Tailoring carbon chain lengths: conversion of two C5 fragments into C8 and C2 on a triruthenium cluster

A process that can yield hydrocarbons of desired chain length has been in continuous demand over the years. Recently, a method for converting lower alkanes into higher alkanes - referred to as alkane metathesis - has attracted considerable attention. Alkane metathesis involves three catalytic reactions: activation of the alkane C-H bond, olefin metathesis, and hydrogenation of the formed olefins. However, the method is hampered by cross-metathesis reactions of alkenes during the recombination process, which result in a molecular weight distribution in the product.

Now, Hiroharu Suzuki and colleagues at Tokyo Institute of Technology have shown that two C₅ hydrocarbyl fragments can be selectively converted into C₈ and C₂ fragments using a triruthenium cluster as a model compound metal surface for the reaction. Thermolysis of the trinuclear µ₃-pentylidyne-µ₃-pentyne complex at 140 °C resulted in the quantitative formation of a µ₃-octyne-µ₃-ethlidyne complex.

Monitoring the reaction using ¹H NMR spectroscopy revealed that the reaction proceeded by way of consecutive recombination of hydrocarbyl ligands on the triruthenium cluster concomitant with cleavage and reforming of the Ru-Ru bond (as shown in the Scheme). Thus the two C₅ ligands coordinated on both faces of Ru₃ plane were selectively converted into C₂-alkylidyne and C₈-alkyne fragments via a µ₃-metallacyclobutenyl intermediate.

The results demonstrate the potential applicability of cluster compounds in synthetic reactions and shed light on the mechanism of reactions on metal surfaces.

• Authors: Atsushi Tahara, Mana Kajigaya, Makoto Moriya, Toshiro Takao, and Hiroharu Suzuki
• Title of original paper: Metathesis reaction of hydrocarbyl ligands across the triruthenium plane
• Journal, volume, pages and year: Angewandte Chemie International Edition 2010, 49, 5898-5901.
• Digital Object Identifier (DOI): 10.1002/anie.201002235
• Affiliations: Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology
• Department website: http://www.apc.titech.ac.jp/index.html

A plausible mechanism for the consecutive recombination of hydrocarbyl ligands on the Ru₃ cluster.