Page 5 - Organogold Reactivity with Palladium, Nickel, and Rhodium: Transmetalation, Cross-Coupling, and Dual Catalysis

Organogold Dual Metal Catalysis Hirner et al.

TABLE 3. Representative Products of the Pd and Au Dual-Catalyzed Rearrangement of Allenoates

allylic (16a), alkyl (16b), and aryl (16c) cross-coupling reac- Transmetalation of neutral vinylgold 22 with the intermedi-
tions. These reactions were efficiently catalyzed by the ate π-allyl Pd complex 23 then regenerates cationic gold
residual Pd present in the crude carboauration reaction catalyst 24. Substituted butenolide 18 is then formed by
mixture. In each case, control reactions established that no reductive elimination from diorganopalladium intermediate
reaction occurred in the absence of Pd. To the best of our 25. Crossover experiments confirmed the intermolecular
knowledge, this is the first example of Au and Pd cross- nature of this reaction. In order to highlight the potential
coupling reactivity to be well-defined in the literature with a generality of Au/Pd dual-catalyzed transformations, we also
19
characterizedorganogoldintermediate. Afterourinitialreport, applied this reaction to the synthesis of substituted isocou-
subsequent publication by the Hashmi group indicated that the marins 28ac from benzoates 26ac (eq 3). Control experi-
idea of a stoichiometric Au/catalytic Pd cross-coupling reactivity ments confirmed that this reactivity could not be attributed
manifold occurred to their group during a similar time frame. 21 to either metal catalyst alone.

V. Au and Pd Dual-Catalyzed Cyclization and
Cross-Coupling Reaction
Through the Au/Pd cross-coupling reaction, we had estab-
lished grounds from which to effectively pursue our goal of
developing dual-metal catalyzed transformations with gold.
The practical considerations of designing a reaction with two
intercepting catalytic cycles are nontrivial at best. 15,22,23
We decided to combine the capacity of Au for cyclizations 79
with the CC bond-forming and oxidative-addition power of
Pd catalysis. In this vein, we were inspired by a timely report
by Hammond et al. detailing the spectroscopic characteriza-
tion of organogold rearrangement product 19b (Scheme 4). 24
We considered that replacement of the ethyl group with allyl
(19a) would prime a rearrangement of allyl allenoates 17ae Our investigation into Pd sources revealed the impor-
tosubstituted butenolides 18ae that proceeded viaa unique tance of selecting the proper ligand for catalyst 20. Specifi-
mechanism that involved simultaneous activation of the cally, phosphine ligands on Pd precatalysts were found to
substrate by gold and palladium (Table 3). 25 have an inhibitory effect on dual catalysis. When the optimal
A likely dual-catalytic cycle is shown in Scheme 4. Catio- precatalyst Pd 2 (dba) 3 was replaced by the precatalyst
nic gold complex 24 cyclizes allenoate 17 to organogold Pd(PPh 3 ) 4 , no formation of 18 was observed. Presumably,
oxocarbenium 19a, 24 activating the allyl group for oxidative this complete inhibition of reactivity was due to the forma-
þ

addition 26 by Pd through bimetallic transition state 21. tion of catalytically inactive [Au(PPh 3 ) 2 ] OTf , which formed
Vol. 44, No. 8 ’ 2011 ’ 603–613 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 607