Communication
methylation via photoinduced oxidative coupling (Scheme 1C).
The working mode of the synthesized dual-functioning
conjugate catalysts was verified by spectroscopic and electro-
chemical analysis. The formation of high-valent charge-transfer
species was found to allow for the CT-induced C−C bond-
forming reductive elimination from the key post-transmetalation
intermediate by reducing the RE energy barrier.
On the basis of our previous result that C−C bond-forming
reductive elimination from a transmetalated intermediate can be
16a,7d,e,16b
permitted by the external chemical oxidants,
we first
questioned whether the same strategy would also be operative
under photoredox conditions (Figure 1A). In the presence of
N,N′-bis(2,6-diisopropylphenyl)perylene diimide as a photo-
catalyst (PC1), compact fluorescent lamp (CFL)-irradiation on
each solution of isolated rhodacycle species Cp*RhBQ-Ar and
7e
Cp*RhBQ-Me afforded reductive elimination products 1 and
in 95% and 87% yield, respectively (Figure 1, eq 1). In stark
2
contrast, no product was formed under thermal (80 °C) or
irradiation conditions in the absence of PC1, suggesting that the
process is mediated by the excited photocatalyst species. We
were pleased to see that this PC1 photoredox system was also
2
operative in the Cp*Rh-catalyzed sp C−H arylation and
methylation (Figure 1, eq 2). For instance, benzo[h]quinoline 3
was readily coupled with aryl and methyl boronic ester providing
C-10 arylation and methylation products 1 and 2, respectively,
I
using Cu transmetalation agent and (tBuO) as a mild terminal
2
oxidant.
It should be noted that the fluorescence emission of PC1 did
not overlap with the absorption bands of the rhodacycle
Cp*RhBQ-Ar and Cp*RhBQ-Me (Figure 1B), thereby ruling
out a dipole−dipole energy transfer mechanism for the above
RE. The triplet energy of PC1 (1.11 eV) was calculated to be
significantly lower than that of Cp*RhBQ-Ar and Cp*RhBQ-
Me (1.42 and 1.43 eV, respectively), suggesting that the RE
process is induced by a photocatalytic oxidation, rather than the
triplet sensitization of the intermediates (Figure 1C). Indeed,
the RE energy barrier from Cp*RhBQ-Ar and Cp*RhBQ-Me
was calculated to decrease substantially from 26.7 and 28.5 kcal/
mol to 6.8 and 8.6 kcal/mol, respectively, upon the single
Figure 1. (A) Photocatalytic C−C bond forming RE from rhodacycle
Absorption spectrum of Cp*RhBQ-Ar and Cp*RhBQ-Me, depicted
with the emission spectrum of PC1. (C) Comparison of triplet energy
of PC1 and the rhodacycle species. S, singlet state; T, triplet state.
Figure 2. (A) Preparation of a series of Rh metallophotocatalyst bearing a mesityl acridinium (PC2) photosensitizer domain. (a) Mesityl lithium (2.0
equiv), TMEDA, Et O, −30 to 25 °C, 26 h, then nBuLi (1.2 equiv), −30 to 25 °C, 3 h, then 2,3,4,5-tetramethyl-2-cyclopentenone (4.0 equiv), 25 °C,
2
2
2 h; (b) Na CO , H O, EtOAc then H O, HClO , CH Cl , 25 °C, 1 h, 60% in two steps; (c) RhCl ·xH O (1.0 equiv Rh), iPrOH, 90 °C, 14 h, 89%;
2 3 2 2 4 2 2 3 2
(
d) AgOTf (6.0 equiv), CH Cl , 25 °C, 14 h, 71%; (e) CH C H , CH NO , AgSbF (6.0 equiv), 25 °C, 14 h, 54%; (f) 3, AgO CCF , Li CO , CH Cl ,
2 2 3 6 5 3 2 6 2 3 2 3 2 2
#
#
2
5 °C, 21 h then 40 °C, 4 h, 65%. (B) Single crystal X-ray diffraction structure of [PC2-Cp RhCl ] and PC2-Cp RhTol. 10% thermal ellipsoids for
2 2
both structures. Hydrogen atoms were omitted for clarity.
B
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX