Question

The organometallic catalyst for the following transformation is (intramolecular olefin closure giving a cyclohexene and ethylene, i.e., ring-closing metathesis):

• A. Grubbs' catalyst \(\mathrm{RuCl_2(=CHPh)(PCy_3)_2}\)
• B. Wilkinson's catalyst \(\mathrm{RhCl(PPh_3)_3}\)
• C. \(\mathrm{HCo(CO)_4}\) (cobalt carbonyl hydride)
• D. \([\mathrm{RhI(CO)_2}]^{-}\) (rhodium carbonyl iodide)

Hint:

If a transformation forms a new ring alkene and expels \(\mathrm{C_2H_4}\), think {RCM} \(⇒\) \(\mathrm{Ru}\)-alkylidene (Grubbs/Hoveyda) catalyst. Wilkinson's = hydrogenation; \(\mathrm{HCo(CO)_4}\) = hydroformylation; Rh carbonyl/iodide = carbonylation.

Answer 1

The Correct Option is A

Step 1: Identify the reaction manifold.
The substrate bears a terminal alkene tether that cyclizes to a cyclohexene with concurrent formation of \(\mathrm{C_2H_4}\). Formation of ethylene as a by-product and construction of a new internal C=C by intramolecular exchange of alkylidenes is diagnostic of ring-closing metathesis (RCM).

Step 2: Match to the appropriate catalyst class.
RCM is catalyzed by \(\mathrm{Ru}\)-alkylidene (Grubbs/Grubbs–Hoveyda) complexes. The classic first-generation Grubbs' catalyst, \(\mathrm{RuCl_2(=CHPh)(PCy_3)_2}\), mediates metathesis to give cyclohexene and releases ethylene, exactly as depicted. Hence option (A).

Step 3: Exclude the other options.
- (B) Wilkinson's catalyst \(\mathrm{RhCl(PPh_3)_3}\) promotes hydrogenation/isomerization of alkenes, not olefin metathesis.
- (C) \(\mathrm{HCo(CO)_4}\) is a hydroformylation catalyst for adding \(\mathrm{CHO}\) under CO/H\(_2\).
- (D) \([\mathrm{RhI(CO)_2}]^{-}\) is associated with carbonylation chemistry (e.g., acylations), not C=C metathesis.