Ethylenebis chiral

The Jacobsen-Katsuki epoxidation reaction is an efficient and highly selective method for the preparation of a wide variety of structurally and electronically diverse chiral epoxides from olefins. The reaction involves the use of a catalytic amount of a chiral Mn(III)salen complex 1 (salen refers to ligands composed of the N,N -ethylenebis(salicylideneaminato) core), a stoichiometric amount of a terminal oxidant, and the substrate olefin 2 in the appropriate solvent (Scheme 1.4.1). The reaction protocol is straightforward and does not require any special handling techniques. 

The enantioselective hydrosilylation of 2-pentylcyclopentenone is effected with PMHS and an active catalyst derived from (R.R)-ethylenebis(tetrahydro-indenyl)titanium difluoride and phenylsilane (EBTHI)Ti (Eq. 3 50).587 The use of diphenylsilane, a rhodium catalyst, and (W, / )-(. ,.S )-BuTRAP as the chiral ligand gives similar results.576 Other related approaches give greatly inferior enantioselectivies, 592 594... 

The hypothesis of stereochemical control linked to catalyst chirality was recently confirmed by Ewen (410) who used a soluble chiral catalyst of known configuration. Ethylenebis(l-indenyl)titanium dichloride exists in two diaste-reoisomeric forms with (meso, 103) and C2 (104) symmetry, both active as catalysts in the presence of methylalumoxanes and trimethylaluminum. Polymerization was carried out with a mixture of the two isomers in a 44/56 ratio. The polymer consists of two fractions, their formation being ascribed to the two catalysts a pentane-soluble fraction, which is atactic and derives from the meso catalyst, and an insoluble crystalline fraction, obtained from the racemic catalyst, which is isotactic and contains a defect distribution analogous to that observed in conventional polypropylenes obtained with heterogeneous catalysts. The failure of the meso catalyst in controlling the polymer stereochemistry was attributed to its mirror symmetry in its turn, the racemic compound is able to exert an asymmetric induction on the growing chains due to its intrinsic chirality. 

An enantioselective version of this allylamine synthesis employs the chiral (S,S)-dimethylzirconium derivative 3, prepared from (S,S)-[l,2-ethylenebis(tetrahydro-l-indenyl]zirconium dichloride.3 Displacement of one methyl group by triflic acid followed by reaction with a lithium anilide results in a zirconaaziridine (a) with loss of methane. Reaction of a with a symmetrical alkyne provides a metallapyrroline (b), which is hydrolyzed to an (S)-allylamine (4) in 90-99% ee. The method tolerates variation in the lithium anilide. Terminal alkynes do not react with a, but 1-trimeth-... 

Figure 3.39 Chiral procatalyst of C2 symmetry containing two -substituents at the cyclopentadienyl rings, rac.-ethylenebis[l-(3-methylindenyl)]zirconium dichloride [rac.-(MeIndCH2)2ZrCl2], which, in combination with [Al(Me)o]x, produces atactic polypropylene. Front side view and side view respectively...

Collins and coworkers have developed two synthetic routes for the preparation of chiral [l,2-ethylenebis(jj -3-alkylcyclopentadienyl)]titanium dichlorides (R = Me, Et, i-Pr, r-Bu) in 80 85% yields as a nuxture of racemic and meso-titanocene dichlorides. In more recent work, the addition of a methyl group to the cyclopentadienyl rings has allowed Collins and coworkers to prepare a series of [1,2-ethylene-l,T-bis(4-R-2-methylcyclopentadienyl)] titanium dichlorides (R = Me, j-Pr, r-Bu) stereoselectively to give the racemic isomers. ... 

The first chiral bridged zirconocene synthesized in 1984 by Brintzinger and used as an isospecific polymerization catalyst was racemic ethylenebis-(4,5,6,7-tetrahydro-l-indenyl)zirconium dichloride (see Structure 9) [45]. Ewen showed that the analogous ethylenebis(l-indenyl)titanium dichloride (a mixture of the meso form and the racemate) produces a mixture of isotactic and atactic polypropylene [46]. The chiral titanocene as well as the zirconocene were shown to work by enantiomorphic site control in the case of the titanocene, the achiral meso structure causes the formation of atactic polymer. 

The first example of catalytic asymmetric hydrogenation of N,N dialkyl enamines was reported by Buchwald and Lee in 1994. By using 5 mol% chiral ansa titanocene catalyst [(S,S,S) (EBTHI)TiO binaphtho] (EBTHI = ethylenebis(tetrahydroindenyl)), they achieved excellent enantioselectivities (up to 98% ee) in the hydrogenation of (1 arylvinyl)amines [50]. In 2000, Boner used chiral rhodium diphosphine complexes for the hydrogenation of 2 N piperidinylethylbenzene and 2 alkyl 1,3,3 trimethyle neindoline and obtained the tertiary amines in moderate enantiomeric excesses [51]. 

