Copper-catalyzed carboxylation of alkylboranes with carbon dioxide: Formal reductive carboxylation of terminal alkenes

Article abstract of DOI:10.1021/ol103128x

Carboxylation of alkylboron compounds (alkyl-9-BBN) with CO₂ proceeded in the presence of catalytic amounts of CuOAc/1,10-phenanthroline and a stoichiometric amount of KO^tBu. The alkylboranes are easily and widely available through the alkene hydroboration, and thus the overall process represents a reductive carboxylation of alkenes with CO₂. The broad functional group compatibility and the inexpensiveness of the Cu/1,10-phenathoroline catalyst system are attractive features of this protocol.(Figure Presented)

Full text of DOI:10.1021/ol103128x

ORGANIC
LETTERS
2011
Vol. 13, No. 5
1086–1088
Copper-Catalyzed Carboxylation
of Alkylboranes with Carbon Dioxide:
Formal Reductive Carboxylation
of Terminal Alkenes
Hirohisa Ohmiya,* Masahito Tanabe, and Masaya Sawamura*
Department of Chemistry, Faculty of Science, Hokkaido University,
Sapporo 060-0810, Japan
ohmiya@sci.hokudai.ac.jp; sawamura@sci.hokudai.ac.jp
Received December 24, 2010
ABSTRACT
Carboxylation of alkylboron compounds (alkyl-9-BBN) with CO2 proceeded in the presence of catalytic amounts of CuOAc/1,10-phenanthroline
and a stoichiometric amount of KOtBu. The alkylboranes are easily and widely available through the alkene hydroboration, and thus the overall
process represents a reductive carboxylation of alkenes with CO2. The broad functional group compatibility and the inexpensiveness of the Cu/
1,10-phenathoroline catalyst system are attractive features of this protocol.
Incorporation of carbon dioxide (CO2), which is an
abundant and renewableC1source, intoorganic molecules
has emerged as an area of attractive research in recent
organic chemistry.1 In this regard, various carbon-carbon
bond formation reactions using CO2 have been achieved
under the influence of transition metal catalysts.2-9 Among
them, the transition-metal-catalyzed carboxylations of less
basic organometallic reagents such as organostannane,
zinc, or boron compounds with CO2 offer versatile methods
for the synthesis of functionalized carboxylic acid deriva-
tives.2-4 Specifically, allylstannane reagents were used for
(1) For reviews, see: (a) Behr, A. Angew. Chem., Int. Ed. 1988, 27,
661–678. (b) Gibson, D. H. Chem. Rev. 1996, 96, 2063–2095.
(c) Arakawa, H.; Aresta, M.; Armor, J. N.; Barteau, M. A.; Beckman,
E. J.; Bell, A. T.; Bercaw, J. E.; Creutz, C.; Dinjus, E.; Dixon, D. A.;
Domen, K.; DuBois, D. L.; Eckert, J.; Fujita, E.; Gibson, D. H.;
Goddard, W. A.; Goodman, D. W.; Keller, J.; Kubas, G. J.; Kung,
H. H.; Lyons, J. E.; Manzer, L. E.; Marks, T. J.; Morokuma, K.;
Nichoras, K. M.; Periana, R.; Que, L.; Rostrup-Nielson, J.; Sachtler,
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B. R.; Tumas, W. Chem. Rev. 2001, 101, 953–996. (d) Sakakura, T.;
Choi, J.-C.; Yasuda, H. Chem. Rev. 2007, 107, 2365–2387. (e) Correa, A.;
Martın, R. Angew. Chem., Int. Ed. 2009, 48, 6201–6204.
(2) For Pd-catalyzed carboxylations of allylstannane compounds
with carbon dioxide, see: (a) Johansson, R.; Wendt, O. F. Dalton Trans.
2007, 488–492. (b) Shi, M.; Nicholas, K. M. J. Am. Chem. Soc. 1997, 119,
5057–5058.
(3) For Ni-catalyzed carboxylations of alkylzinc compounds with
CO2, see: (a) Ochiai, H.; Jang, M.; Hirano, K.; Yorimitsu, H.; Oshima,
K. Org. Lett. 2008, 10, 2681–2683. (b) Yeung, C. S.; Dong, V. M. J. Am.
Chem. Soc. 2008, 130, 7826–7827.
