Chemistry Letters Vol.35, No.3 (2006)
289
situ from [RhCl(cod)]2 and DPPP8 furnished not 4a but olefin 8
from 3a through a decarbonylation process involving ꢂ-hydride
elimination (Scheme 3).6d,12
Furthermore, no chemoselectivity was observed in the de-
carbonylation reaction of 6 in the presence of the Rh(I)–DPPP
complex; both the aldehydic and ketonic carbonyl groups were
decarbonylated to afford 1-isopropenyl-4-propoxymethylben-
zene (9) in 84% yield (Eq 4).
much more rapidly than 3d (Entry 4). During decarbonylation
of 3e with 2, ꢂ-carbon elimination did not follow the insertion
step of rhodium, unlike the case of a ring-expansion reaction
of an analogous spiro compound catalyzed by [Rh(dppp)2]Cl.14
In summary, the present study provides an intriguing exam-
ple of preferential activation of a C–C bond over a C–H bond.
The unique potential of NHC complexes for synthetic purposes
is inferred from the contrasting chemoselectivies observed in
decarbonylation. Mechanistic explanation of the marked con-
trast and application to other catalytic process are the subjects
of further studies in our laboratory.
[RhCl(cod)]2 (2.5 mol %)
DPPP (10 mol %)
6
(4)
m-xylene, reflux, 6 h
O
H
References and Notes
9 84%
1
a) A. E. Shilov, G. B. Shul’pin, Chem. Rev. 1997, 97, 2879.
b) R. H. Crabtree, J. Chem. Soc., Dalton Trans. 2001, 2437.
c) V. Ritleng, C. Sirlin, M. Pfeffer, Chem. Rev. 2002, 102,
1731. d) F. Kakiuchi, N. Chatani, Adv. Synth. Catal. 2003, 345,
1077. e) W. D. Jones, Inorg. Chem. 2005, 44, 4475.
A chemoselectivity opposite to that provided by 2 was
observed when 6 was treated with a stoichiometric amount of
the Wilkinson’s complex at room temperature. Only the aldehy-
dic carbonyl group was decarbonylated with the cyclobutanone
carbonyl remaining intact in the product 10 (Eq 5).13 Thus, it
proved that aldehydes and cyclobutanones possess similar reac-
tivities toward rhodium-mediated decarbonylation and that an
appropriate choice of the ligand system can result in opposite
chemoselectivity.
2
a) M. Murakami, Y. Ito, in Topics in Organometallic Chemistry,
ed. by S. Murai, Springer, Berlin, 1999, Vol. 3, p. 97. b) B.
Rybtchinski, D. Milstein, Angew. Chem., Int. Ed. 1999, 38, 870.
c) C.-H. Jun, Chem. Soc. Rev. 2004, 33, 610. d) T. Kondo, T.
Mitsudo, Chem. Lett. 2005, 34, 1462.
3
4
For thermodynamic preference for C–C bond activation, see:
a) R. A. Periana, R. G. Bergman, J. Am. Chem. Soc. 1986, 108,
7346. b) C. Perthuisot, W. D. Jones, J. Am. Chem. Soc. 1994,
116, 3647.
a) H. M. Colquhoun, D. J. Thompson, M. V. Twigg, Carbonyla-
tion: Direct Synthesis of Carbonyl Compounds, Plenum, New
York, 1991, p. 407. b) K. Ohno, J. Tsuji, J. Am. Chem. Soc.
1968, 90, 99. c) H. M. Walborsky, L. E. Allen, J. Am. Chem.
Soc. 1971, 93, 5465.
O
[RhCl(PPh3)3] (1.1 equiv.)
6
(5)
CH2Cl2, rt, 10 days
O
H
10 83%
Other examples of the decarbonylation of cyclobutanones
using the Rh–NHC complex 2 are listed in Table 1. An active hy-
drogen of 3b remained intact during decarbonylation (Entry 1).
