A kinetic study on the pairwise competition reaction of α-diazo esters with rhodium(II) catalysts: Implication for the mechanism of Rh(II)-carbene transfer

Article abstract of DOI:10.1021/jo0107352

The relative rate constants for the Rh(II)-mediated diazo decomposition of a series of para- or mete-substituted diazophenylacetates were measured through intermolecular competition. The kinetic data were further subjected to Hammett correlation analysis

Full text of DOI:10.1021/jo0107352

J . Org. Chem. 2001, 66, 8139-8144
8139
A Kin etic Stu d y on th e P a ir w ise Com p etition Rea ction of r-Dia zo
Ester s w ith Rh od iu m (II) Ca ta lysts: Im p lica tion for th e Mech a n ism
of Rh (II)-Ca r ben e Tr a n sfer
Zhaohui Qu, Weifeng Shi, and J ianbo Wang*
Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education,
Department of Chemical Biology, College of Chemistry, Peking University, Beijing 100871, China
wangjb@pku.edu.cn
Received J uly 23, 2001
The relative rate constants for the Rh(II)-mediated diazo decomposition of a series of para- or meta-
substituted diazophenylacetates were measured through intermolecular competition. The kinetic
data were further subjected to Hammett correlation analysis and were found to have better linear
correlation with σ+. Reaction constants for four Rh(II) catalysts have been obtained, Rh2(OAc)4
(F ) -1.29 with σ+, r ) -0.99), Rh2(Ooct)4 (F ) -1.31 with σ+, r ) -0.99), Rh2(acam)4 (F ) -1.18
with σ+, r ) -0.99), Rh2(O2CCF3)4 (F ) -1.46 with σ+, r ) -0.99). The mechanistic implications of
these data are discussed.
In tr od u ction
or no information could be obtained concerning the
interaction of the diazo substrate with the transition
metal catalyst from those studies.
Reactions of rhodium(II) complex mediated carbenoids
have gained wide application in synthetic organic chem-
istry in the past decades.1 Recent advances in asymmetric
cyclopropanation, C-H bond insertion, and ylide reac-
tions with chiral Rh(II) complex have reached a high level
of enantiocontrol.1,2 Along with the development in
synthesis, the reaction mechanism of the the transforma-
tion of these carbenoids, especially the nature of the
Rh(II) carbene intermediate, has aroused considerable
interest. Previous investigation through intra- and inter-
molecular competition reactions has shown that Rh(II)
carbene is electrophilic in nature,3 while the ligands of
the Rh(II) catalyst and the structure of the diazo com-
pounds can significantly affect the reaction selectivity.3,4
However, these mechanistic investigations were mainly
carried out through the analysis of the final product
distribution, which gave only the information on the
reaction between metal carbene and the substrate. Little
Direct mechanistic investigation on the Rh(II)-cata-
lyzed diazo decomposition process is sparse.5 Pirrung and
Morehead have recently demonstrated the application of
saturation kinetics method in the study of diazo decom-
position, from which informative kinetic parameters have
been obtained.6 In this paper, we describe our study on
the diazo decomposition mechanism based on the linear
free energy correlation analysis of the electronic effects
of the Rh(II)-catalyzed diazo decomposition process.
Resu lts a n d Discu ssion
Our approach is to measure the relative rate constants
of the diazo decomposition of a series of para- or meta-
substituted diazo phenylacetates 1a -g. These diazo
esters are employed in this investigation because the
substituent in the phenyl ring can effectively interact
with the reaction center through the relatively polariz-
able aromatic π-electron system. Since steric and con-
formational factors are minimized, pure electronic effects
can be evaluated. The diazo compounds could be easily
prepared by conventional diazo transfer reaction with
TsN3 and DBU as base.
(1) For reviews, see: (a) Doyle, M. P.; McKervey, M. A.; Ye, T.
Modern Catalytic Methods for Organic Synthesis with Diazo Com-
pounds; Wiley-Interscience: New York, 1998. (b) Ye, T.; McKervey,
M. A. Chem. Rev. 1994, 44, 1091. (c) Padwa, A.; Austin, D. J . Angew.
Chem., Int. Ed. Engl. 1994, 33, 1797. (d) Padwa, A.; Krumpe, K. E.
Tetrahedron 1992, 48, 5385. (e) Adams, J .; Spero, D. M. Tetrahedron
1991, 47, 1765. (f) Doyle, M. P. In Homogeneous Transition Metal
Catalysts in Organic Synthesis; Moser, W. R., Slocum, D. W., Eds.;
ACS Advanced Chemistry Series 230; American Chemical Society:
Washington, DC, 1992. (g) Taber, D. F. In Comprehensive Organic
Synthesis; Pattenden, G., Ed.; Pergamon Press: Oxford, 1991; Vol. 3,
p 1045. (h) Doyle, M. P. Chem. Rev. 1986, 86, 919. (I) Padwa, A. J .
