1,2-Migration in rhodium(II) carbene transfer reaction: Remarkable steric effect on migratory aptitude

Article abstract of DOI:10.1021/jo0605391

A series of diazo carbonyl compounds bearing different substituents have been prepared in order to investigate the steric effect in 1,2-migration reaction of rhodium(II) carbene. Through the investigation on the diazo decomposition of these compounds with Rh₂(OAc)₄, it was found that the steric effect could dramatically influence the migratory aptitude. In many cases, the steric effect could override the inherent electronic effect of the substituent.

Full text of DOI:10.1021/jo0605391

SCHEME 1
1,2-Migration in Rhodium(II) Carbene Transfer
Reaction: Remarkable Steric Effect on
Migratory Aptitude
Fengping Xiao and Jianbo Wang*
Beijing National Laboratory of Molecular Sciences (BNLMS),
Key Laboratory of Bioorganic Chemistry and Molecular
Engineering of the Ministry of Education, College of Chemistry,
Peking UniVersity, Beijing 100871, China
wangjb@pku.edu.cn
ReceiVed March 11, 2006
metal carbenes similar migratory aptitude as in the case of
singlet carbene has been known,2 a dramatic nonmigrating
(bystander) effect has been observed. For example, the 1,2-aryl
migration becomes completely predominant over 1,2-H migra-
tion when there is a nonmigrating substituent NHCOCl3 attached
to the migration origin (Scheme 1).4d This effect of the bystander
has been attributed to their electronic effects. As shown by the
transition state in Scheme 1, the migration origin is supposed
to have more positive charge development in 1,2-H migration
than in 1,2-Ph migration. Thus, a strong π-electron-donating
group, such as hydroxy group, will accelerate 1,2-H migration,
while relatively less electron-donating group such as Cl3CC-
(O)NH has the opposite effect. As a continuation of this
investigation, we report in this paper that steric factors also have
an important effect on the 1,2-migratory aptitude in the reactions
of rhodium(II) carbene.
A series of diazo carbonyl compounds bearing different
substituents have been prepared in order to investigate the
steric effect in 1,2-migration reaction of rhodium(II) carbene.
Through the investigation on the diazo decomposition of
these compounds with Rh2(OAc)4, it was found that the steric
effect could dramatically influence the migratory aptitude.
In many cases, the steric effect could override the inherent
electronic effect of the substituent.
A series of diazo carbonyl compounds 1a-f, 2a-e, and 3a-
e, which bear different substituents, have been prepared ac-
cording to known methods.4c,5 In the diazo compounds 1a-f
and 2a-e, the â substituents are NHTs and Cl3CC(O)NH,
respectively. The R is changed from small group to large group
(from Me to tBu, and from Ph to 2,6-Cl2C6H3, respectively). It
is easy to expect that with the increase of the bulkiness of R, a
large group, in these cases the phenyl group, will become
gradually difficult to migrate. In the series of 3a-e, the size of
the â substituent is gradually changed from small to large. It is
expected that a large â substituent will push a large group (the
phenyl) to migrate in order to relieve the steric congestion.
1,2-Migration represents a fundamental reaction pathway for
both free carbenes and metal-associated carbenes.1,2 In the cases
of singlet carbene, the 1,2-migratory aptitude has been inves-
tigated extensively in the past decades. The inherent migratory
aptitude is in the order of H > Ar > alkyl. This migratory
preference has also been known to be affected by steric and
conformational factors.3 We have recently studied the problem
of 1,2-migratory aptitude in the case of rhodium(II)-carbene,
which is generated through Rh(II) complex catalyzed reaction
of R-diazocarbonyl compounds.4 Although in 1,2-migration of
(1) For recent comprehensive reviews, see: (a) Doyle, M. P.; McKervey,
M. A.; Ye, T. Modern Catalytic Methods for Organic Synthesis with Diazo
Compounds; Wiley-Interscience: New York, 1998. (b) Carbene Chemistry;
Bertrand, G., Ed.; Fontis Media: Lausanne, Switzerland, 2002.
