Cyclobutadiene Arene Complexes of Rhodium and Iridium

Abstract

Reactions of [(C2H4)2RhCl]2 or [(coe)2RhCl]2 (coe = cyclooctene) with AgPF6 and arenes, followed by addition of 3-hexyne, give the cyclobutadiene complexes [(C4Et4)­Rh­(arene)]+ in 40–65% yield (arene = tert-butylbenzene, p-xylene, mesitylene, 4-mesitylbutanoic acid). In the absence of arenes, the hexaethylbenzene complex [(C4Et4)­Rh­(C6Et6)]+ is formed in 70% yield as a result of cyclotrimerization of 3-hexyne in the coordination sphere of rhodium. Similar reaction of [(coe)2IrCl]2 with AgPF6 and 3-hexyne leads to [(C4Et4)­Ir­(C6Et6)]+, which is apparently the first reported cyclobutadiene iridium complex. DFT calculations suggest that formation of the model cyclobutadiene complex [(C4Me4)­Rh­(C6H6)]+ from bis­(alkyne) intermediate [(C2Me2)2Rh­(C6H6)]+ can proceed via a metallacycle transition state with a low energy barrier of 14.5 kcal mol–1.

A Planar‐Chiral Rhodium(III) Catalyst with a Sterically Demanding Cyclopentadienyl Ligand and Its Application in the Enantioselective Synthesis of Dihydroisoquinolones

“…This motivated us to propose an alternative approach to chiral rhodium catalysts that is based on the separation of racemic mixtures of complexes through crystallization of their diastereomeric adducts with naturally available amino acids. [12] Driven by our interest in rhodium-catalyzed transformations of alkynes, [23] we chose an on-classical route for the synthesis of our catalyst, namely a[2+ +2+ +1] cyclotrimerization of terminal alkynes in the coordination sphere of am etal. It is noteworthy that despite the success of planar-chiral complexes in other catalytic reactions, [22] they have apparently not been used for enantioselective CÀHa ctivation thus far.…”

“…It is noteworthy that despite the success of planar-chiral complexes in other catalytic reactions, [22] they have apparently not been used for enantioselective CÀHa ctivation thus far. [12] Driven by our interest in rhodium-catalyzed transformations of alkynes, [23] we chose an on-classical route for the synthesis of our catalyst, namely a[2+ +2+ +1] cyclotrimerization of terminal alkynes in the coordination sphere of am etal. [24] Ther eaction of the commercially available rhodium precursor [(cod)RhCl] 2 with tert-butylacetylene in the presence of AgPF 6 gave the cationic fulvene complex (AE)-1 in 64 %yield (Scheme 1).…”

Cyclobutadiene Rhodium Complexes as Catalysts for the Synthesis of Amides from Electron‐rich Arenes, Tosyl Azide and CO

“…We started our investigation with the model reaction of N‐methylindole with tosyl azide in MeCN in the presence of 1 atm of CO at 80 °C (Table 1). The readily available [6] complex [(C 4 Et 4 )Rh(xylene)]PF 6 ( Rh1 ) with strongly‐bound cyclobutadiene and loosely‐bound xylene ligand indeed promoted this reaction at 1 mol % catalyst loading giving the desired amide 1 a in 10 % yield (entry 1). Treatment of the complex Rh1 with [Et 3 NBn]Cl led to the displacement of xylene ligand and the formation of the neutral chloride complex [(C 4 Et 4 )RhCl] 2 ( Rh2 ) in almost quantitative yield [5] .…”

Hexaethylbenzene: A Sterically Crowded Arene and Conformationally Versatile Ligand

“…Arenes react with [(C 2 H 4 ) 2 RhCl] 2 and AgPF 6 in nitromethane to form the cationic species [(η 6 ‐arene)Rh(C 2 H 4 ) 2 ] + ( 35 ); upon addition of 3‐hexyne the two alkenes are displaced by alkynes that couple to form the corresponding arene–cyclobutadiene sandwich compounds [(η 6 ‐arene)Rh(η 4 ‐C 4 Et 4 )][PF 6 ] ( 36 ). However, if the [(C 2 H 4 ) 2 RhCl] 2 –AgPF 6 starting material is treated directly with excess 3‐hexyne, the fully ethylated version [(C 6 Et 6 )Rh(η 4 ‐C 4 Et 4 )][PF 6 ] ( 37 ) is produced in 70 % yield (Scheme ) . Likewise, the iridium analogue [(C 6 Et 6 )Ir(η 4 ‐C 4 Et 4 )][PF 6 ] ( 38 ) has also been prepared; however, crystallographic disorder has thus far thwarted attempts to obtain the structure…”

“…However,i ft he [(C 2 H 4 ) 2 RhCl] 2 -AgPF 6 startingm ateriali st reated directly with excess 3-hexyne, the fully ethylated version [(C 6 Et 6 )Rh(h 4 -C 4 Et 4 )][PF 6 ]( 37)i sp roduced in 70 %y ield (Scheme5). [40] Likewise, the iridium analogue[ (C 6 Et 6 )Ir(h 4 -C 4 Et 4 )] [PF 6 ]( 38)h as also been prepared;h owever,c rystallographic disorder has thus far thwarted attempts to obtaint he structure. [41] Treatment of [(1,5-COD)IrCl] 2 with n-propyllithium forms non-isolable [(1,5-COD)IrH] species, the reactions of which with alkynes depend markedlyo nt he nature of the alkyne substituents (Scheme6).…”