Carbonyl titanium-catalyzed

For a review encompassing titanium-catalyzed hydrosilylation of alkenes and carbonyl compounds, see Harrod, J. F. Coord. Chem. Rev. 2000, 206-207, 493-531. 

TITANIUM-CATALYZED ENANTIOSELECTIVE ENYNE B1CYCLIZATION-CARBONYLATION 185... 

TITANIUM-CATALYZED ENANTIOSELECTIVE ENYNE BICYCLIZATION-CARBONYLATION... 

Since the introduction of the titanocene chloride dimer 67a to radical chemistry, much attention has been paid to render these reactions catalytic. This field was reviewed especially thoroughly for epoxides as substrates [123, 124, 142-145] so only catalyzed reactions using non-epoxide precursors and a few very recent examples of titanium-catalyzed epoxide-based cyclization reactions, which illustrate the principle, will be discussed here. A very useful feature of these reactions is that their rate constants were determined very recently [146], The reductive catalytic radical generation using 67a is not limited to epoxides. Oxetanes can also act as suitable precursors as demonstrated by pinacol couplings and reductive dimerizations [147]. Moreover, 5 mol% of 67a can serve as a catalyst for the 1,4-reduction of a, p-un saturated carbonyl compounds to ketones using zinc in the presence of triethylamine hydrochloride to regenerate the catalyst [148]. 

If the reaction between enol silyl ethers and a,/ -unsaturated ketones is attempted in the presence of a titanium Lewis acid, the mode of the reaction switches to 1,4-addition with reference to the unsaturated ketone [109-113]. The reaction of an enol silyl ether is shown in Eq. (30) [114]. Ketene silyl acetals react with a,j8-unsaturated ketones in similar 1,4-fashion, as exemplified in Eq. (31) [115]. Acrylic esters, which often tend to polymerize, are also acceptable substrates for a, -unsaturated carbonyl compounds [111]. A difluoroenol silyl ether participated in this cationic reaction (Eq. 32) [116], and an olefinic acetal can be used in place of the parent a-methylene ketone [111] to give the 1,5-diketone in good yield (Eq. 33) [117]. More results from titanium-catalyzed 1,4-addition of enol silyl ethers and silyl ketene acetals to a,f -unsaturated carbonyl compounds are summarized in Table 4. 

Carreira et al. [4] have discovered a titanium-catalyzed asymmetric carbonyl-ene reaction of aldehydes with the cheap commodity chemical 2-methoxypropene (Scheme 1). 

The titanium-catalyzed diastereoselective nucleophilic allylation of carbonyl compounds has been well studied. In this respect, the titanium Lewis acid catalyzed... 

Asymmetric activation concept has been proposed by Mikami on the basis of titanium-catalyzed carbonyl-ene reactions [132a]. The catalysis of carbonyl-ene... 

Optically active cyanohydrins can be easily transformed to P-hydroxy amines, a-hydroxy and a-amino carboxylic acids, which represent versatile intermediates for the synthesis of biologically important compounds, including insecticides and medicines [189, 190]. Asymmetric cyanation of carbonyl compounds catalyzed by chiral metal complexes, particularly titanium compounds, has provided one of the most convenient protocols to the access of these type of compounds. The first example of catalytic asymmetric cyanation of aliphatic aldehydes was realized in Reetz s group using BINOL-Ti complex as the catalyst to give the cyanohydrins in up to 82% ee [104] (Scheme 14.85). 

Similar to the results discussed for the silylcarbocyclizations of carbon-carbon multiple bonds, reductive cyclizations in the presence of carbonyl compounds are readily achieved. Crowe has developed a titanium-catalyzed procedure for the intramolecular reductive coupling of i5, -unsaturated carbonyl compounds in the presence of triethoxysilane (eq 18).The electronic advantage of triethoxysilane is demonstrated by the lack of reductive coupling in the presence of less reactive silanes, such as triethylsilane and diphenylsilane. With this method, Mori has utilized nickel(O) catalysts to generate five- and six-membered carbocycles and pyrrolidine derivatives. Furthermore, coordination of a chiral phosphine ligand to the nickel catalyst renders the reaction moderately enantioselective. ... 

The interest in chiral titanium(IV) complexes as catalysts for reactions of carbonyl compounds has, e.g., been the application of BINOL-titanium(IV) complexes for ene reactions [8, 19]. In the field of catalytic enantioselective cycloaddition reactions, methyl glyoxylate 4b reacts with isoprene 5b catalyzed by BINOL-TiX2 20 to give the cycloaddition product 6c and the ene product 7b in 1 4 ratio enantio-selectivity is excellent - 97% ee for the cycloaddition product (Scheme 4.19) [28]. 