(4) For transition-metal (Rh or Cu)-catalyzed carboxylations of aryl-
and alkenylboron compounds with carbon dioxide, see: (a) Ukai, K.;
Aoki, M.; Takaya, J.; Iwasawa, N. J. Am. Chem. Soc. 2006, 128, 8706–
8707. (b) Takaya, J.; Tadami, S.; Ukai, K.; Iwasawa, N. Org. Lett. 2008,
10, 2697–2700. (c) Ohishi, T.; Nishiura, M.; Hou, Z. Angew. Chem., Int.
Ed. 2008, 47, 5792–5795.
(5) For transition-metal-free carboxylation of organozinc com-
pounds, see: Kobayashi, K.; Kondo, Y. Org. Lett. 2009, 11, 2035–2037.
(6) For Cu- or Au-catalyzed carboxylations of (hetero)arenes, see:
(a) Boogaerts, I. I. F.; Nolan, S. P. J. Am. Chem. Soc. 2010, 132, 8858–
8859. (b) Boogaerts, I. I. F.; Fortman, G. C.; Furst, M. R. L.; Cazin,
C. S. J.; Nolan, S. P. Angew. Chem., Int. Ed. 2010, 49, 8674–8677.
(c) Zhang, L.; Cheng, J.; Ohishi, T.; Hou, Z. Angew. Chem., Int. Ed.
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2010, Advance Articles, DOI: 10.1039/c0cc03890c.
r
10.1021/ol103128x
Published on Web 01/28/2011
2011 American Chemical Society

the palladium-catalyzed carboxylations.2 More recently,
nickel- and palladium-catalyzed carboxylations of aryl-
(Ni, Pd) or alkylzinc (Ni) reagents have been developed.3
The organoboron compounds are particularly attractive
reagents due to their broad availability and functional
group compatibility. Iwasawa and co-workers developed
a rhodium-catalyzed carboxylation of aryl- and alkenyl-
boronates.4a Copper-catalyzed reactions were also reported
by Iwasawa and co-workers and Hou and co-workers.4b,c
However, alkylboron compounds have not been used for
the transition-metal-catalyzed carboxylation.3,5,10
Scheme 1
Earlier, we showed that alkylcopper species can catalyti-
cally be formed from alkylboron compounds (alkyl-9-BBN)
through B/Cu transmetalation in the copper-catalyzed allylic
substitution reaction with allylic phosphates and conjugate
addition to imidazolyl R,β-unsaturated ketones.11a,12 Here
we report an extension of our work to copper-catalyzed
carboxylation of alkylboron compounds with CO2 to pro-
duce alkanoic acids.11,12 The wide and easy availability of
alkylboranes via the established alkene hydroboration is an
attractive feature of this transformation, and thus the overall
molecular transformation represents a formal reductive
carboxylation of terminal alkenes with CO2.8e,9 A variety
of functional groups are tolerated in the alkenes.
Specifically, a solution of alkylborane in toluene was
prepared via hydroboration of alkene (1a) (0.5 mmol) with
a 9-borabicyclo[3.3.1]nonane (9-BBN-H) dimer (1a/B 1:1)
at 60 °C (Scheme 1). Subsequently, the solution was added
to a vial containing CuOAc (10 mol %), 1,10-phenanthro-
line (1,10-phen) (10 mol %), and KOtBu (1 equiv to 1, 0.5
mmol) under an atomospheric pressure of CO2 (balloon),
and the resulting reddish brown opaque solution was
heated at 100 °C for 12 h. As the reaction proceeded, a
small amount of black solid was precipitated. Hydrolytic
workup afforded carboxylic acid 3a in 71% isolated yield
(based on1a). A smaller amountof deborylated product4a
(21%) was also obtained as a side product.
Ligand screening showed that 1,10-phenanthroline was
most effective. Other diamines such as 2,20-bipyridyl (7%)
and TMEDA (5%) and phosphine ligands such as PPh3
(0%) and DPPE (19%) were much less efficient. The reac-
tion occurred without a ligand but with a decreased product
yield (31%). The reaction with CuCl(IPr) (10 mol %)/KO
tBu (1.1 equiv)/toluene at 100 °Cresultedina35%yield[IPr:
1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene].10
Several observations concerning the optimum reaction
conditions are to be noted. Controlling the reaction tem-
perature was crucial for the reproducibility in the product
yield; the reaction temperature over 100 °C (oil bath temp)
resulted in the formation of a significant amount of the
deborylated product 4a. Cheaper CuCl was as effective as
CuOAc, giving 3a in 61% yield. The use of Cu(OAc)2
resulted in a lower reaction efficacy (56%). No carboxyla-
tion occurred in the absence of KOtBu.
