Today’s grad school guinea pig is Dean Toste of Berkeley. His recent work has dealt with gold-based catalysts for cyclization reactions. This communication describes an interesting reaction his lab discovered: conversion of an “allenene” (allene with a neighboring double bond) to a cyclobutane through a gold(I)-catalyzed [2+2] cycloaddition. His group started running reactions with Ph3PAuCl, then tested the catalyst shown below. Check it out!
Toste’s proposed mechanism involves a series of carbocationic intermediates, as shown below.
40 is the more quickly formed intermediate, and flies to 41 in the presence of methanol (or any nucleophile, really). In the absence of an external nucleophile, the last remaining pi bond does the job, attacking the cation to generate the cyclobutane ring. The reaction is fairly enantioselective as well–the Ar group tries to stay exo throughout the reaction (and particularly at the last step) to avoid bad pi-pi interactions.
If I end up at Berkeley, remind me to sift through the water to strike it rich… 🙂 Then again, it’s a catalyst, so they probably reuse most of it…