Organomet. Chem. 2001, 617-618, 3.
(2) For reviews, see: (a) Doyle, M. P.; Forbes, D. C. Chem. Rev. 1998,
98, 911. (b) Davies, H. M. L.; Antoulinakis, E. G. J . Organomet. Chem.
2001, 617-618, 47. (c) Timmons, D. J .; Doyle, M. P. J . Organomet.
Chem. 2001, 617-618, 98. (d) Hodgson, D. M.; Pierard, F. Y. T. M.;
Stupple, P. A. Chem. Soc. Rev. 2001, 30, 50.
(3) (a) Taber, D. F.; Ruckle, R. E., J r. J . Am. Chem. Soc. 1986, 108,
7686. (b) 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. (c) Wang, P.; Adams, J . J . Am. Chem. Soc. 1994, 116, 3296.
(d) Pirrung, M. C.; Morehead, A. T., J r. J . Am. Chem. Soc. 1994, 116,
8991. (e) Taber, D. F.; Song, Y. J . Org. Chem. 1996, 61, 6706. (f) Doyle,
M. P.; Kalinin, A. V.; Ene, D. G. J . Am. Chem. Soc. 1996, 118, 8837.
(g) Taber, D. F.; You, K. K.; Rheingold, A. L. J . Am. Chem. Soc. 1996,
118, 547. (h) Wang, J .; Chen, B.; Bao, J . J . Org. Chem. 1998, 63, 1853.
In 1952, Yates proposed a mechanism for the diazo
decomposition catalyzed by Cu catalyst.7 For Rh(II)-
catalyzed reaction, the previous investigation has shown
that the most likely model of the diazo decomposition
process involves the initial complexation of the negatively
polarized carbon of the diazo compound to the axial site
of the Rh(II) catalyst, which is coordinatively unsaturated
10.1021/jo0107352 CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/26/2001

8140 J . Org. Chem., Vol. 66, No. 24, 2001
Qu et al.
Sch em e 1
Sch em e 2
(Scheme 1).6 Subsequent irreversible extrusion of N2 from
the intermediate 2 generates the Rh(II) carbene inter-
mediate 3. In this model, if the first step is a fast
equilibrium and the second step is slow and rate limiting,
as suggested by a previous study,6 then the overall
rate-limiting step.11 In the diazo compounds, the carbon
bearing the diazo group is negatively polarized, while the
same carbon in the resulting Rh(II) carbene is positively
polarized. An electron-withdrawing group like NO2 will
stabilize the diazo compound but destabilize the Rh(II)
carbene intermediate. An electron-donating group will
operate in the opposite manner. Thus, the diazo com-
pounds bearing electron-withdrawing substituents will
decompose more slowly than those bearing electron-
donating substituents. Consequently, we should be able
to observe significant electronic effects.
observed rate of the diazo decomposition will be kobs
)
Kkcat[Rh(II)], where [Rh(II)] is the concentration of the
catalyst.8 In a pairwise intermolecular competition in
which a substituted phenyldiazoacetate 1a -c,e-g and
diazo phenylacetate 1d compete for the Rh(II) catalyst
in the same reaction solution, the catalyst concentration
is the same for the two diazo substrates and is assumed
to keep constant within the period that the kinetic
measurement is performed.9 Thus, the relative rate
constant for a substituted diazo phenylacetate to the
diazo phenylacetate 1d can be calculated as follows.10
Since the Rh(II) carbene intermediate is highly reactive
and the subsequent reaction is fast, to perform a kinetic
measurement it will be necessary to choose a suitable
clean carbenoid reaction to complete a catalytic cycle. We
initially used styrene to trap the Rh(II) carbene inter-
mediate. However, the diazo decomposition of p-nitro-
phenyldiazoacetate 1g in the presence of excess styrene
gave exclusively coupling product 6 (Scheme 2). There-
fore, cyclopropanation is not suitable for our purpose.
After several screenings, we were delighted to find Si-H
insertion12 is a highly efficient process and give clean
insertion products for all the diazo substrates to be used
in the kinetic study.
krel ) kobs(X)/kobs(H) ) (KX/KH)[kcat(X)/kcat(H)
]
(1)
The Hammett equation can be expressed in the fol-
lowing way.
log [kobs(X)/kobs(H)] ) log [(KX/KH)(kcat(X)/kcat(H))] )
F1σ + F2σ (2)
Therefore, the measured electronic effects actually
involve both the preequilibrium step and the following
(9) Although the Rh(II) catalyst has a very high turnover number
when decomposing diazo compounds, the catalyst itself does decompose
and lose activity as the reaction proceeds. Our experiment showed that
if the reaction is completed within a relatively short period of time (in
60 min) the catalyst does not show a significant lose of activity.
