Styrenes asymmetric

A Z>2-symmetric chiral /ra/7,v-dioxoruthenium( VI) porphyrin, [RuVI(L1)02], bifa-cially encumbered by four threitol units can effect enantioselective epoxidation of (fs)- 3-methylstyrene in up to 70% ee. For the asymmetric styrene oxidation, a lower enantioselectivity of 40 % ee was obtained (c.f. 62 % ee, see Table 6.3) when... 

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. 

The effects of bisphosphite ligand structure on regioselectivity and enantio-selectivity in asymmetric styrene hydroformylation are shown in Table 1. Catalytic reactions were preformed at ambient temperature and 130 psi CO/H2. Hydroformylation regioselectivity was determined by GC of the product aldehydes. Enantioselectivity was determined by chiral GC after conversion to the carboxylic acid (eqn 1). The, i -enantiomer of the bisphosphites in Figure 1 all produced the... 

Figure 3.4 Asymmetric styrene/ butadiene biock copoiymers with different moieouiar architecture as iiiustrated on the ieft side of each micrograph note that the sampies have aimost identioai net ohemicai composition (74 voi.% PS), but differ drasticaiiy in their morphoiogy on the right-hand side are TEM micrographs, PB phase stained dark with OSO4...

Catalytic, enantioselective cyclopropanation enjoys the unique distinction of being the first example of asymmetric catalysis with a transition metal complex. The landmark 1966 report by Nozaki et al. [1] of decomposition of ethyl diazoacetate 3 with a chiral copper (II) salicylamine complex 1 (Scheme 3.1) in the presence of styrene gave birth to a field of endeavor which still today represents one of the major enterprises in chemistry. In view of the enormous growth in the field of asymmetric catalysis over the past four decades, it is somewhat ironic that significant advances in cyclopropanation have only emerged in the past ten years. 

Bartoli recently discovered that by switching from azide to p-anisidine as nucleophile, the ARO of racemic trans- 3-substituted styrene oxides could be catalyzed by the (salen)Cr-Cl complex 2 with complete regioselectivity and moderate selectivity factors (Scheme 7.36) [14]. The ability to access anti-P-amino alcohols nicely complements the existing methods for the preparation of syn-aryl isoserines and related compounds [67] by asymmetric oxidation of trans-cinnamate derivatives [68]. 

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... 

Analogous results were obtained for enol ether bromination. The reaction of ring-substituted a-methoxy-styrenes (ref. 12) and ethoxyvinylethers (ref. 10), for example, leads to solvent-incorporated products in which only methanol attacks the carbon atom bearing the ether substituent. A nice application of these high regio-and chemoselectivities is found in the synthesis of optically active 2-alkylalkanoic acids (ref. 13). The key step of this asymmetric synthesis is the regioselective and chemoselective bromination of the enol ether 4 in which the chiral inductor is tartaric acid, one of the alcohol functions of which acts as an internal nucleophile (eqn. 2). 

Chelucci et al. [41] synthesized further chiral terpyridines derived from (-)-yd-pinene, (-i-)-camphor, and (-l-)-2-carene and tested their ability to chelate copper or rhodium for the asymmetric cyclopropanation of styrene. The copper catalysts were poorly efficient and selective in this reaction. The corresponding rhodium complexes led to the best result (64% ee) with the ligand derived from (-l-)-2-carene (ligand 33 in Scheme 17). 

Woo et al. [54] prepared new chiral tetraaza macrocyclic hgands (48 in Scheme 23) and their corresponding iron(II) complexes and tested them, as well as chiral iron(II) porphyrin complexes such as Fe (D4 -TpAP) 49, in asymmetric cyclopropanation of styrene. 

Hydroformylation has been extensively studied since it produces optically active aldehydes which could be important precursors for pharmaceutical and fine chemical compounds. Thus, asymmetric hydroformylation of styrene (Scheme 27) is a model reaction for the synthesis of ibuprofen or naproxen. Phosphorus ligands were used for this reaction with excellent results, espe-... 

Some chiral mono-, acyl- and di-thioureas have been used as ligand for the Rh-catalysed asymmetric hydroformylation of styrene. Although thiourea ligands form inactive systems with [Rh(COD)Cl]2 as the catalyst precursor, in standard conditions (40 °C, 40 bar CO -l- H2 1/1), the cationic Rh complex [Rh(COD)2]Bp4 combined with monothioureas as the ligand showed moderate to good activity (Scheme 29) [114]. 

In 2008, Que and coworkers reported an asymmetric version of the dihydroxylation with a new type of ligands bearing bipyrrolidine as the chiral backbone [71]. The corresponding iron(II) complex showed general activity in the dihydroxylation of various olefins using H202- Satisfactory results are obtained with aliphatic as well as with aromatic olefins. For example, dihydroxylation of styrene gave styrene oxide and 1-phenylethane-1,2-diol in <1% and 65% yield, respectively (Scheme 10). 

In 2004, ruthenium-catalysed asymmetric cyclopropanations of styrene derivatives with diazoesters were also performed by Masson et al., using chiral 2,6-bis(thiazolines)pyridines. These ligands were prepared from dithioesters and commercially available enantiopure 2-aminoalcohols. When the cyclopropanation of styrene with diazoethylacetate was performed with these ligands in the presence of ruthenium, enantioselectivities of up to 85% ee were obtained (Scheme 6.6). The scope of this methodology was extended to various styrene derivatives and to isopropyl diazomethylphosphonate with good yields and enantioselectivities. The comparative evaluation of enantiocontrol for cyclopropanation of styrene with chiral ruthenium-bis(oxazolines), Ru-Pybox, and chiral ruthenium-bis(thiazolines), Ru-thia-Pybox, have shown many similarities with, in some cases, good enantiomeric excesses. The modification... 

In 1999, Casado et al. developed heterotetranuclear complexes (TiRh3) depicted in Scheme 10.3 with bridging sullido ligands combined with P-donor ligands. These complexes were further tested as catalysts for the asymmetric hydroformylation reaction of styrene. In this process, [CpTi((/i3-S)3 Rh(tfbb 3] was efficiently active under mild conditions (10 bar, CO/H2 = 1 atm, 353 K). In order to explore the effect of the added phosphorus ligand and the possibilities of this system for the asymmetric hydroformylation of styrene, achiral diphosphines such as dppe (l,2-bis(diphenylphosphine)ethane) and... 

In 2000, better results were obtained by Bonnet et al. by using readily available chiral thioureas as new ligands in the asymmetric rhodium-catalysed hydroformylation of styrene. In general, the conversion of styrene and enantioselectivities were modest, but when the reaction was carried out in heptane as the solvent, an enantioselectivity of 41% ee was obtained (Scheme 10.6). 

In addition, various chiral (p-A -sulfonylaminoalkyl)phosphine ligands were earlier employed by Achiwa et al. for the asymmetric palladium-catalysed hydrosilylations of cyclopentadiene and styrene, affording the corresponding... 

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