P-Bidentates

Although (R, R )-( )- and (R, S )-( )-93 epimerize at phosphorus when heated at 140 °C for ca 2 h, epimerization at arsenic was not effected by the usual racemizing conditions for tertiary arsines (methanolic HCl) indeed, optically active salts protonated at phosphorus were isolated. 

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Finally, the participation of the Ar,P-bidentated phosphinous amides 61-65 (Scheme 45) as chiral ligands in the catalytic enantioselective hydrosilylation of acetophenone with diphenylsilane is worth mentioning, despite low ee (2-20%) [52]. 

In 2001, Imamoto et al. reported the preparation of novel chiral S/P-bidentate ligands containing a chirogenic centre at the phosphorus atom and their stereoinduction capability in palladium-catalysed asymmetric allylic substitution reactions (Scheme 1.14)." ... 

In 2005, Carretero et al. reported a second example of chiral catalysts based on S/P-coordination employed in the catalysis of the enantioselective Diels-Alder reaction, namely palladium complexes of chiral planar l-phosphino-2-sulfenylferrocenes (Fesulphos). This new family of chiral ligands afforded, in the presence of PdCl2, high enantioselectivities of up to 95% ee, in the asymmetric Diels-Alder reaction of cyclopentadiene with A-acryloyl-l,3-oxazolidin-2-one (Scheme 5.17). The S/P-bidentate character of the Fesulphos ligands has been proved by X-ray diffraction analysis of several metal complexes. When the reaction was performed in the presence of the corresponding copper-chelates, a lower and opposite enantioselectivity was obtained. This difference of results was explained by the geometry of the palladium (square-planar) and copper (tetrahedral) complexes. 

Actually, the reactions led to the formation of the corresponding acyclic Mannich-type addition products, which were further transformed into their corresponding Diels-Alder adducts by treatment with TFA (Scheme 5.18). The 5, P-bidentate character of the ligands was proven by isolation of one complex and its X-ray analysis. Complexes prepared with CuCl afforded the expected product in up to 80% ee, whereas the use of CuBr as the catalytic precursor allowed an enantioselectivity of 97% ee to be obtained with the similar ligand. 

Stereospecific replacement of 12CO by 13CO in LMo (CO)4 complexes (L = P-N or P-P bidentate ligand) followed by IR (some spectral data given)... 

The amino(phosphino)carbene 12a with a pyramidalized phosphino substituent behaves as a,p-bidentate ligand to give highly stable Rh(I) [53] and Pd(II) [54] complexes. 

Melaimi, M. and Gabbai, F. P., Bidentate group 13 Lewis acids with ortho-phenylene and peri-naphthalenediyl backbones , Adv. Organomet. Chem., 2005, 53, 61-99. 

Other types of N,P-donor ligand systems (chelate included) were recently reviewed [13,703,704], In this respect, it is worth noting that the examined ligand systems with N,P,0-donor centers participate in coordination with metals as N,P-bidentate ligands [704-706],... 

Complex compounds obtained on the basis of phosphoranimine 409 [712a] and diaminophosphine 410 [520] ligands are of considerable interest. Complex compounds of the types 408 and 410 are represented in general by chelates, and 409 by cluster structures. N,P-Bidentate ligands and their rhodium and palladium complexes are described in a recent review [712b],... 

The syntheses and structures of frons-[Ir(Bu2PC6H40)2] and fra/tt-[Ir Bu2PC, H3(0Me)0 2] (79) have been reported by Mason and Thomas.185 The iridium(II) state is presumably stabilized by the O.P bidentate ligands. X-Ray analysis of these complexes reveal C, symmetry and planarity. 

In the arylative ring opening of 3-substituted cyclobutanols, enantioselective cleavage of the C-C bond has been achieved by using a palladium catalyst with a chiral ligand [33-35]. Particularly, the use of the chiral ferrocene-containing N,P-bidentate ligand shown in Eq. 17 leads to excellent enantioselectivity. 

Aryltrialkoxy- and diaryldialkoxytitaniums are relatively stable (some of them are even distillable) compounds which readily undergo palladium-catalysed reaction with aryl iodides, bromides and chlorides, as well as triflates to furnish the biaryls in very high yields. For instance, the reaction of 2-naphthyl triflate (165) with phenyltitanium triisopropoxide (212, in 20 mol% excess) catalysed by palladium-complex of N,P-bidentate ligand 213 gave 2-phenylnaphthalene (214) in 98% yield [143], respectively, Scheme 40. 

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