Styrene enantioselective

Homochiral pyridyl, bipyridyl, and phosphino derivatives of 2,2-dimethyl-1,3-dioxolane (113)-(115) were prepared from T-(+)-tartrate. These compounds were assessed in metal-catalyzed asymmetric hydroformylation of styrene enantioselectivity was generally low.342... 

Chiral diphosphites based on (2R,3R)-butane-2,3-diol, (2R,4R)-pentane-2,4-diol, (25, 5S)-hexane-2,5-diol, (lS -diphenylpropane-hS-diol, and tV-benzyltartarimide as chiral bridges have been used in the Rh-catalyzed asymmetric hydroformylation of styrene. Enantioselectivities up to 76%, at 50% conversion, have been obtained with stable hydridorhodium diphosphite catalysts. The solution structures of [RhH(L)(CO)2] complexes have been studied NMR and IR spectroscopic data revealed fluxional behavior. Depending on the structure of the bridge, the diphosphite adopts equatorial-equatorial or equatorial-axial coordination to the rhodium. The structure and the stability of the catalysts play a role in the asymmetric induction.218... 

For a standard substrate such as styrene, enantioselective diamination could be mediated by several hypervalent iodine reagents (Scheme 21) [65,68]. For example. 

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. 

A number of studies have recently been devoted to membrane applications [8, 100-102], Yoshikawa and co-workers developed an imprinting technique by casting membranes from a mixture of a Merrifield resin containing a grafted tetrapeptide and of linear co-polymers of acrylonitrile and styrene in the presence of amino acid derivatives as templates [103], The membranes were cast from a tetrahydrofuran (THF) solution and the template, usually N-protected d- or 1-tryptophan, removed by washing in more polar nonsolvents for the polymer (Fig. 6-17). Membrane applications using free amino acids revealed that only the imprinted membranes showed detectable permeation. Enantioselective electrodialysis with a maximum selectivity factor of ca. 7 could be reached, although this factor depended inversely on the flux rate [7]. Also, the transport mechanism in imprinted membranes is still poorly understood. 

Figure 5.4-3 shows the results of a lifetime study for Wilke s catalyst dissolved, activated, and immobilized in the [EMIM][(CF3S02)2N]/compressed CO2 system. Over a period of more than 61 h, the active catalyst showed remarkably stable activity while the enantioselectivity dropped only slightly. These results clearly indicate - at least for the hydrovinylation of styrene with Wilke s catalyst - that an ionic liquid catalyst solution can show excellent catalytic performance in continuous product extraction with compressed CO2. 

Unfortunately, the highest enantioselectivity so far obtained for the synthesis of styrene oxide by this route is only 57 % ee with Goodman s sulfide 30 [21]. Thus methylidene transfer is not yet an effective strategy for the synthesis of terminal epoxides. 

These two compounds with S configuration on their oxazohne rings were tested as copper(I) catalysts for the cyclopropanation of styrene, the hgand 9 with S axial chirality being much more enantioselective than 10 with the R configuration. Thus, the catalytic system CuOTf-(S,S)-bis(oxazolyl)-binaphthyl (9, R = Bu) led to excellent enantioselectivities, particularly for the cyclopropanation of styrene with (-menthyldiazoacetate 95% ee for the trans-cyclopropane and 97% ee for the cis, with trans/cis = 68/32. 

Davies [30] studied the PyBOx-induced conformational effects by testing several ligands sterically hindered on the oxazoUne moieties (Scheme 11, structures 18 and 19). However, these new ligands gave poorer results in terms of yields and enantioselectivities than ligand 16 for the Ru-catalyzed cyclopropanation reaction, indicating unfavorable steric interactions between styrene and the carbene complex. 

In dichloromethane, this complex with alow catalyst loading (1 mol%) achieved the cyclopropanation of styrene with ethyldiazoacetate in high di-astereoselectivity (trans/cis = 90/10) and enantioselectivity (up to 97% ee for the major isomer). 

Metalloporphyrins have proved their efficiency as ruthenium carbonyl hg-ands for the enantioselective cyclopropanation of styrene [50,51]. 

