Rhodium(II)-Catalyzed Hydroamination of Propargylguanidines

Significance

Following a previous lanthanum(III)-catalyzed report on an addition-hydroamination sequence of propargylcyanamides to 2-aminoim­idazoles (R. L. Giles, J. D. Sullivan, A. M. Steiner, R. E. Looper Angew. Chem. Int. Ed. 2009, 48, 3116), Looper and co-workers have developed selective 5-exo-dig versus 6-endo-dig hydroamination-cyclization reactions of propargylguanidine derivatives 1 to give products 2 and 3, respectively. Thus, dirhodium(II) carboxylates, such as [Rh₂(oct)₄], as catalysts effect the highly 6-endo-dig-selective hydroamination of propargyl­guanidines 1 to give products 2, whereas silver(I) in combination with acetic acid proved to be the optimal catalyst for the selective formation of 5-exo-dig products 3. For R^³ = alkyl, catalytic rhodium(II) gave very high selectivity towards the 6-endo-dig products 2, in contrast with the poor ­selectivity observed for the 5-exo-dig products 3 using catalytic silver(I). For R^³ = aryl, the selectivity is completely reversed depending on the EDG or EWG nature of R^³. Bulky R^³ substituents were not tolerated for the rhodium(II)-catalyzed cyclization, but alkyl alcohols and chlorides were compatible. The nature of R^² on the guanidine nitrogen has modest impact on the selectivity.