C O M M U N I C A T I O N S
Scheme 1
stereochemical results are consistent with an intramolecular process
that occurs through neutral intermediates and syn addition of the
metal and alkoxo units across the C-C double bond.14,15 These
data are consistent with the migratory insertion path A.
In summary, we report the preparation of a series of bis-
(phosphine) rhodium(I) alkoxides containing an accompanying
olefin ligand. The 5-hydroxy-1-alkene complexes undergo cycliza-
tions to afford 5-methylenetetrahydrofurans and the corresponding
Rh hydride as products. In contrast, the complexes of 4-hydroxy-
1-alkene are more stable and undergo â-allyl elimination at elevated
temperatures to afford a Rh allyl complex and the corresponding
ketones. The intramolecularity, lack of evidence for ionic intermedi-
ates, and syn stereochemistry of the cyclizations are all character-
istics of a migratory insertion mechanism. Future studies will focus
on the scope of these reactions, extension into catalytic processes,
and detection of the initial product from cyclization.
Acknowledgment. Financial support for this work was provided
by the Department of Energy, Office of Basic Energy Sciences.
Supporting Information Available: Experimental details and full
structural characterization of 2e (CIF and PDF). This material is
Scheme 1 shows pathways for the C-O bond-forming process
of compounds 2a-d (illustrated for deuterium-labeled, dimethyl-
substituted 2a) involving migratory insertion (path A) or dissociation
of alkoxide and backside attack on the olefin (path B).13 These
pathways were distinguished from each other and from alternative
mechanisms by kinetic experiments that revealed whether the
process was intramolecular or intermolecular, solvent effects that
probed for charged intermediates, and stereochemical labeling
experiments that distinguished between syn and anti addition across
the olefin.
Rate constants for the reaction of diphenyl-substituted 2c were
measured by 1H NMR spectroscopy at 35 °C with an initial 0.040
M concentration of 2c and concentrations of PEt3 varying from
0.12 to 1.20 M. A clear exponential decay of 2c indicated that the
reaction was first-order in rhodium (see Supporting Information).
The rate constants for reactions conducted with these concentrations
of added PEt3 were indistinguishable and are consistent with the
direct, unimolecular mechanisms of Scheme 1.
References
(1) Henry, P. M. Handbook of Organopalladium Chemistry for Organic
Synthesis; Negishi, E., Ed.; Wiley & Sons: New York, 2002; p 2119.
(2) For a recent review, see: (a) Muzart, J. Tetrahedron 2005, 61, 5955. (b)
Kondo, T.; Tsunawaki, F.; Sato, R.; Ura, Y.; Wada, K.; Mitsudo, T.-a.
Chem. Lett. 2003, 32, 24. (c) Hayashi, T.; Yamasaki, K.; Mimura, M.;
Uozumi, Y. J. Am. Chem. Soc. 2004, 126, 3036. (d) Trend, R. M.;
Ramtohul, Y. K.; Stoltz, B. M. J. Am. Chem. Soc. 2005, 127, 17778. (e)
Wolfe, J. P.; Rossi, M. A. J. Am. Chem. Soc. 2004, 126, 1620. (f) Hay,
M. B.; Wolfe, J. P. J. Am. Chem. Soc. 2005, 127, 16468. (g) Hay, M. B.;
Hardin, A. R.; Wolfe, J. P. J. Org. Chem. 2005, 70, 3099.
(3) Stoichiometric insertion of perfluorinated olefins into a Pt(II) methoxide:
Bryndza, H. E. Organometallics 1985, 4, 406.
(4) Coleman, J. P.; Hegedus, L. S. Principles and Applications of Organo-
metallic Chemistry; University Science Books: Mill Valley, CA, 1980;
pp 401-424.
(5) (a) Pd and Pt phenoxides with intramolecular olefin coordination: Aresta,
M.; Nyholm, R. S. J. Organomet. Chem. 1973, 56, 395. (b) Observation
of a Rh(I) carboxylate intermediate chelated by an activated olefin moiety
during catalytic hydrogenation: Burk, M. J.; Bienewald, F.; Challenger,
S.; Derrick, A.; Ramsden, J. A. J. Org. Chem. 1999, 64, 3290.
(6) Zr(IV) alkoxides stabilized by intramolecular olefin coordination: (a) Wu,
Z.; Jordan, R. F.; Petersen, J. L. J. Am. Chem. Soc. 1995, 117, 5867. (b)
Carpentier, J.-F.; Wu, Z.; Lee, C. W.; Stroemberg, S.; Christopher, J. N.;
Jordan, R. F. J. Am. Chem. Soc. 2000, 122, 7750.
(7) Zhao, P.; Krug, C.; Hartwig, J. F. J. Am. Chem. Soc. 2005, 127, 12066.