Compounds lb and 2b were the Urst fluorinated ligands tested in Mn-catalyzed alkene epoxidation [5,6]. The biphasic Uquid system perfluorooc-tane/dichloromethane led to excellent activity and enantioselectivity (90% ee) in the epoxidation of indene with oxygen and pivalaldehyde (Scheme 1, Table 1). In addition, the fluorous solution of the catalyst was reused once and showed the same activity and selectivity. This represents a considerable improvement over the behavior in the homogeneous phase, where the used catalyst was bleached and reuse was impossible. Unfortunately, indene was the only suitable substrate for this system, which failed to epoxidize other alkenes (such as styrene or 1,2-dihydronaphthalene) with high enantioselectivity. The system was also strongly dependent on the oxidant and only 71% ee was obtained in the epoxidation of indene with mCPBA at - 50 °C. 

The copper complexes of these ligands were tested in the cyclopropanation of styrene with ethyl diazoacetate (Scheme 7) and the ene reaction between a-methylstyrene and ethyl glyoxylate (Scheme 8). hi both cases moderate enantioselectivities were obtained but these were lower than those foimd with the parent hgand. 

The solids were used as catalysts in the benchmark cyclopropanation reaction between styrene and ethyl diazoacetate (Scheme 7). As far as the nature of the clay is concerned, laponite was foimd to be the best support for the catalytic complexes. The best enantioselectivity results (Table 7) were obtained with ligand 6b (69% ee in trans cyclopropanes and 64% ee in cis cyclopropanes) but the recovered solid showed a lower activity and enantioselectivity, which was attributed to partial loss of the chiral ligand from the support. In general, the use of the three chiral ligands led to enantioselectivity results that were intermediate between those obtained in homogeneous phase with CuCl2 and Cu(OTf)2 as catalyst precursors. This seemed to indicate that the sohd behaved as a counterion with an intermediate coordinating abihty to the copper centers. 

It is concluded from these results that with this kind of non-C2 symmetric ligand (that led necessarily to poor enantioselectivities in homogeneous phase), it is possible to exploit support effects to change the trans/cis selectivity and to improve the enantioselectivity. This is demonstrated for the trans-cyclopropanes obtained with ligand 10a in styrene. Due to the relative disposition of the ester and phenyl groups in the transition state, support ef-... 

The reaction used to test these solid catalysts was the aziridination of styrene with AT-tosyliminophenyliodinane (Phi = NTos) (Scheme 10). In most cases, enantioselectivities were low or moderate (up to 60% ee). The loss of enantioselectivity on changing from ligand 11a to ligand 12 was attributed to the fact that ligand 12 is too big for the copper complex to be accommodated into the zeolite supercages. Further studies carried out with hgands 11a and 11b [62] demonstrated that the reaction is more enantioselective with the supported catalyst (82% ee with 11a and 77% ee with 11b) than in solution (54% ee with 11a and 31% ee with 11b). This trend supports the confinement effect of the zeolite structure on the stereoselectivity of the reaction. 

Very few examples have been described for the non-covalent immobilization of chiral porphyrin complexes (Fig. 26). In the first case, the porphyrin-dichlororutheninm complex was encapsulated in silica, which was prepared around the complex by a sol-gel method [78], in an attempt to prevent deactivation observed in solution in the epoxidation of different alkenes with 2,6-dichloropyridine N-oxide. In fact, the heterogeneous catalyst is much more active, with TON up to 10 800 in the case of styrene compared to a maximum of 2190 in solution. Enantioselectivities were about the same imder both sets of conditions, with values aroimd 70% ee. 

Diboration of terminal alkenes has also been studied with other d " metals (Fig. 2.12) including the Ag and Au complexes 75-77 and the Pt" complexes 78-79. Styrene is diborylated with 100% selectivity and good conversions in THF (46% for 75 and 94% for 77 at 5 mol%, 60 h) using equimolecular amounts of (Bcat)j. The difference in activity between the Ag and Au complexes has been ascribed to the increased lability of the Ag-NHC bond, which may lead to catalyst decomposition under the reaction conditions, hi both catalytic systems it is believed that the active species involves only one coordinated NHC ligand. Complex 77 is less active than 74 and 75, possibly due to steric reasons. The enantioselectivity of 77 in the diboration of prochiral alkenes is very low [63]. 

Concerning enantioselective processes, Fujihara and Tamura have proved that palladium NPs containing (S)-BINAP (2,2 -bis(diphenylphosphino)-l,l -binaphthyl) as chiral stabiliser, catalyse the hydrosilylation of styrene with trichlorosilane, obtaining (S)-l-phenylethanol as the major isomer (ee = 75%) [42]. In contrast, the palladium complex [Pd(BINAP)(C3H5)]Cl is inactive for the same reaction [43]. 

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