position of 3a with several transition metal catalysts com-
monly used in diazo carbonyl chemistry. First, two other
typical Rh(II) catalysts, Rh2(O2CCF3)4 and Rh2(acam)4, were
compared with Rh2(OAc)4. The data show that although they
all promote the aryl migration efficiently, the ligands of the
catalyst do have influence over the migratory aptitude of the
phenyl group over the hydride group (Table 2). The electron-
withdrawing trifluoacetate ligand slightly enhances the
hydride migration.12 For copper catalysts, the phenyl migra-
tion is again predominant, although the cis isomer of the
phenyl migration product 6a was formed in considerable
amount in these cases (entries 4-6).
d, 5f-i and the cis isomer of the phenyl migration products
6a-i could only be detected on TLC and in 1H NMR spectra
of the crude product, but they were not be isolated because
of their tiny amounts.
A possible reaction mechanism could be tentatively
proposed on the basis of the information now available. The
aryl migration promoted by Rh(II) or copper catalysts should
occur through a metal-bound carbene 8 as intermediate
(Scheme 2). It has been suggested that in this metal carbene
Scheme 2
Finally, silver benzoate AgO2CPh, which is an efficient
catalyst for Wolff rearrangement in R-diazo carbonyl chem-
istry, was investigated. Under typical Wolff rearrangement
conditions, AgO2CPh/Et3N,13 3a decomposed to give hydride
migration isomer 5a as main product (entry 7). It is likely
that the hydride migration occurs through a free carbene
intermediate rather than a silver complexed carbene species.
To confirm this point, a photolysis experiment was con-
ducted. However, contrary to our prediction, when the
R-diazoketamine was photolyzed (λ > 300 nm) at room
temperature, the phenyl migration product was formed as
major product (entry 8).
Having confirmed that Rh2(OAc)4 is the effective catalyst
to promote phenyl migration, we then carried out the diazo
decomposition of N-tosyl diazoketamines 3b-i with this
catalyst. As shown by the data collected in Table 3, the aryl
intermediate, the carbon attached to the metal has a partial
positive charge.15 It is possible that aryl migration may go
through a “bridged” intermediate or transition state, in which
the orbital system of the aryl group is expected to assist in
the stabilization of a bridged carbonium ion or partial
carbonium ion through delocalization.16 This situation is
similar as the well-known 1,2-shift in carbonium ions, in
which the phenyl group also has higher migratory aptitude
than an alkyl group.17 The 1,2-shift in carbonium ions has
been extensively studied, and the aryl participation through
the formation of phenonium ion has been experimentally
confirmed. However, since electronic structure of the Rh-
(II) carbene or Cu(I) carbene is still controversial,15,18 further
experiments will be needed to confirm if a “bridged”
phenonium ion is truly involved in the migration pro-
cess.
a
Table 3. 1,2-Aryl Migration Catalyzed by Rh2(OAc)4
yield (%)a
entry
N-tosyl R-diazoketamines
4
5
1
2
3
4
5
6
7
8
9
3a , X ) H, R ) H
71b
89b
83b
86b
53b
81d
84d
82d
90d
c
3b, X ) p-MeO, R dH
3c, X ) p-Cl, R ) H
3d , X ) p-Ph, R ) H
3e, X ) m-Br, R ) H
3f, X ) o-Me, R ) H
3g, X ) 2,4-Cl, R dH
3i,X ) 3,5-OMe, R ) H
3j, X ) H, R ) Me
29b
a For general procedure, see ref 14. b Yields after column chromatog-
In the end, it is worthwhile to mention that the presence
of the X group in diazo compound 1 has a significant impact
raphy. c 1,2-Hydride migration products 5 and the cis isomer of the aryl
1
migration products 6 were detectable in TLC and the H NMR spectra of
the crude products, but they were not isolated because of their tiny amounts.
d Yields after single recrystallization of the crude product.
(14) General procedure for the Rh2(OAc)4-catalyzed diazo decomposi-
tion of N-tosyl diazoketamines 3a-j. A solution of 3 (0.5 mmol) in CH2-
Cl2 (20 mL) was stirred at 0 °C under N2, and then Rh2(OAc)4 (1 mg) was
added. The reaction mixture was stirred at 0 °C for 10 min. The solvent
was removed under vacuum, and the residue was purified by chromatog-
raphy or recrystallization.
(15) 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.
migration products were predominant for all these substrates.
Except in one case the hydride migration 5e was isolated as
minor product (entry 5), the hydride migration product 5a-
(16) However, we could not rule out the possibility that migration
concerted with rhodium dissociation, as one of the referees suggested.
(17) Lowry, T. H.; Richardson, K. S. Mechanism and Theory in
Organic Chemistry, 3rd ed.; Harper Collins Publishers: New York,
1987.
(18) Pirrung, M. C.; Morehead, A. T., Jr. J. Am. Chem. Soc. 1994, 116,
8991.
(11) Wang, J.; Chen, B.; Bao, J. J. Org. Chem. 1998, 63, 1853 and
references therein.
(12) Rh2(O2CCF3)4 has been reported to be more effective in promoting
1,2-hydride migration than Rh2(OAc)4; see ref 2c.
(13) Wang, J. Hou, Y. J. Chem. Soc., Perkin Trans. 1 1998, 1919 and
references therein.
Org. Lett., Vol. 3, No. 19, 2001
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