with arylboronic acids.6 However, thismethod requires the
preparation of the diarylmethyl coupling partners and uses
stoichiometric organometallic reagents. The reaction in
some cases only gives moderate yields of the desired prod-
ucts with formation of homocoupling byproducts. In 2012,
Walsh and co-workers reported an elegant approach to
triarylmethanes by the coupling of aryl bromides and
diarylmethanes (Scheme 1a).7 Notably, this deprotonative-
cross-coupling process employs diarylmethanes as diaryl-
methyl metallic reagent equivalents, transforming the
benzylic CÀH bonds into the C(sp3)ÀC(sp2) bonds.
Recently, N-tosylhydrazones have emerged as a new
type of organometallic reagent equivalent in transition
metal-catalyzed cross-coupling reactions.9,10 Since N-tosyl-
hydrazones are easily prepared through the correspond-
ing carbonyl compounds, these reactions can be formally
considered astransformation ofCdO bonds. The previous
work in this area has employed N-tosylhydrazone as a
vinyl metallic reagent equivalent to form C(sp2)ÀC(sp2)
double bonds, as first demonstrated by Barluenga and co-
workers.10a Tothe bestof ourknowledge, there isnoreport
on using N-tosylhydrazone as an alkyl metallic reagent
equivalent in Pd-catalyzed C(sp3)ÀC(sp2) single bond
formations.11 Here, we demonstrate that under reductive
conditions, N-tosylhydrazones derived from diarylmetha-
nones can be served as diarylmethyl metallic reagent equiva-
lent in Pd-catalyzed coupling reactions (Scheme 1b). Since
diarylmethanones are easily available, this transformation
constitutes a practical and general approach toward various
triarylmethanes.
At the outset of this investigation, benzophenone
N-tosylhydrazone 1a and 4-tert-butylphenyl bromide 2a
were used as the substrates in the reductive Pd-catalyzed
coupling reaction (Table 1). In the presence of 5 mol %
Pd(OAc)2, 15 mol % PPh3, and 1.2 equiv of HCO2NH4
as the reductant, we first tested two bases: LiOtBu and
Cs2CO3,12 and we found Cs2CO3 could afford the desired
product triarylmethane 3 in 29% yield (entry 2). With this
initial result, we then screened the ligand of this transform-
tion. We found that XPhos, SPhos, and tri(2-furyl)phosphine
were ineffective ligands in this reaction (entries 3À5). Change
of PPh3 to more electron-rich ligand CyPPh2 resulted in a
similar yield (entry 6). Then, it was found that the less
hindered ligand than XPhos or SPhos, such as L3, could
increasethe yield to34% (entry 7). Further modification of
the substituents on the phosphine led us find that L5 was a
more efficient ligand (entry 9).
We then turned our attention to alternative hydride
sources, however, both triethylsilane13a and isopropanol13b
were found ineffective in this reaction (entries 10 and 11).
We next inspected the effect of solvent (entries 12 and 13).
Interestingly, it was found that tert-pentanol was a more
suitable solvent, increasing the yield to 61% (entry 13).
Under such conditions, the major byproduct was the direct
reduction of 2a to tert-butylbenzene. We were delighted to
find that ammonium acetate could significantly suppress
the side reaction, and the desired product 3 could be
obtained in 85% yield (entry 14). Finally, by increasing
the amount of ammonium acetate to 1.5 equiv, the yield
was further improved (entry 15).
With the optimized reaction conditions, the scope of this
palladium-catalyzed reductive coupling reaction was then
examined by using a series of diarylmethanone N-tosylhy-
drazones 1aÀj and aryl bromides 2aÀt. As illustrated in
Scheme 2, this transformation proceeds smoothly over a
wide range of substrates, providing the triarylmethane
with moderate to excellent yields. For the aryl bromides,
the substituents on the para or meta position of the aryl
ring marginally affect this reaction, affording the corre-
sponding triarylmethanes in 66À95% yields (3À15).
