Phosphines bulky

With some bulky phosphines, OsH2C12(PR3)2 have been obtained (PR3, e.g. PPr3, PMeBu2) [167]... 

With less bulky phosphines, 18 electron OsH2C12(PR3)3 are known wer-OsCl3(PEt2Ph)3 ----> OsH2Cl2(PEt2Ph)3... 

The 4 1 complexes tend to be formed only by less bulky phosphines and even then tend to be coordinatively saturated. 

Further evidence of steric crowding owing to bulky phosphines is found in RhCl(CO)(PBu2 alkyl)2. Study of the 3IP NMR spectra at low temperature, freezing in the rotational conformers shows separate signals for each (Figure 2.25) [62],... 

Ir(PMe2Ph)J + H2 -> IrH2(PMe2Ph)J With bulky phosphines, 6-coordinate structures are not possible [157]... 

With less bulky phosphines, 5-coordinate M(PR3)3X2 can be obtained... 

Complexes of bulky phosphines and internal metallation reactions... 

An interesting parallel was found while the microwave-enhanced Heck reaction was explored on the C-3 position of the pyrazinone system [29]. The additional problem here was caused by the capability of the alkene to undergo Diels-Alder reaction with the 2-azadiene system of the pyrazinone. An interesting competition between the Heck reaction and the Diels-Alder reaction has been noticed, while the outcome solely depended on the substrates and the catalyst system. Microwave irradiation of a mixture of pyrazinone (Re = H), ethyl acrylate (Y = COOEt) and Pd(dppf)Cl2 resulted in the formation of a mixture of the starting material together with the cycloaddition product in a 3 1 ratio (Scheme 15). On the contrary, when Pd(OAc)2 was used in combination with the bulky phosphine ligand 2-(di-t-butylphosphino)biphenyl [41-44], the Heck reaction product was obtained as the sole product. When a mixture of the pyrazinone (Re = Ar) with ethyl acrylate or styrene and Pd(dppf)Cl2 was irradiated at 150 °C for 15 min, both catalytic systems favored the Heck reaction product with no trace of Diels-Alder adduct. 

The hydrosilylation of alkynes has also been studied using as catalysts Pt, Rh, Ir and Ni complexes. The improvement of the regioselectivity of the catalyst and the understanding of stereoelectronic factors that control it have been major incentives for the ongoing research. From numerous studies involving non-NHC catalysts, it has been established that there is a complex dependence of the product ratio on the type of metal, the aUcyne, the metal coordination sphere, the charge (cationic versus neutral) of the catalytic complex and the reaction conditions. In the Speier s and Karstedt s systems, mixtures of the thermodynamically more stable a- and -E-isomers are observed. Bulky phosphine ligands have been used on many occasions in order to obtain selectively P-f -isomers. 

The P-H oxidative addition, acrylonitrile insertion, and C-H reductive elimination steps were observed directly with the dcpe catalyst, and the potential intermediates Pt(diphos)(PHMes )(CH2CH2CN) (7, diphos = dppe, dcpe) were shown not to undergo P-C reductive elimination. The generality of this proposed mechanism for less bulky phosphine substrates, or for Pt catalysts supported by monodentate ligands, remains to be investigated [9]. 

Sulfur dioxide and carbon disulfide react with bis(bulky phosphine)platinum complexes to give monomeric trigonal planar [Pt(PR3)2(S02)] and trimeric complexes such as [Pt3(/u-S02)(PR3)2] and [Pt3(/i-CS2)(PR3)2] where R = Cy or But70 71... 

Asymmetric hydrophosphination has been utilized as a route for preparing chiral phosphines. The Pt° complex [(Me-DUPHOS)Pt(t/ tf/ ,s-PhCII ClIPh)] (73) brings about the catalytic P-H addition of bulky secondary phosphines to activated alkenes with modest enantioselectivity. The most promising substrate combinations for further development appear to be bulky alkenes and less bulky phosphines (Scheme 46).195... 

From the results discussed above, it appears that the main influence of phosphines in controlling the course of propene dimerization consists in hindering, through steric interference, the second insertion step from proceeding according to Ni — C2 type addition. An explanation for the predominant Ni — C2 type addition in the absence of steric hindrance in terms of the polarity of metal-carbon, metal-H. and C=C bonds has been recently presented (104). The extremely bulky phosphine (t-C4H9)2P-i-C3H7 is even able to reverse the direction of Ni—H addition to propene, a step which is obviously less sensitive toward steric hindrance. 

An interesting decarbamoylative arylation is observed when the reaction of secondary 2-thiophenecarboxamides with bromobenzene is conducted in the presence of Pd(OAc)2 and a bulky phosphine [P(<9-biphenyl)(/-Bu)2] (Equation (61 )).70 However, the arylation occurs at the 2-position without the removal of the ester moiety when the reaction of 3-carboethoxyfuran with an aryl bromide is conducted.71... 

Based on the insight that a dissociative mechanism plays the major role along the metathesis pathway [11], these catalysts have been designed such that only one bulky phosphine, one chloride and one cumulenylidene ligand are attached to a Ru(II) center. Because arene ligands are known to be labile on such a metal fragment, they will easily liberate free coordination sites ( ) for the interaction with the alkene substrate. Although the precise mode of action of such allenyli-... 

The catalytic system of [Ir(COD)Cl]2 with an excess of the bulky phosphine P(o-MeOPh)3 under transfer hydrogenation conditions of propan-2-ol and KOH was used successfully in the selective hydrogenation of cinnamaldehyde (Scheme 15.4) [4]. Selectivity and activity were found to increase with increasing P/Ir ratios, and complete conversion was achieved in as little as 5 minutes (turnover frequency (TOF) 6000 IT1). 

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