A stable dirhodium tetracarboxylate carbenoid: Crystal structure, bonding analysis, and catalysis [15]

Full text of DOI:10.1021/ja016928o

11318
J. Am. Chem. Soc. 2001, 123, 11318-11319
mational effects15 inherent within a particular system can override
electronic factors. When the latter operate, the selectivity of the
rhodium carbenoid intermediate is less predictable.
A Stable Dirhodium Tetracarboxylate Carbenoid:
Crystal Structure, Bonding Analysis, and Catalysis
James P. Snyder,* Albert Padwa, and Thomas Stengel
Department of Chemistry, Emory UniVersity
Atlanta, Georgia 30322
Anthony J. Arduengo, III,* Alexander Jockisch, and
Hyo-Joong Kim
Department of Chemistry, The UniVersity of Alabama
Tuscaloosa, Alabama 34587-0336
While resonance forms representing metal-stabilized carboca-
tions and metal carbenes in the classical σ/π framework have
been invoked to explain Rh-carbenoid chemistry,11 the more recent
formulation of a double “half-bond” model12,16 appears to better
describe the Rh-C linkage. To substantiate the independent
existence of rhodium carbenoids and to more fully characterize
the Rh-Rh-C bonding, we report a combined X-ray crystal-
lographic and quantum chemical evaluation of the first stable
rhodium carbenoid intermediate.
Since the observation of an imidazol-2-ylidene by the Arduengo
group in 1991,17 a number of stable crystalline carbenes have
been described.18 We reasoned that combination of a derivative
of the latter and a dirhodium(II) tetracarboxylate catalyst would
deliver the desired metal complex. Accordingly, admixture of
carbene 419 (R5 ) Me) and the Rh(II) pivaloate 5 delivered
compound 6 as wine-red crystals suitable for X-ray structure
determination.
ReceiVed August 23, 2001
ReVised Manuscript ReceiVed September 20, 2001
Transition metal catalyzed reactions of R-diazo ketones (1) are
a powerful means to synthesize complex polycyclic organic
frameworks.1 In particular, dirhodium(II) carboxylate and car-
boxamide catalysts mediate a wide range of synthetic transforma-
tions such as cyclopropanation,2 C-H and X-H insertion,3,4
aromatic substitution,5 and ylide formation.6 Enantioselective
transformations promoted by chiral Rh(II) complexes allow
construction of both carbocyclic and heterocyclic systems in
optically active form,7-10 and much effort has been directed
toward understanding the factors that control both regio- and
enantioselectivities.7,11,12
The critical intermediate presumed to unify this body of
chemistry is an electrophilic rhodium carbenoid in which a metal-
axial carbene adopts a linear Rh-Rh-C arrangement (3, R2 )
COR4). A growing number of examples demonstrate that the
reactivity of rhodium carbenoids is greatly affected by the
electronic nature of the bridging ligand attached to the metal.1
Accordingly, the reactivity-determining Rh-C interaction is
regarded as highly polarizable,7,13 although steric14 and confor-
(1) Doyle, M. P.; McKervey, M. A.; Ye, T. Modern Catalytic Methods for
Organic Synthesis with Diazo Compounds; John Wiley and Sons: New York,
1998.
The molecular geometry is depicted in Figure 1 with selected
variables given in Table 1. The overall structure of the dirhodium
cage is faithful to that determined by X-ray crystallography for a
series of Rh2-tetracarboxylates.20 One noteworthy difference is
an expanded Rh-Rh distance of 0.05 Å in 6 by comparison with
the average for 2, R ) H and C3H7 (2.424 vs 2.37 Å, respectively).
A similar elongation upon formation of rhodium carbenoids has
been predicted and rationalized by DFT calculations.12,16
To gain insight into the nature of the metal-carbon bonding
in 6, we have performed additional DFT calculations on alkyl-
truncated variations of the structure (i.e. tetraformate or tetraac-
etate) with two different basis sets (Table 1). All geometry
optimizations predict the overall structure and the bisection of
the carboxylate bridges by the imidazoyl ring (N-C-Rh-O‡ )
ca. 0°, Table 1). The latter feature is clearly steric in origin as
indicated by the alternative energy minimum DFT conformers
of the bis-N-demethyl analogue (6, N-R, R ) H) and the parent
carbenoid (3, R1 ) R2 ) H), both of which orient the carbene
moiety in a plane common to two of the bridging carboxylates.
(2) Burke, S. D.; Grieco, P. A. Org. React. (N.Y.) 1979, 26, 361.
(3) Taber, D. F. ComprehensiVe Organic Synthesis; Trost, B. M., Fleming,
I., Eds.; Pergamon Press: Oxford, UK, 1991; Vol. 3, p 1045.
(4) Buck, R. T.; Doyle, M. P.; Drysdale, M. J.; Ferris, L.; Forbes, D. C.;
Haigh, D.; Moody, C. J.; Pearson, N. D.; Zhou, Q. L. Tetrahedron Lett. 1996,
37, 7631.
