Asymmetric hydroformylation, platinum catalysts

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 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... 

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. 

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). 

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... 

The above results were reviewed in 1974 (5). Since then the main advances in the field have been the achievement of asymmetric hydro-carbalkoxylation (see Scheme I, X = -OR) using palladium catalysts in the presence of (-)DIOP (6), the use of other diphosphines as asymmetric ligands in hydroformylation by rhodium (7), and the achievement of the platinum-catalyzed asymmetric hydroformylation (8, 9). Further work in the field of asymmetric hydroformylation with rhodium catalysts has been directed mainly towards improving optical yields using different asymmetric ligands (10), while only very few efforts were devoted to asymmetric hydroformylation catalyzed by cobalt or other metals (11, 12) and it will be discussed in a modified form in this chapter. 

Asymmetric hydroformylations of all the above types have been achieved with rhodium catalysts enantioface- and enantiomer-discriminating hydroformylations also occur with cobalt and platinum catalysts whereas with ruthenium or iridium complexes only enantioface-discriminating synthesis has been reported up to now (see Sect. 2.1.4.). 

The first asymmetric hydroformylation with platinum catalysts was carried out42 using NMDPP as the asymmetric ligand. An optical yield of 9% was obtained in the hydroformylation of 2-methyl-l-butene to 3-methylpentanal. Subsequently, different types of olefins were asymmetrically hydroformylated using a catalytic system formed from [(—)-DIOP]PtCl2 and SnCl2 2 H20 in situ 42,45) (Table 4). 

Despite high volumes of aldehyde products that are manufactured and a deep understanding of the catalytic mechanistic process, no successful industrial asymmetric hydroformylation process has been achieved. Rhodium and platinum catalysts that contain chiral bisphosphine and bisphos-phinite ligands have provided the highest enantioselectivies in the study of asymmetric hydroformylation and are chronicled in several reviews.10-222 224... 

An interesting step forward in asymmetric hydroformylation is the development of chiral catalysts on a platinum/tin basis [12], which surpass... 

Table 12.2 Asymmetric hydroformylation of styrene with platinum and rhodium catalysts modified by (2, 4R)-2,4-bis[(4,R,6R)-4,6-dimethyl-l,3,2-dioxaphosphorinane-2-yloxy]-pentane.

Optically-active aldehydes are very important as precursors not only for biologically active compounds but also for new materials. Asymmetric hydroformylation is an attractive catalytic approach to the synthesis of a large number of chiral aldehydes. With the platinum precursor (Pt(PhCN)2Cl2), anhydrous tin(II) chloride was used as cocatalyst (SnCl2/Pt 1), which is essential for catalytic activity. In case of rhodium systems an excess amount (P/Rh = 4) of diphosphite ligand was always added to the catalyst precursor to exclude the formation of HRh(CO)4, which is an active achiral hydroformylation catalyst. 

New Diop-type ligands are derived from tartaric acid and were originally developed for asymmetric hydrogenation. Most important is the phosphole analog 2.2-dimethyl-4,5-bis(5//-dibenzophosphol-5-ylmethyl)-l,3-dioxolane (DTPHOL or Diop-DBP)57 l6°, which is successfully applied to asymmetric hydroformylation mainly if used with rhodium catalysts i r 7i. 119, ifio kut ajso platinum complexes47,88,124,138. 

A polymer-supported rhodium catalyst modified with Diop attached to non-cross-linked polystyrene, first used in the asymmetric hydroformylation of styrene, gives 95 % branched aldehyde, however with only 2% ee9. Further developments in the preparation and use of cross-linked polymers with attached chiral phosphane ligands (Diop, DIPHOL, BPPM) in rhodium- and platinum-catalyzed asymmetric hydroformylation have led to good to excellent results with respect to the asymmetric induction62-124 157,159 and arc described in Section 1.5.8 2.2.3.2. The results arc integrated in Table 4. 

An active catalyst for asymmetric hydroformylation is created by combining [pt(C2H4)-(-t-)-Diop] and [PtCl,-(+)-Diop]l23, while the platinum(O) catalyst alone turns out to be a poor hydroformylation catalyst. 

Asymmetric hydroformylation of prochiral olefins has been investigated both for the elucidation of reaction mechanism and for development of a potentially useful method for asymmetric organic synthesis. Rhodium and platinum complexes have been extensively studied, and cobalt complexes to a lesser extent. A variety of enantiopure or enantiomerically enriched phosphines, diphosphines, phosphites, diphosphites, phosphine-phosphites, thiols, dithiols, P,A-ligands, and P,5-ligands have been developed as chiral modifiers of rhodium and platinum catalysts. - " ... 

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