Steric hindrance phosphine ligands

The percentage of linear product increases greatly by replacing CO with the much bulkier phosphine ligands. Due to the increased steric hindrance the catalyst shows a distinct preference for the n- over the tw-isomer. Tkatchenko (1991) has reported a detailed analysis of this system in terms of the correlation between catalyst performance (activity and selectivity) and detailed structure. 

Thermochemistry has been used to determine heats of formation of metal-phosphine adducts. When electronic effects are small, the heats measured are a measure of the steric hindrance in the complexes heats of formation decrease with increasing steric bulk of the ligand. Recent thermochemistry concerning ligand effects can be found in reference [18],... 

On mixed ligated complexes of palladium(II) and platinum(II), the cis coordination of the NHC and a phosphine ligand is thermodynamically favored. i27,261,262 Thermal isomerization was reported for tra i -[(NHC)2Cr(CO)4] (M = Cr, Mo) to the ds-complex. For [(NHC)2Mo(CO)4] this proceeds even in the solid state. A barrier of rotation due to a double-bond character of the metal-NHC bond could not be determined so far. This is in agreement with the single-bond character of the metal-carbon bond. Barriers of rotation determined so far are due to steric hindrance. 

The sole coordination of the phosphine sites has also been observed with PBs featuring the flexible (CH2)2 linker. The coordination of ligands 40e and 40f to the [Ru(p-cymene)Cl2] fragment affords complexes 74a and 74b (Scheme 44). Despite the modest steric hindrance around boron, neither Cl-B (coordination mode F) nor Ru-B (coordination mode FI) interaction was found. Coordination mode E was established spectroscopically (8nB = 82 and 87 ppm for 74a and 74b, respectively) and confirmed crystallographically.55... 

The diboration of allenes afforded another series of allylboron compounds that each have a boryl group at the vinyl carbon. The addition had a strong tendency to occur at the internal double bond of terminal allenes such as 1,2-heptadiene (Equation (33)).237 However, steric hindrance in both allenes and phosphine ligands forced the addition toward the terminal double bond of dimethylallene (Equation (34)).237 On the other hand, addition to the terminal double bond occurred selectively for both monosubstituted and 1,1-disubstituted allenes when Pd(dba)2 was used in the presence of a co-catalyst (RI) such as I2, Arl, and iodoalkenes (Equation (35)).238 The role of co-catalyst was attributed to in situ formation of I-Bpin intermediate, which undergoes oxidative addition and insertion leading to 2-boryl-7r-allylpalladium intermediate. 

In recent years, polycyclic P-Si and P-Ge clusters respectively have been obtained by reaction of organosilyltrihalides v th threefold functionalized phosphines [1-3]. A decisive influence on the nature and the steric hindrance of the ligand R of the adequately substituted RECI3 on the reaction can be observed. 

Phosphines that are tightly chelated to the metal center often achieve high selectivity of secondary over tertiary amine products in reactions of primary alkylamines with unhindered aryl halides. The chelation helps prevent competing /3-hydrogen elimination of the aryl palladium amido intermediate (vide infra). Additionally, the greater steric hindrance of bisphosphine palladium complexes, when compared to mono phosphine palladium complexes, prevents diarylation. Some ligands originally introduced by... 

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