(8) Isolation of Rh(I) tert-alkoxides that undergo â-aryl eliminations: Zhao,
P.; Incarvito, C. D.; Hartwig, J. F. J. Am. Chem. Soc. 2006, 128, 3124.
(9) See for example: Kegley, S. E.; Schaverien, C. J.; Freudenberger, J. H.;
Bergman, R. G. J. Am. Chem. Soc. 1987, 109, 6563.
(10) Nishihara, Y.; Yoda, C.; Osakada, K. Organometallics 2001, 20, 2124.
(11) Added PEt3 improved the cyclization yields, apparently by suppressing
hydride-mediated olefin isomerization (see Supporting Information).
(12) Added PEt3 did not affect the yield or rate of the reaction. These data are
consistent with direct â-allyl eliminations from alkoxides 2e and 2f. (a)
For analogous â-aryl elimination from Rh(I) arylmethoxides, see ref 6.
(b) â-Aryl eliminations from Rh(I) iminyl complexes: Zhao, P.; Hartwig,
J. F. J. Am. Chem. Soc. 2005, 127, 11618. (c) Ru-catalyzed C-C cleavage
proposed to proceed via â-allyl elimination from Ru(II) alkoxides: Kondo,
T.; Kodoi, K.; Nishinaga, E.; Okada, T.; Morisaki, Y.; Watanabe, Y.;
Mitsudo, T.-a. J. Am. Chem. Soc. 1998, 120, 5587.
The solvent effects were inconsistent with the alkoxide dissocia-
tion step of path B to form a zwitterionic intermediate. The solvent
effect was small, and the reaction was slightly faster in less polar
solvents (kTHF-d ) 0.50 × 10-3 s-1, kbenzene-d ) 0.84 × 10-3 s-1
,
6
k
) 81.44 × 10-3 s-1).
cyclohexane-d12
The stereochemical results were also inconsistent with the
backside attack of alkoxide on the coordinated olefin in path B.
Scheme 1 shows the predicted stereochemical outcome for reaction
of the alkoxide derived from trans-5-d-1a by syn (path A) and anti
(path B) addition of the rhodium and alkoxo units across the olefin,
followed by â-hydrogen elimination through the usual syn coplanar
transition state. Syn addition of the Rh-O bond across the olefin
(path A) would afford diastereomer 4a. Subsequent rotation about
the C-C bond to create a syn coplanar transition state for
â-hydrogen elimination would generate E-5-d-3a. Anti addition after
dissociation of the alkoxide ligand (path B), followed by outer-
sphere nucleophilic attack of the pendant alkoxide, would generate
the opposite diastereomeric intermediate 4b. Rotation about the
C-C bond, followed by â-hydrogen elimination, would then form
Z-5-d-3a.
(13) (a) For related deuterium labeling studies, see refs 2c, 2d, and 2f. (b) For
a Pd-catalyzed stereospecific enol cyclization proposed to proceed via
chelation-directed syn-oxypalladation, see: Uenishi, J.; Ohmi, M.; Udea,
A. Tetrahedron: Asymmetry 2005, 16, 1299.
2
Stereochemically defined, H-labeled (45% deuterium) alcohol
(14) We also considered a third pathway involving initial formation of an
allylrhodium(III) intermediate by allylic C-H activation because Pd-
catalyzed allylic acetoxylation has been shown to involve π-allyl
intermediates (see ref 15). The resulting allyl intermediate could form
the final product by dissociation of alkoxide, followed by attack of the
alkoxide at the central carbon of the allyl unit, C-H reductive elimination
from the resulting metallacycle, and â-hydrogen elimination from the
resulting alkyl to form the methylenetetrahydrofuran. However, the lack
of solvent effect argues against dissociation of alkoxide, and an alternative
pathway involving reductive elimination between the alkoxide and the
central carbon of the allyl group lacks precedent.
trans-5-d-1a was prepared as described in Supporting Information,
and from this alcohol was generated the alkoxo olefin complex
trans-5-d-2a. The stereoselectivity of the cyclization of trans-5-
1
2
d-2a was studied by H and H NMR spectroscopy (Scheme 1).
2
The H NMR spectrum of the cyclization products contained a
single resonance for a deuterium located trans to the oxygen, and
the 1H NMR spectrum contained a signal of appropriately decreased
intensity corresponding to the hydrogen trans to the oxygen. The
chemical shifts of the olefinic hydrogens were assigned by NOESY
NMR spectroscopy. Thus, the kinetic data, solvent effects, and
(15) Grennberg, H.; Simon, V.; Ba¨ckvall, J.-E. J. Chem. Soc., Chem. Commun.
1994, 265.
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