Rhodium-Catalyzed Intramolecular Alkyne-carbodiimide Pauson-Khand-Type Reaction

Link: http://pubs.acs.org/cgi-bin/abstract.cgi/orlef7/asap/abs/ol063123i.html

From Prof. Takao Saito's group at Tokyo University of Science, Tokyo, Japan

This is a method which applies the Pauson-Khand cyclization strategy to a different system using a different metal catalyst, rather than the usual C02(CO)8. The substrate being studied in this report (Org Lett ASAP) is the a system consisting of carbodiimide moiety (as referred to as heterocumulenic portion) tethered to an alkynic system, which cyclizes intramolecularly in the presence of CO in a [2+2+1] fashion.

In their earlier findings, it was discovered that this Pauson-Khand-type process could be accomplished using Mo(CO)6 in the presence of DMSO as the promoter in the reaction conducted in toluene, albeit stoichiometric molybdenum compound was required.

However in their latest findings using substrate 1a during the screening, they found that this process could be catalyzed by a Rh-catalyst. The catalyst of choice was found to be [RhCl(CO)dppp]2 (5 mol %), generated in situ by mixing [RhCl(COD)]2 and 1,3-bis(diphenylphosphino)propane (dppp, 11 mol %). This marks the first known Rh-catalyst system able to catalyze Pauson-Khand-type reaction of carbodiimide and alkyne. Other results from catalyst screening are summarized in the following table.

It should be noted that dppp is more superior than other ligands studied and the Rh catalyst could be lowered to 2.5 mol % and still afforded 58% of 2a (compare entries 10 and 11).

Next, the scope of this new catalyst system was explored in a series of substrates and the results are included in the following table.

In general, the catalyst seems to function well across the table giving the desired products in moderate to good yields, except in substrates 1b and 1g (entries 2 and 7) where the R2 group being cyclohexyl group, where the yields were lower. This may be due to the steric hindrance during the cyclization.

The scope of the catalyst was explored further in its application to the substrate where the carbodiimide is tethered through ethylene (CH2-CH2) group to the alkyne (3a-g), Table 3. The Rh catalyst was still able to function well, yielding the desired products in acceptable yields in most cases. The only exception was 3a where the subtituent R2 on the carbodiimide is less bulky (n-Pr group), which made it prone to dimerize or deteriorate under the harsh reaction conditions.