Page 2 - Organogold Reactivity with Palladium, Nickel, and Rhodium: Transmetalation, Cross-Coupling, and Dual Catalysis

Organogold Dual Metal Catalysis Hirner et al.

TABLE 1. Representative Pd- and Au-Cocatalyzed Carbostannylation Products

goldcarbon σ-bond remains an underdeveloped field, 14,15
SCHEME 1. Contrast between the Pd-Only Catalyzed and Au/Pd
limited primarily to electrophilic trapping 79 and carbene Dual-Catalyzed Carbostannylation of Alkynes 13
insertion. 10 Concurrent with our work, Au-only cross-
coupling reactivity has expanded the functionalization pos-
sibilites by employing Au(I)/Au(III) catalysis. 11 We reasoned
that dual transition metal reactions would provide reactivity
complementary to that which is already available though
conventional functionalization methods.
In many instances, the challenges inherent in turning
over two catalysts simultaneously have necessitated first
studying related processes that are stoichiometric in gold.
Developing the fundamental reactivity of organogold com- Hiyama provides predominately double-addition pro-
pounds with other transition metals has therefore become ducts, 14,15 while our dual-catalyzed reaction is completely
the pivotal step in connecting two catalytic cycles in the selective for monoaddition (Scheme 1). Attempted carbo-
design of new dual-catalytic reactivity. Mechanistic investi- stannylation of alkynes in the presence of gold catalyst but
gations have accompanied the development of these reac- in the absence of palladium did not produce any product. 16
tions to provide broader insight into dual transition metal These results provided our first demonstration that dual-
turnover and activation strategies, knowledge which facil- metal catalyzed reactions involving gold allowed for reac-
itates the design of future reactions. tivity and selectivity different from that available to their
single-metal catalyzed counterparts.
II. Au and Pd Dual-Catalyzed Carbostannyla- The treatment of alkynes 1ae with stannanes 2ae in
tion of Alkynes the presence of Pd and Au cocatalysts afforded tri- and
The CC bond-forming capability of Pd suggested this metal tetrasubstituted olefin carbostannylation products 3ae with
as an appropriate starting point for our study of the reactivity excellentregio-andstereoselectivity(Table1).Ourstudiesinto
of Au with other transition metals. The wealth of knowledge the mechanism of this transformation later led to a proposal
regarding Pd organometallic transformations 12 allowed for that involved a AuC σ-bond intermediate.
fewer unknown steps while maximizing the opportunities to
predict and intercept well-established intermediates. We III. Mechanism of Dual-Catalyzed Carbostan-
postulated that the dual usage of Au and Pd catalysts could nylation: Intermediacy of a AuC σ -Bond
effect a carbostannylation reaction wherein a SnC bond After developing the Pd and Au dual-catalyzed carbostan-
could be broken and added across a CC multiple bond. 13 nylation reaction, we elucidated the specific role of each
We tested our hypothesis by examining the Pd/Au dual- metal catalyst building on a previous working hypothesis. 17
catalyzed addition of organostannanes across electron- Our proposed mechanism elaborates substrate 1 in three
deficient alkynes (Table 1). Notably, the Pd-only catalyzed steps: (i) migratory insertion of alkyne 1 into organopalladium
vinylstannylation of alkynes developed by Shirakawa and intermediate 5; (ii) transmetalation to Au to afford vinylgold

604 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 603–613 ’ 2011 ’ Vol. 44, No. 8