2442
J. Am. Chem. Soc. 2001, 123, 2442-2443
Rhodium(I)-Catalyzed Homologation of Aromatic
Aldehydes with Trimethylsilyldiazomethane
Eric L. Dias, Maurice Brookhart,* and Peter S. White
Department of Chemistry
UniVersity of North Carolina at Chapel Hill
Chapel Hill, North Carolina 27599-3290
ReceiVed October 6, 2000
One-carbon homologation reactions are of broad utility in
organic synthesis. In the case of aromatic aldehyes, homologation
to the corresponding arylacetaldehydes has previously been
accomplished via conversion to the epoxides1 and subsequent
rearrangement, or phosphorane transfer2 followed by acid hy-
drolysis of the resulting alkyl enol ethers. In this communication,
we report the single-step transformation of aromatic aldehydes
to cis-1-aryl-2-trimethylsilyloxy ethylenes, that is the phenyl
acetaldehydes protected as their cis-TMS-enol ethers, employing
trimethylsilyldiazomethane and the rhodium(I) Lewis acid catalyst
1 (eq 1). This reaction is equivalent to three separate steps
Figure 1. X-ray crystal structure of 2. Thermal ellipsoids are drawn at
the 50% probability level. Selected bond lengths and angles are: Rh-
(1)-N(7), 2.007(3); Rh(1)-N(1), 2.0203(25); Rh(1)-N(11), 1.0952(24);
N(7)-N(8), 1.067(5); N(8)-C(41), 1.348(6); Rh(1)-N(7)-N(8), 142.1-
(3); N(7)-N(8)-C(41), 176.2(4); N(11)-Rh(1)-N(7), 175.56(12).
in dichloromethane solution at room temperature, decomposing
to unidentified products with a half-life of approximately 12 h.
employing standard organic techniques3 and is selective for the
Z-isomer. Furthermore, experimental evidence supports the
involvement of a unique, stable rhodium(I)-trimethylsilyldiaz-
omethane complex, 2, which has been isolated and crystallized.
Treatment of an orange solution of the three-coordinate, 14-
electron rhodium complex 14 in dichloromethane at -78 °C with
trimethylsilyldiazomethane (eq 1) produced a dark green solution,
which upon addition to cold pentane gave a microcrystalline solid
that was determined by spectroscopic and crystallographic
methods to be the N-bound adduct of TMS-diazomethane, 2.5
Complex 2 is the first example of a transition metal η1-N-bound
diazoalkane complex that contains a proton at the R-carbon atom,
and it is the only diazoalkane complex that lacks either the
resonance stabilization provided by esters or aromatic groups,5,6
or the additional steric and electronic stabilization provided by a
second TMS group.5,7 The isolated product 2 is moderately stable
(1) (a) Lemini, C.; Ordonez, M.; Perez-Flores, J.; Crus-Almanza, R. Synth.
Commun. 1995, 25, 2695-2702. (b) Cannon, J. G.; True, C. D.; Long, J. P.;
Bhatnagar, R. K.; Leonard, P.; Flynn, J. R. J. Med. Chem. 1989, 32, 2210-
2214. (c) Haridas, K. Indian J. Chem., Sect. B 1987, 26B, 1018-1020. (d)
Capon, R. J.; Ghisalberti, E. L.; Jefferies, P. R. J. Chem. Res. Synop. 1987,
118-119. (e) Sinha, J.; Singh, R. P.; Srivastava, J. N. J. Indian Chem. Soc.
1986, 63, 907-909. (f) Hanlon, B.; John, D. I. J. Chem. Soc., Perkin Trans.
1 1986, 2207-2212.
(2) (a) Sardessai, M. S.; Abramson, H. N. Org. Prep. Proced. Int. 1991,
23, 419-424. (b) Nair, M. G.; Murthy, B. R.; Patil, S. D.; Kisliuk, R. L.;
Thorndike, J.; Gaumont, Y.; Ferone, R.; Duch, D. S.; Edelstein, M. P. J. Med.
Chem. 1989, 32, 1277-1283. (c) Doad, J. G. S. J. Chem. Res., Synop. 1987,
370-371.
(3) Two steps are required to homologate an aryl aldehyde (see refs 1 and
2) and a third to protect it as the silyl enol ether.
On the basis of spectroscopic data, the resonance structure
depicted in eq 2 is the more accurate description of 2. In the
infrared spectrum, ν(N-N) ) 2057 cm-1 in dichloromethane
(2030 cm-1 KBr), as compared with 2069 cm-1 for free TMS-
diazomethane. Not only does this confirm the triply bonded
structure, but it also suggests that there is little to no back-bonding
1
from rhodium to nitrogen. In the H NMR spectrum (CD2Cl2),
the TMS-diazomethane proton appears as a doublet at δ ) 2.44
4
ppm, with a surprising JRh-H ) 2 Hz. The R-carbon resonance
(4) Dias, E. L.; Brookhart, M.; White, P. S. Chem. Commun. In press.
(5) For a recent review of transition metal complexes of diazoalkanes,
see: Dartiguenave, M.; Menu, M. J.; Deydier, E.; Dartiguenave, Y.; Siebald,
H. Coord. Chem. ReV. 1998, 180, 623-663.
appears as a broad singlet at δ ) 38.2 ppm in the 13C NMR
spectrum, comparable to chemical shifts observed for the two
bis-trimethylsilyldiazomethane complexes that have been previ-
ously reported.7
(6) (a) Werner, H.; Schneider, M. E.; Bosch, M.; Wolf, J. et al. Chem.
Eur. J. 2000, 6, 3052-3059. (b) Wolf, J.; Lutz, B.; Fries, A.; Werner, H.
Angew. Chem., Int. Ed. Engl. 1990, 29, 510-512.
The X-ray crystal structure of 2 (Figure 1) reinforces these
observations. The Rh(1)-N(7) distance of 2.01 Å is comparable
to that for the imine nitrogens (∼2.03 Å), indicating that the
TMS-diazomethane acts as a simple two-electron donor ligand.
The N(7)-N(8) distance of 1.07 Å confirms the triple bond
character, and is by far the shortest N-N bond length observed
for this type of complex (∼1.16 Å for other structures).5-7,8 While
(7) Menu, J. M.; Crocco, G.; Dartiguenave, M.; Dartiguenave, Y.; Bertrand,
G. J. Chem. Soc., Chem. Commun. 1988, 1598-1599.
(8) It should be noted that this distance may be somewhat artificially
shortened by vibrational disorder about N(8).
(9) One methyl of the TMS group does in fact have van der Waals contacts
with an isopropyl group of the neighboring molecule, which is manifested in
an uncharacteristic twisting of the isopropyl group with respect to the plane
of the aromatic ring.
10.1021/ja003608g CCC: $20.00 © 2001 American Chemical Society
Published on Web 02/20/2001