179.3 (NdCMe), 175.3
(NdCMe), 148.2 (CH2dCHOtBu), 139.4, 139.0, 138.2, 137.8,
137.6, 137.1, 128.09, 128.06, 124.65, 124.62, 124.13, 124.11, 83.6
(OCMe3), 54.4 (CH2dCHOtBu), 28.8, 28.6, 28.41, 28.36, 27.5,
24.4, 24.0 (2C), 23.9, 23.5, 23.4, 22.9, 22.8, 21.6, 16.7 (PdMe).
Generation of [(r-Diimine)PdMe(CH2dCHOtBu)][B(C6F5)4]
(3a[B(C6F5)4]). An NMR tube was charged with (R-diimine)PdMeCl
(12.0 mg, 0.0214 mmol) and [Li(Et2O)2.8][B(C6F5)4] (20.2 mg,
0.0226 mmol). CD2Cl2 (0.4 mL) was added by vacuum transfer at
-196 °C. The tube was warmed to 20 °C and shaken for 10 min.
2a (0.04 mmol) was added by vacuum transfer at -196 °C. The
tube was kept at 0 °C for 10 min. The volatiles were evacuated,
and CD2Cl2 (0.4 mL) was added by vacuum transfer at -196 °C.
The tube was warmed to 20 °C, shaken vigorously, and monitored
periodically by NMR. NMR analysis showed a mixture of [{(R-
diimine)PdMe}2(µ-Cl)]+ (8%), 3a[B(C6F5)4] (78%), [(R-
diimine)Pd{CH2CHMe(OtBu)}][B(C6F5)4] (4a[B(C6F5)4], 10%), and
[(R-diimine)Pd{CMe2(OtBu)}][B(C6F5)4] (5a[B(C6F5)4], 4%) after
10 min. The NMR spectra of 3a[B(C6F5)4] are very similar to those
of 3a[SbF6].
Generation of 4a[B(C6F5)4] and 5a[B(C6F5)4]. An NMR tube
was charged with (R-diimine)PdMeCl (12.0 mg, 0.0214 mmol) and
[Li(Et2O)2.8][B(C6F5)4] (20.2 mg, 0.0226 mmol). CD2Cl2 (0.4 mL)
was added by vacuum transfer at -196 °C. The tube was warmed
to 20 °C and shaken for 10 min. 2a (0.04 mmol) was added by
vacuum transfer at -196 °C. The tube was kept at 0 °C for 10
min. The volatiles were evacuated, and CD2Cl2 (0.4 mL) was added
by vacuum transfer at -196 °C. The tube was warmed to 20 °C,
shaken vigorously, and monitored periodically by NMR. NMR
analysis showed that, after 2 h, a mixture of 3a[B(C6F5)4] (3%),
4a[B(C6F5)4] (66%), and 5a[B(C6F5)4] (25%) was present. After
22 h, the consumption of 3a[B(C6F5)4] was complete, and
4a[B(C6F5)4] (59%), 5a[B(C6F5)4] (22%), and [(R-diimine)Pd(η3-
C3H5)][B(C6F5)4] (6[B(C6F5)4], 16%) were present. ESI-MS of (R-
diimine)Pd{CH2CH(OtBu)Me}+ and (R-diimine)Pd{CMe2(OtBu)}+:
1
calcd m/z ) 625.3, found 625.2. The aromatic H NMR resonances
of 4a[B(C6F5)4] and 5a[B(C6F5)4] overlap, and the 13C NMR reso-
nances of these species are very similar and therefore only key NMR
data are listed.67 Key NMR data for 4a[B(C6F5)4]. 1H NMR (CD2Cl2):
δ 4.86 (m, 1H, PdCH2CHMe(OtBu)), 3.79 (sept, J ) 7, 1H,
CHMe2), 3.29 (sept, J ) 7, 1H, CHMe2), 2.93 (sept, J ) 7, 1H,
CHMe2), 2.61 (sept, J ) 7, 1H, CHMe2), 2.20 (s, 3H, NdCMe),
2.17 (s, 3H, NdCMe), 0.93 (t, J ) 7, 1H, PdCHH′CHMe(OtBu)),
0.85 (s, 9H, OCMe3), 0.40 (dd, J ) 7, 4, 1H, PdCHH′CHMe(OtBu)).
