Chelate wide bite angle

Bidentate ligands with very wide bite angles (like dpephos or xantphos, Scheme 4) are likely to form unstable chelates prone to dechelation at elevated temperatures, thus giving another route to monophosphine species. The application of such ligands to the crosscoupling of sterically hindered bromides and arylboronic acids under strictly anhydrous conditions enforced by the addition of molecular sieves has been shown to be advantageous (118).398... 

The influence of the bite angle in chelating bis-phosphine ligands was investigated by Bo et al. revealing that wide bite angle ligands destabilize the four-coordinated intermediate and stabilize the transition state, which reflects in the acceleration of reductive elimination [117]. 

In Rh-catalyzed hydroformylation, the n iso ratio increases with the bite angle = (preferred P-M-P angle) of a chelate phosphine, probably because these ligands facilitate the RE step in the mechanism. The Rh complex (9.27) of the wide bite angle ligand, BISBI, has proved particularly useful. ... 

Phosphacyclic diphosphines (73a) and (73b) with wide natural bite angles were synthesized and the effect of the phosphacyclic moieties on the coordination chemistry in the [(diphosphine) Rh(CO)2H] complexes was studied. Both NMR and IR spectroscopy showed that the phosphacyclic xantphos ligands exhibit an enhanced preference for diequatorial chelation compared to the diphenylphosphino-substituted parent compound. In the hydroformylation of 1-octene the introduction of the phosphacyclic moieties leads to higher reaction rates. The dibenzophospholyl- and phenoxaphosphino-substituted xantphos ligands exhibit a high activity and selectivity in the hydroformylation of trans-2- and 4-octene to linear nonanal. CO dissociation rates from the... 

The development of supported aqueous-phase catalysis (SAPC) [275, 276] is a new and efficient way to facilitate the hydroformylation of longer olefins. Most of the SAP catalysts described in the literature use TPPTS as ligand. Only a few sulfonated diphosphine ligands were examined [277]. A water-soluble chelating diphosphine ligand with a wide natural bite angle, based on a xanthene backbone, was studied as a SAP aqueous catalyst. This ligand showed a much better selectivity than the SAP catalysts known so far [278]. 

That tropolone is such an effective chelating agent contributes to its wide application in chemistry. The tropolonate ion forms a five-membered chelating ring of which the bite , that is, the oxygen-to-oxygen distance, is smaller than that in the S-diketonate, i.e. acetylacetonate. The bite is also well illustrated by the bite angle, O-M-0, in metal complexes. 

Yamamoto and Casey separately reported that 2,5-dppm-nor 21 (Figure 9) which has a wide natural bite angle (126 ) [47a,b], gave an unexpectedly low 1 b ratio of only 2.6, which was attributed to its inability to form stable chelates. Indeed, no monomeric metal complexes of 21 have been reported. Molecular mechanics calculations show that the Rh 21 chelate has 12 kcal more strain than the fi ee ligand in contrast, BISBI and DPPE chelates are less than 3 kcal more strained than the free ligands. In designing new wide... 

A water-soluble chelating diphosphine ligand (12) with a wide natural bite angle has been prepared [19a]. These hgands are known to form very selective catalysts for the hydroformylation of 1 -aUcenes, 5delding mainly linear aldehyde as the product. These ligands were also studied as supported aqueous phase catalysts and it was shown that these ligands performed well... 

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