Insight into the Activation of In Situ Generated Chiral Rh^I Catalysts and Their Application in Cyclotrimerizations

Abstract

We report a detailed study concerning the efficient generation of highly active chiral rhodium complexes of the general structure [Rh(diphosphine)(solvent) ] as well as their exemplary successful utilization as catalysts for cyclotrimerizations. Such solvent complexes could likewise be prepared from novel ammonia complexes of the type [Rh(diphosphine)(NH ) ] . A valuable, feasible approach to generate novel chiral Rh complexes was found by in situ generation from Wilkinson's catalyst [RhCl(PPh ) ] with chiral P,N ligands. The generated catalysts led to moderate to good enantioselectivities and excellent yields in the cyclotrimerizations of triynes, showcasing their usefulness in the synthesis of axially chiral benzene derivatives.

Two precatalysts for application in propargylic CH activation

“…As described in the literature (Drexler et al, 2007;Meissner, Pribbenow et al, 2014;Thiel et al, 2017), NBD complex (I) is more easily activated by hydrogenation than the corresponding COD complex (II). However, both complexes can be used as precursors for catalysis in chloride-free systems.…”

Activation, Deactivation and Reversibility Phenomena in Homogeneous Catalysis: A Showcase Based on the Chemistry of Rhodium/Phosphine Catalysts

“…An in-depth discussion of the issues concerning diolefin hydrogenation (COD vs. NBD) can be found in references [22,95]. In Table A1 [97], and [Rh(PPh 3 ) 2 (COD)]BF 4 [98], have also been measured.…”

“…An in-depth discussion of the issues concerning diolefin hydrogenation (COD vs. NBD) can be found in references [22,95]. In Appendix 1 a list of related (pseudo) rate constants for ca.…”

“…The problem can be solved using the precatalyst [Rh(Me-BPE)(NBD)] + as hydrogenation of NBD is faster than COD and faster than the formation of polynuclear rhodium hydride. An alternative entry to the solvent complexes, which circumvents the prehydrogenation step required for diolefin-containing precatalysts and the possible related problems, is represented by ammonia complexes of the general formula [Rh(diphosphine)(NH3)2] + which are very stable species and therefore easy to handle [95]. The solvent complex can be generated in situ by the addition of stoichiometric amounts of acid such as HBF4 which displace NH3 as the corresponding ammonium salt (using HBF4, BF4 -remains the sole counteranion for any cationic rhodium species in solution).…”

“…This and [Rh((S,S)-Et-Ferrotane)(NH 3 ) 2 ] + proved to be competent catalysts for the [2+2+2] cycloaddition of selected triynes, providing equal and, in some cases, better selectivity than the corresponding independently generated solvent complexes (Scheme 9) [95]. Catalysts 2018, 8, x FOR PEER REVIEW 17 of 45This and [Rh((S,S)-Et-Ferrotane)(NH3)2] + proved to be competent catalysts for the [2+2+2] cycloaddition of selected triynes, providing equal and, in some cases, better selectivity than the corresponding independently generated solvent complexes (Scheme 9)[95].…”