The PdCH2CHMe(OtBu) signal is obscured but was identified by
COSY NMR at δ 1.22. 1H-1H COSY correlations (CD2Cl2, -40 °C):
δ/δ 4.83 (PdCH2CHMe(OtBu))/1.22 (PdCH2CHMe(OtBu)),
4.83(PdCH2CHMe(OtBu))/0.81 (PdCHH′CHMe(OtBu)), 4.83 (Pd-
CH2CHMe(OtBu))/0.27 (PdCHH′CHMe(OtBu)), 0.81 (PdCHH′-
CHMe(OtBu))/0.27 (PdCHH′CHMe(OtBu)). 1H NMR (CDCl2F,
-130 °C): δ 4.85 (m, 1H, PdCH2CHMe(OtBu)), 3.96 (m, J ) 7,
1H, CHMe2), 3.32 (sept, J ) 7, 1H, CHMe2), 2.84 (sept, J ) 7,
1H, CHMe2), 2.46 (sept, J ) 7, 1H, CHMe2), 0.78 (s, 9H, OCMe3).
13C{1H} NMR (CD2Cl2, -40 °C): δ 88.9 (OCMe3), 88.3
(PdCH2CHMe(OtBu)), 9.3 (PdCH2CHMe(OtBu)). Key NMR data
1
for 5a[B(C6F5)4]. H NMR (CD2Cl2): δ 3.05 (sept, J ) 7, 4H,
CHMe2), 2.21 (s, 3H, NdCMe), 2.20 (s, 3H, NdCMe), 1.11 (s,
1
9H, OCMe3), 0.64 (s, 6H, PdCMe2(OtBu)). H NMR (CDCl2F,
-130 °C): δ 3.02 (sept, J ) 7, 4H, CHMe2), 1.07 (s, 9H, OCMe3),
0.63 (s, 6H, CMe2). 13C{1H} NMR (CD2Cl2, -40 °C): δ 90.3
(OCMe3), 82.8 (PdCMe2(OtBu)).
The first-order rate constant for the consumption of 3a[B(C6F5)4]
(kinsert, 3a) was measured by the disappearance of the PdMe resonance
of 3a[B(C6F5)4] and increase of the PdCH2CHMe resonance of
4a[B(C6F5)4] plus the PdCMe2 resonance of 5a[B(C6F5)4]. The
equilibrium constant K5/4 was determined from the ratio of the
integrated intensities of the PdCMe2 resonance of 5a[B(C6F5)4] and
the PdCH2CHMe resonance of 4a[B(C6F5)4].
Reaction of 4a[B(C6F5)4]/5a[B(C6F5)4] with MeCN. An NMR
tube containing a CD2Cl2 solution (0.4 mL) of 4a[B(C6F5)4] (0.016
mmol) and 5a[B(C6F5)4] (0.0055 mmol) was frozen at -196 °C,
and MeCN (0.026 mmol) was added by vacuum transfer. The tube
was warmed to -78 °C, agitated to mix the components, placed in
an NMR probe that had been precooled to -60 °C, and monitored
by NMR. 1H NMR spectra showed that, after 5 min, 4a[B(C6F5)4]
had been consumed, and a mixture of 5a[B(C6F5)4] (24%) and [4a-
MeCN][B(C6F5)4] (76%) was present. The tube was then warmed
to -40 °C and monitored periodically by NMR. After 140 min, a
mixture of 5a[B(C6F5)4] (5%) and [4a-MeCN][B(C6F5)4] (91%) was
present. Exchange between free and coordinated MeCN is slow on
the NMR time scale at -40 °C. Compound 4a[B(C6F5)4] was not
1
detected during this reaction. Data for [4a-MeCN][B(C6F5)4]. H
NMR (CD2Cl2, -40 °C): δ 7.42-7.26 (m, 6H), 3.18 (m, 1H,
PdCH2CHMe(OtBu)), 2.90 (m, 2H, CHMe2), 2.83 (m, 2H, CHMe2),
2.21 (s, 3H, NdCMe), 2.19 (s, 3H, NdCMe), 1.71 (s, 3H, MeCN),
1.53 (m, 1H, PdCHH′CHMe(OtBu)), 1.31 (d, J ) 6, 12H, CHMe2),
1.15 (d, J ) 6, 6H, CHMe2), 1.10 (3H, CHMe2), 1.08 (3H, CHMe2),
0.97 (d, J ) 6, 3H, PdCH2CHMe(OtBu)), 0.84 (s, 9H, OCMe3).
