Phosphine ligands sterically demanding

Pd-catalyzed C-C-coupling reactions of the Heck type, e. g., Suzuki [48c] and Stille [49c] cross-coupling, are excellent tools for the preparation of biaryls (see also Section 3.1.9). The activity and stability of the catalyst is highly dependent on the steric and electronic properties of the ligands. Sterically demanding basic alkylphosphines e. g., tri(/-butyl)phosphine, have proven to be very effective ligands in the Heck reaction [50] as well as in the Suzuki cross-coupling [51]. NHCs resemble those basic phosphines (see above) and therefore were tested... 

Isolated mononuclear palladium(O) carbonyl complexes are relatively rare, although more examples have been identified in solution. This appears to be due to facile loss of GO from the palladium center. Those identified or isolated recently are based almost exclusively on monodentate or bidentate phosphine ligands, and result from studies of the alkoxycarbonylation reaction. The stability depends on the nature of the phosphine ligand, with bidentate chelating phosphines containing sterically demanding substitution affording the most stable complexes. 

The crystal structure of the K(18-crown-6) salt shows a fac-octahedral structure (Ru—H 1.59-1.71 A, Ru—P 2.312-2.331 A) with a large distortion from regular octahedral geometry (H-Ru-H 70-88° P-Ru-P 102-111°) owing to the disparate steric demands of the hydride and tertiary phosphine ligands [95]. 

The versatile starting material lCp RuCI 4 (1) reacts rapidly with sterically demanding phosphines (PCy and P Pr ) as well as with the nucleophilic carbene ligands (L) to give deep blue, coordinatively unsaturated Cp Ru(L)CI complexes 2-8 (L= l,.Tbis(2,4,6-lrimethylphenyl) (IMes. 2) 1,3-R2-imidazol-2-ylidene = cyclohcxyl (ICy, 3) 4-methylphenyl (ITol, 4) 4-chlorophenyl (IPCl, 5) adamanlyl (lAd, 6) 4..5-dichloro-1,3-bis(2.4,6-trimethylphenyl) (IMesCI, 7) and 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidenc (IPr. 8) in high yields according to Eq. (4). 

Sterically demanding, water-soluble alkylphosphines 6.10 and 6.11 as ligands have been found to have a high activity for the Suzuki coupling of aryl bromides in aqueous solvents (Eq. 6.35).115 Turnover numbers up to 734,000 mmol/mmol Pd have been achieved under such conditions. Glucosamine-based phosphines were found to be efficient ligands for Suzuki cross-coupling reactions in water.116... 

The research group of Van Leeuwen has focused on catalysis at the core of a carbosilane dendrimer in an effort to be able to control stereoselectivity [10]. To this end, a ferrocenyl diphosphine backbone was functionalized with different generations of carbosilane dendrons producing a series of dendrimer phosphine ligands with an increasing steric demand (see 7 for an example, Scheme 6). In situ... 

Non-activated aryl bromides (but not fluorides) can be used as substrates for palla-dium(0)-catalyzed aromatic nucleophilic substitutions with aliphatic or aromatic amines. These reactions require sodium alcoholates or cesium carbonate as a base, and sterically demanding phosphines as ligands. Moreover, high reaction temperatures are often necessary to achieve complete conversion (Entries 7 and 8, Table 10.4 Experimental Procedure 10.1). Unfortunately, the choice of substituents on the amine... 

Bagnall and Xing-fu205,206 in their study of lanthanide and actinide complexes developed a more sophisticated approach to the analysis of the steric demands of ligands. The emphasis of their work, in contrast to that previously discussed, was on ligands other than phosphines. 

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