Palladium-catalyzed dimerization of vinyl ethers to acetals

Article abstract of DOI:10.1021/ja104523y

(α-Diimine)PdCl⁺ species catalytically dimerize alkyl and silyl vinyl ethers to β,γ-unsaturated CH₂=CHCH ₂CH(OR)₂ acetals, and they cyclize divinyl ethers to analogous cyclic acetals. A plausible mechanism comprises in situ generation of an active PdOR alkoxide species, double vinyl ether insertion to generate Pd{CH₂CH(OR)CH₂CH(OR)₂} species, and β-OR elimination to generate the acetal product. In the presence of vinyl ethers, (α-diimine)PdCl⁺ species can be used to initiate ethylene polymerization.

Full text of DOI:10.1021/ja104523y

Published on Web 07/13/2010
Palladium-Catalyzed Dimerization of Vinyl Ethers to Acetals
Changle Chen and Richard F. Jordan*
Department of Chemistry, The UniVersity of Chicago, 5735 South Ellis AVenue, Chicago, Illinois 60637
Received May 25, 2010; E-mail: rfjordan@uchicago.edu
polymerization is observed in dry solvent, even in the absence of
Abstract: (R-Diimine)PdCl+ species catalytically dimerize alkyl and
DTBP. Phenyl vinyl ether does not react with 1a or 1b, even at elevated
silyl vinyl ethers to ꢀ,γ-unsaturated CH2dCHCH2CH(OR)2 acetals,
temperature. {(2,6-iPr2-C6H3)NdCMeCMedN(2,6-iPr2-C6H3)}PdCl2
and they cyclize divinyl ethers to analogous cyclic acetals. A
plausible mechanism comprises in situ generation of an active PdOR
alkoxide species, double vinyl ether insertion to generate Pd{CH2-
CH(OR)CH2CH(OR)2} species, and ꢀ-OR elimination to generate
the acetal product. In the presence of vinyl ethers, (R-diimine)PdCl+
species can be used to initiate ethylene polymerization.
and (CH3CN)2PdCl2 also catalytically dimerize 2a when activated by
[Li(OEt2)2.8][B(C6F5)4].
(R-Diimine)PdR+ alkyl complexes1 react with vinyl ethers by two
pathways.2,3 First, (R-diimine)PdR+ can initiate the cationic polym-
erization of vinyl ethers with concomitant formation of Pd0. This
reaction proceeds via (R-diimine)PdR(CH2dCHOR′)+ π-complexes
Table 1. Dimerization of Vinyl Ethers by
[{(R-Diimine)Pd(µ-Cl)}2][B(3,5-(CF3)2-C6H3)4]2 (1a)a
in which the CdC bond is polarized with carbocation character at the
substituted carbon (Cint). Electrophilic attack of Cint on the monomer
c
CH2dCHOR
mol % cat time (h) yield (%) TONb TOF (h-1
)
initiates polymerization. Cationic polymerization is the dominant
process for alkyl vinyl ethers. Alternatively, (R-diimine)PdR-
(CH2dCHOR′)+ adducts can undergo insertion to form O-chelated
(R-diimine)Pd(CH2CHROR′)+ species, which can react further by
ꢀ-OR′ elimination to generate (R-diimine)Pd(OR′)(CH2dCHR)+ (not
observed) and subsequent allylic C-H activation to yield (R-
diimine)Pd(allyl)+ species and R′OH. For aryl-silyl vinyl ethers,
insertion out-competes cationic polymerization, enabling copolymer-
ization of these substrates with olefins and multiple vinyl ether insertion
reactions.4 For aryl vinyl ethers, both insertion and ꢀ-OAr elimination
are fast, and the latter reaction precludes copolymerization or multiple
insertion processes. Here we report that (R-diimine)PdCl+ chloride
complexes catalyze a third reaction, the dimerization of vinyl ethers
to ꢀ,γ-unsaturated acetals.