The other PdCHH′ signal is obscured but was identified by COSY
NMR at δ 1.15. Key 1H-1H COSY correlations (CD2Cl2, -40 °C,
NMR 500-2, 45-126) δ/δ: 3.18 (PdCH2CHMe(OtBu))/1.53
(PdCHH′CHMe(OtBu)), 3.18 (PdCH2CHMe(OtBu))/1.15 (PdCH-
H′CHMe(OtBu)), 3.18 (PdCH2CHMe(OtBu))/0.97 (PdCH2CH-
Me(OtBu)), 1.53 (PdCHH′CHMe(OtBu))/1.15 (PdCHH′CH-
Me(OtBu)). 13C{1H} NMR (CD2Cl2, -40 °C): δ 179.8 (NdCMe),
172.1 (NdCMe), 139.7, 139.4, 138.4, 138.2, 137.4 (2C), 128.9,
128.1, 124.7, 124.6, 124.1, 124.0, 121.8, 72.8 (OCMe3), 67.2
(PdCH2CHMe(OtBu)), 36.9 (PdCH2CHMe(OtBu)), 28.95, 28.92,
28.7, 28.6, 27.8, 25.6, 23.8, 23.6, 23.3, 23.2, 23.1, 23.0 (2C), 22.8,
22.2, 20.0, 2.0 (MeCN).
(67) 13C{1H} NMR of [(R-diimine)Pd{CH2CHMe(OtBu)}][B(C6F5)4] and
[(R-diimine)Pd{CMe2(OtBu)}][B(C6F5)4]: δ 178.1, 174.5, 171.6, 143.4,
142.8, 141.6, 140.9, 138.5, 137.8, 137.5, 137.0, 136.5, 136.2, 128.7,
128.1, 127.9, 127.6, 124.54, 124.48, 124.41, 124.36 (2C), 124.2, 90.3,
88.9, 88.3, 82.9, 30.6, 29.7, 29.04, 28.99, 28.95, 28.75, 28.65, 28.6,
26.2, 25.5, 24.5, 24.0, 23.8, 23.7, 23.29, 23.25, 22.9, 22.8, 22.6, 22.5,
22.3, 21.2, 21.1, 20.0, 19.5, 9.3. One of the NdCMe resonances of
[(R-diimine)Pd{CMe2(OtBu)}][B(C6F5)4] was obscured, and the OC-
Me3 resonances for [(R-diimine)Pd{CH2CHMe(OtBu)}][B(C6F5)4] and
[(R-diimine)Pd{CMe2(OtBu)}][B(C6F5)4] overlap.
9
J. AM. CHEM. SOC. VOL. 132, NO. 14, 2010 5283

A R T I C L E S
Chen et al.
Conversion of 4a[B(C6F5)4]/5a[B(C6F5)4] to 6[B(C6F5)4]. An
NMR tube containing a CD2Cl2 solution of 4a[B(C6F5)4] (0.038
mmol) and 5a[B(C6F5)4] (0.014 mmol) was maintained at 20 °C
and monitored by NMR periodically. Over the course of 15 days,
90% of 4a[B(C6F5)4] and 5a[B(C6F5)4] were converted to
6[B(C6F5)4] and HOtBu. The first-order rate constant for consump-
tion of the total of 4a[B(C6F5)4] and 5a[B(C6F5)4] was measured
by the disappearance of the sum of the PdCH2CHMe resonance of
4a[B(C6F5)4] and the PdCMe2 resonance of 5a[B(C6F5)4].
6[B(C6F5)4] was prepared by the procedure reported by Risse for
the analogous compound [(2,2′-bipyridyl)Pd(η3-C3H5)][SbF6] and
characterized by X-ray diffraction (see Supporting Information).68
low fragmentor volatage, while 1H PGSE NMR experiments suggest
a tetrameric structure (n ) 4), referencing [(tmeda)Pd(OPh)-
(NCMe)][B(C6F5)4] as a monomeric analogue.70 These results
indicate that 7 is most likely a labile oligomer in CD2Cl2 solution.
1
Key NMR data for 7. H NMR (CD2Cl2): δ 7.73 (d, J ) 8, 2H,
H
ortho), 7.45 (t, J ) 8, 2H, Hmeta), 7.27 (t, J ) 8, 1H, Hpara), 2.46
(m, 4H, -CH2-), 2.08 (s, 12H, NMe2). 13C{1H} NMR (CD2Cl2):
δ 158.8, 131.8, 126.1, 124.3, 64.0, 50.6. ESI-MS of (tmeda)P-
d(OPh)+: calcd m/z ) 315.1, found 315.0.
Reaction of 7 with Propylene. The above solution of in situ
generated 7 was frozen at -196 °C, and propylene (1 equiv) was added
by vacuum transfer. The tube was warmed to -30 °C. H NMR
1
1
NMR data for 6[B(C6F5)4]. H NMR (CD2Cl2): δ 7.41-7.32 (m,
showed no reaction occurred after 30 min at -30 °C. The sample
was then warmed to 20 °C. After 10 h, 1H NMR showed that 7 was
completely converted to [(tmeda)Pd(η3-C3H5)][B(C6F5)4] and HOPh.
