Ligands catalyst effects

Cyclodimerization of 1,3-Diolefins. The nickel-ligand catalysts effective in the cyclodimerization of 1,3-diolefins are composed of preformed complexes such as [Ni(CO) L4 ] or [Ni(cod)2]/L [L = PR3, P(OR)3] or are prepared in situ by reducing a Ni(II) compound such as [Ni(acac)2] in the presence of L and the diolefin . Orga-noaluminum compounds are most commonly used reducing agents, but various other systems have also been investigated . ... 

Tetrakisligand nickel(0) complexes have tetrahedral stmctures. Electronic stmctures have been studied and conformational analysis performed. Quantitative equiUbria measurements of the ligands in these complexes imply a dominant role for ligand steric effects when the complexes are employed as catalysts (94). 

To overcome these problems with the first generation Brmsted acid-assisted chiral Lewis acid 7, Yamamoto and coworkers developed in 1996 a second-generation catalyst 8 containing the 3,5-bis-(trifluoromethyl)phenylboronic acid moiety [10b,d] (Scheme 1.15, 1.16, Table 1.4, 1.5). The catalyst was prepared from a chiral triol containing a chiral binaphthol moiety and 3,5-bis-(trifluoromethyl)phenylboronic acid, with removal of water. This is a practical Diels-Alder catalyst, effective in catalyzing the reaction not only of a-substituted a,/ -unsaturated aldehydes, but also of a-unsubstituted a,/ -unsaturated aldehydes. In each reaction, the adducts were formed in high yields and with excellent enantioselectivity. It also promotes the reaction with less reactive dienophiles such as crotonaldehyde. Less reactive dienes such as isoprene and cyclohexadiene can, moreover, also be successfully employed in reactions with bromoacrolein, methacrolein, and acrolein dienophiles. The chiral ligand was readily recovered (>90%). 

The electroreductive carbonylation and carboxylation of organic halides catalyzed by different metal complexes is summarized in Section 9.10.5. Electrogenerated Ni° complexes containing bpy193-196 or pentamethyldiethylenetriamine196-199 ligands are effective catalysts for the electrosynthesis of... 

Though the models discussed above have great mechanistic value, the catalytic properties are complicated by the presence of cofactors, such as the structure of cocatalyst and the nature of the phosphine ligands. The effect of these cofactors on the catalytic properties can be used to optimize the catalyst activity or to direct the course of the reactions. To... 

Eleven aromatic and aliphatic aldehydes have been alkylated with Et2Zn in the presence of homoannularyl bridged hydroxyamino ferrocene (—>123. The resulting carbinols have ee values varying from 66% to 97%. This new ferro-cenyl catalyst has been used successfully to alkylate aromatic and linear or branched chain aliphatic aldehydes to secondary alcohols with up to 97% ee. This ligand is effective even for -branched aliphatic substrate. 

Corey and Ishihara29 report the synthesis of a new bis(oxazoline). This catalyst effects Diels-Alder reaction via a tetracoordinated metal complex. Ligand (.S )-8I is synthesized from (iS )-phenylglycine, as depicted in Scheme 5-25. Treatment of 81 with Mgl2 L gives a dark solution of complex 82, which can be utilized as a Diels-Alder reaction catalyst. Thus, reaction of cyclopentadiene with 71 in the presence of 82 yields product 72a with an enantiomeric ratio of over 20 1 (Scheme 5-26). 

Highly crosslinked polymer-supported-BINAPHOS ligands were effective for the hydroformylation of styrene and other functionalized olefins (ee s up to 89%). The catalyst could be recovered and reused at low stirring conditions [61,62]. 

A chiral dinuclear Ti(IV) Lewis acid catalyst 18 can be prepared in situ from a 1 2 molar mixture of (7 )-3,3 -di(2-mesitylethynyl)binaphthol and Ti(Oi-Pr)4 at ambient temperature. The 3- and 3 -substituents on the chiral ligand are effective for preventing undesired aggregation between Ti(IV) complexes and increasing the enantioselectivity (up to 82% ee) in the Diels-Alder reaction of methacrolein with cyclopentadiene (Scheme 12.16). ... 

The chelating behavior was also evident from H P-NM R experiments. The addition of triphenylphosphine (a) to a catalyst solution of [HRh(CO)2(13 b)] did not affect the complex. Moreover, the addition of 1 equiv. of b to a solution of HRh(CO)2(13) PPhj resulted in the exclusive formation of [HRh(CO)2(13 b)] upon release of free triphenylphosphine. The chelating effect of the supramolecular ligand assembly effectively competes with triphenylphosphine, leading to exclusive formation of the rhodium complex of 13 b. In the complex [HRh(CO)2(13 b) the supramolecular ligand 13 b coordinates in an equatorial-equatorial fashion to the rhodium metal center, whereas the HRh(CO)2(13)PPh3 exists in a mixture of complexes (ee and ea). 

In 1992, Crabtree and co-workers reported the first nickel catalyst effective for silane alcoholysis.161 The complex, [Ni(tss)]2 (tss = salicylaldehyde thiosemicarbazone), bears a ligand that contains O and N donor groups and a semicarbazide sulfur. Alcoholysis of Et3SiH with ethanol or methanol occurs at room temperature in 50% dimethyl sulfoxide-benzene. However, the reaction is inhibited in the presence of strong donor ligands, H2, or atmospheric pressure of CO. 

The current hydrocyanation process can be broken down into two major steps. In the first, HCN is added to butadiene in the presence of an NiL4 catalyst to give 3-pentenenitrile (3PN) and 2-methyl-3-butenenitrile (2M3BN) [Eq. (7)]. Fortunately the branched 2M3BN may be isomerized to the linear 3PN isomer [Eq. (8)]. In the second step, a Lewis acid promoter is added to the NiL4 (L = a phosphorus ligand) catalyst to effect the double bond isomerization of 3PN to 4-pentenenitrile (4PN) concurrently with the... 

A chiral bis(oxazoline) ligand forms effective catalysts in combination with magnesium salts for enantioselective Diels-Alder addition of acryloyl oxazolidinone L32] (Eq. 8A.17), Recendy, Desimoni found that both Diels-Alder adduct enantiomers could be obtained by using the same bis(oxazoline)/MgC104 catalyst, that is, in the presence of water (2 equiv.), the other enantiomer was obtained selectively, which can be rationalized by taking into account the coordination of two water molecules to the metal, changing its geometry to octahedral [33],... 

As discussed in Section III J, in general, catalytic asymmetric aldol reactions have been studied using enol silyl ethers, enol methyl ethers, or ketene silyl acetals as a starting material. So far several types of chiral catalysis have been reported.75-85 The chiral lanthanoid complex prepared from Ln(OTf)3 and a chiral sulfonamide ligand was effective in promoting an asymmetric Mukaiyama aldol reaction with a ketene silyl acetal.86 The preparation of the catalyst and a representative reaction are shown in Figure 45. 

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