(5) Ye, T.; McKervey, A. Chem. ReV. 1994, 94, 1091.
(6) Padwa, A.; Hornbuckle, S. F. Chem. ReV. 1991, 91, 263. Padwa, A.;
Weingarten, M. D. Chem. ReV. 1996, 96, 223.
(7) Doyle, M. P. Aldrichim. Acta 1996, 29, 3. Doyle, M. P.; McKervey,
M. A. J. Chem. Soc., Chem. Commun. 1997, 983. Doyle, M. P.; Forbes, D.
C. Chem. ReV. 1998, 98, 911-935.
(8) Davies, H. M. L.; Ahmed, G.; Calvo, R. L.; Churchill, M. R.; Churchill,
D. G. J. Org. Chem. 1998, 63, 2641. Davies, H. M. L.; Huby, N. J. S.; Cantrell,
W. R., Jr.; Olive, J. L. J. Am. Chem. Soc. 1993, 115, 9468.
(9) McCarthy, N.; McKervey, M. A.; Ye, T.; McCann, M.; Murphy, E.;
Doyle, M. P. Tetrahedron Lett. 1992, 33, 5983. Pierson, N.; Fernandez-Garcia,
C.; McKervey, M. A. Tetrahedron Lett. 1997, 38, 4705.
(10) Hodgson, D. M.; Stupple, P. A.; Johnstone, C. Chem. Commun. 1999,
2185. Hodgson, D. M.; Stupple, P. A.; Johnstone, C. Tetrahedron Lett. 1997,
38, 6471. Kitagaki, S.; Anada, M.; Kataoka, O.; Matsuno, K.; Umeda, C.;
Watanabe, N.; Hashimoto, S. J. Am. Chem. Soc. 1999, 121, 1417. Kitagaki,
S.; Yasugahira, M.; Anada, M.; Nakajima, M.; Hashimoto, S. Tetrahedron
Lett. 2000, 41, 5931.
(11) Doyle, M. P. Acc. Chem. Res. 1986, 19, 348. Doyle, M. P. Chem.
ReV. 1986, 86, 919.
(15) Doyle, M. P.; Pieters, R. J.; Tauton, J.; Pho, H. Q.; Padwa, A.; Hertzog,
D. L.; Precedo, L. J. Org. Chem. 1991, 56, 820.
(16) Sheehan, S. M.; Padwa, A.; Snyder J. P. Tetrahedron Lett. 1998, 39,
949.
(12) Padwa, A.; Snyder, J. P.; Curtis, E. A.; Sheehan, S. M.; Worsencroft,
K. J.; Kappe, C. O. J. Am. Chem. Soc. 2000, 122, 8155.
(13) Pirrung, M. C.; Morehead, A. T., Jr. J. Am. Chem. Soc. 1994, 116,
8991.
(17) Arduengo, A. J., III; Harlow, R. L.; Kline, M. J. Am. Chem. Soc.
1991, 113, 361.
(14) Hashimoto, S.-I.; Watanabe, N.; Ikegami, S. Tetrahedron Lett. 1992,
33, 2709. Hashimoto, S.-I.; Watanabe, N.; Ikegami, S. J. Chem. Soc., Chem.
Commun. 1992, 1508. Doyle, M. P.; Westrum, L. J.; Wolthuis, W. N. E.;
See, M. M.; Boone, W. P.; Bagheri, V.; Pearson, M. M. J. Am. Chem. Soc.
1993, 115, 958. Cane, D. E.; Thomas, P. J. J. Am. Chem. Soc. 1984, 106,
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(18) For some leading references, see: Arduengo, A. J., III; Krafczyk, R.
Chem. Z. 1998, 32, 6.
(19) Arduengo, A. J., III; Rasika Dias, H. V.; Harlow, R. L.; Kline, M. J.
Am. Chem. Soc. 1992, 114, 5530.
(20) Cotton, F. A.; Walton, R. A. Multiple Bonds between Metal Atoms;
Clarenden Press: Oxford,UK, 1993; pp 435-443.
10.1021/ja016928o CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/17/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 45, 2001 11319
with or without an all-electron treatment23 shows slight polariza-
tion of the 4d electron shell, but uniformly places 7.8-8.1
electrons in it. Both metal sites are thus predicted to be be Rh(II)
with clear-cut d8 configurations.
Is rhodium carbenoid 6 capable of catalyzing the transforma-
tions of R-diazo ketones analogous to ligand-free catalyst 2? An
intramolecular C-H insertion reaction (7 f 8) and two intramo-
lecular alkyne cyclizations for diazo ketone substrates were
performed separately (9 f 10; 11 f 12) in the presence of 1
mol % of 2 (R3 ) CH3), 5, and 6 under the same reaction
conditions. Products and yields were identical in all three cases.
The most plausible mechanistic scenario is that the weak Rh-C
bond engages in a dissociative equilibrium as pictured above
followed by subsequent reaction of 5 and diazo ketone. The
reactions then proceed normally. Although we have not recovered
elements of complex 6, we presume that the small quantitity of
freed carbene 4 (R ) Me) reacts with diazo ketone to form the
corresponding diazine as observed in related systems.24
Figure 1. KANVAS drawing21 of the X-ray structure of dirhodium(II)
carbenoid 6.