(2) For 1,2-migration in rhodium(II) carbene, see the following. 1,2-
Hydride migration: (a) Pellicciari, R.; Fringuelli, R.; Ceccherelli, P.; Sisani,
E. J. Chem. Soc., Chem. Commun. 1979, 959-960. (b) Ikota, N.; Takamura,
N.; Young, S. D.; Ganem, B. Tetrahedron Lett. 1981, 22, 4163-4166. (c)
Hoffmann, K.-L.; Regitz, M. Tetrahedron Lett. 1983, 24, 5355-5358. (d)
Hudlicky, T.; Olivo, H. F.; Natchus, M. G.; Umpierrez, E. F.; Pandolfi, E.;
Volonterio, C. J. Org. Chem. 1990, 55, 4767-4770. (e) Taber, D. F.;
Hoerrner, R. S. J. Org. Chem. 1992, 57, 441-447. (f) Ohno, M.; Itoh, M.;
Umeda, M.; Furuta, R.; Kondo, K.; Eguchi, S. J. Am. Chem. Soc. 1996,
118, 7075-7082. 1,2-Alkyl or 1,2-aryl migration: (g) Nagao, K.; Chiba,
M.; Kim, S.-W. Synthesis, 1983, 197-199. (h) Ye, T.; McKervey, M. A.
Tetrahedron 1992, 48, 8007-8022. (i) Kanemasa, S.; Kanai, T.; Araki, T.;
Wada, E. Tetrahedron Lett. 1999, 40, 5055-5058. 1,2-Acetoxy migration:
(j) Lopez-Herrera, F. J.; Sarabia-Garcia F. Tetrahedron Lett. 1994, 35,
6705-6708. (k) Lopez-Herrera, F. J.; Sarabia-Garcia, F. Tetrahedron 1997,
53, 3325-3346.
10.1021/jo0605391 CCC: $33.50 © 2006 American Chemical Society
Published on Web 06/27/2006
J. Org. Chem. 2006, 71, 5789-5791
5789

TABLE 1. Rh2(OAc)4-Catalyzed Reaction of 1a-fa
TABLE 2. Rh2(OAc)4-catalyzed Reaction of 2a-ea
entry
R
ratio (4:5)b
yieldc (%)
entry
R
ratio (6:7)b
yieldc (%)
1
2
3
4
5
6f
Me
iPr
17:83
91:9
100:0
15:85
38:62
73:27
74
90
91
1
2
3
4
5
Me
iPr
0:100
65:35
100:0
2:98
87
53
85
68
97d
tBu
Ph
tBu
Ph
81
2-ClC6H4
2,6-Cl2C6H3
93d,e
56
2-ClC6H4
68:32
a If not specifically noted, the reaction was carried out in CH2Cl2 at room
temperature with 0.5 mol % Rh2(OAc)4. b Ratio was determined by 1H NMR
(400 MHz) of the crude product. c If not specifically noted, refer to the
isolated yield of the major products, which have Z configurations. d Both
(Z)- and (E)-isomers of 5e were isolated. e Refer to the combined yield of
the two products. f The reaction was carried out in refluxing 1,2-dichloro-
ethane.
a The reaction was carried out in CH2Cl2 at room temperature with 0.5
mol % Rh2(OAc)4. b Ratio was determined by 1H NMR (400 MHz) of the
crude product. c If not specifically noted, refer to the isolated yield of the
major product. d Refer to the combined yield of the two products.
TABLE 3. Rh2(OAc)4-Catalyzed Reaction of 3a-ea
With these diazo compounds in hand, we then proceeded to
examine the diazo decomposition under Rh2(OAc)4-catalyzed
conditions. First, diazo compounds 1a-f were investigated. The
diazo decomposition occurred smoothly with Rh2(OAc)4 in
CH2Cl2 at room temperature for 1a-e to afford the 1,2-
migration products in excellent yields (Table 1, entries 1-5).
For 1f, the Rh2(OAc)4-catalyzed reaction at room temperature
was sluggish, presumably due to the steric hindrance around
the diazo group which blocks the approach of Rh(II) catalyst.
A similar problem has been encountered in our previous
studies.4b To substantiate this reasoning, 1f was treated with
catalytic TsOH with the expectation that protonic acid-catalyzed
diazo decomposition should not be sensitive to steric hindrance.