(10) Panek et al. have recently reported an investigation of a
competitive Si-H insertion of a series of para-substituted methyl
aryldiazoacetates catalyzed by chiral Cu(I) complex. In the Hammett
correlation, the kX/kH ratio was simply determined by the relative
product yields. This situation could only be valid when the reaction is
zero-order or pseudo zero-order both to the diazo substrate and to the
silane. A reaction constant of -1.12 with σ was obtained. See: Dakin,
L. A.; Ong, P. C.; Panek, J . S.; Steples, R. J .; Stavropoulos, P.
Organometallics 2000, 19, 2896.
(11) For an example of Hammett analysis in multistep reactions,
see: Miller, L. L.; Watkins, B. F. J . Am. Chem. Soc. 1976, 98, 1515.
(12) For example of Rh(II) carbene Si-H insertion, see: (a) Bagheri,
V.; Doyle, M. P.; Taunton, J .; Claxton, L. E. J . Org. Chem. 1988, 53,
6158. (b) Landais, Y.; Parra-Rapado, L.; Planchenault, D.; Weber, V.
Tetrahedron Lett. 1997, 38, 229.
(4) (a) Padwa, A.; Austin, D. J .; Hornbuckle, S. F.; Semones, M. A.;
Doyle, M. P.; Protopopova, M. N. J . Am. Chem. Soc. 1992, 114, 1874.
(b) 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. (c) Wee, A. G. H.; Yu, Q. J . Org. Chem. 1997, 62,
3324. (d) Padwa, A.; Austin, D. J .; Hornbuckle, S. F. J . Org. Chem.
1996, 61, 63. (e) Doyle, M. P.; Dyatkin, A. B. J . Org. Chem. 1995, 60,
3035. (f) Davies, H. M. L.; Panaro, S. A. Tetrahedron 2000, 56, 4871.
(g) Estevan, F.; Lahuerta, P.; Perez-Prieto, J .; Sanau, M.; Stiriba, S.-
E.; Ubeda, M. A. Organometallics 1997, 16, 880.
(5) (a) Aniciaux, A. J .; Hubert, A. J .; Noels, A. F.; Petiniot, N.;
Teyssie, P. J . Org. Chem. 1980, 45, 695. (b) Alonso, M. E.; Garcia, M.
Del C. Tetrahedron 1989, 45, 69.
(6) Pirrung, M. C.; Morehead, A. T., J r. J . Am. Chem. Soc. 1996,
118, 8162.
(7) Yates, P. J . Am. Chem. Soc. 1952, 74, 5376.
(8) A detailed kinetic treatment is submitted as Supporting Infor-
mation.

Competition Reaction of R-Diazo Esters with Rhodium(II) Catalysts
J . Org. Chem., Vol. 66, No. 24, 2001 8141
Ta ble 1. Rela tive Ra te Con sta n ts of Dia zo
Decom p osition a
Ta ble 2. Cor r ela tion of log kr el w ith Su bstitu en t
Con sta n tsa
F (r)b
b
krel
subst
constants Rh2(OAc)4
Rh2(Ooct)4
Rh2(acam)4
Rh2(tfa)4
entry
X
Rh2(OAc)4 Rh2(Ooct)4 Rh2(acam)4 Rh2(tfa)4
σ+
σ
-1.29 (-0.99) -1.31 (-0.99) -1.18 (-0.99) -1.46 (-0.99)
-1.72 (-0.94) -1.69 (-0.92) -1.60 (-0.96) -1.94 (-0.94)
-1.11 (-0.91) -1.07 (-0.87) -1.03 (-0.93) -1.23 (-0.89)
1
2
3
4
5
6
7
p-MeO
p-Ph
m-Me
H
m-MeO
p-Br
p-NO2
10.5
1.35
12.3
1.73
7.58
1.65
1.25
1.00
1.12
0.74
0.099
16.4
1.32
σ-
1.20
1.00
1.00
0.84
0.084
1.19
1.00
0.62
0.90
0.102
1.54
1.00
0.98
0.65
0.072
a
b
Substituent constants were taken from ref 15. F ) reaction
constant, r ) correlation coefficient.
a
Experiments were preformed in CH2Cl2 at 0 °C, except when
Rh2(acam)4 was the catalyst, in which case the reaction was run
b
at room temperature. Error limits are less than 5% with average
deviation of two runs.
