Attempted deprotonation of the cationic C ~S -[ R ~( P M ~, ) ~( H ) ( C ~C R ) ] C I salts with BunLi leads t o the formation of both [Rh(PMe3)4(C=_CR)] and [Rh(PMe3)4H] whereas the use of KOH/H20 yields exclusively the Rh' o-acetylides; the molecular structures of the trigonal-bipyramidal [Rh(PMe3)4(~CPh)] and square-planar [Rh(PMe3)3(CrCSiMe3)] complexes have been determined by X-ray diffraction.
The mechanism of formation of mer,trans-[(PMe 3 ) 3 Rh(-CtC-R) 2 H] from [(PMe 3 ) 4 Rh(Me)] and terminal alkyne has been studied. The initial step of the reaction is the elimination of methane and the formation of the trigonal bipyramidal complex [(PMe 3 ) 4 Rh(-CtC-R)], a reaction that is complete in time of mixing at -78 °C. This intermediate undergoes an oxidative addition reaction with a second equivalent of alkyne to give fac-[(PMe 3 ) 3 Rh(-Ct C-R) 2 H] as the kinetic product. This fac isomer is not stable above -20 °C and isomerizes to the thermodynamic product mer,trans-[(PMe 3 ) 3 Rh(-CtC-R) 2 H]. fac-[(PMe 3 ) 3 Rh(-CtC-R) 2 H] will exchange alkynyl groups with free alkyne, a reaction that has a lower energetic barrier than the isomerization to mer,trans-[(PMe 3 ) 3 Rh(-CtC-R) 2 H]. Density functional theory studies on all stages of the formation of mer,trans-[(PMe 3 ) 3 Rh(-CtC-R) 2 H] have been carried out and give ground state energies in line with those experimentally observed. Once formed, mer,trans-[(PMe 3 ) 3 Rh(-CtC-R) 2 H] is configurationally stable and not prone to scrambling, although it will react with chloroform, whereupon the hydride is replaced by chloride. The initial product of this reaction is mer,trans-[(PMe 3 ) 3 Rh(-CtC-R) 2 Cl], and this compound has been studied by single-crystal X-ray diffraction. In solution mer,trans-[(PMe 3 ) 3 -Rh(-CtC-R) 2 Cl] isomerizes slowly to mer,cis-[(PMe 3 ) 3 Rh(-CtC-R) 2 Cl]. Scheme 2 Scheme 3 Mechanism of Formation of [(PMe 3 ) 3 Rh(-CtC-R) 2 (H)]
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