Stereoselective coordination catalysts

The addition to 1,3-dienes afforded a new class of allylboron compounds. The platinum(0)-phosphine catalysts stereoselectively yielded or-1,4-addition products 131233 234 and 133216 235 for 2,3-disubstituted butadiene, 1,3-cyclohex-adiene, and 1,3-pentadiene by Txr-coordination of a diene to a platinum catalyst (Equations (30) and (31)). In contrast, phosphine-free Pt(dba)2 resulted in the selective formation of a 1,2-addition product 134216 for 1,3-pentadiene (Equation (31)). The corresponding chiral allyl boronates were synthesized by diboration of dienes with 123 or 124.234 235... 

Most stereoselective coordination catalysts polymerize propylene oxide to yield polymers that contain high ratios of isotactic to syndiotactic sequences. Laige portions of amorphous materials, however, are also present in these same products. These amorphous portions contain head-to-head units that are imperfections in the structures. For every head-to-head placement, one (R) monomer is converted to an (S) unit in the polymer. This shows that at the coordination sites abnormal ring openings occur at the secondary carbon with an inversion of the configuration and result in head-to-head placements. Also, erythro and threo isomers units are present. The isotactic portion consists almost exclusively of the erythro isomer while the amorphous fraction contains 40-45% erythro and 55-60% threo ... 

Elastomers based on biological monomers can be obtained by the stereoselective coordination polymerization of /S-myrcene (14). By the combination of a neodymium pre-catalyst with triisobutyl aluminmn in the presence of a boron activator results in a polymer in good yield, but there is evidence for crosslinking since the material displays a low solubility. 

One of the key features of the stereoselective coordination catalysts for propylene oxide polymerization is that, while they can give Isotactic polymer with a very high ratio of Isotactic to syndiotactic sequences (e.g. > 370) (34). in the same reaction mixture a large part of the polymer is amorphous. Even vdien R-propylene oxide is used as starting material, much of the product is amorphous polymer of low optical rotation containing many S-propylene oxide units (30). 

In fact, Basset et al. ) suggest that catalysts of low stereoselectivity might exhibit an empty site of coordination at the metal while those with high stereoselectivity coordinatively might be more saturated. This would also account for the observation of Ivin et at.22) vho found a higher cis-contend when some coordinating species such as ethyl acrylate is added to the catalyst. 

The (E)-a-alkyl-/5-silylacrylate 22 is prepared by regio- and stereoselective car-bony lation of the trimethylsilylalkyne 21 using a Pd catalyst coordinated by SnClj and dppf[20]. 

A model for the mechanism of the highly enantioselective AlMe-BINOL-cata-lyzed 1,3-dipolar cycloaddition reaction was proposed as illustrated in Scheme 6.13. In the first step nitrone la coordinates to the catalyst 11b to form intermediate 12. In intermediate 13, which is proposed to account for the absolute stereoselectivity of this reaction, it is apparent that one of the faces of the nitrone, the si face, is shielded by the ligand whereas the re face remains available... 

Dipolar cydoadditions are one of the most useful synthetic methods to make stereochemically defined five-membered heterocydes. Although a variety of dia-stereoselective 1,3-dipolar cydoadditions have been well developed, enantioselec-tive versions are still limited [29]. Nitrones are important 1,3-dipoles that have been the target of catalyzed enantioselective reactions [66]. Three different approaches to catalyzed enantioselective reactions have been taken (1) activation of electron-defident alkenes by a chiral Lewis acid [23-26, 32-34, 67], (2) activation of nitrones in the reaction with ketene acetals [30, 31], and (3) coordination of both nitrones and allylic alcohols on a chiral catalyst [20]. Among these approaches, the dipole/HOMO-controlled reactions of electron-deficient alkenes are especially promising because a variety of combinations between chiral Lewis acids and electron-deficient alkenes have been well investigated in the study of catalyzed enantioselective Diels-Alder reactions. Enantioselectivities in catalyzed nitrone cydoadditions sometimes exceed 90% ee, but the efficiency of catalytic loading remains insufficient. 

Agbossou E., Carpentier J. E. Hapiot E., Suisse I., Mortreux A. The Aminophos-phine-Phosphinites and Related Ligands Synthesis, Coordination Chemistry and Enantioselective Catalysis Coord. Chem. Rev. 1998 I78-I80 1615-1645 Keywords stereoselective Diels-Alder reaction catalysts, aminophosphine-phosphinites, enantioselective catalysts... 

The high catalytic activity also enabled aza-Claisen rearrangements to form Al-substituted quaternary stereocenters (Fig. 26) [71]. The catalyst does not need to distinguish between differently sized substituents on the double bond of 49 (e.g., R = CDa, R = CHs, ee = 96%), indicating that coordination of the olefin is the stereoselectivity predetermining step. The imidate-N-atom subsequently attacks intermediate 47-1 from the face remote to the Pd-center totally resulting in a... 

In 2005, Nguyen et al. reported the first example of asymmetric cyclopropa-nation of olefins with EDA mediated by a combination of a (salen) ruthenium(II) catalyst and a catalytic amount of a chiral sulfoxide (Scheme 6.7). These authors proposed that the mechanism explaining the asymmetric induction involved the axial coordination of the chiral sulfoxide to the ruthenium centre as the key induction step in the reaction stereoselectivity. 