CH2dCHOEt (2a)
0.5
0.5
0.5
0.5
2
2.5
5
3
0.25
1
48
0.25
0.25
0.25
10
4
10
60
300
70
84
98
69
35
10
50
59
10
47
85
140
168
196
140
17
4
10
20
2
560
168
4
560
70
16
1
5
0.2
0.15
0.05
CH2dCHOnBu (2b)
CH2dCHOtBu (2c)d
CH2dCHOSiMe3 (2d)
CH2dCHOSiMe2Ph (2e)
CH2dCHOSiMePh2 (2f)
CH2dCHOSiPh3 (2g)
5
5
5
9
15
a 23 °C, CH2Cl2, with 3 equiv of DTBP (vs Pd), [Pd] ) 4 mM.
b Turnover number. c Average turnover frequency for the reaction
period. d Ca. 35% of 2c was dimerized, and 65% of 2c was cationically
polymerized.
The reaction of (R-diimine)PdCl2 with Na[B(3,5-(CF3)2-C6H3)4] in
CH2Cl2 yields the dinuclear dicationic species [{(R-diimine)Pd(µ-
Cl)}2][B(3,5-(CF3)2-C6H3)4]2 (1a, R-diimine ) 1,2-(2,6-iPr2-C6H3)Nd
CAnCAndN(2,6-iPr2-C6H3); An ) acenaphthyl). 1a catalytically
dimerizes alkyl and silyl vinyl ethers CH2dCHOR (2a-g: R ) Et
(a), nBu (b), tBu (c), SiMe3 (d), SiMe2Ph (e), SiMePh2 (f), SiPh3 (g))
to CH2dCHCH2CH(OR)2 acetals (3a-g, eq 1, Table 1). No Pd0
formation is observed in these reactions, except for the case of 2c.
After complete consumption of the vinyl ether (2a,g), catalysis resumes
if more substrate is added. A small amount (<5%) of cationic
polymerization of 2a,b,d-g is observed, and this competing process
becomes more significant as the concentration of H2O in the solvent
is increased. However, for 2a,b,d-g, the addition of 2,6-di-tert-
butylpyridine (DTBP, 3 equiv vs Pd) prevents cationic polymerization
but does not affect the dimerization reaction. This result suggests that
the cationic polymerization is proton-initiated. In contrast, for 2c,
cationic polymerization and Pd0 formation are observed even with 20
equiv of DTBP. This result is indicative of initiation by a cationic Pd
species and reflects the high susceptibility of 2c to cationic polymer-
ization.5
1a also catalytically cyclizes divinyl ethers 2h-k to cyclic acetals
3h-k (eq 2). No intermolecular dimerization or oligomerization
products were observed in these reactions.6 The TOF for the generation
of 3h-k is ca. 1 h-1 (room temperature, CH2Cl2, 5 h). NMR yields
are shown in eq 2.
The catalytically active Pd species responsible for vinyl ether
dimerization has not yet been identified.7 However, a plausible
mechanism involving in situ generation of an active Pd-OR species
(A) and double vinyl ether insertion, ꢀ-OR elimination, and ligand
substitution steps is shown in Scheme 1.
The dinuclear monocationic species [{(R-diimine)Pd-Cl}2(µ-
Cl)][B(C6F5)4] (1b), which is generated in situ by the reaction of (R-
diimine)PdCl2 with [Li(OEt2)2.8][B(C6F5)4] in CD2Cl2, behaves similarly
to 1a in the dimerization of 2f,g. However, in this case, no cationic
The insertion of olefins into M-OR bonds has been observed in
Pt, Rh, and Pd systems and proposed to occur in several catalytic
processes.8–10 In particular, the insertion of vinyl ethers into Pd-OR
bonds was proposed to be a key step in transfer vinylation,11 vinyl
9
10254 J. AM. CHEM. SOC. 2010, 132, 10254–10255
10.1021/ja104523y  2010 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1a
The cyclization of divinyl ethers likely proceeds by an analogous
mechanism (Scheme 2).