Table 1. Selected Bond Lengths (Å) and Angles (deg) for
Rhodium Carbenoid 6 Derived from X-ray Crystallography and
Alkyl-Truncated DFT Calculations with Two Basis Setsa
Rh-C
Rh-Rh
N-C-N
N-C-Rh-O‡
X-ray
2.057
2.089
2.110
2.424
2.465
2.461
102.0
104.4
104.3
0.5-2.8
-0.1
-0.2
3-21G/ECPc,d
6-31G*/ECPc,d
a All geometry optimizations were performed with G-98 (Frisch, M.
J. et al. Gaussian 98, Revision A.3; Gaussian Inc.: Pittsburgh, PA,
1998). b O‡ is the midpoint between two oxygens bound to a given
Rh. c 3 (R3 ) Me, R5 ) H). d B3LYP on all atoms, LANL2DZ basis
set and ECP-2 core potentials on Rh as implemented in G-98.
The effective core potential calculations slightly overestimate
the Rh-C separation and the N-C-N bond angle (Table 1).
Nonetheless, by comparison with previously optimized Rh-C
bond distances for less stable carbene fragments (3, R1 ) H, R2
) H, COR, 1.92-1.96 Å),12,16 it is clear that the X-ray derived
2.057 Å metal-carbon distance in 6 is rather long. An under-
standing of this phenomenon follows from the double “half-bond”
model in which half of the Rh-C bonding electrons in 3 (R1 )
R2 ) H) derive from the σ-donation of the H2C: lone pair to Rh
and half from π-back-donation at Rh to the empty carbon
p-orbital.12,16 In 6, π-electron repulsion by the flanking nitrogen
p-type lone pairs counteracts significant Rh-back-donation to the
carbene center and is expected to weaken the Rh-C bond. NBO
population analysis22 confirms this viewpoint. At the 6-31G*/
LANL2DZ level, 6 registers an Rh-C bond order of 0.5 (vs 1.0
for 3, R1 ) R2 ) H) reflecting a bond-antibond interaction
between σ-CLP and a virtual orbital centered on both Rh atoms.
The well-developed π Rh-CH2 interaction in 3 (R1 ) R2 ) H)16
is, however, seriously diminished in 6 compatible with the 50%
reduction in bond order. By contrast, both 4 and 6 exhibit partial
π-character around the carbenoid carbon with identical N-C bond
orders of 1.3.
In conclusion, the first stable dirhodium(II) carbenoid 6 has
been isolated and its structure determined by X-ray crystal-
lography. The compound exhibits an increase in the Rh-Rh
distance expected for carbene complexation, along with a
somewhat long and weak Rh-C bond. NBO population analysis
indicates the latter to be similar in character to that in systems
incorporating more reactive carbenes, but with much reduced
metal back-bonding. Catalytically, complex 6 most likely frag-
ments to carbene 4 and the chemically active ligand-free Rh2-
carboxylate 5. The postulated ease of dissociation is entirely
consistent with previous proposals for catalytic action by 3 and
its analogues.
Acknowledgment. A.P. gratefully acknowledges the National Insti-
tutes of Health (GM59384-22) for generous support. A.J.A. is grateful
to the University of Alabama for support through the Saxon Endowment.
The assistance of William J. Marshall and Grant A. Broker enabled
solution of the X-ray structure of 6. J.P.S. thanks Dennis Liotta (Emory
University) for support and encouragement.
An additional issue concerns the oxidation state of the Rh atoms
in 6. Formally, the two metal centers can be regarded as two
Rh(II) atoms or a Rh(I)/Rh(III) pair. DFT/NBO assessment of
electron configuration in 3 (R1 ) R2 ) H) and alkyl-truncated 6
Supporting Information Available: Synthesis and properties for 6
along with tables of X-ray structural data (CCDC No. 168865) (PDF).
This material is available free of charge via the Internet at http://pubs.acs.org
(21) This drawing was made with the KANVAS computer graphics program
based on the program SCHAKAL of E. Keller (Kristallographisches Institut
der Universita¨t Freiburg, Germany) and modified by A. J. Arduengo, III to
produce the back and shadowed planes. The planes depict a 50-pm grid. The
lighting source is at infinity so that shadow size is meaningful.
(22) Foster, J. P.; Weinhold, F. J. Am. Chem. Soc. 1980, 102, 7211. Reed,
A. E.; Curtis, L. A.; Weinhold, F. Chem. ReV. 1988, 88, 899.
JA016928O
(23) (a) All-electron on Rh: (23s16p13d)/[10s6p5d]; Huzinaga, S.; Klo-
bukowski, M. J. Mol. Struct. (THEOCHEM) 1988, 167, 1. (b) Relativistic
core potentials, see footnote d, Table 1.
(24) Arduengo, A. J.; Goerlich, J. R. Unublished results.