Cp2Zr(H)(Cl) (8). The apparent record for catalyzed double bond movement is on 9-decene-l-ol to decanal (nine positions) using Fe3(CO)i2 (9). However, 30 mol % was required, which means that nearly a mole of metal was used per mole of alkenol. Herein we expand upon our initial report (10) of a very active catalyst (1) which has been shown to move a double bond over 30 positions. Catalyst 1 appears to have an intriguing and useful mode of action, in which the pendant base ligand performs proton transfer on coordinated alkene and Ti-allyl intermediates in a stereoselective fashion. 

Presumably, the stereoselectivity in these cases is the result of coordination of iridium by the functional group. The crucial property required for a catalyst to be stereodirective is that it be able to coordinate with both the directive group and the double bond and still accommodate the metal hydride bonds necessary for hydrogenation. In the iridium catalyst illustrated above, the cyclooctadiene ligand (COD) in the catalysts is released by hydrogenation, permitting coordination of the reactant and reaction with hydrogen. 

The most famous mechanism, namely Cossets mechanism, in which the alkene inserts itself directly into the metal-carbon bond (Eq. 5), has been proposed, based on the kinetic study [134-136], This mechanism involves the intermediacy of ethylene coordinated to a metal-alkyl center and the following insertion of ethylene into the metal-carbon bond via a four-centered transition state. The olefin coordination to such a catalytically active metal center in this intermediate must be weak so that the olefin can readily insert itself into the M-C bond without forming any meta-stable intermediate. Similar alkyl-olefin complexes such as Cp2NbR( /2-ethylene) have been easily isolated and found not to be the active catalyst precursor of polymerization [31-33, 137]. In support of this, theoretical calculations recently showed the presence of a weakly ethylene-coordinated intermediate (vide infra) [12,13]. The stereochemistry of ethylene insertion was definitely shown to be cis by the evidence that the polymerization of cis- and trans-dideutero-ethylene afforded stereoselectively deuterated polyethylenes [138]. 

The polymerization mechanism for the dual-side catalysts is totally different from the C2-symmetric complexes. Due to their geometry, the dual-side complexes show different stereoselectivities for monomer coordination and insertion. It was shown that the introduction of the stereoerror formation by the 5-substituted asymmetric catalysts originates predominately from the kinetic competition between chain back-skip and monomer coordination at the aspecific side of the catalyst [9],... 

Chien already postulated that C,-symmetric ansa-bridged complexes exist in two isomeric states, which interconvert during the course of the polymerization reaction [14, 15, 21, 22], Different stereoselectivities for monomer coordination and insertion are found for the two coordination sites of the asymmetric metallocene catalysts (Fig. 6,1 and IV). The migration of the polymer chain to the monomer, coordinated at the isoselective site f I—>11), followed by a consecutive chain back-skip (at higher temperatures) to the sterically less hindered side (II >111) leads to isotactic [mmmm] sequences [11],... 

The trans/cis ratio of the product must, therefore, be determined at an earlier reaction stage and most probably by the ratio of species 27a and 27b. Steric or electronic factors affecting this ratio will influence the trans/cis ratio of the resulting 1,4-hexadiene. The phosphine and the cocatalyst effect on the stereoselectivity can thus be interpreted in terms of their influence on the mode of butadiene coordination. Some earlier work on the stereospecific synthesis of polybutadiene by Ni catalyst can be adopted to explain the effect observed here, because the intermediates that control the stereospecificity of the polymerization should be essen-... 

The extreme stereoselectivity toward the synthesis of cis-1,4-hexadiene is attributed to the fact that only cisoid-coordinated 1,3-diene can undergo the addition reaction (65, 66). 1,3-Dienes whose cisoid conformations are stoically unfavorable do not react with ethylene under the dimerization conditions. For example, Hata (65) was able to show that, using an Fe-based catalyst system, l-tra/is-3-pentadiene (40) and 2-methyl-1 -trans-3-pentadiene (41) reacted readily with ethylene to form the expected 1 1 addition products, while l-c/s-3-pentadiene (42) and 4-methyl- 1,3-penta-diene (43) failed to interact with ethylene. The explanation is that the cisoid conformations of 40 and 41 are stoically favorable while those for 42 and 43 are not. 

The 1,3-dipolar cycloaddition of nitrones to vinyl ethers is accelerated by Ti(IV) species. The efficiency of the catalyst depends on its complexation capacity. The use of Ti( PrO)2Cl2 favors the formation of trans cycloadducts, presumably, via an endo bidentate complex, in which the metal atom is simultaneously coordinated to the vinyl ether and to the cyclic nitrone or to the Z-isomer of the acyclic nitrones (800a). Highly diastereo- and enantioselective 1,3-dipolar cycloaddition reactions of nitrones with alkenes, catalyzed by chiral polybi-naphtyl Lewis acids, have been developed. Isoxazolidines with up to 99% ee were obtained. The chiral polymer ligand influences the stereoselectivity to the same extent as its monomeric version, but has the advantage of easy recovery and reuse (800b). 

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