Ligands trialkylphosphines

R = aryl, benzyl, vinyl (alkenyl), alkyl (no p hydrogen) R, R , R" = alkyl, aryl, alkenyl X = Cl, Br, I, OTf, OTs, N2 ligand = trialkylphosphines, triarylphosphines, chiral phosphines base = 2° or 3° amine, KOAc, NaOAc, NaHCOa... 

Ligand-Modified Cobalt Process. The ligand-modified cobalt process, commercialized in the early 1960s by Shell, may employ a trialkylphosphine-substituted cobalt carbonyl catalyst, HCo(CO)2P( -C4H2)3 [20161 -43-7] to give a significantly improved selectivity to straight-chain... 

AT-heterocyclic carbenes show a pure donor nature. Comparing them to other monodentate ligands such as phosphines and amines on several metal-carbonyl complexes showed the significantly increased donor capacity relative to phosphines, even to trialkylphosphines, while the 7r-acceptor capability of the NHCs is in the order of those of nitriles and pyridine [29]. This was used to synthesize the metathesis catalysts discussed in the next section. Experimental evidence comes from the fact that it has been shown for several metals that an exchange of phosphines versus NHCs proceeds rapidly and without the need of an excess quantity of the NHC. X-ray structures of the NHC complexes show exceptionally long metal-carbon bonds indicating a different type of bond compared to the Schrock-type carbene double bond. As a result, the reactivity of these NHC complexes is also unique. They are relatively resistant towards an attack by nucleophiles and electrophiles at the divalent carbon atom. 

Thereafter, however, P-chirogenic phosphine ligands were the subject of less investigation since the synthesis of highly enantiomerically enriched P-stereo-genic phosphines often proves difficult. Another reluctance Hes in the fact that this class of phosphines, especially diaryl- and triarylphosphines, is conforma-tionally unstable and gradually racemize at high temperature [57,58]. In contrast, optically active trialkylphosphines are known to be optically stable even at considerably elevated temperature. 

At least partial substitution of other inherently strong ligands by phosphines can be achieved. Trialkylphosphines substitute partially into arylisocyanide complexes [Co(CNR)5)]+, with monosubstitution and at best disubstitution observed, influenced by the steric bulk of the phosphine.152... 

The cross-coupling of aryl or alkenylsilanes with aryl chlorides bearing electron-withdrawing substituents has been reported to require trialkylphosphine (P Pr3) ligands.407... 

The other use of the bulky trialkylphosphines is in the cases where PPh3 or similar ligands can interfere with the cross-coupling reaction by forming by-products (115).412... 

A major advance in homogeneous catalysis was the introduction of a trialkyl-phosphine to supplement the role of carbon monoxide in catalyst stabilization. [3] A ligand modifier such as trialkylphosphine serves three principal roles in a homogeneous catalytic process. It stabilizes the metal, it influences the reaction rate, and it influences process selectivity. 

High ligand concentrations and/or low partial pressures of carbon monoxide cause a predominance of species substituted by more than one phosphorus ligand. These species containing multiple ligands present a greater sterically hindered environment for the olefin substrate and favor the linear product (24). Trialkylphosphines, the more basic ligands of the... 

Silylstannanes add to ethylene in the presence of palladium catalysts (Equation (111)). A trialkylphosphine with moderate steric bulkiness is suitable as the ligand for the reaction.276... 

The first fluorous phosphine ligands for application in fluorous phase catalysis were fluorous trialkylphosphines 1 [7-9]. 

Optically active trialkylphosphines are known to be configurationally stable toward racemization, and ligands of type 3 have been synthesized and applied in the asymmetric catalytic hydrogenation of amidoacrylic acids (Scheme 6 5). 

The comparison of NHCs with various other monodentate ligands such as phosphines and amines on a [MoLsfCOls], a tra/w-[RhL2(CO)X], and various other [M(CO)nLm] complexes shows the significantly increased donor capacity relative to phosphines, even to trialkylphosphines (Table The... 

Promising applications for metal-NHC compounds in materials science and medicinal chemistry are based on the strong metal-carbon bond and the high donor capability of the NHC. The most extensive investigations have been carried out in the field of homogeneous catalysis. Here, NHCs have to be considered as typical directing or innocent spectator ligands, best comparable to trialkylphosphines. The recent successful applications of NHCs in ruthenium-catalyzed olefin metathesis... 

The effect of varying the triorganophosphine ligands in the palladium-catalyzed carbonylations has been looked at briefly. In general, triarylphos-phines are much better than triorgano phosphites and trialkylphosphines. Changes in the substituents in the triarylphosphine have only minor effects upon reaction rates (//). 

Simple trialkylphosphines, R3P, can be prepared relatively easily by using excess Grignard or alkyllithium reagents.1,2 However, the synthesis of mixed R2 R P monophosphines or chelating aliphatic ligands of the type of R2 P(CH2 )n A (where A = a donor group and n - 2,3,4) have been almost prohibitively difficult, owing to the toxicity, chemical reactivity, oxidation sensitivity, and commercial unavailability of likely precursors. 

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