[(tmeda)Pd(OPh)(NCMe)][B(C6F5)4] ([7-MeCN][B(C6F5)4]). A
solution of in situ generated 7 in CD2Cl2 was frozen at -196 °C,
and MeCN (1.1 equiv) was added by vacuum transfer. The tube
was warmed to 20 °C and shaken. 1H NMR showed that
[7-MeCN][B(C6F5)4] had formed quantitatively in 5 min. 1H NMR
(CD2Cl2): δ 7.11-7.06 (m, 4H), 6.69 (m, 1H, Hpara), 2.78 (m, 2H,
-CH2-), 2.72 (s, 6H, NMe2), 2.68 (s, 6H, NMe2), 2.55 (m, 2H,
-CH2-), 2.08 (MeCN). 13C{1H} NMR (CD2Cl2) δ 165.9, 156.4,
129.6, 119.8, 117.2, 63.3, 62.3, 51.7, 50.9, 3.1 (MeCN). ESI-MS
of (tmeda)Pd(OPh)(NCMe)+: calcd m/z ) 356.1, found 356.0.71
[(tmeda)Pd(η3-C3H5)][B(C6F5)4]. This species was prepared
following the procedure reported by Risse for the analogous
compound [(2,2′-bipyridyl)Pd(η3-C3H5)][SbF6].72 1H NMR
(CD2Cl2): δ 5.66 (m, 1H, Hint), 3.79 (d, J ) 7, 2H, Hsyn), 3.00 (d,
J ) 13, 2H, Hanti), 2.91 (s, 6H, NMe2), 2.73 (s, 6H, NMe2), 2.80
(m, 2H, -CH2-), 2.71 (m, 2H, -CH2-). 13C{1H} NMR (CD2Cl2):
δ 119.8 (allyl CH), 61.9, 61.1, 52.5 (NMe2), 51.9 (NMe2). ESI-MS
of (tmeda)Pd(η3-C3H5)+: calcd m/z ) 263.1, found 263.0. Anal.
Calcd for C33H21BF20N2Pd: C, 42.04; H, 2.25; N, 2.97. Found: C,
42.13; H, 2.31; N, 2.89.
6H), 5.65 (m, 1H, Hint), 3.36 (d, J ) 7, 2H, Hsyn), 3.04 (d, J ) 13,
2H, Hanti), 2.96 (sept, J ) 7, 2H, CHMe2), 2.70 (sept, J ) 7, 2H,
CHMe2), 2.26 (s, 6H, NdCMe), 1.35 (d, J ) 7, 6H, CHMe2), 1.26
(d, J ) 7, 6H, CHMe2), 1.23 (d, J ) 7, 6H, CHMe2), 1.21 (d, J )
7, 6H, CHMe2). 13C{1H} NMR (CD2Cl2): δ 176.9 (NdCMe), 144.0,
137.1, 137.0, 129.1, 125.0, 125.0, 121.1, 65.8 (allyl CH2), 29.8,
29.5, 23.7, 23.6, 23.4, 23.3, 20.1. ESI-MS of (R-diimine)Pd(η3-
C3H5)+: calcd m/z ) 551.3, found 551.2. Anal. Calcd for
C55H45BF20N2Pd: C, 53.66; H, 3.68; N, 2.28. Found: C, 53.78; H,
3.67; N, 2.19.
The first-order rate constant for consumption of the total of
t
4a[B(C6F5)4] and 5a[B(C6F5)4], kꢀ-O Bu,obs, was measured by the
disappearance of the PdCH2CHMe 1H NMR resonance of
4a[B(C6F5)4] plus the PdCMe2 1H NMR resonance of 5a[B(C6F5)4]
and the increase in the Hint resonance of 6[B(C6F5)4].
Insertion of 3a[SbF6] and Reactions of 1 with 2b-g. The
insertion rate constant of 3a[SbF6], the equilibrium constant between
5a[SbF6] and 4a[SbF6], and the ꢀ-OtBu elimination rate constant
of 5a[SbF6]/4a[SbF6] were measured by the methods described
-
above for the B(C6F5)4 salts. The reactions of 1[SbF6] and
1[B(C6F5)4] with 2b-g were studied using the procedures described
above for 2a. Full details are provided in the Supporting Informa-
tion.
