Vinyl C-H activation reactions of vinyl esters mediated by B(C6F5)3

Article abstract of DOI:10.1021/om010515e

The vinyl C-H activation reactions of vinyl esters mediated by B(C₆F₅)₃ were described. B(C₆F₅)₃ reacts with simple vinyl esters by electrophilic addition to the C=C bond and subsequent pro

Full text of DOI:10.1021/om010515e

© Copyright 2002
Volume 21, Number 5, March 4, 2002
American Chemical Society
Communications
Vin yl C-H Activa tion Rea ction s of Vin yl Ester s
Med ia ted by B(C6F 5)3
Aswini K. Dash and Richard F. J ordan*
Department of Chemistry, 5735 South Ellis Avenue, The University of Chicago,
Chicago, Illinois 60637
Received J une 14, 2001
-
Summary: B(C6F5)3 reacts with simple vinyl esters by
electrophilic addition to the CdC bond and subsequent
proton transfer and elimination of C6F5H to yield the
chelated vinylborane products (C6F5)2B{κ2-CHdCHOC-
(dO)R} (R ) Me, Ph).
degradation by X- transfer than are XBF3 anions.8
Here we describe an unusual reaction in which electro-
philic addition of B(C6F5)3 to the CdC bond of vinyl
esters and subsequent B-C6F5 bond cleavage results
in net vinylic C-H activation and the formation of
vinylborane products.
Tris(perfluorophenyl)borane, B(C6F5)3,1 has been used
extensively as an activator for metallocene and other
single-site olefin polymerization catalysts.2 In this ap-
plication, B(C6F5)3 abstracts a hydrocarbyl group from
an LnMR2 precursor to form an active [LnMR][RB-
(C6F5)3] ion pair. B(C6F5)3 has also been used as a Lewis
acid catalyst for the hydrosilation of carbonyl com-
pounds,3 silation and reduction of alcohols and cleavage
of ethers with silanes,4 addition of silyl enol ethers to
carbonyl compounds and other electrophiles,5 hydrostan-
nation of allenes,6 and a variety of other reactions.7 The
Lewis acidity of B(C6F5)3 is comparable to that of BF3,1b
The new chemistry is summarized in Scheme 1.9 At
23 °C in benzene-d6, B(C6F5)3 reacts immediately with
vinyl acetate to form the carbonyl adduct CH2dCHOC-
{dOB(C6F5)3}Me (1a ). Complex 1a was characterized
by multinuclear NMR but was not isolated. Key NMR
parameters for 1a include a low-field 13C carbonyl
resonance at δ 179.9 (vs 167.0 for free vinyl acetate),
19F NMR resonances at δ -133.1, -151.6, -161.9, and
an 11B NMR resonance at δ 15.6 characteristic of a four-
coordinate B(C6F5)3L species.10 These data are very
similar to the data for the ethyl benzoate adduct EtOC-
{dOB(C6F5)3}Ph (δC, 173.5; δB, 19.2) reported by Piers
-
and XB(C6F5)3 anions are generally more resistant to
(7) (a) Ishihara, K.; Hanaki, N.; Yamamoto, H. Synlett 1995, 721.
(b) Ishihara, K.; Hanaki, N.; Yamamoto, H. Synlett 1993, 577. (c)
Ishihara, K.; Funahashi, M.; Hanaki, N.; Miyata, M.; Yamamoto, H.
Synlett 1994, 963.
(1) (a) Massey, A. G.; Park, A. J . J . Organomet. Chem. 1964, 2, 245.
(b) Massey, A. G.; Park, A. J . J . Organomet. Chem. 1966, 5, 218. (c)
Piers, W. E.; Trivers, T. Chem. Soc. Rev. 1997, 26, 345.
(2) (a) Chen, E. Y.-X.; Marks, T. J . Chem. Rev. 2000, 100, 1391 and
references therein. (b) Yang, X.; Stern, C. L.; Marks, T. J . J . Am. Chem.
Soc. 1991, 113, 3623. (c) Ewen, J . A.; Edler, M. J . U.S. Pat. Appl. 419,-
017, 1989; Chem. Abstr. 1991, 115, 136998g.
(3) Parks, D. J .; Piers, W. E. J . Am. Chem. Soc. 1996, 118, 9440.
(4) (a) Blackwell, J . M.; Foster, K. L.; Beck, V. H.; Piers, W. E. J .