Scheme 2a
a Pd ) (R-diimine)Pd.
interchange,12 and isomerization reactions.13 Such insertions can have
lower barriers than analogous insertions into Pd-R bonds due to the
destabilization of the Pd-OR complex by repulsion between the filled
metal d and oxygen p orbitals and the fact that the Pd-OR bond need
not be fully cleaved during the insertion since O-chelated products
are formed.10b DFT calculations show that the insertion barrier of the
model complex (HNdCHCHdNH)Pd(OMe)(CH2dCH2)+ (9.7 kcal/
mol) is much lower than that for (HNdCHCHdNH)Pd(Me)-
(CH2dCH2)+ (17 kcal/mol). The failure of phenyl vinyl ether to be
dimerized by 1a,b may be due to fast ꢀ-OPh elimination of B to re-
form A.2,3 For comparison, (tmeda)Pd(OPh)+ does not react with 2a
or 3a under these conditions. The proposed insertion of C is analogous
to the multiple insertion of 2g into (R-diimine)PdMe+, which, following
allylic C-H activation, yields Pd allyl products.4 No Pd allyl species
have been detected in 1a,b-catalyzed vinyl ether dimerizations. Allylic
C-H activation of E may be inhibited by the geminal bis-alkoxide
unit, so that ligand exchange/product release dominates.
a Pd ) (R-diimine)Pd.
This work shows that (R-diimine)PdCl+ species catalytically dimer-
ize vinyl ethers to CH2dCHCH2CH(OR)2 acetals and cyclize divinyl
ethers to analogous cyclic acetals. In situ-generated (R-diimine)-
Pd(OR)+ alkoxide complexes may be the active species in these
reactions. An interesting application of this reaction is olefin polym-
erization by (R-diimine)PdCl+/vinyl ether mixtures.
Acknowledgment. This work was supported by the National
Science Foundation (Grant CHE-0911180).
The addition of olefins to 1a-catalyzed vinyl ether dimerization
reactions was studied to probe the proposed intermediates in Scheme
1. The addition of 1-hexene (60 equiv) to the 1a-catalyzed dimerization
of 2f (40 equiv) resulted in termination of catalysis after 4 h, with
50% conversion of 2f to 3f and complete conversion of 1a to a mixture
of (R-diimine)Pd(η3-C6H11)+ allyl species. This result can be explained
by reaction of A or E with hexene to form (R-diimine)Pd(OR)(hex-
ene)+ and allylic C-H activation.14 The reaction of 1a with 2f (12
equiv) under 150 psi of ethylene pressure (CH2Cl2, 1 h, 23 °C) yields
branched polyethylene (113 br/103 C, Mn ) 5100, Mw/Mn ) 1.97).15
This result can be explained by trapping of A-E by ethylene and
further ethylene insertion.1 No polymer was generated in the absence
of 2f.
A similar dimerization of vinyl ethers to acetals by Hg(OAc)2/
BF3 ·Et2O has been reported.16 This system produces a significant
amount of trimer, tetramer, and polymer side products and also
dimerizes substituted vinyl ethers such as EtCHdCHOEt and
CH2dCMeOEt. These results are most consistent with a cationic
oligomerization mechanism. In contrast, 1a/DTBP does not produce
detectable oligomer or polymer byproducts in eq 1,6 and it does not
react with MeCHdCHOEt or CH2dCHOPh, even at elevated tem-
peratures. Furthermore, as noted above, DTBP does not influence the
Pd-catalyzed dimerization reaction. These results argue that the 1a,b-
catalyzed dimerization process is mechanistically distinct from the Hg
system and does not proceed by a cationic mechanism.
As shown in Table 1, turnover frequencies in eq 1 decay with time.
Product inhibition due to binding of the product to the active Pd species
may contribute to this decay. For example, the reaction of 1a with 13
equiv of 2g produces 1.2 and 5.2 equiv of 3g after 4 and 26 h,
respectively. In contrast, in the presence of 20 equiv of 3g under the
same conditions, only 0.1 and 1.9 equiv of 2g are consumed after 4
and 26 h, respectively. To model the competitive binding of 2g and
3g to the active Pd species, the reaction of 2g/3g with (R-diimine)-
PdMe+ was studied. The equilibrium constant (Keq) for eq 3 was
determined to be 0.12(1) at -20 °C. Assuming ∆S ) 0, Keq ) 0.16
at 23 °C.
Supporting Information Available: Experimental procedures and
characterization of compounds (PDF, CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.
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