(7) For Cu-catalyzed reactions with carbon dioxide, see: (a) Fukue,
Y.; Oi, S.; Inoue, Y. J. Chem. Soc., Chem. Commun. 1994, 2091. (b) Oi,
S.; Fukue, Y.; Nemoto, K.; Inoue, Y. Macromolecules 1996, 29, 2694–
€
2695. (c) Laitar, D. S.; Muller, P.; Sadighi, J. P. J. Am. Chem. Soc. 2005,
127, 17196–17197. (d) Zhang, W.-Z.; Li, W.-J.; Zhang, X.; Zhou, H.; Lu,
X.-B. Org. Lett. 2010, 12, 4748–4751. (e) Fujihara, T.; Xu, T.; Semba, K.;
Terao, J.; Tsuji, Y. Angew. Chem., Int. Ed. 2011, 50, Early view, DOI:
10.1002/anie.201006292. See also refs 4b, c.
The formal reductive carboxylation protocol can trans-
form various terminal alkenes into the corresponding alka-
noic acids. Functional groups such as methoxy, siloxy, ester,
acetal, phthalimide, bromo, and benzyloxy groups were
tolerated (Table 1, entries 1-7 and 10). The terminal alkene
1d bearing a tertiary alkyl substituent served as a substrate
to afford the corresponding carboxylic acid in a reasonable
yield. The β-branched alkylboranes 2j and 2k prepared from
1,1-diphenylethylene (1i) and 1k also underwent carboxyla-
tion, giving the corresponding branched carboxylic acids
(entries 9 and 10). Notably, the carboxylation of styrene (1i)
afforded hydrocinnamic acid (3i): the regioselectivity is
complementary to that of the Ni-catalyzed carboxylation
of styrenes developed by Rovis and co-workers.8e The use of
secondary alkylborane reagents prepared from internal
alkenes resulted in no reaction (data not shown).
(8) For other examples on transition-metal-catalyzed carboxylations
with carbon dioxide, see: (a) Takimoto, M.; Kawamura, M.; Mori, M.;
Sato, Y. Synlett 2005, 2019–2022. (b) Takimoto, M.; Nakamura, Y.;
Kimura, K.; Mori, M. J. Am. Chem. Soc. 2004, 126, 5956–5957.
(c) Takimoto, M.; Mori, M. J. Am. Chem. Soc. 2002, 124, 10008–
10009. (d) Louie, J.; Gibby, J. E.; Farnworth, M. V.; Tekavec, T. N.
J. Am. Chem. Soc. 2002, 14, 15188–15189. (e) Williams, C. M.; Johnson,
J. B.; Rovis, T. J. Am. Chem. Soc. 2008, 130, 14936–14937. (f) Takaya, J.;
Iwasawa, N. J. Am. Chem. Soc. 2008, 130, 15254–15255. (g) Correa, A.;
Martın, R. J. Am. Chem. Soc. 2009, 131, 15974–15975.
(9) (a) Hoberg, H.; Gross, S.; Milchereit, A. Angew. Chem., Int. Ed.
€
1987, 26, 571–572. (b) Hoberg, H.; Peres, Y.; Kruger, C.; Tsay, Y.-H.
Angew. Chem., Int. Ed. 1987, 26, 771–773. (c) Hoberg, H.; Peres, Y.;
Milchereit, A. J. Organomet. Chem. 1986, 307, C38–C40.
(10) Hou and co-workers presented the carboxylation of alkyl-9-
BBN derivatives with carbon dioxide catalyzed by an N-heterocyclic
carbene-copper complex: Ohishi, T.; Nishiura, M.; Hou, Z. The 90th
Annual Meeting of Chemical Society of Japan, Osaka, March 26-30,
2010, 1F1-46.
(11) For Cu-catalyzed γ-selective and stereospecific allyl-alkyl and
allyl-aryl couplings with organoboron compounds, see: (a) Ohmiya,
H.; Yokobori, U.; Makida, Y.; Sawamura, M. J. Am. Chem. Soc. 2010,
132, 2895–2897. (b) Ohmiya, H.; Yokokawa, N.; Sawamura, M. Org.
Lett. 2010, 12, 2438–2440. (c) Whittaker, A. M.; Rucker, R. P.; Lalic, G.
Org. Lett. 2010, 12, 3216–3218.