Org. Chem. 1999, 64, 4887. (b) Gevorgyan, V.; Rubin, M.; Benson, S.;
Liu, J .-X.; Yamamoto, Y. J . Org. Chem. 2000, 65, 6179.
(5) Ishihara, K.; Hanaki, N.; Funahashi, M.; Miyata, M.; Yamamoto,
H. Bull. Chem. Soc. J pn. 1995, 68, 1721.
(8) However, B(C6F5)3 and B(C6F5)3X- anions undergo B-C6F5 bond
cleavage and other reactions under some conditions. For representative
examples see: (a) Chernega, A. N.; Graham, A. J .; Green, M. L. H.;
Haggit, J .; Lloyd, J .; Mehnert, C. P.; Metzler, N.; Souter, J . J . Chem.
Soc., Dalton Trans. 1997, 2293. (b) Pindado, G. N.; Lancaster, S. J .;
Thornton-Pett, M.; Bochmann, M. J . Am. Chem. Soc. 1998, 120, 6816.
(c) Barlow, G. K.; Boyle, J . D.; Cooley, N. A.; Ghaffar, T.; Wass, D. F.
Organometallics 2000, 19, 1470. (d) Dagorne, S.; Guzei, I. A.; Coles,
M. P.; J ordan, R. F. J . Am. Chem. Soc. 2000, 122, 274. (e) Vagedes,
D.; Fro¨hlich, R.; Erker, G. Angew. Chem., Int. Ed. 1999, 38, 3362.
(9) Characterization data for new compounds are given in the
Supporting Information.
(6) Gevorgyan, V.; Liu, J .-X.; Yamamoto, Y. J . Org. Chem. 1997,
62, 2693.
10.1021/om010515e CCC: $22.00 © 2002 American Chemical Society
Publication on Web 02/07/2002

778 Organometallics, Vol. 21, No. 5, 2002
Communications
Sch em e 1
fer to the carbonyl oxygen to generate the carbonyl-
protonated species 5a , and protonolysis of a B-C6F5
bond.
The reaction of vinyl acetate with B(C6F5)3 to produce
2 and 3a is much faster in CD2Cl2 than in benzene-d6,
and in this case intermediate 5a can be detected by
NMR. Monitoring the reaction in CD2Cl2 by NMR at 23
°C revealed the initial formation of carbonyl adduct 1a ,
subsequent conversion to 5a , and ultimate formation
of 2 and 3a . The 1a /5a /3a ratio was 1.0/0.72/0.27 after
5 h, and the conversion to 2 and 3a (1/1 ratio) was
complete after 30 h. Key NMR parameters for 5a
1
include (i) a H resonance at δ 12.87 and a 13C carbonyl
resonance at δ 199.1, which are correlated in the 2D-
HMBC spectrum and are assigned to the protonated
carbonyl group,12 (ii) two coupled doublets (J ) 4.4 Hz)
1
in the H spectrum, which are correlated in the COSY
spectrum and are assigned to the cis-vinyl CH groups,
and (iii) 19F resonances at δ -134.6, -156.7, and -163.8
and a 11B resonance at δ 2.15, for the (vinyl)B(C6F5)3
-
unit. The close proximity of the protonated carbonyl
1
group and the methyl group was established by a H-
1H NOESY spectrum, which exhibited a strong cross-
peak between the O-H (δ 12.87) and the Me (δ 2.47)
resonances. The acceleration of the reaction in CD2Cl2
versus benzene-d6 is ascribed to stabilization of the
zwitterionic intermediates 4a and 5a by the more polar
solvent.
