Terminal Alkyne Coupling Reactions Through a Ring: Effect of Ring Size on Rate and Regioselectivity

Abstract

Terminal alkyne coupling reactions promoted by rhodium(I) complexes of macrocyclic NHC-based pincerl igands-which feature dodecamethylene,t etradecamethylene or hexadecamethylene wingtip linkers viz. [Rh(CNCn)(C 2 H 4)][BAr F 4 ](n = 12, 14, 16;A r F = 3,5-(CF 3) 2 C 6 H 3)-have been investigated, using the bulky alkynes HCCtBu and HCCAr'(Ar' = 3,5-tBu 2 C 6 H 3)a ss ubstrates. These stoichiometric reactions proceed with formation of rhodium(III) alkynyl alkenyl derivatives andp roduce rhodium(I) complexes of conjugated 1,3-enynes by CÀCbond reductiveelimination through the annuluso ft he ancillary ligand. The intermediates are formed with orthogonal regioselectivity,w ith E-alkenyl complexes derived fromH C CtBu and gem-alkenyl complexes derived from HCCAr', and the reductive elimination step is appreciably affected by the ring size of the macrocycle. For the homocoupling of HCCtBu, E-tBuCCCH= CHtBu is produced via direct reductivee limination from the corresponding rhodium(III)a lkynyl E-alkenyl derivatives with increasing efficacy as the ring is expanded. In contrast, direct reductivee limination of Ar'CCC(= CH 2)Ar'i se ncumbered relative to head-to-head coupling of HCCAr'a nd it is only with the largest macrocyclic ligand studied that the two processesa re competitive.T hese results showcaseh ow macrocyclic ligands can be used to interrogate the mechanism and tune the outcomeo ft erminal alkyne coupling reactions, and are discussed with reference to catalytic reactions mediated by the acyclic homologue [Rh(CNC-Me)(C 2 H 4)][BAr F 4 ] and solventeffects. Scheme1.Metal catalysed reactionsoft erminal alkynes. Mes = 2,4,6-Me 3 C 6 H 2 .

Rh Complexes with Pincer Carbene CNC Lutidine-Based Ligands: Reactivity Studies toward H2Addition

“…In particular, we found out that there are just a few examples regarding lutidine-based bis­(carbene) ligands, in combination with rhodium, where highly active catalysts for the C–C coupling reactions have been recently described. , Herein, we report on the utilization of pincer CNC-based ligands with rhodium that leads to the preparation of a number of cationic complexes. In order to understand the different reactivity observed for CNC-based ligands with rhodium that leads to the preparation of a number of cationic complexes, we study their reactivity toward hard bases and dihydrogen.…”

Oxidative Addition of a Mechanically Entrapped C(sp)–C(sp) Bond to a Rhodium(I) Pincer Complex

“…[5] In these isolated examples the substrate substituents play a decisive role and attempts to replicate this reactivity using diphenylbutadiyne proved unsuccessful. [6] Inspired by reports of unique metal-based reactivity using interlocked ligands [7,8] and as part of our research exploring the organometallic chemistry of macrocyclic pincer complexes, [9,10] we speculated that mechanical entrapment could be exploited to overcome the unfavorable kinetics associated with C(sp)-C(sp) bond oxidative addition relative to substitution of the diyne, and enable onward reactivity of the resulting bis(alkynyl) products to be explored. We herein describe work evaluating this hypothesis using rhodium complex 1, which features a macrocyclic phosphinite pincer ligand (POCOP-14) [9] and bulky aryl terminated 1,3-diyne (Ar'C 4 Ar', Ar' = 3,5-tBu 2 C 6 H 3 ; Scheme 1).…”

The Fascinating Flexibility and Coordination Modes of a Pentamethylene Connected Macrocyclic CNC Pincer Ligand

“…Notably, Chaplin and coworkers showed very interesting examples of macrocyclic CNC ligands that contain alkyl tethers between 8 and 16 C-atoms [ 3 , 4 , 5 ]. Their transition metal complexes showed particular properties depending on the ring size [ 6 , 7 , 8 ], but no reactivity at the tether itself.…”