The pairwise competition kinetic measurement was
performed by following the decrease of the diazo sub-
strate concentration with HPLC.13 Thus, a substituted
diazo compound (1, X) p-NO2, p-Br, m-OMe, p-Ph, m-Me,
p-OMe) together with the phenyldiazoacetate (1d , X )
H) was dissolved in CH2Cl2 at 0 °C or at room temper-
ature, followed by the addition of a solution of the Rh(II)
catalyst dissolved in CH2Cl2 or chlorobenzene. Aliquots
were removed from the reaction solution at 5 min
intervals. To terminate the diazo decomposition before
analysis, the catalyst was either removed by a fast silica
gel column [for Rh2(OAc)4] or was blocked by adding
pyridine [for Rh2(acam)4, Rh2(Ooct)4, Rh2(O2CCF3)4], which
can strongly bind to the catalytic sites.14 The aliquots
thus treated were immediately subjected to HPLC analy-
sis to determine the concentration of the diazo substrates.
Kinetic plots of the data have revealed that the rate
of the diazo compound consumption does obey the first-
order kinetics in the concentration of diazo substrates.
This is consistent with the mechanism shown in Scheme
1.
The relative rate constant calculated according to eq
1 is summarized in Table 1. As shown by the kinetic data,
the diazo substrates with electron-donating substituents
do decompose faster than those with electron-withdraw-
ing substituents. For example, when Rh2(O2CCF3)4 is the
catalyst, p-methoxyphenyldoazoacetate 1a decomposes
226 times faster than p-nitrophenyldiazoacetate 1g.
Davies and Panaro have recently compared the chemo-
selectivity of p-methoxyphenyldiazoacetate 1a and
phenyldiazoacetate 1d in Rh(II)-mediated carbenoid
reactions. They observed that the diazoacetate with the
most electron-donating functionality gave the highest
chemoselectivity.4f Therefore, we can conclude here that
the fastest reaction is actually most selective, because
the step to generate the Rh(II) carbene intermediate is
the rate-limiting step in overall transformation. The more
stable Rh(II) carbene, which is consequently more
chemoselective, will form faster.
F igu r e 1. Plot of log krel against σ+ for the reaction with Rh2-
(OAc)4.
there is a difference in rates between the catalysts. The
diazo decomposition is faster with Rh2(O2CCF3)4 than
with Rh2(OAc)4 and then with Rh2(acam)4. It is known
that the carbenoids generated from Rh2(O2CCF3)4 are less
stable and more reactive,3 but they formed faster. A
reasonable explanation for this superficial contradiction
with the above-mentioned substituent effects is that for
Rh2(O2CCF3)4, the complexation constant is larger than
that for Rh2(OAc)4 or Rh2(acam)4, because trifluoroacetate
ligand is more electron-withdrawing. Although the sub-
sequent nitrogen extrusion reaction maybe slower due
to the formation of a less stable carbenoid, the overall
rate constant of the diazo decomposition is larger.
The relative rate constants were further applied to the
Hammett equation. The results are summarized in Table
2 and in Hammett plots (Figures1-4, with σ+ only). In
all cases, we found better correlation with σ+ than with
σ or σ- was obtained. As expected, the Hammett treat-
ment gives the negative value of the reaction constants.
As shown by eq 2, the observed reaction constant F is
the sum for the two individual steps. In the preequilib-
rium step, an electron-donating group should push the
equilibrium to the right, since the partial negative charge
will disappear when the substrate goes to complex with
the catalyst. Therefore, F1 should be negative. Because
partial negative charge is involved in this equilibrium,
F1 may have better correlation with σ- or σ, rather than
with σ+. In the second step, partial positive charge
develops in the transition state, so the electron-donating
group should speed up the nitrogen extrusion and F2 is
again negative. Although in multistep reactions like this
the composition nature of F means that it may not give
a direct measure of electronic effects on the transition
state in the rate-limiting step, the observed overall F of
Although the absolute rate constants are not measured
in this study, it is apparent from the experiments that
(13) (a) We initially attempted to follow the reaction by 1H NMR
(400 MHz). However, we found it did not give well reproducible results,
probably due to the difficulty in determining the time of precise
intervals when collecting the data, and also due to the error of the
NMR integration in the spectra. (b) We also tried to measure the
absolute rate constants by following the consumption of a single diazo
compound. But for some reason that is still unclear, the measurement
did not give reproducible data.
(14) The first and second equilibrium constants for binding of
pyridine to rhodium(II) butyrate are 1.6 × 108 and 2.4 × 104; see:
Drago, R. S.; Long, J . R.; Cosmano, R. Inorg. Chem. 1981, 20, 2920.

8142 J . Org. Chem., Vol. 66, No. 24, 2001
Qu et al.
F igu r e 2. Plot of log krel against σ+ for the reaction with Rh2-
(Ooct)4.