Similarly, vinyl benzoate reacts with B(C6F5)3 in CD2-
Cl2 at room temperature to generate 5b in 50% yield
after 15 min along with 50% of unreacted starting
materials (Scheme 1). After 12 h, 93% conversion to a
1/1 mixture of 2 and 3b was observed.13 The NMR data
for 3b and 5b are similar to the data for 3a and 5a .9 To
corroborate the structure of intermediates 5a ,b and in
particular to confirm the presence of a protonated
carbonyl group in these species, the 1/1/1 mixture of 5b,
vinyl benzoate, and B(C6F5)3 generated at 50% conver-
sion was treated with Proton Sponge (1,8-bis(dimethyl-
amino)naphthalene, PS). An immediate reaction oc-
curred to produce [PS-H][{cis-(C6F5)3BCHdCHOC{d
OB(C6F5)3}Ph}] ([PS-H][6b]) quantitatively (along with
unreacted vinyl benzoate). The unreacted B(C6F5)3
present in the solution reacts with deprotonated 5b to
form 6b-. Key NMR parameters for 6b- include (i) a
low-field 13C carbonyl resonance at δ 196.4, (ii) two sets
of 19F signals for the two four-coordinate -B(C6F5)3
groups, and (iii) a broad 11B signal centered at δ -1.5
et al.10a Adduct 1a is stable at room temperature in
benzene-d6 for at least 48 h. However, heating a
benzene-d6 solution of 1a at 60 °C for 12 h yields a 1/1
mixture of C6F5H (2) and the chelated vinylborane
(C6F5)2B{κ2-CHdCHOC(dO)Me} (3a ) with a conversion
of 93%. Compounds 2 and 3a were characterized by
multinuclear NMR and GC-MS and, for 3a , elemental
analysis. Key NMR parameters for 3a include (i) a 13C
carbonyl resonance at δ 196.0, characteristic of carbonyl
oxygen coordination to a Lewis acid, (ii) two doublets
for the cis-vinyl CH groups (J ) 4.0 Hz) in the 1H
spectrum, and (iii) 19F resonances at δ -136.2, -155.5
and -163.4 and a 11B resonance at δ 5.2, consistent with
a four-coordinate RB(C6F5)2L species. Very similar NMR
data (19F, δ -134.5, -158.1, -163.8; 11B, δ 6.8) were
reported for (C6F5)2B{κ2-CH2(CH2)3C(dO)OEt}, in which
the ester carbonyl group is coordinated to boron.11
A plausible mechanism for this reaction is shown in
Scheme 1. The key steps leading to 2 and 3a are
electrophilic attack of B(C6F5)3 at the CdC bond to
generate the zwitterionic intermediate 4a , proton trans-
1
for the two B centers. The H NMR spectrum of [PS-
H][6b] contains a signal at δ 19.49 for the PS-H+
bridging proton which is correlated with the NMe2
resonance at δ 3.14 in the COSY spectrum.14
Vinyl esters are not generally susceptible to electro-
philic attack at the vinyl group, due to the weak ability
(12) The HMBC (heteronuclear multiple-bond correlation) experi-
ment enables determination of two- and three-bond 1H-13C connectiv-
ity: Bax, A.; Summers, M. J . J . Am. Chem. Soc. 1986, 108, 2093.
(13) Compounds 2 and 3b (1/1 ratio) are generated quantitatively
by heating a CD2Cl2 solution of B(C6F5)3 and vinyl benzoate to 80 °C
for 3 h (sealed tube). In benzene-d6 solution, 86% conversion to 2 and
3b is observed after 5 days at 80 °C.
(10) (a) Parks, D. J .; Piers, W. E.; Parvez, M.; Atencio, R.; Zaworotko,
M. J . Organometallics 1998, 17, 1369. (b) J acobsen, H.; Berke, H.;
Do¨ring, S.; Kehr, G.; Erker, G.; Fro¨hlich, R.; Meyer, O. Organometallics
1999, 18, 1724. (c) Galsworthy, J . R.; Green, J . C.; Green, M. L. H.;
Mu¨ller, M. J . Chem. Soc., Dalton Trans. 1998, 15.
(11) Parks, D. J .; Piers, W. E.; Yap, G. P. A. Organometallics 1998,
17, 5492.
(14) This assignment is consistent with literature data: (a) Pietrzak,
M.; Wehling, J .; Limbach, H.-H.; Golubev, N. S.; Lo´pez, C.; Claramunt,
R.; Elguero, J . J . Am. Chem. Soc. 2001, 123, 4338. (b) Grech, E.;
Stefaniak, L.; Ando, I.; Yoshimizu, H.; Webb, G. H.; Sobczyk, L. Bull.
Chem. Soc. J pn. 1990, 63, 2716. (c) Brzezinski, B.; Schroeder, G.;
J arczewski, A.; Grech, E.; Nowicka-Scheibe, J .; Stefaniak, L.; Klimk-
iewicz, J . J . Mol. Struct. 1996, 377, 149.

