Triphenylmethyl ligands

The crystal structure of the red triphenylmethylsodium TMEDA complex (compound XVI in Fig. 3), published by Weiss and Koster (41), resembles that of the red triphenylmethyllithium-TMEDA complex (42) and can be described as a n complex between a triphenylmethyl carbanion with an sp2-hybridized central carbon atom and a sodium cation coordinated to the bidentade ligand TMEDA. The sodium atom has close contacts to several carbon atoms of the triphenylmethyl ligand, which possesses twisted phenyl groups. An additional short distance exists between sodium and a p-C (phenyl) atom of a neighboring n system. 

With the accessibility of the fairly stable pentaphenylphosphorane (12) and its even more stable homologues, there arose the intriguing question of the stereochemistry of such species. The first speculations were of course based on a trigonal bipyramidal model, since this structure type had already been proved for phosphorus penta-halides21). It was not possible, however, to differentiate in chemical reactions between the unlike ligands in compounds such as tetraphenyl(triphenylmethyl)-phosphorane (22) and tetraphenyl-p-tolylphosphorane 33) or in pentaphenylphosphorane derivatives appropriately labeled with deuterium or 14C 39). 

One of the most studied polymerization systems employs alkyllithium initiators that are modified by chiral amine ligands for the polymerization of sterically bulky methacrylates [8,38,39,40,41], acrylates [42],crotonates [43], and acrylamides [44]. A primary example is the reaction of triphenylmethyl methacrylate with an initiator derived from 9-fluorenyllithium and (-)-sparteine (3) at -78 °C (Scheme 4). The resultant isotactic polymer is optically active, and is postulated to adopt a right-handed helix as it departs from the polymerization site. This polymer has been particularly successful as a chiral stationary phase for the chromatographic resolution of atropisomers [8]. Many modifications of the or-ganolithium initiator/chiral ligand system have been explored. Recently, Okamo-to has applied enantiopure radical initiators for the enantioselective polymerization of bulky methacrylate monomers [45]. 

Triphenylcarbinol (PI13COH), 281 Triphenylmethyl carbocation, 106, 146, 281 Triphenylphosphine (PPI13) effect on Ao, 181 as L ligand, 176 trails effect, 181... 

The direction of the reaction shown in Eq. 9 indicates that the cation (LXXI), which might formally be regarded as a carbonium ion complexed to Fe(CO)3, is more stable than the triphenylmethyl cation. The opposite is true when comparing the relative stabilities of the noncomplexed cyclo-hexadienyl cation and the triphenylmethyl cation. Viewed in this way the decreased electrophilicity of the cation (LXXI) may be ascribed to the large extent of electron transfer from the metal to the ligand via back donation. In line with this is the fact that cationic complexes of this type show abnormally high carbonyl absorption frequencies in the infrared. 

Triphenylmethyl tetrafluoroborate abstracts a hydride ligand RuH2(PPh3)4 + (Ph3C)Bp4 [RuH(PPh3)4] BF4 + Ph3CH... 

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