In the case of 2-(2-naphthyl)cyclobutanone (3c), 9% of (E)-1-(2-
naphthyl)propene (11), formed through ꢂ-hydride elimination,
was obtained as another decarbonylation product together with
the major product, cyclopropane 4a (83%) (Entry 2). 3,3-Disub-
stituted cyclobutanone 3d requires a longer reaction time to
reach full conversion, probably due to steric reasons (Entry 3).
On the other hand, spiro[3.3]heptan-2-one 3e, with constrained
geminal disubstituents at the 3-position, was decarbonylated
5
6
a) D. H. Doughty, L. H. Pignolet, J. Am. Chem. Soc. 1978, 100,
7083. b) J. M. O’Connor, J. Ma, J. Org. Chem. 1992, 57, 5075.
c) C. M. Beck, S. E. Rathmill, Y. J. Park, J. Chen, R. H. Crabtree,
L. M. Liable-Sands, A. L. Rheingold, Organometallics 1999, 18,
5311. d) T. Shibata, N. Toshida, K. Takagi, Org. Lett. 2002, 4,
1619. e) T. Morimoto, K. Kakiuchi, Angew. Chem., Int. Ed.
2004, 43, 5580.
ˇ
a) A. Rusina, A. Vlcek, Nature 1965, 206, 295. b) E. Muller,
¨
A. Segnitz, Liebigs Ann. Chem. 1973, 1583. c) K. Kaneda, H.
Azuma, M. Wayaku, S. Teranishi, Chem. Lett. 1974, 215. d) M.
Murakami, H. Amii, K. Shigeto, Y. Ito, J. Am. Chem. Soc.
1996, 118, 8285. e) N. Chatani, Y. Ie, F. Kakiuchi, S. Murai,
J. Am. Chem. Soc. 1999, 121, 8645. f) M. Murakami, T. Itahashi,
Y. Ito, J. Am. Chem. Soc. 2002, 124, 13976. g) O. Daugulis, M.
Brookhart, Organometallics 2004, 23, 527.
Table 1. Decarbonylation of cyclobutanones 3b–3e using 2a
Enrty
3
Time/h
4 (%yieldb)
7
For reviews on transition metal–NHC complexes, see: a) W. A.
Herrmann, Angew. Chem., Int. Ed. 2002, 41, 1290. b) M. C. Perry,
K. Burgess, Tetrahedron: Asymmetry 2003, 14, 951. c) C. M.
Crudden, D. P. Allen, Coord. Chem. Rev. 2004, 248, 2247.
Abbreviations: cod = cycloocta-1,5-diene, DPPP = 1,3-bis(di-
phenylphosphino)propane.
O
EtO2C
CO2Et
EtO2C
1
6
CO2Et
4b (92)
3b
8
9
F. E. Hahn, M. Paas, D. Le Van, T. Lugger, Angew. Chem., Int.
¨
O
2
8
4ac (83)
Ed. 2003, 42, 5243.
10 N. Kuhn, T. Kratz, Synthesis 1993, 561.
11 Decarbonylation of 3a with [RhCl(cod)(1,3-dimethylimidazol-2-
ylidene)] under otherwise identical conditions gave 4a in 82%
yield.
3c
3d
3e
Ph
Ph
Ph
O
3
4
84
2
4Pdh (90)
12 Catalyst with 1/2 ratio of Rh/DPPP exhibited a higher activity
than that with 1/1 ratio.
O
13 Decarbonylation of 3a in the presence of 1.1 equiv. of [RhCl-
(PPh3)3], which failed to occur at rt, proceeded in refluxing
m-xylene (4 days) to afford a mixture of 4a (7%), 8 (25%), and
11 (23%, E=Z ¼ 90=10).
4e (86)
aCyclobutanone 3 (0.60 mmol) and Rh–NHC complex 2 (0.03
mmol, 5 mol %) were heated in refluxing m-xylene (3.0 mL).
bIsolated yield. cObtained as a mixture with (E)-1-(2-naphthyl)-
propene (11) (9%).
14 M. Murakami, K. Takahashi, H. Amii, Y. Ito, J. Am. Chem. Soc.
1997, 119, 9307.