Communications
Organometallics, Vol. 21, No. 5, 2002 779
of the -OC(dO)R group to stabilize the carbocation
intermediate. Thus, while vinyl ethers undergo facile
cationic polymerization, vinyl esters do not.15 Neverthe-
less, electrophilic attack at the CdC bond of vinyl esters
has been established in several cases. For example,
Noyce and Pollack showed by kinetic, substituent effect,
and solvent isotope effect studies that acid hydrolysis
of vinyl esters proceeds by two competing mechanisms:
(i) initial protonation at the carbonyl oxygen followed
by H2O attack and collapse to products (AAC2 mecha-
nism), analogous to the mechanism for saturated esters,
or (ii) initial protonation at the CdC bond followed by
H2O attack and collapse to products (ASE2 mechanism),
analogous to the normal mechanism for vinyl ethers.16
The latter process is important under highly acidic
conditions and when the carbocation resulting from
protonation at carbon is strongly stabilized by substit-
uents (e.g. R-acetoxy styrenes). Landgrebe showed by
NMR H/D exchange studies that isopropenyl acetate
undergoes fast reversible protonation at the CdC bond
in concentrated H2SO4/D2SO4 solution.17-19 In the present
case, the kinetic product of the reaction of B(C6F5)3 with
vinyl acetate is the carbonyl adduct, but formation of
the vinylborane product derived from CdC attack is
driven by the irreversible protonolysis of the B-C6F5
bond.
The formation of 5a ,b from B(C6F5)3 and the ap-
propriate vinyl ester is a net electrophilic substitution
of a vinyl hydrogen by a B(C6F5)3 group. This reaction
bears some similarity to the reaction of metal cyclopen-
tadienyl complexes with electrophilic boranes to yield
M{C5H4B-X3} products.20 For example, the reaction of
the zirconacyclopentadiene complex Cp2Zr(C4Me4) with
B(C6F5)3 yields Cp{η5-C5H4B-(C6F5)3}Zr+(σ-CMedCMe-
CMedCHMe), presumably via electrophilic attack of
B(C6F5)3 at a Cp ligand to generate Cp{(1-exo-B-(C6F5)3-
cyclopentadiene)Zr+(C4Me4) followed by protonolysis of
a Zr-C σ bond by the endo C-H group.20b
Su p p or tin g In for m a tion Ava ila ble: Text giving syn-
thetic procedures and characterization data for new com-
pounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
OM010515E
(19) The -OC(dO)R group is a mildly activating ortho, para director
in electrophilic aromatic substitution: Smith, M. B.; March, J .
Advanced Organic Chemistry, 5th ed.; Wiley: New York, 2001; p 684.
(20) (a) Braunschweig, H.; Wagner, T. Chem. Ber. 1994, 127, 1613.
(b) Ruwwe, J ,; Erker, G.; Fro¨hlich, R. Angew Chem., Int. Ed. Engl.
1996, 35, 80. (c) Burlakov, V. V.; Pellny, P.; Arndt, P.; Baumann, W.;
Spannenberg, A.; Shur, V. B.; Rosenthal, U. Chem. Commun. 2000,
241. (d) Doerrer, L. H.; Graham, A. J .; Haussinger, D.; Green. M. L.
H. J . Chem. Soc., Dalton Trans. 2000, 813. (e) Burlakov, V. V.;
Troyanov, S. I.; Strunkina, L. I.; Minacheva, M. Kh.; Letov, A. V.;
Furin, G. G.; Rosenthal, U.; Shur, V. B. J . Organomet. Chem. 2000,
598, 243. (f) Piers, W. E. Chem. Eur. J . 1998, 4, 13.
(15) Odian, G. Principles of Polymerization, 3rd ed.; Wiley: New
York, 1991; p 200.
(16) (a) Noyce, D. S.; Pollack, R. M. J . Am. Chem. Soc. 1969, 91,
7158. (b) Noyce, D. S.; Pollack, R. M. J . Am. Chem. Soc. 1969, 91, 119.
(c) Euranto, E. Pure Appl. Chem. 1977, 49, 1009 and references therein.
(17) Landgrebe, J . A. J . Org. Chem. 1965, 30, 2105.
(18) See also: (a) Gebelein, C. G.; Swern, D. J . Org. Chem. 1968,
33, 2758. (b) Abley, P.; Byrd, J . E.; Halpern, J . J . Am. Chem. Soc. 1972,
94, 1985. (c) Morgan, P. E.; McCague, R.; Whiting, A. Tetrahedron Lett.
1999, 40, 4857.