1,2-Vinyl and 1,2-Acetylenyl Migration
SCHEME 6. Rh2(OAc)4-Catalyzed Reaction of
â-Tosylamino r-Diazo Carbonyl Compounds 20a,b
FIGURE 2. Conformations leading to 1,2-vinyl and 1,2-
acetylenyl migrations.
group is a strong π-electron-donating functionality, which
can stabilize the positive charge in the transition state
of 1,2-hydrogen migration, it accelerates the 1,2-hydrogen
migration. Alkoxy and alkyl groups have similar effects.4a,15
Conversely, since electron-withdrawing NHCOCCl3 has
less ability to stabilize positive charge, it retards the 1,2-
hydrogen migration. Because C-C bond migration was
less affected by the substituent due to the relatively small
extent of charge separation in the transition state, the
1,2-vinyl and 1,2-acetylenyl migrations become competi-
tive, completely suppressing the 1,2-H migration.
On the other hand, an interesting feature in the 1,2-
vinyl and 1,2-acetylenyl bond migrations is that thermo-
dynamically less stable products, that is to say, NHCOC-
Cl3 and ester groups with E configurations, are observed
to be overwhelmingly predominant in all cases. All the
reactions were analyzed with 1H NMR of the crude
products before column chromatography, and the (Z)-
isomer could not be identified. However, the originally
formed (E)-products could isomerize to their (Z)-counter-
parts on silica gel column. Moreover, we have confirmed
that the Z/E isomerization does not occur under the
Rh(II)-mediated reaction conditions, thus indicating that
the predominant formation of (E)-products is due to the
1,2-migration reaction itself but not isomerization.
We have observed similar results in the previous study
on 1,2-aryl migrations.6b The stereoselectivity implies
that conformational factors may play a role in the
migration process. For the migration to occur, it is
necessary that the migrating σ-bond orient in parallel
to the empty p orbital of the carbene carbon in the
transition states (Figure 2). Considering conformations
A and B, which will lead to (Z)- and (E)-1,2 vinyl or
acetylenyl migration products, respectively, one may
predict that (Z)-product would predominate because of
the repulsion between NHCOCCl3 and Rh(II)/ligands.
However, one can argue that the migration concerts with
dissociation of the Rh(II) catalyst. If this is the case, then
in the transition state from the conformation B the C-Rh
bond has departed to a considerable extent. As a result,
the steric interaction between NHCOCCl3 and Rh(II)/
ligands is reduced, and the E migration products formed
preferentially. This argument is supported by the invari-
ability of 13c to 14c with the change of Rh(II) catalyst
ligands (Table 2).16
which the migrating group is originally attached.6b,13 The
following experiments clearly indicate the importance of
electronic effects in affecting the migratory aptitude
(Scheme 6). The electron-withdrawing NO2 substituent
in the aromatic ring of 20a completely interdicts aryl
migration, while the electron-donating MeO group has
the opposite effect.
Theoretical treatment of 1,2-hydrogen and 1,2-phenyl
migrations in singlet carbene at the B3LYP/6-311G*//
B3LYP16-31G* level indicates that in the transition state
of 1,2-hydrogen migration 23a, more positive charge
developed in the migration origin than in the transition
state of 1,2-phenyl migration 23b (Figure 1).14 It is thus
FIGURE 1. Transition states in singlet carbene and Rh(II)
carbene reactions.
concluded that the 1,2-hydrogen migration can be viewed
as largely resembling a hydride shift with significant
charge separation in the transition state. If one accepts
that the 1,2-migration in Rh(II)-carbene is similar to
that in singlet carbene, as shown in transition states 24a
and 24b, then the bystander substituent effects observed
in this study can be easily understood. Because hydroxyl
(11) Padwa et al. has reported the same reaction of diazo compound
8c. Padwa, A.; Kulkarni, Y. S.; Zhang, Z. J. Org. Chem. 1990, 55, 4144.
(12) (a) Doyle, M. P.; Griffin, J. H.; Bagheri, V.; Dorow, R. L.
Organometallics 1984, 3, 53. (b) Doyle, M. P. Chem. Rev. 1986, 86,
919. (c) Doyle, M. P. Acc. Chem. Res. 1986, 19, 348. (d) Taber, D. F.;
Ruckle, R. E., Jr. J. Am. Chem. Soc. 1986, 108, 7686. (e) 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. (f) Wang, P.;
Adams, J. J. Am. Chem. Soc. 1994, 116, 3296. (g) Pirrung, M. C.;
Morehead, A. T., Jr. J. Am. Chem. Soc. 1994, 116, 8991. (h) Taber, D.
F.; Song, Y. J. Org. Chem. 1996, 61, 6706. (i) Doyle, M. P.; Kalinin, A.
V.; Ene, D. G. J. Am. Chem. Soc. 1996, 118, 8837. (j) Taber, D. F.;
You, K. K.; Rheingold, A. L. J. Am. Chem. Soc. 1996, 118, 547. (k)
Wang, J.; Chen, B.; Bao, J. J. Org. Chem. 1998, 63, 1853.
(13) For comparison, in the 1,2-hydrogen migration of singlet
arylchlorocarbene, Hammett analysis gave a reaction constant of -1.0
with σ-constants; see: Liu, M. T. H.; Bonneau, R. J. Am. Chem. Soc.
1992, 114, 3604.
In summary, we have observed the 1,2-double-bond
and 1,2-triple-bond migration in Rh(II) carbene reaction.
The investigation demonstrates a remarkable substituent
(15) Sarabia Garc´ıa, F.; Pedraza Cebria´n, G. M.; Heras Lo´pez, A.;
Lo´pez Herrera, F. J. Tetrahedron 1998, 54, 6867.
(14) According to the calculation by Keating et al., NPA partial
charge in the carbon of 1,2-shift origin increases by +0.23 for 1,2-phenyl
shift of benzylchlorocarbene (PhCH2CCl), while for 1,2-H shift of
methylchlorocarbene (MeCCl) the corresponding charge increase is
+0.33. See ref 3c.
(16) Ligands of Rh(II) catalysts have been known to affect the
chemoselectivity in competitive Rh(II) carbene transformations of diazo
compounds. For a detailed study, see: Padwa, A.; Austin, D. J.; Price,
A. T.; Semones, M. A.; Doyle, M. P.; Protopopova, M. N.; Winchester,
W. R.; Tran, A. J. Am. Chem. Soc. 1993, 115, 8669.
J. Org. Chem, Vol. 70, No. 11, 2005 4321