These observations were explained in terms of an SE reaction between the 2-butenyl-(tributyl)stannane and tin(IV) chloride, which competes with Lewis acid catalyzed carbonyl attack. The 1 -methyl-2-propenyltin trichloride so formed reacts with the aldehyde to give linear products, or isomerizes to give the more stable ( )- and (Z)-2-butenyltin trichlorides which then react74. Similar results were obtained with titanium(lV) chloride, except that the anti-ad-duct was the major product from the butenyltitanium intermediate74. 

There is evidence to suggest that the unexpected anti selectivity found in several cases of Lewis acid catalyzed carbonyl additions can be explained by transmetalation of 2-butenylstannanes by titanium(IV) chloride (Sections 1.3.3.3.8.2.3. and 1.3.3.3.6.1.3.) to form the reactive allyltri-chlorotitanium46-49. 

Several aluminum- and titanium-based compounds have been supported on silica and alumina [53]. Although silica and alumina themselves catalyze cycloaddition reactions, their catalytic activity is greatly increased when they complex a Lewis acid. Some of these catalysts are among the most active described to date for heterogeneous catalysis of the Diels-Alder reactions of carbonyl-containing dienophiles. The Si02-Et2AlCl catalyst is the most efficient and can be... 

Nitroalkenes are also reactive Michael acceptors under Lewis acid-catalyzed conditions. Titanium tetrachloride or stannic tetrachloride can induce addition of silyl enol ethers. The initial adduct is trapped in a cyclic form by trimethylsilylation.316 Hydrolysis of this intermediate regenerates the carbonyl group and also converts the ad-nitro group to a carbonyl.317... 

R)-Benzoins and (/ )-2-hydroxypropiophcnonc derivatives are formed on a preparative scale by benzaldehyde lyase (BAL)-catalyzed C-C bond formation from aromatic aldehydes and acetaldehyde in aqueous buffer/DMSO solution with remarkable ease in high chemical yield and high optical purity (Eq. 8.112).303 Less-stable mixed benzoins were also generated via reductive coupling of benzoyl cyanide and carbonyl compounds by aqueous titanium(III) ions.304... 

Titanium silicate molecular sieves not only catalyze the oxidation of C=C double bonds but can be successfully employed for the oxidative cleavage of carbon-nitrogen double bonds as well. Tosylhydrazones and imines are oxidized to their corresponding carbonyl compounds (243) (Scheme 19). Similarly, oximes can be cleaved to their corresponding carbonyl compounds (165). The conversion of cyclic dienes into hydroxyl ketones or lactones is a novel reaction reported by Kumar et al. (165) (Scheme 20). Thus, when cyclopentadienes, 1,3-cyclohexadiene, or furan is treated with aqueous H202 in acetone at reflux temperatures for 6 h in the presence of TS-1, the corresponding hydroxyl ketone or lactone is obtained in moderate to good yields (208). 

Several methods for the anti-selective, asymmetric aldol reaction recorded in the literature include (i) the use of boron, titanium, or tin(ll) enolate carrying chiral ligands, (ii) Lewis acid-catalyzed aldol reactions of a metal enolate of chiral carbonyl compounds, and (iii) the use of the metal enolate derived from a chiral carbonyl compound. Although many of these methods provide anti-aldols with high enantioselectivities, these methods are not as convenient or widely applicable as the method reported here, because of problems associated with the availability of reagents, the generality of reactions, or the required reaction conditions. 

Cyclopropylamines and cyclopropanols can be prepared from alkyknagnesium ha-lides. The reaction is catalyzed by titanium alcoholates and its mechanism includes the formation of a dialkoxytitanacyclopropane 270, which reacts with a carbonyl compound or nitrile (Scheme 22). The use of chiral titanium alcoholates allows the reaction to be performed with up to 78% ee (equation 171) . 

A full account5 describes the enantioselective carbonyl-ene reaction of glyoxylate esters catalyzed by a binaphthol-derived chiral titanium complex that is potentially useful for the asymmetric synthesis of a-hydroxy esters of biological and synthetic importance.6 The present procedure is applicable to a variety of 1,1-disubstituted olefins to provide ene products in extremely high enantiomeric purity by the judicious choice of the dichloro or dibromo chiral catalyst (see Table). In certain glyoxylate-ene reactions involving removal of a methyl hydrogen, the dichloro catalyst... 

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