Chiral metallocene complex [(5 )-l,2-ethylenebis(j7 -tetrahydroindenoyl)]Ti(OTf)2 422a and its zirconium analog 422b efficiently catalyzed the cycloadditions of 1,3-oxazolidin-2-one based dienophiles 17a and 404 with cyclopentadiene which gave 421 and 405, respectively . The endo selectivity was highest in dichloromethane, whereas the enan-tioselectivity was higher in nitroalkane solvents (equation 127, Table 24). 

The first chiral bridged zirconocene used as an isospecific polymerization catalyst was the racemic mixture of ethylenebis(4,5,6,7-tetrahydro-l-indenyl)ZrCl2 . Other chiral metallocenes have dimethylsilyl bridges, indenyl, or substituted cyclopentadienyl... 

A new type of enantioselective diene polymerization is found with cyclopolymerization of 1,5-hexadiene which leads to polymers with a saturated chiral main chain28,58>109. As catalyst, (—)-(7 )-[l,T-ethylenebis(4,5,6,7-tetrahydro-l-indenyl)]zirconium (/ )-binaphtholate is used in the presence of methylalumoxane to give optically active poly(methylene-1,3-cyclopentane) (3) with 68% trans configuration in the five-membered ring (diisotacticity). If the (S)-enantiomer of the ansa-metallocene with (ft)-binaphthol is used as catalyst then the opposite rotation of the polymer is observed58. 

Chiral zirconocene complexes have also been studied as catalysts for the hydrogenation of nonfunctionalized olefins115. Using homogeneous Ziegler Natta-type catalyst systems derived from [ethylenebis(4,5.6,7-tetrahydro-l-indenyl)]zirconium complexes and methyl aluminoxane [A1(CH3)0] , 2-phenyl-l-butene was hydrogenated in 36% optical yield (20 bar H2, benzene, 25 °C). Under the same conditions, the reaction of styrene with D2 gave optically active 1,2-dideuteroethylbenzene with 65% ee. 

Metal complexes of enantiomericaUy pure N,N -ethylenebis(salicylideneaminato) (salen) complexes in combination with stoichiometric oxidants currently provide the most selective method for the catalytic asymmetric epoxidation of unfunctionalised alkenes. The use of C2-symmetric salen complexes of manganese(lll) were reported independently in 1990 by Jacobsen and coworkers and Katsuki and coworkers. The first generation catalysts are represented by the general structure (4.33). The complex with R = Bu is known as Jacobsen s catalyst. All of the first generation catalysts are composed of a enantiopure diamine core and possess large substituents at the 3/3 and 5/5 positions. Subsequently Katsuki and coworkers developed second generation catalysts such as (4.34) with axially chiral groups at the 3/3 positions. 

With these C2-symmetric homogeneous catalysts at hand, the groups of John Ewen at Exxon and Walter Kaminsky at the University of Hamburg almost simultaneously published revolutionary studies on the formation of isotactic polypropylene upon activation of chiral flni fl-metallocenes with Ewen used a 56 44 mixture of rac- and meso-ethylenebis(indenyl)titanium... 

On the basis of this experiment, Pino and coworkers were able to determine that catalysts derived from the (/ )-ethylenebis(tetrahydroindenyl)zirconium binaptholate preferentially selected the Re enantioface of propylene. These results led to a model for the transition state where the polymer chain is forced into an open region of the metallocene, thereby relaying the chirality of the metallocene to the incoming monomer through the orientation of the p-carbon of the aUcyl chain (Scheme IIA).43 Here, the role of the C2-symmetry of the catalyst site can be readily appreciated since as the polymer chain migrates to the coordinated olefin, the coordination site available for binding of the olefin alternates between two coordination sites (A -> B -> C). Because these two sites are related by a C2-symmetry axis, they are homotopic and therefore selective for the same olefin enantioface. The result is polymerization to yield an isotactic polyolefin. 

Highly isotactic polypropylene is obtained when chiral ethylenebis(4,5,6,7-tetrahydro-l-indenyl)-zirconium dichloride is employed in combination with methylalumoxane (MeAlO) as the... 

The literature describes several cases of the copolymerization of ethylene with cyclopentene [31b], cyclo-heptene [31b], norbornene [31c], 2-methylnorbornene [31c], dicyclopentadiene [31d], dimethanooctahydro-naphthalene [31a,e], and 2-methyldimethanooctahy-dronaphthalene [31a]. When chiral catalysts were employed (e.g., ethylenebis(indenyl)zirconium dichloride associated with methylaluminoxane), the activity was much higher than that of the achiral catalyst. In addition, the ehiral catalyst increased the polymer stereoselectivity and the amount of the cycloolefin incorporated into the copolymer. Materials having exeellent transpareney, thermal stability, and ehemical resistance—properties that are useful for optical disks—may be produced efficiently by this method. 

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