F igu r e 4. Plot of log krel against σ+ for the reaction with Rh2-
(tfa)4.
decomposition with this catalyst gave an appreciably
larger magnitude of reaction constant (F ) -1.46 with
σ+, r ) -0.99), which suggests more positive charge build-
up in the Rh(II) carbene intermediate. This is consistent
with the argument that electron-withdrawing ligand like
trifluoroacetate will have little back-bonding. On the
other hand, the Rh2(acam)4, in which the ligand is
electron-donating, is just in the opposite (F ) -1.18 with
σ+, r ) -0.99). Rh2(OAc)4 has a reaction constants in
between (F ) -1.29 with σ+, r ) -0.99). As expected,
rhodium octanoate Rh2(Ooct)4 has a reaction constant
close to that of Rh2(OAc)4 (F ) -1.31 with σ+, r ) -0.99).
There has been considerable controversy concerning
the structure of the Rh(II) carbene intermediate. Doyle
proposed that the Rh(II) carbene is more likely to be a
metal-stabilized cation,3b while Pirrung suggests the
importance of back-bonding in the Rh(II)-carbon bond.3d
A theoretical calculation favors neither the structure of
Rh(II) carbene nor a metal-stabilized cation, instead a
neutral or negatively charged rhodium bond carbon is
suggested.17 However, there has been little direct experi-
mental evidence to clarify the exact nature of the Rh(II)
carbene structure.18 The electronic effect analysis in the
present study do provide some experimental evidence
favoring the partial positive charge buildup in the Rh-
(II) carbene.
F igu r e 3. Plot of log krel against σ+ for the reaction with Rh2-
(acam)4.
better correlation with σ+ may suggest that F2 is domi-
nant. That means there are little electronic effects in the
preequilibrium step; the significant electronic effects of
the overall reaction may largely due to the second step,
which is closely related to the nature of the Rh(II)
carbene intermediate.
Better correlation with σ+ than with σ or σ- suggests
the involvement of positive charge or partial positive
charge in the mechanism. Since the polarity of the
reaction center is reversed, one might expect larger
magnitude of the reaction constants. The moderately
large reaction constant values obtained from this study
indicate that either the positive charge buildup in the
Rh(II) carbene is only moderate, or the nitrogen extrusion
step has a relatively earlier transition state.
It has been known that the ligands of the Rh(II)
catalysts have remarkable effect on the reaction selectiv-
ity.4 This ligand effect has been explained being due to
their electron-donating or -withdrawing ability. The
Rh2(O2CCF3)4, which has a strong electron-withdrawing
ligand, has been found in many cases behave quite
differently with Rh2(OAc)4 or Rh2(acam)4.4,16 The diazo
In conclusion, we have carried out the first Hammett
correlation study on the Rh(II) complex catalyzed diazo
carbonyl decomposition and obtained quantitative data
on the electronic effects. The results provide useful
insights into the mechanism of the decomposition process,
as well as the nature of the Rh(II) carbene intermediate.
The limitation of the current analysis is that the overall
electronic effects involve two individual steps. The effort
(15) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165.
(16) For a recent example, see: Moniz, G. A.; Wood, J . L. J . Am.
Chem. Soc. 2001, 123, 5095.
(17) Sheehan, S. M.; Padwa, A.; Snyder, J . P. Tetrahedron Lett. 1998,
39, 949.
(18) White and Hrnciar recently reported an unusual Rh(II) car-
benoid reaction, which implies a zwitterionic mechanism; see: White,
J . D.; Hrnciar, P. J . Org. Chem. 1999, 64, 7271. Also see: Clark, J . S.;
Dossetter, A. G.; Russell, C. A.; Whitingham, W. G. J . Org. Chem. 1997,
62, 4910.

Competition Reaction of R-Diazo Esters with Rhodium(II) Catalysts
J . Org. Chem., Vol. 66, No. 24, 2001 8143
column chromatography with petroleum ether/EtOAc (30:1)
to give pure compounds 4a -g for analysis.
to obtain the reaction constants for both steps is currently
underway, and the results will be reported in due course.
Meth yl r-tr ieth ylsilyl-p-m eth oxyph en ylacetate 4a: 78%;
Rf ) 0.37 (petroleum ether/ethyl acetate ) 30:1); 1H NMR (200
MHz, CDCl3) δ 0.52-0.64 (m, 6H), 0.86-0.94 (m, 9H), 3.46 (s,
1H), 3.66 (s, 3H), 3,78 (s, 3H), 6.82 (d, J ) 8 Hz, 2H), 7.25 (d,
J ) 8 Hz, 2H); 13C NMR (50 MHz, CDCl3) δ 2.71, 7.02, 41.62,
51.24, 55.17, 113.56, 128.54, 129.42, 157.62, 173.45; IR 2953,
2877, 1723, 1511, 1247, 1148 cm-1; MS (m/z, relative intensity)
294 (M+, 12), 148 (100), 120 (16), 87 (25), 59 (21); HRMS calcd
for C16H26O3Si 294.1651, found 294.1639.