Two reaction pathways for the carboxylation of alkyl-
boranes can be postulated as illustrated in Scheme 2 (paths
a and b). In path a, a trialkyl(alkoxo)borate B is initially
formed by the stoichiometric reaction beween an alkylborane
2 and KOtBu.13 Subsequently, the B/Cu transmetalation
(12) For Cu-catalyzed conjugate additions with alkylboron com-
pounds (alkyl-9-BBN) to imidazol-2-yl R,β-unsaturated ketones, see:
Ohmiya, H.; Yoshida, M.; Sawamura, M. Org. Lett. 2010, 12, ASAP,
DOI: 10.1021/ol102819k.
Org. Lett., Vol. 13, No. 5, 2011
1087

Table 1. Cu-Catalyzed Carboxylationa
Scheme 2. Possible Reaction Pathways
a The reaction was carried out with CO2 (1 atm, balloon), alkylbor-
ane 2 (0.5 mmol), CuOAc (10 mol %), 1,10-phen (10 mol %), and KOtBu
(1 equiv to 1, 0.5 mmol) in toluene at 100 °C for 12 h. Alkylborane 2 was
prepared in advance by hydroboration of 1 with 9-BBN dimer in toluene
at 60 °C for 1 h and used without purification. b Isolated yield. c After
hydrolytic workup and methylation with TMSCHN2, the product was
isolated as its methyl ester. d NMR yield.
the coexistence of the alkylborane 2 and KOtBu as in path b is
improbable. In fact, when a toluene-d8 solution of PhCH2-
CH2-9-BBN was treated with t-BuOK (1:1) at 100 °C for 10
min, a complete conversion of the borane PhCH2CH2-9-BBN
(δ87.5, rt) into a tetravalent borate (B) occurred as confirmed
by 11B NMR spectroscopy (δ -1.6, rt). Subsequent mixing of
the borate (1:1) with CuOAc/1,10-phen at 100 °C for 1 h
resulted in the formation of 9-BBN-OtBu (δ 54.7, rt).13
In summary, we have developed a copper-catalyzed
carboxylation of alkylboron compounds (alkyl-9-BBN)
with CO2 to afford alkanoic acids.10 The alkylboranes are
easily and widely available through the alkene hydrobora-
tion, and thus the overall process represents a reductive
carboxylation of alkenes with CO2.8e,9 The broad functional
group compatibility and the inexpensiveness of the Cu/1,10-
phen catalyst system are attractive features of this protocol.
between the borate B and a 1,10-phenanthroline-ligated
copper carboxylate A (or an alkoxocopper D shown in
pathb forthe initial cycle) occursto formanalkylcopper(I)
species C in the same way as the recently reported Cu-
catalyzed allyl-alkyl coupling between allylic phosphates
and alkylboranes.11a Finally, the addition of the alkylcop-
per species C across the CdO bond of CO2 (in place of the
CdC bond of the allylic phosphates) occurs to afford the
carboxylate complex A.
In path b, the reaction of the copper carboxylate A and
KOtBu initially gives an alkoxocopper D.4c Next, the
transmetalation between the alkylborane 2 and the alkox-
ocopper D forms the alkylcopper C. The addition of the
Cu-C bond of C across the CdO bond of CO2 affords the
carboxylate complex A.
Acknowledgment. This work was supported by Grants-
in-Aid for Scientific Research (B) and for Young Scientists
(B), JSPS. We thank MEXT for financial support in the
form of a Global COE grant (Project No. B01: Catalysis as
the Basis for Innovation in Materials Science). Dr. Yasuhiro
Kumaki (High-Resolution NMR Laboratory, Graduate
School of Science, Hokkaido University) is acknowledged
for conducting the NMR spectroscopy measurements.
At present, neither pathways can be ruled out. Never-
theless, the former (path a) seems to be more plausible because
(13) See Supporting Information for details of the NMR studies. In
addition, the 11B NMR study in our previous work on the Cu-catalyzed
allyl-alkyl coupling between allylic phosphates and alkylboranes sup-
ports the formation of the borate B (path a) (see ref 11a). Thus, the
reaction of PhCH2CH2-9-BBN with t-BuOK (1:1) in THF-d8 at rt for 5
min resulted in a complete conversion of PhCH2CH2-9-BBN (δ 65.7)
into a tetravalent borate (δ -1.4). Subsequent treatment of the solution
with CuOAc (B/Cu 1:1) afforded 9-BBN-OtBu (δ 55.1).
Supporting Information Available. Experimental de-
tails and characterization data for new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
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Org. Lett., Vol. 13, No. 5, 2011