However, the reaction only gave a complex mixture under these
conditions. On the other hand, the Rh(II)-catalyzed reaction of
1f occurred efficiently in refluxing 1,2-dichloroethane (Table
1, entry 6).
entry
3a-e, SiRR′R′′
ratio (8:9)b
yieldc (%)
1
2
3
4
5
3a, SiMe3
11:89
10:90
9:91
9:91
4:96
57d
72d
79
3b, SiMe2Ph
t
3c, SiMe2 Bu
3d, SiEt3
82d
92
3e, Si tBuPh2
a The reaction was carried out in CH2Cl2 at room temperature with 0.5
mol % Rh2(OAc)4. b Ratio was determined by 1H NMR (300 MHz) of the
crude product. c Refer to the isolated yield of the major product. d The
corresponding enol was obtained after separation with silica gel column.
3). For the series of 1d-f, the trend is similar, although the
product ratio change is less sensitive than that for 1a-c.
Next, the Rh2(OAc)4-catalyzed reaction of 2a-e was inves-
tigated (Table 2). Our previous study has demonstrated that
Cl3CC(O)NH is more efficient than TsNH in promoting 1,2-
aryl migration.4b,d As shown in Scheme 1, only the 1,2-phenyl
migration product is observed in the case of ethyl diazoester.
In the Rh2(OAc)4-catalyzed reaction of 2a-e, a similar change
of migratory aptitude was observed, as shown by the data
collected in Table 2. Again, the steric bulkiness effectively
overrides the electronic effects.
Finally, the steric effect of â substituent on the 1,2-migration
was investigated through Rh2(OAc)4-catalyzed reaction of 3a-e
(Table 3). The previous study has demonstrated that 1,2-H
migration predominates when the bystander is an electron-
donating group, such as hydroxyl or alkyl group.4 In the reaction
of 3a, in which the â substituent is trimethylsiloxy, the ratio of
1,2-phenyl migration vs 1,2-hydride migration is 8:1 (Table 3,
entry 1). Since a siloxy substituent is considered an electron-
donating group in general, this result is not in accordance with
the previous observations. Here, the steric bulkiness of the
trimethylsiloxy group might be more determinant in affecting
the migratory aptitude. This rationalization is supported by the
Rh2(OAc)4-catalyzed reaction of 3b-e, in which the size of
siloxy substituent is gradually increased. As shown by the data
in Table 3, more 1,2-phenyl migration is observed when the
â-siloxy substituent becomes bulkier.
The configuration of the products could be easily identified
1
as indicated by H NMR the existence of hydrogen bonding
between N-H and the carbonyl group. In most cases, only (Z)-
isomers were obtained for both 1,2-H and 1,2-Ph migrations.
Our previous study has shown that the NHTs group switches
the 1,2-migration from 1,2-H to 1,2-Ph. In the case of 1a, in
which the R is a relatively small methyl group, the 1,2-Ph
migration predominates (Table 1, entry 1). When R is switched
i
to a bulkier Pr group, the ratio is completely reversed (Table
1, entry 2). When the size of R is further increased to tBu, only
the 1,2-H migration product could be obtained (Table 1, entry
(3) For reviews, see: (a) Nickon, A. Acc. Chem. Res. 1993, 26, 84-89.
(b) Liu, M. T. H. Acc. Chem. Res. 1994, 27, 287-294. For recent study,
see: (c) Farlow, R. A.; Thamattoor, D. M.; Sunoj, R. B.; Hadad, C. M. J.
Org. Chem. 2002, 67, 3257-3265. (d) Keating, A. E.; Garcia-Garibay, M.
A.; Houk, K. N. J. Phys. Chem. A 1998, 102, 8467-8476.
(4) (a) Jiang, N.; Qu, Z.; Wang, J. Org. Lett. 2001, 3, 2989-2992. (b)
Jiang, N.; Ma, Z.; Qu, Z.; Xing, X.; Xie, L.; Wang, J. J. Org Chem. 2003,
68, 893-900. (c) Shi, W.; Jiang, N.; Zhang, S.; Wu, W.; Du, D.; Wang, J.
Org. Lett. 2003, 5, 2243-2246. (d) Shi, W.; Xiao, F.; Wang, J. J. Org.
Chem. 2005, 70, 4318-4322. (e) Xu, F.; Shi, W.; Wang, J. J. Org. Chem.