Meth yl r-tr ieth ylsilyl p-p h en ylp h en yla ceta te 4b: 81%;
Rf ) 0.31 (petroleum ether/ethyl acetate ) 30:1); 1H NMR (200
MHz, CDCl3) δ 0.56-0.68 (m, 6H), 0.89-0.96 (m, 9H), 3.57 (s,
1H), 3.69 (s, 3H), 7.24-7.60 (m, 9H); 13C NMR (50 MHz,
CDCl3) δ 2.77, 7.04, 42.55, 51.33, 126.77, 126.89, 127.00 128.69,
128.86, 135.75, 138.41, 140.94, 174.6; IR 2952, 2877, 1723,
1487, 1148 cm-1; MS (m/z, relative intensity) 340 (M+, 16), 194
(100), 165 (15), 117 (14), 87 (28), 59 (25); HRMS calcd for
Exp er im en ta l Section
Gen er a l Meth od s. Melting points were determined with
a Yanaco micro melting point apparatus and are uncorrected.
All reactions with air- and moisture-sensitive components were
performed under a nitrogen atmosphere in a flame-dried
reaction flask, and the components were added via syringe.
All solvents were distilled prior to use. CH2Cl2 and CH3CN
were freshly distilled from CaH2 before use. For chromatog-
raphy, 100-200 mesh silica gel (Qindao, China) was employed.
1H and 13C NMR spectra were recorded at 200 and 50 MHz
with a Varian Mercury 200 spectrometer or at 400 and 100.6
MHz with a Bruker ARX400 spectrometer. Chemical shifts are
reported in ppm using tetramethylsilane as internal standard.
IR spectra were recorded with a Nicolet 5MX-S infrared
spectrometer. Mass spectra were obtained on a VG ZAB-HS
mass spectrometer. Rhodium(II) acetate, rhodium(II) trifluo-
roacetate and rhodium(II) octanoate were purchased from
Aldrich. Rhodium(II) acetamide was prepared according to the
literature procedure.19 HPLC analysis was performed on an
HP 1100 apparatus.
Gen er a l P r oced u r e for th e Syn th esis of Meth yl Ar yl-
d ia zoa ceta te.20 To a solution of methyl aryl acetate (10 mmol)
in anhydrous CH3CN (20 mL) was added 1,8-diazabicyclo-
[5.4.0]undec-7-ene (DBU) (15 mmol). A solution of p-toluene-
sulfonyl azide (TsN3) (12 mmol) in anhydrous CH3CN (10 mL)
was added dropwise at room temperature. The solution was
stirred at room temperature overnight. Solvent was evapo-
rated, and 5% aqueous KOH solution (∼30 mL) was added to
the residue. The mixture was extracted with ether (3 × 30
mL) and the combined ethereal layer washed with H2O, brine,
and then dried over anhydrous MgSO4. Column chromatog-
raphy with silica gel provided the desired diazo compounds
1a -g.
C
21H28O2Si 340.1858, found 340.1855.
Meth yl r-tr ieth ylsilyl m -m eth ylp h en yla ceta te 4c: 83%;
Rf ) 0.40 (petroleum ether/ethyl acetate ) 30:1); 1H NMR (200
MHz, CDCl3) δ 0.52-0.64 (m, 6H), 0.87-0.94 (m, 9H), 2.32 (s,
3H), 3.48 (s, 1H), 3.66 (s, 3H), 6.95-6.99 (m, 1H), 7.13-7.16
(m, 3H); 13C NMR (50 MHz, CDCl3) δ 2.67, 6.98, 21.46, 41.61,
51.21, 125.52, 126.27, 127.91, 129.12, 136.36, 137.53, 173.64;
IR 2953, 2877, 1725, 1605, 1458, 1146 cm-1; MS (m/z, relative
intensity) 278 (M+, 12), 132 (100), 117 (22), 87 (26), 57 (23);
HRMS calcd for C16H26O2Si 278.1702, found 278.1705.
Meth yl r-tr ieth ylsilyl p h en yla ceta te 4d : 89%; Rf ) 0.54
(petroleum ether/ethyl acetate ) 30:1); 1H NMR (200 MHz,
CDCl3) δ 0.52-0.65 (m, 6H), 0.86-0.94 (m, 9H), 3.52 (s, 1H),
3.67 (s, 3H), 7.15-7.38 (m, 5H); 13C NMR (50 MHz, CDCl3) δ
2.70, 7.00, 42.80, 51.30, 125.55, 128.10, 128.28, 136.58, 173.64;
IR 2953, 2878, 1725, 1496, 1454, 1433, 1147 cm-1; MS (m/z,
relative intensity) 264 (M+, 18), 118 (100), 117 (41), 87 (56),
59 (42); HRMS calcd for C15H24O2Si 264.1545, found 264.1537.
