metathesis of PhI(OAc)2 may consume some AgOTs, and (3) in the light of our recent
work, the role of silver to benefit transmetalation of arylorganometallics to gold cen-
ter cannot be ruled out.49,50
Interestingly, to achieve gold-catalyzed cross-coupling of organometallics, the use
of suitable arylboronates appears to be a very important factor (Figure 2E). Among
six aryl boronic derivatives examined, Ar-Bneop (3a) can furnish the desired product
(4a) in 72% yield in 2 h, whereas other aryl boronic derivatives (3b–3f) are reluctant to
undergo such cross-coupling. This suggests that the transmetalation rate of two
organometallics to gold center should match well each other. Too rapid or too
slow transmetalation of one coupling partner may result in the failure of cross-
coupling. For example, when arylboronic acids were employed, a 38% yield of
homocoupling by-product (6a) was obtained, but replacement of 3a with Ar-Bpin
(3b) gave only 14% yield of the desired product (4a) along with homocoupling by-
products 5a (27%) and 6a (5%). Significantly, our developed gold-catalyzed cross-
coupling of organometallics can occur smoothly without an external base.
The scope of gold-catalyzed cross-coupling with regard to arylsilanes was investi-
gated, using 5-(3-methoxyphenyl)-2,2-dimethyl- 1,3,5-dioxaborinane (3a) as one
coupling partner. Electron-donating and electron-withdrawing para-substituents
are amenable to the reaction, affording structurally diverse biaryls in useful yields
(4a–4k). The versatile para-iodo, -bromo, and -OTf substituents are tolerated well
in this protocol. When meta- and ortho-substituted arylsilanes were subjected to
the standard conditions, the desired products (4l–4v) were isolated in 41%–82%
yield. The compatibility of (pseudo)halogen substituents, as in 4a–4c, 4l, 4m, 4u,
and 4v provides a promising opportunity for downstream diversification of the
coupling reactions and functional-group installation. The alcohol and alkyne units
(4r and 4w) are also compatible with the reaction and a product (4p) containing
pinacol boronic ester can be obtained in 65% yield without competing for cross-
coupled products. The success of product 4p further implies the specificity of
transmetalation with less sterically hindered Ar-Bneop unit between two different
arylboronates (-Bpin and -Bneop). This result supports the excellent functional-
group tolerance and predictable site-selectivity. The electron-rich naphthalenylsi-
lanes can undergo the desired cross-coupling smoothly to afford the product
(4x) in 51% yield. When (9,9-dimethyl-9H-fluorene-2,7-diyl)bis-(trimethylsilane)
was used, only monoarylation product (4y) was formed. The formation of these
products clearly competes against the existing gold-catalyzed electronic effect
controlling C–H bond arylation (ortho- and para-selectivity to the electron-
donating group).
Different arylboronates were examined (Figure 3B). In general, arylboronates bearing
different substituents at the para-position (4aa–4ff) provide the corresponding products
in good yield. A promising feature of this catalytic coupling is that the useful (pseudo)
halogen substituents both on the arylsilanes (4a–4c, 4l, 4m, 4u, and 4v) and the arylbor-
onates (4aa–4cc, 4gg, 4nn, 4oo, and 4qq–4ss) are very compatible. In addition to
halogen substituents, a wide range of versatile functional groups, such as ꢀSO2Me
(4ee), ꢀCOOEt (4ff), ꢀCHO (4hh), ꢀCN (4ii), acetal (4kk), and vinyl (4ll) are also toler-
ated. The substituents at the meta- and ortho-positions on phenyl rings have little influ-
ence on the formation of cross-coupling products (4gg–4uu). Significantly, unprotected
secondary amides (4jj) and carboxylic acids (4mm) tolerate the conditions well. The
base-free reaction conditions allow for successful coupling of base-sensitive electron-
withdrawing arylboronic esters (4pp, 4tt, and 4uu) and dibenzo[b,d]thiophenylboro-
nates (4ww and 4xx). Unfortunately, both aromatic silanes and boronates bearing
2722 Chem 5, 2718–2730, October 10, 2019