Carbonylation of Alkyl Halides Pd, Rh

Sometimes a Pd(II) compound such as (Ph3P)2PdCl2 or Pd(OAc)2 is added to the reaction mixture as catalyst. The Pd(II) must be reduced to Pd(0) before the catalytic cycle can proceed. EtsN can act as the reducing agent. Coordination to Pd(II) and j8-hydride elimination gives a Pd(II) hydride, which is deprotonated to give Pd(0). Triarylphosphines can also reduce Pd(II) to Pd(0) by electron transfer. 

Problem 6.5. Draw a reasonable mechanism for the following alkoxycar-bonylation reaction. 

The Monsanto process, one of the most successful industrial homogeneous catalytic processes, uses a Rh complex and catalytic HI to carbonylate MeOH to MeC02H. A Rh precatalyst (almost any Rh complex will do) is converted into Rh(CO)2l2, the active catalyst, under the reaction conditions. The mechanism of the reaction involves three steps. In the first step, MeOH and HI are converted to Mel and H2O by an Sn2 mechanism. In the second step, Mel and CO are converted to MeCOI under Rh catalysis. In the third step, H2O (generated in the first step) hydrolyzes MeCOI to afford MeC02H and to regenerate HI. 

The catalytic cycle of the second step proceeds by oxidative addition of Mel to Rh(I) to give a MeRh(III) complex, insertion of CO to give the (MeCO)Rh(III) complex, coordination of more CO, and reductive elimination of MeCOI to regenerate the Rh(I) complex. 

---