Meth yl r-tr ieth ylsilyl m -m eth oxyp h en yla ceta te 4e:
78%; Rf ) 0.29 (petroleum ether/ethyl acetate ) 30:1); 1H NMR
(200 MHz, CDCl3) δ 0.53-0.66 (m, 6H), 0.87-0.95 (m, 9H),
3.51 (s, 1H), 3.67 (s, 3H), 3,79 (s, 3H) 6.68-7.25 (m, 4H); 13C
NMR (50 MHz, CDCl3) δ 2.67, 7.02, 42.78, 51.29, 55.11, 111.00,
114.15, 120.97, 128.97, 138.03, 159.39, 173.49; IR 2953, 2877,
1723, 1611, 1511, 1247, 1148 cm-1; MS (m/z, relative intensity)
294 (M+, 14), 148 (100), 117 (18), 87 (28), 59 (21); HRMS calcd
for C16H26O3Si 294.1651, found 294.1639.
Meth yl p-p h en ylp h en yld ia zoa ceta te 1b: 89%; mp 96-
97 °C; Rf ) 0.19 (petroleum ether/ethyl acetate ) 30:1); 1H
NMR (200 MHz, CDCl3) δ 3.88 (s, 3H), 7.33-7.65 (m, 9H); 13
C
NMR (50 MHz, CDCl3) δ 52.00, 124.33, 124.38, 126.88, 127.36,
127.60, 128.82, 138.71, 140.35, 165.59; IR 2080, 1700, 1340,
1240, 1150 cm-1; MS (m/z, relative intensity) 252 (15), 224 (16),
194 (52), 181 (100), 165 (47), 152 (56), 76 (20). Anal. Calcd for
C
15H12N2O2: C, 71.42; H, 4.79; N, 11.10. Found: C, 71.40; H,
4.79; N, 11.07.
Meth yl r-tr ieth ylsilyl p-br om op h en yla ceta te 4f: 82%;
Rf ) 0.44 (petroleum ether/ethyl acetate ) 30:1); 1H NMR (200
MHz, CDCl3) δ 0.51-0.63 (m, 6H), 0.86-0.94 (m, 9H), 3.49 (s,
1H), 3.67 (s, 3H), 7.23 (d, J ) 8 Hz, 2H), 7.39 (d, J ) 8 Hz,
2H); 13C NMR (50 MHz, CDCl3) δ 2.63, 8.98, 42.23, 51.40,
119.36, 130.10, 131.14, 135.71, 173.20; IR 2953, 2878, 1724,
1488, 1151 cm-1; MS (m/z, relative intensity) 342 (M+, 13), 196
(99), 117 (52), 87 (100), 57 (84); HRMS calcd for C15H2379BrO2-
Si 342.0650, found 346.0644.
Meth yl r-tr ieth ylsilyl p-n itr op h en yla ceta te 4g: 83%;
Rf ) 0.15 (petroleum ether/ethyl acetate ) 30:1); 1H NMR (200
MHz, CDCl3) δ 0.53-0.65 (m, 6H), 0.88-0.96 (m, 9H), 3.69 (s,
1H), 3.71 (s, 1H), 7.54 (d, J ) 8 Hz, 2H), 8.14 (d, J ) 8 Hz,
2H); 13C NMR (50 MHz, CDCl3) δ 2.47, 6.83, 43.22, 51.55,
123.22, 128.80, 144.82, 145.74, 172.33; IR 2957, 1735, 1602,
1513, 1346, 1173 cm-1; MS (m/z, relative intensity) 195 (81),
136 (47), 120 (76), 90 (83), 59 (100).
Gen er al P r ocedu r e for th e Kin etic Com petition Stu dy.
A. With Rh 2(OAc)4 a s Ca ta lyst. A three-necked round-
bottom flask was flashed with N2 three times, and then
anhydrous CH2Cl2 (21.6 mL) was introduced. After the solvent
was cooled with an ice bath for 30 min, phenyldiazoacetate
1d (1 mL, 50 mM solution in CH2Cl2) and a meta- or para-
substituted phenyldiazoacetate (1 mL, 50 mM solution in CH2-
Cl2), internal standard biphenyl (1 mL, 50 mM solution in
CH2Cl2), and triethylsilane (0.3 mL) were added. Rh2(OAc)4
(0.1 mL, 1.0 mM solution in chlorobenzene) was then rapidly
introduced, and the time was set as zero. Aliquots of the
reaction solution (1.0 mL) were removed from the flask at 5
Meth yl m -m eth ylp h en yld ia zoa ceta te 1c: 73%; Rf ) 0.34
(petroleum ether/ethyl acetate ) 30:1); 1H NMR (200 MHz,
CDCl3) δ 2.35 (s, 3H), 3.85 (s, 3H), 6.94-7.01 (m, 1H), 7.23-
7.27 (m, 3H); 13C NMR (50 MHz, CDCl3) δ 21.43, 51.79, 63.04,
121.09, 124.53, 125.20, 126.64, 128.73, 138.58, 165.54; IR 2080,
1700, 1590, 1430, 1340, 1240, 1140, 1050 cm-1; MS (m/z,
relative intensity) 190 (M+, 19), 162 (15), 119 (100), 104 (24),
91 (84), 77 (21), 65 (24), 51 (19); HRMS calcd for C10H10N2O2
190.0742, found 190.0742.
