whereas the total positive charge (+0.8816) of
the PhBr moiety in 4 decreases to +0.3754, +0.4524,
and +0.4896, respectively (Fig. 2). These results
suggest transfer of alkane C–H s-bonding elec-
trons to iminobromane 4 with some hydride-
transfer character. Although the C21-H22 bond
Table 1. Metal-free C–H amination at room temperature with imino-l3-bromane 1. Unless otherwise noted,
0.01 M of 1 (with 20 equivalents of HFIP, where indicated) was stirred in neat alkane under Ar for 10 to 24
hours. Yields and ratios of the indicated products 3 (major isomer on the left) were determined by GC
(numbers in parentheses are isolated yields of the major product; N = NHTf). Relative reactivities of C–H
bonds compared with cyclohexane (2b) after statistical correction (per H atom) are shown in red (methine
groups) and blue (methylene groups).
activation and the N13-C21 bond formation take
place as a single event, TS-1 to TS-3 indicate
a highly asynchronous transition state, with
N13-H22 bond formation being much more advanced than N13-C21 bond formation (Fig. 2).
These transition-state structures are reminiscent
of those calculated for the concerted alkane C–H
insertions by dimethyldioxirane (25) and rhodium-carbene/nitrene complex (26, 27). As observed
for trans- (2p) and cis-1,4-dimethylcyclohexane
(2q), the aminations proceed with full retention
of stereochemistry, indicating that long-lived car-bocation or free radical intermediates are not involved in the aminations.
Deuterium kinetic isotope effects (KIEs) were
evaluated by the competitive reaction of imino-
l3-bromane 1 with a 1:1 mixture of cyclohexane
(2b) and cyclohexane-d12 (2b-d12) at 25°C: A
primary deuterium KIE kH/kD (kH, rate constant
of 2b; kD, rate constant of 2b-d12) of 3.72 for 2°
C–H insertion was determined by gas chromatography (GC) (Fig. 3). Comparison of k2 values
(table S1) for 3° C–H insertion of adamantane
(2v) and 1,3,5,7-tetradeuterioadamantane (2v-d4)
in tetramethylpentane 2m at 30°C afforded a
smaller kH/kD value of 1.49 (28). These moderate
KIE values suggest only partial C–H bond breaking in the transition state and are not compatible
with a hydrogen atom abstraction/radical-rebound
mechanism, as proposed for Ru- and Cu-nitrenoid–
based C–H aminations with large KIEs of 6.1 to
6.6 (29, 30). The smaller KIE value observed for
the tertiary C–H bonds of 2v relative to the secondary C–H bonds of 2b is consistent with the
calculations, which show greater extension of the
breaking C21-H22 and N13-Br1 bonds in TS-2
than in TS-3 (Fig. 2); the latter structure reflects
the more early character of the transition state.
We feel that these KIEs—together with the second-order kinetics, kinetic parameters (negative activation entropy), retention of stereochemistry, and
theoretical calculations—strongly implicate a concerted asynchronous pathway for metal-free C–H
amination with sulfonylimino-l3-bromane 1. The
driving force for this room-temperature uncata-lyzed insertion into unactivated alkane C–H
bonds is probably the greatly enhanced nucleofu-gality of aryl-l3-bromanyl groups compared with
aryl-l3-iodanyl groups, which correlates with
increased instability of hypervalency at bromine
relative to iodine (31–33).
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