Stereochemistry phosphine complexes, determination

In phosphine complexes, the v MP modes (around 400 cm-1)36,64,20s are often obscured by aryl-group vibrations. Main efforts for the determination of stereochemistry are concentrated on 31P nmr spectroscopy, especially in the case of platinum(II) and platinum(IV) complexes36,371 (195Pt nucleus ... 

Of at least as great importance to the chemistry of PX3 compounds as the electronic factors are steric factors.6 Indeed these may be more important or even dominant in determining the stereochemistries and structures of compounds Steric factors also affect rates and equilibria of dissociation reactions and the stereochemistry of phosphine ligands is the prime factor in many highly selective catalytic reactions of phosphine complexes, such as hydroformylation and asymmetric hydrogenation. 

In many reactions, particularly of phosphine complexes, the above reaction scheme still applies, but with a rapidly established dissociative equilibrium followed by rate-determining reaction of ML3 with Y. This is the case in oxidative elimination reactions, which are discussed in the oxidative addition section. Sometimes the mechanism and products depend on conditions, as in reaction of Pt(PPha)4 with hydroxyacetylenes, where the nature of the solvent, the presence of excess of phosphine, and particularly the stereochemistry of the alkyne, all affect the course of the reaction. ... 

Competitive experiments of allyl phenyl sulfide and allyl phenyl selenide with this system indicate that oxidative addition of the organic selenide is faster. The equilibrium displacement between the allylic sulfide and 7 -allylic complex depends on the actual precursor for Pd(0) phosphine complexes (Equation (41)). The stereochemistry of the reverse reaction on 7/ -cyclohexenyl palladium thiolate dimers (nucleophilic attack of sulfide) has been determined to be trans. According to the principle of microscopic reversibility, the oxidative addition of allylic sulfides must occur with inversion of configuration. 

For platinum(II) complexes with alkylphosphine ligands there is a small but marked dependence of the values (PC) on the nature of the group tram to phosphine.1353 By analogy with the earlier method using 3H NMR, 13C 1H NMR techniques using virtual coupling have been used for phosphite complexes of platinum(II), but it does not appear that the method can be generally used to determine stereochemistry.1354... 

In an attempt to resolve this question of stereochemistry and also to determine whether or not the decarbonylation of an acid chloride containing a f3 hydrogen takes place stereospecifically, erythro- (XI) and fhreo-2,3-diphenylbutanoyl chlorides (XII), obtained by the reaction of the known acids (13, 14) with oxalyl chloride, were synthesized. The reaction of these acid chlorides (see Reaction 8) with chlorotris( triphenyl-phosphine) rhodium gave the corresponding acyl complexes of type lib [R = C6H5CH(CH3)CH(C6H5)]. Decarbonylation of the erythro- cy complex in benzene at 30 °C gave a 90% yield of frans-a-methylstilbene while decarbonylation of the threo-acyl complex under similar reaction... 

The stereochemistry of ligands, especially that of tertiary phosphines is well described by Tolman s cone angle concept99. The cone angle describes the angle between the outer substituent atoms, determined by their van der Waals radii, and the metal atom in mononuclear complexes (see Fig. 15). 

The use of NMR in determining the stereochemistry of group V complexes has lead to a controversy. Shaw et al. (153) examined the utility of C NMR in defining stereochemistry of tertiary phosphine and tertiary arsine compounds. The NMR is not always suitable in studying these complexes because of the great complexity of the spectra. The great utility of NMR appeared to be in the simplicity of the interpretation of the decoupled spectra. 1-2-1 triplets are, for example, observed for six of the eight carbon... 

Characterization Hydrides are usually detected by H NMR because they resonate to high field of TMS in a region (0-605) normally free of other ligand resonances. They couple with the metal, where this has i spin, and with cis (7 = 15-30 Hz) and trans (J = 90-150 Hz) phosphines, which is often useful for determining the stereochemistry of the complex. Inequivalent... 

Increasing the steric requirement of the substituent at the benzylic carbon ( Bu for Me) produced clear improvement in the resolution of monodentate phosphines chiral at the P atom [29]. The stereochemistry and conformation of these complexes (Figure 4.10) was determined by monodimensional NOE experiments and by analysis of J values. The palladacycle with configuration for the stereogenic C atom showed exclusively the X conformation with both enantiomers of the phosphine. 

Reactions of cationic [(2,4-dimethylpentadienyl)Fe(CO)3]+ complexes with sodium methoxide and with phosphines have been reported. Use of (5)-neomenthyldiphenylphosphine under appropriate conditions afforded a single optically pure diastereomer, the stereochemistry of which was determined by single crystal X-ray methods. Treatment of [(Ti4-2-alkoxy-4-vinyl-2,5-dihydrofuran)Fe(CO)3] complexes with nucleophiles in the presence of Lewis acids afforded297 exo-addition products with high diastereoselectivity. Hydrolysis of the products afforded hydroxyaldehyde complexes. 

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