Met h yl m -m et h oxyp h en yld ia zoa cet a t e 1e: 76%; Rf )
0.22 (petroleum ether/ethyl acetate ) 30:1). 1H NMR (200
MHz, CDCl3) δ 3.79 (s, 3H), 3.83 (s, 3H), 6.69-7.31 (m, 4H);
13C NMR (50 MHz, CDCl3) δ 51.83, 55.14, 63.37, 109.66,
111.44, 115.91, 126.88, 129.76, 160.06, 165.36; IR 2080, 1700,
1600, 1250, 1040 cm-1; MS (m/z, relative intensity) 206 (M+,
39), 178 (22), 135 (99), 120 (100), 107 (52), 92 (41), 77 (61);
HRMS calcd for C10H10N2O3 206.0691, found 206.0682.
Gen er a l P r oced u r e for Rh 2(OAc)4-Med ia ted Si-H In -
ser tion s. Methyl arydiazoacetate (0.5 mmol) was dissolved in
anhydrous CH2Cl2 (5 mL). To the solution were added triethyl-
silane (0.6 mmol) and Rh2(OAc)4 (0.2 mg, 0.1% mmol). The
solution was stirred at room temperature until the diazo
compound disappeared according to TLC. Solvent was removed
by evaporation, and the residue was subjected to silica gel
(19) Doyle, M. P.; Bagheri, V.; Wandless, T. J .; Ham, N. K.; Brinker,
D. A.; Eagle, L. T.; Loh, K.-L. J . Am. Chem. Soc. 1990, 112, 1906.
(20) Nakano, H.; Ibata, T.Bull. Chem. Soc. J pn. 1995, 68, 1393.

8144 J . Org. Chem., Vol. 66, No. 24, 2001
Qu et al.
min intervals and were immediately subjected to flash silica
gel (5 g) column and eluted with ether (25 mL) or CH2Cl2 (25
mL) to gave catalyst-free solution. Solvent was removed by
evaporation. The residue was dissolved in CH3CN (HPLC
grade, 1.0 mL) and then was analyzed with HPLC.
B. Wit h R h 2(Oct )4, R h 2(a ca m )4, or R h 2(O2CCF 3)4 a s
Ca ta lyst. The reaction system was same as the case when
Rh2(OAc)4 was the catalyst. To remove the catalyst from
aliquots, flash column column chromatography was not effec-
tive for these catalysts. Therefore, pyridine (1.0 mL, 0.4 mM
solution in CH2Cl2) was added to each aliquot to block the
catalyst. Control experiments showed that no appreciable diazo
decomposition was observed for the pyridine-treated aliquots
at 0 °C after 5 h. The aliquots were then similarly analyzed
with HPLC. For Rh2(acam)4, the reaction was run at room
temperature, because the diazo decomposition at 0 °C with
this catalyst was too slow to be practically measured.
The elution was continued with 100% MeCN for 6 min, and
then MeCN was decreased to 70% within 1.5 min. Elution was
continued for 6.5 min with MeCN-H2O (70:30). The retention
time for the diazo compound under these conditions is as
follows: 1a , 5.54 min; 1b, 10.73 min; 1c, 7.53 min; 1d , 5.93
min; 1e, 5.71 min; 1f, 8.78 min; 1g, 5.08 min. Biphenyl has a
retention time of 9.74 min. The diazo compound consumption
could be accurately quantified against the internal standard
biphenyl. The experiment was repeated at least two times until
the deviation was within 5%.
Ack n ow led gm en t. The project is generously sup-
ported by Natural Science Foundation of China (Grant
No. 29972002).
Su p p or t in g In for m a t ion Ava ila b le: 1H NMR and 13C
NMR spectra for compounds 1b,c,e, 4a -g, the kinetic plots,
and the Hammett plots with σ and σ-. This material is
available free of charge via the Internet at http://pubs.acs.org.
HP LC Con d ition s. The diazo compound was analyzed by
HP1100 with a MonoChrom 5 µm C18 column (MetaChem.,
Inc.) and UV detector at 250 nm. After injection, the column
was eluted with MeCN-H2O (70:30) for 6.5 min, and the
MeCN was then increased gradually to 100% within 1.5 min.
J O0107352