Scheme 1. Triarylmethane Synthesis by Pd-Catalyzed Coupling
Reactions
ꢀ
(10) For selected examples, see: (a) Barluenga, J.; Moriel, P.; Valdes,
C.; Aznar, F. Angew. Chem., Int. Ed. 2007, 46, 5587. (b) Zhou, L.; Ye, F.;
Zhang, Y.; Wang, J. J. Am. Chem. Soc. 2010, 132, 13590. (c) Barluenga,
ꢀ
J.; Escribano, M.; Aznar, F.; Valdes, C. Angew. Chem., Int. Ed. 2010, 49,
6856. (d) Brachet, E.; Hamze, A.; Peyrat, J.-F.; Brion, J.-D.; Alami, M.
Org. Lett. 2010, 12, 4042. (e) Zhou, L.; Ye, F.; Ma, J.; Zhang, Y.; Wang,
J. Angew. Chem., Int. Ed. 2011, 50, 3510. (f) Chen, Z.-S.; Duan, X.-H.;
Wu, L.-Y.; Ali, S.; Ji, K.-G.; Zhou, P.-X.; Liu, X.-Y.; Liang, Y.-M.
Chem.;Eur. J. 2011, 17, 6918. (g) Ojha, D. P.; Prabhu, K. R. J. Org.
Chem. 2012, 77, 11027. (h) Roche, M.; Hamze, A.; Provot, O.; Brion,
J.-D.; Alami, M. J. Org. Chem. 2013, 78, 445.
(11) While this manuscript is under preparation, we noticed a report
on Pd-catalyzed reductive coupling of R-diazoester with aryliodides, see:
Titanyuk, I. D.; Beletskaya, I. P. Synlett 2013, 24, 355.
(12) For a detailed discussion on the effect of base in transition-
metal-catalyzed coupling reactions, see: Ouyang, K.; Xi, Z. Acta Chim.
Sinica 2013, 71, 13.
(13) (a) Triethylsilane has been used as the hydride source in palladium-
catalyzed carbonylation/acyl migratory insertion sequence. For an exam-
ple, see: Zhang, Z.; Liu, Y.; Gong, M.; Zhao, X.; Zhang, Y.; Wang, J.
Angew. Chem., Int. Ed. 2010, 49, 1139. (b) For a recent report on using
alcohol as hydride source in palladium-catalyzed processes, see:
Greenaway, R. L.; Campbell, C. D.; Chapman, H. A.; Anderson,
E. A. Adv. Synth. Catal. 2012, 354, 3187.
(6) Yu, J. Y.; Kuwano, R. Org. Lett. 2008, 10, 973.
(7) (a) Zhang, J.; Bellomo, A.; Creamer, A. D.; Dreher, S. D.; Walsh,
P. J. J. Am. Chem. Soc. 2012, 134, 13765. (b) Bellomo, A.; Zhang, J.;
Trongsiriwat, N.; Walsh, P. J. Chem. Sci. 2013, 4, 849.
(8) For other related reports, see: (a) Niwa, T.; Yorimitsu, H.;
Oshima, K. Org. Lett. 2007, 9, 2373. (b) McGrew, G. I.; Temaismithi,
J.; Carroll, P. J.; Walsh, P. J. Angew. Chem., Int. Ed. 2010, 49, 5541. (c)
Taylor, B. L. H.; Harris, M. R.; Jarvo, E. R. Angew. Chem., Int. Ed.
2012, 51, 7790. (d) Harris, M. R.; Hanna, L. E.; Greene, M. A.; Moore,
C. E.; Jarvo, E. R. J. Am. Chem. Soc. 2013, 135, 3303. (e) Zhou, Q.;
Srinivas, H. D.; Dasgupta, S.; Watson, M. P. J. Am. Chem. Soc. 2013,
135, 3307.
ꢀ
(9) For reviews, see: (a) Barluenga, J.; Valdes, C. Angew. Chem., Int.
Ed. 2011, 50, 7486. (b) Shao, Z.; Zhang, H. Chem. Soc. Rev. 2012, 41,
560. (c) Xiao, Q.; Zhang, Y.; Wang, J. Acc. Chem. Res. 2013, 46, 236.
Org. Lett., Vol. 15, No. 7, 2013
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