Table 2 Intramolecular competition reactions using 0.1 mol% of
catalyst 2a
We have made some attempts to perform intermolecular C–H
amination reactions using 2a. Experiments were conducted using
the highly activated H2NTces (1,1,1-trichloroethylsulfamate ester)
as the nitrogen atom source and diacetoxyiodobenzene as the
oxidant in either dichloromethane or benzene. With several
substrates, 2a can achieve full conversion to the C–H amination
products with catalyst loadings as low as 0.1 mol%, but the yield
of the reaction depends critically on the concentration of the
C–H substrate (see ESI,w Table S1). Unlike with catalyst 1, which
offers superior performance in intermolecular C–H amination, 2a
appears to catalyze a fast background reaction between H2NTces
and PhI(OAc)2, yielding 2,2,2-trichloroethanol and sulfamic
acid, among other unidentified and undesired products. Optimum
yields with catalyst 2 are therefore only obtained when the
reaction is carried out in neat hydrocarbon substrate.
Substrate
Product
%Conv. A : B Ratio
100%
1 : 4
100%
100%
1.4 : 1
In conclusion, Rh2(espn)2Cl (2a) is a new effective and
highly efficient catalyst for intramolecular C–H amination
reactions. This complex is unique because it is a mixed-valent
II,III
1 : 7
Rh2
species when it is introduced to catalytic reactions,
unlike its predecessor Rh2(esp)2 (1), which becomes oxidized
II,III
in situ. The effectiveness of 2a suggests that cationic Rh2
species may be key in designing even better catalysts for C–H
amination. The parallels between 1+ and 2a are striking; given
the indefinite stability of 2a as a mixed-valent dimer, studies
are ongoing in our lab to better understand the mechanistic
groundwork that allows cationic complexes 1+ and 2a to
perform so favorably in the C–H amination transformation.
We thank the Chemical Sciences, Geosciences, and Biosciences
Division, Office of Basic Energy Sciences, Office of Science, U.S.
Department of Energy for support (DE-FG02-10ER16204). We
also wish to acknowledge helpful discussions with Justin Du Bois
and other members of the CCI Center for Selective C–H
Functionalization supported by NSF (CHE-1205646).
Scheme 2 Possible pathways for C–H amination.
Notes and references
catalyst (3, with hp = 2-hydroxypyridinate) has opposite selec-
tivity, and has been the subject of experimental and computa-
tional studies that indicate a two-step mechanism.8
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The two limiting mechanisms for C–H amination via a
dimetal nitrenoid that may be contemplated are shown in
Scheme 2. A concerted, but asynchronous reaction mechanism
has a great deal of experimental and computational support in
Rh2II,II catalyzed carbenoid reactions,1a and Rh2II,II nitrenoid
reactions are proposed to behave similarly.3b Alternatively, a
two-step C–H abstraction/radical recombination mechanism
may be considered that is similar to the classic mechanistic
profile of Cytochrome P450.10 We note that the selectivity of
2a towards S3 and S5 is opposite that of 3, and that we obtain
identical selectivity for S5 to that observed for 1. These
preliminary data suggest that 2a likely utilizes a concerted
asynchronous mechanism as in 1, though the selectivity differ-
ences seen in S4 suggest that the nitrene in 2a is more electron
deficient than the 1-nitrene, consistent with the cationic nature
of the former. We should note, however, that the concerted
mechanism may be considered a special case of the C–H
abstraction/radical rebound mechanism in which the rate
of the radical rebound is infinitely faster than the rate of
abstraction. Further experiments to determine where on the
continuum between these two limiting cases catalyst 2a lies
are needed.
9 L. Villalobos, Z. Cao, P. E. Fanwick and T. Ren, Dalton Trans.,
2012, 41, 644–650.
10 B. Meunier, S. I. P. de Visser and S. Shaik, Chem. Rev., 2004, 104,
3947–3980.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 12097–12099 12099