Computational Methods. DFT computations were performed
using Gaussian 03.73 The geometries of the stationary points were
found using the B3LYP functional.74 C, H, N, and O atoms were
modeled using the 6-31G* basis set,75 and Pd and Si were modeled
using the LANL2DZ basis set including the relativistic pseudopo-
tentials of Hay and Wadt.76
Competitive Binding of Ethylene and CH2dCHOR (2a-c)
to 1[SbF6] (eq 4). The procedure for 2a is described here; an
identical procedure was used for 2b,c. An NMR tube was charged
with 1[SbF6] (15.0 mg, 0.0179 mmol). CD2Cl2 (0.4 mL) was added
by vacuum transfer at -196 °C. The tube was warmed to 20 °C
and shaken. Ethylene (0.062 mmol) and 2a (0.040 mmol) were
added by vacuum transfer at -196 °C. The tube was warmed to
-78 °C, shaken, and placed in an NMR probe that had been
precooled to -60 °C. The reaction was monitored periodically by
Acknowledgment. This work was supported by the U.S.
Department of Energy (DE-FG-02-00ER15036).
Supporting Information Available: Experimental procedures,
characterization of complexes, details of kinetic studies, and
complete ref 73. This material is available free of charge via
the Internet at http://pubs.acs.org.
1H NMR at -60 °C until the reaction quotient Q2a/ethylene
)
[3a][CH2dCH2][(R-diimine)PdMe(CH2dCH2)+]-1[2a]-1 reached a
constant value. Additional 2a (0.062 mmol) was added by vacuum
transfer to change the ethylene/2a ratio, and the tube was monitored
1
by H NMR at -60 °C until Q2a/ethylene again reached a constant
JA100491Y
value. This process was repeated one more time, and the average
K2a/ethylene (SbF6-) value is reported in Table 2. The competitive
binding of 2d-g and 2c to 1[SbF6] at -20 °C (eq 5) and of ethylene
and CH2dCHOR (2a-c, 2g) to 1[B(C6F5)4] at -60 °C (eq 4) was
studied in a similar manner.
Generation of [{(tmeda)Pd(OPh)}n][B(C6F5)4]n (7). An NMR
tube was charged with (tmeda)Pd(OPh)2 (10.9 mg, 0.0267 mmol),
[HNMe2Ph][B(C6F5)4] (21.6 mg, 0.0270 mmol), and CD2Cl2 (0.4
mL) was added by vacuum transfer at -196 °C. The tube was
warmed to 20 °C and shaken vigorously. NMR spectra showed
that [{(tmeda)Pd(OPh)}n][B(C6F5)4]n, NMe2Ph, and HOPh had
formed quantitatively in 10 min.69 The ESI-MS contains signals
for only the mononuclear monocation (tmeda)Pd(OPh)+ even at
(70) For the related tetrameric compound [(cod)4Pt4(µ2-OH)4](OTf)4, see:
Szuromi, E. Ph.D. Thesis, University of MissourisColumbia, De-
cember 2005.
(71) The major peak is (tmeda)Pd(OPh)+ calcd. m/z ) 315.1, found
315.0.
(72) For the related compound [(tmeda)Pd(η3-C3H5)]Br, see: Graff, W.;
Boersma, J.; Koten, G. Organometallics 1990, 9, 1479.
(73) Frisch, M. J.; et al. Gaussian 03, revision C.02; Gaussian, Inc.:
Wallingford, CT, 2004. See the Supporting Information for the
complete citation.
(74) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang,
W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (c) Miehlich, B.; Savin,
A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989, 157, 200.
(75) (a) Rassolov, V. A.; Ratner, M. A.; Pople, J. A.; Redfern, P. C.; Curtiss,
L. A. J. Comput. Chem. 2001, 22, 976. (b) Rassolov, V. A.; Pople,
J. A.; Ratner, M. A.; Windus, T. L. J. Chem. Phys. 1998, 109, 1223.
(c) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54,
724.
(68) (a) Rush, S.; Reinmuth, A.; Risse, W. Macromolecules 1997, 30, 7375.
(b) For [(R-diimine)Pd(η3-C3H5)][3,5-(CF3)2C6H3)] see: Li, W.; Zhang,
X.; Meetsma, A.; Hessen, B. J. Am. Chem. Soc. 2004, 126, 12246.
(69) NMR spectra showed that NMe2Ph and HOPh are both free and do
not coordinate to the [{(tmeda)Pd(OPh)}n][B(C6F5)4]n in CD2Cl2 at
20 °C.
(76) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (b) Wadt,
W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (c) Hay, P. J.; Wadt,
W. R. J. Chem. Phys. 1985, 82, 299.
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5284 J. AM. CHEM. SOC. VOL. 132, NO. 14, 2010