Styrene catalysts, platinum complexes

Styrene, a-ethyl-asymmetric hydroformylation catalysts, platinum complexes, 6, 266 asymmetric hydrogenation catalysts, rhodium complexes, 6, 250 Styrene, a-methyl-asymmetric carbonylation catalysis by palladium complexes, 6, 293 carbonylation... 

Platinum complexes [PtCl2(diphosphine)] and [PtCl(SnCl3)(diphosphine)] of the ferrocenyl diphosphine ligands (35a), (35b), and (36) have been synthesized. Complexes [PtCl2(35)] and [PtCl2(36)] have been structurally characterized by XRD. Both the preformed and the in situ catalysts have been used in the hydroformylation of styrene.112... 

Platinum complexes with chiral phosphorus ligands have been extensively used in asymmetric hydroformylation. In most cases, styrene has been used as the substrate to evaluate the efficiency of the catalyst systems. In addition, styrere was of interest as a model intermediate in the synthesis of arylpropionic acids, a family of anti-inflammatory drugs.308,309 Until 1993 the best enantio-selectivities in asymmetric hydroformylation were provided by platinum complexes, although the activities and regioselectivities were, in many cases, far from the obtained for rhodium catalysts. A report on asymmetric carbonylation was published in 1993.310 Two reviews dedicated to asymmetric hydroformylation, which appeared in 1995, include the most important studies and results on platinum-catalogued asymmetric hydroformylation.80,81 A report appeared in 1999 about hydrocarbonylation of carbon-carbon double bonds catalyzed by Ptn complexes, including a proposal for a mechanism for this process.311... 

Chiral bis-(binaphthophosphole) (bis(BNP)) ligands have been used in the asymmetric hydroformylation of styrene. In solution, the free diphospholes display fluxional behavior. Consistent with their structure, the reaction of the bis(BNP) compounds with platinum(II) derivatives gives either cis chelate mononuclear complexes or trans phosphorus-bridged polynuclear derivatives. Coordination to platinum enhances the conformational stability of bis(BNP)s and diastereomeric complexes can be detected in solution. In the presence of SnCl2, the platinum complexes give rise to catalysts that exhibit remarkable activity in the hydroformylation of styrene. Under optimum conditions, reaction takes place with high branched selectivity (80-85%) and moderate enantio-selectivity (up to 45% ee). 

Rhodium (I) complexes of chiral phosphines have been the archetypical catalysts for the hydrocarbonylation of 1-alkenes, with platinum complexes such as (61) making an impact also in the early 1990s[1461. More recently, rhodium(I)-chiral bisphosphites and phosphine phosphinites have been investigated. Quite remarkable results have been obtained with Rh(I)-BINAPHOS (62), with excellent ee s being obtained for aldehydes derived for a wide variety of substrates1 471. For example, hydroformylation of styrene gave a high yield of (R)-2-phenylpropanal (94% ee). The same catalyst system promoted the conversion of Z-but-2-ene into (5)-2-methylbutanal (82% ee). 

The majority of studies of asymmetric hydroformylation with rhodium and platinum complexes have made use of DIOP (49) as a ligand. With either the complex [RhCl(CO)(DIOP)] or [RhCl(C2H4)2]2 plus DIOP, styrene was hydroformylated to 2-phenylpropanal with optical yields of only 16%.366 When a-monodeuterostyrene was used as substrate, with DIOP and complex (34) as catalyst, essentially the same optical yield was obtained.367 The same catalyst with non-deuterated styrene under different conditions gave an optical yield of 25%.368... 

In spite of extensive studies on the asymmetric hydroformylation of olefins using chiral rhodium and platinum complexes as catalysts in early days, enantioselectivity had not exceeded 60% ee until the reaction of styrene catalyzed by PtCl2[DBP-DIOP (l)]/SnCl-> was reported to attain 95% ee in 1982 [8]. Although the value was corrected to 73% ee in 1983 [9], this result spurred further studies of the reaction in connection to possible commercial synthesis of antiinflammatory drugs such as (S)-ibuprofen and (S)-naproxen. The catalyst PtCl2[BPPM... 

Platinum complexes, [PtCl2 (l ,l )-XantBino ] (2) and its S,S analogue, were treated with tin(II) chloride to form the pre-catalyst for chemo-, regio-, and enantio-selective hydroformylation of styrene, vinyl acetate and allyl acetate. Although the reaction showed good chemo- and regio-selectivities, only moderate ee was obtained.103... 

T-Olefin platinum(O) complexes are important starting materials for oxidative addition see Oxidative Addition) or catalysts. Karstedt s catalysts, which are the most active ones for hydrosilylation, have been structurally characterized and found to show the structure of Pt2(M y M y )3 (9), wherein = divinyltetramethyldisiloxane." A styrene analogue Pt°(styrene)3 provides a convenient route to get an r-alkyne platinum complex by displacement (Scheme 27). DFT calculations indicate that aUcyne in the... 

Since the reaction of styrene with o-bis(deuteriodimethylsilyl)benzene produced the expected disilacyclopentene with no deuterium incorporation, the mechanism of this reaction is believed to involve the intermediacy of the cyclic bis(silyl)platinum complex (116) first reported by Eaborn et al. <73JOM(63)107> from the reaction of (113) and the platinum catalyst. However, complex (116) itself did not react with styrene but when 1.1 equivalents of (113) was added, an 83% yield of (114 Ri = H, R2 = Ph) was obtained. 

Zeise s salt, modified with chiral aminophosphane-phosphinites (AMPP), can also be used as a catalyst precursor in asymmetric styrene hydroformylation113. Other catalytic platinum systems for the hydroformylation of styrene are platinum(O)- alkene complexes of the type [Pt(C2H4)(L2)] L2 = l,2-bis[(diphenylphosphino)methyl]benzene and ( + )-Diop 24. When activated with methanesulfonic acid, catalysts for styrene hydroformylation are formed, which give total yields of aldehydes ranging from 44 to 67% and selectivities towards linear 3-phenyl-propanal ranging from 80 to 88%. Smaller amounts of the corresponding alcohols (3-18%) are also obtained with a pronounced selectivity towards 3-phenylpropanol of 94-96%. However, when Diop complexes of this type are used, no asymmetric induction in hydroformylation can be detected24. 

Platinum(II) complexes with diphosphines based on DIOP (85),315-321 CHIRAPHOS (86),316,320 and bdpp (87)322-325 backbones have been prepared to be used, in the presence of SnCl2, as catalyst precursors in asymmetric hydroformylation of styrene and other alkenes. 

With respect to polymer-bound platinum catalysts, a dinuclear amidate-bridged platinum(III) complex was supported on pol3winylpyridine [95] and tested as catalyst in the epoxidation of cyclohexene and styrene by isobutyralde-hyde/O2. Its catalytic activity was found to be significantly lower compared to the same complex immobilized on inorganic supports. 

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