2005, 70, 4191-4194.
(5) For the experimental details of the synthesis of 1a-f, 2a-e, and
3a-e and the characterization data for new compounds, see Supporting
Information. (a) Jiang, N.; Wang, J. Tetrahedron Lett. 2002, 43, 1285-
1287. (b) Bardot, V.; Gelas-Mialhe, Y.; Gramain, J.-C.; Remuson, R.
Tetrahedron: Asymmetry 1997, 8, 1111-1114.
5790 J. Org. Chem., Vol. 71, No. 15, 2006

SCHEME 2
proceeds through a rather late transition state, as discussed in
our previous study.4d The transition states resemble the olefin
products with the Rh(II) catalyst being departed to a considerable
extent. Thus, the migratory aptitude is largely governed by the
stability of the products. The steric bulkier group in the
migrating origin will obviously favor a large group, such as
the phenyl group, to migrate, because the steric hindrance can
thus be relieved. On the other hand, when there is a bulkier
group attached to the carbene carbon, a small group (H) will
migrate in order to relieve steric congestion in the product.
In conclusion, we have demonstrated that, in addition to
electronic factors, steric factors play an even more important
role in affecting the 1,2-migratory aptitude in rhodium(II)-
carbene reactions.7 These results provide useful information in
understanding the mechanism of 1,2-migration reaction of
rhodium(II) carbene.
Experimental Section
The above results clearly demonstrate that steric effects play
an important role in affecting the migratory aptitude. In general,
the steric effects can override the electronic effects of the
bystander group. As previously proposed, the 1,2-migration
proceeds through the overlap of the migrating bond with the
empty p orbital of the carbene center.4b,d The migration is
supposed to be a concerted process with a transition state shown
in Scheme 1. In the transition state, there is a positive charge
development in the migrating origin. This explains the electronic
effects of the bystander group. More positive charge is
developed in 1,2-H migration than that in 1,2-phenyl migration.6
Thus, an electron-donating bystander group that is attached to
the migrating origin will promote 1,2-H migration, while an
electron-withdrawing group, such as Cl3CCONH, will work in
an opposite way.
Caution! Diazo compounds are generally toxic and potentially
explosive. They should be handled with care in a well-ventilated
fume hood.
General Procedure for Rh2(OAc)4-Catalyzed Reactions of
1a-f, 2a-e, and 3a-e. To a stirring solution of anhydrous CH2Cl2
(10 mL) containing Rh2(OAc)4 (0.5 mol %) was added the diazo
compound 1a-f, 2a-e, or 3a-e (1.0 mmol). The reaction mixture
was stirred at room temperature. When the diazo compound
disappeared as indicated by thin-layer chromatography, solvent was
removed under reduced pressure to give a crude residue, which
was purified by silica gel column chromatography.
Acknowledgment. The project is generously supported by
Natural Science Foundation of China (Grant Nos. 20572002,
20521202, 20225205, and 20390050) and the Ministry of
Education of China (Cheung Kong Scholars Program).
For the migration to occur, the migrating bond needs to be
parallel to the p orbital of the carbene carbon. The conformations
10 and 11 that will lead to 1,2-H or 1,2-Ph migration are
depicted in Scheme 2. However, it is difficult to get a clear
insight into the substituent effect on migratory aptitude through
conformation 10 or 11. The steric effect observed in this study
can be simply rationalized by assuming that the 1,2-migration
Supporting Information Available: Experimental details,
including the preparation of the diazo compounds and the charac-
terization data for all new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
JO0605391
(7) For other examples in which the 1,2-migration is affected by steric/
conformational factors, see: (a) Vitale, M.; Lecourt, T.; Sheldon, C. G.;
Aggarwal, V. K. J. Am. Chem. Soc. 2006, 128, 2524-2525. (b) Aggarwal,
V. K.; Sheldon, C. G.; Macdonald, G. J.; Martin, W. P. J. Am. Chem. Soc.
2002, 124, 10300-10301.
(6) Keating, A. E.; Garcia-Garibay, M. A.; Houk, K. N. J. Phys. Chem.
A 1998, 102, 8467-8476.
J. Org. Chem, Vol. 71, No. 15, 2006 5791