Discussion
The organometallic catalyst for the following transformation is
- A.
- B.
- C.
- D.
![The anion cis-diiododicarbonylrhodate(I), [RhI2(CO)2] with a negative charge](/uploads/question-images/gate-cy-2023/q26-d.png)
Answer: A
The catalyst is (A), the first-generation Grubbs catalyst RuCl2(PCy3)2(=CHPh).
The transformation joins the two ends of an α,ω-diene into a cycloalkene and releases ethylene: ring-closing metathesis. It needs a metal carbene (alkylidene) that undergoes [2+2] cycloaddition with an alkene to a metallacyclobutane and cycloreversion, exchanging alkylidene partners; the escape of ethylene drives the reaction. The Grubbs ruthenium benzylidene is the classic catalyst.
The other complexes have no carbene and catalyse other reactions: RhCl(PPh3)3 (Wilkinson's catalyst) is for hydrogenation, HCo(CO)4 for hydroformylation, and [RhI2(CO)2]− for methanol carbonylation (the Monsanto acetic acid process).
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