Page 3 - Organogold Reactivity with Palladium, Nickel, and Rhodium: Transmetalation, Cross-Coupling, and Dual Catalysis

Organogold Dual Metal Catalysis Hirner et al.

SCHEME 2. Revised Mechanism for the Au/Pd Dual-Catalyzed Carbostannylation of Alkynes a

a
Generation of the active catalysts is omitted for clarity. Starting materials are demarcated with black boxes. The gold-shaded box demarcates a palladium-catalyzed
carboauration reaction that operates as a subcycle within the broader carbostannylation cycle.
intermediate 8; (iii) transmetalationtoSn togeneratethe final withthestannylatingreagentBu 3 SnOTf(eq2).Transmetalation
1
product 3 (Scheme 2). The mechanism involved a series of product 3b was observed by H NMR spectroscopy in accor-
transmetalation reactions that shuttled the organic fragments dance with the mechanistic hypothesis. The addition of PPh 3
between all three metals. Evidence for each these mechanistic was necessary in order to sequester the gold triflate coproduct
steps were provided by independent experiments demon- as [Au(PPh 3 ) 2 ] in a way that mimicked catalytic conditions in
þ
strating their viability. which cationic gold would be consumed by catalyst turnover.
In order to prime the metal catalysts for reactivity, this
mechanistic proposal invoked two successive transmetala-
tion reactions of the organic fragment that started on the
stannane (i.e., Sn-to-Au and then Au-to-Pd) rather than a
direct Sn-to-Pd transfer in order to explain the fact that gold
was required to be present in order to produce the observed
product selectivity. Consistent with this proposed role of
We noticed that the mechanism proposed in Scheme 2
gold as a transmetalation “shuttle”, tributylvinylstannane
suggested a previously unknown reaction: palladium-
transmetalated rapidly with triphenylphosphinegold triflate
catalyzed carboauration of alkynes. This Pd-only catalyzed
to produce (triphenylphosphine)vinylgold, a process which
carboaurationreactionformedpartofthedual-catalyticcyclefor
1
we could observe separately by H NMR spectroscopy.
carbostannylation, highlighted by the yellow box in Scheme 2.
We next tested the possible transmetalation of an orga-
Indeed, we discovered that catalytic organogold intermediate 6
1720
nic fragment from Pd to Au, as proposed in step (ii).
could be replaced by a stoichiometric organogold reagent in
Treatment of known palladacycle 11 with methylgold 12 17
order to isolate carboauration products 8ae (Table 2) with
afforded dienylgold complex 14 in reasonable yield (eq 1).
equally high and analogous regio- and stereoselectivity to that
The formation of 14 is consistent with the intermediacy of
which occurs in the overall carbostannylation reaction.
bimetallic transmetalation intermediate 13, derived from
Through our investigation of the carbostannylation reac-
transmetalation from methyl(triphenylphosphine)gold 12 to
tion mechanism, the Pd/Au and Sn/Au transmetalation
palladacycle 11. This result demonstrates the plausibility of
reactions and a palladium-catalyzed carboauration reaction
step (ii) of the mechanism.
were revealed. Knowledge of these fundamental processes
has formed a basis for other studies within our laboratory.
IV. Pd-Catalyzed Cross-Coupling of Vinylgold
Reagents
Inspired by the Au/Pd transmetalation that was uncovered
In order to probe the viability of the previously unknown during the course of our investigation into the carbostannyla-
transmetalation of an organic fragment from Au to Sn as tionmechanism,wedevelopedaPd-catalyzedcross-couplingof
proposed in step (iii), vinylgold compound 8a was treated vinylgold reagents. 17 After the completion of the carboauration

Vol. 44, No. 8 ’ 2011 ’ 603–613 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 605