Fig. 1. (A) Generalized equation for an Ullmann coupling to form an arylamine. (B) Outline of two of the possible mechanisms for Ullmann C–N bond
formation. (C) Outline of a possible pathway for photoinduced Ullmann C–N bond formation via a copper–carbazolide complex.
ligands are replaced with P(m-tol)3 (25). A single-crystal x-ray diffraction study confirmed that
copper–carbazolide complex 1 maintains a three-coordinate trigonal-planar geometry in the solid
state (Fig. 2A). Complex 1 is colorless and is not
visibly luminescent in acetonitrile; however, emission and excitation spectra confirm that it has accessible excitations in the near ultraviolet (Fig. 2B).
When an acetonitrile solution of copper–
carbazolide complex 1 and iodobenzene is irradiated with a standard 13-W compact fluorescent
light bulb (CFL) at room temperature for 10 hours,
C–N bond formation proceeds in good yield (77%)
(Table 1A, entry 1); an even higher yield is obtained in CD3CN (84%) (Table 1A, entry 2). Irradiation with a 100-W mercury lamp results in
C–N bond formation even at –40°C (Table 1A,
entry 3). Previously described couplings of carbazole with iodobenzene in the presence of copper
have used temperatures of at least 90°C (26).
Under otherwise identical conditions in the
absence of light, no N-phenylcarbazole is observed (<1%) (Table 1A, entry 4), and negligible
coupling occurs in the dark even upon heating
at 65°C for 12 hours. Irradiation of a mixture of
carbazole and iodobenzene (without 1) leads to
no detectable N-phenylcarbazole (<1%).
For these photoinduced Ullmann C–N coupling reactions, we postulate that upon photoexcitation, copper complex 1 transfers an electron
to iodobenzene to produce a radical ion pair
(Fig. 1C). The higher yield obtained in CD3CN
(Table 1A, entry 2) as compared with that in
CH3CN (Table 1A, entry 1) can be attributed to
Fig. 2. (A) X-ray structure of copper complex 1 (thermal ellipsoids drawn at 50% probability and H
atoms omitted for clarity). (B) Excitation and emission spectra of copper complex 1 in CH3CN. (C)
Chemical oxidation of copper complex 1.
a kinetic isotope effect for undesired abstraction of a hydrogen/deuterium from the solvent
by the phenyl radical or by radical cation 3
(Fig. 1C). Consistent with this hypothesis, we
observed benzene and unsubstituted NH/ND
carbazole as side products in these photoinduced
couplings (25), and we have established that when
we independently generate radical cation 3 via
chemical oxidation of 1 in CH3CN, the unsubstituted NH carbazole is formed (Fig. 2C) (27).
Bromobenzene also undergoes Ullmann cou-
pling when irradiated with a 13-W CFL in the
presence of copper–carbazolide complex 1. As
would be expected on the basis of relative re-
duction potentials [PhI, –1.91 V; PhBr, –2.43 V;
PhCl, –2.76 V versus saturated calomel electrode
(SCE) in dimethylformamide (DMF) on a plat-
inum electrode] (28), photoinduced C–N bond
formation is considerably slower for bromobenzene
(Table 1A, entry 5) than for iodobenzene (Table
1A, entry 1). Nevertheless, a good yield of the
desired product can be obtained at room temper-
ature if a 100-W mercury lamp is used (Table 1A,
entry 6), and a moderate yield is observed even at
–40°C (Table 1A, entry 7). In the absence of light,
no Ullmann coupling occurs (Table 1A, entry 8).