Platinum-catalyzed phenyl and methyl group transfer from tin to iridium: evidence for an autocatalytic reaction pathway with an unusual preference for methyl transfer.

Transition-metal-catalyzed cross-coupling reactions (cycle A in Scheme 1) have enjoyed wide application in organic synthesis and are now extensively used for the efficient formation of carboncarbon and carbon-heteroatom bonds.1 The synthesis of these complexes is believed to occur via intermediates containing metalheteroatom and metal-carbon bonds.1 In principle, analogous coupling reactions could be devised in which a metal complex could be used to catalyze the formation of a bond between a different metal center and a carbon or heteroatom (cycle B in Scheme 1), generating products potentially analogous to the above proposed intermediates. The ability to catalytically synthesize these types of complexes could have a wide impact on research areas in which such complexes are employed or are postulated as intermediates, especially if mechanistic insight can be gained to guide new synthetic developments. However, the application of transition-metal-catalyzed coupling reactions to the synthesis of organometallic complexes remains relatively rare. A few recent examples have come from the laboratories of Lo Sterzo2 and Komiya.3 In both cases, it has been proposed that insertion of a transition metal catalyst, M1, into a transition metal complex M2-X bond (X ) halide, H) plays a key role in the overall transformation. The M2-M1-X intermediate is analogous to the R1-M1-X intermediate implicated in typical organic cross-coupling reactions (Scheme 1).4 This M2-M1-X intermediate is then assumed to react via a transmetalation/insertion followed by reductive elimination, yielding the organometallic product. It seems likely that many organometallic reactions that are catalyzed by a second metal complex remain to be discovered. To investigate this, we have explored the possibility that Stille-type reactions can be developed to promote the metal-catalyzed transfer of an alkyl or aryl group from a tin reagent to the iridium center in Cp*(PMe3)IrCl2 (1) (Cp* ) η-C5Me5; eq 1). The resulting aryl/ alkyl chloride complexes are important precursors to catalysts for H/D exchange5 and for the hydration of alkenes6 and C-H bond activation.7 We have found that such reactions can be catalyzed by coordinatively unsaturated platinum complexes. However, the mechanism is much more complicated than the type of routes proposed previously.2,3 Our initial experiments targeted the transmetalation of a phenyl group from Bu3SnPh (2) to 1 to generate [Ir]PhCl (3) ([Ir] ) Cp*(PMe3)Ir; eq 1), resulting in the formation of a new iridium-carbon bond. A variety of potential transition metal catalysts were examined to effect the desired cross-coupling reaction (Table 1). The most promising complexes were platinum-based. While catalysts derived from Pt(dba)2 gave the best yields,8 we chose to use Pt(PBu3)2 (4) for our subsequent studies due to its solubility and the ability to monitor 4 by both 1H and 31P NMR. In an effort to lower the reaction temperature, the use of Me3SnPh (5a) was explored with the hope that decreased steric repulsion would increase the reaction rate. Surprisingly, use of this compound did not furnish 3. Instead, exclusive methyl transfer was observed at 75 °C, yielding [Ir]MeCl (6). The selectivity for methyl transfer contrasts with the preferential aryl transfer from tin reagents observed in organic cross-coupling reactions,9 as well as with the organometallic reactions studied by Lo Sterzo where selective alkynyl transfer from R3Sn-R′ (R ) Bu, Me, R′ ) alkyne) is observed.10 Tin reagents bearing both Ph and alkyl groups commonly transfer the Ph rather than the Me group in electrophilic reactions.11 In our system, only methyl transfer is observed with the tin reagents Me2SnPh2 (5b) and MeSnPh3 (5c), in addition to SnMe4 (5d), where only methyl transfer is possible (Table 2). In the absence of the Pt catalyst 4, a slow background reaction is observed. However, in this thermal process, selective phenyl transfer occurs at 105 °C from 5c, while both phenyl and methyl transfers occur from 5a (Table 2).12 At 75 °C, these reactions are extremely sluggish. Thus, 4 is crucial to controlling the rate and chemoselectivity of the reaction. Our first evidence for mechanistic complexity appeared when reaction monitoring revealed that all of the cross-coupling reactions studied showed the gradual formation of a new Pt-H complex, identified as (PBu3)2Pt(H)(Cl) (7). Since the source of the HCl in this complex was unclear, we sought to determine whether the presence of adventitious acid could be responsible for the catalytic † University of California, Berkeley. ‡ Colorado State University. Scheme 1. Representative Mechanism for Metal-Catalyzed Cross-Coupling Reactions Published on Web 01/17/2008