Carbonyls titanium

While the majority of group 4B metal carbonyl complexes contain 7r-bonded hydrocarbon ligands, most notably 17-cyclopentadienyl, recent studies by Wreford and co-workers have led to the identification and isolation of three novel phosphine-stabilized titanium carbonyl complexes (12,13). 

Treatment of 4 with either PF3 or 13CO results in CO substitution believed to proceed via a dissociative process yielding Ti(CO)2(PF3)-(dmpe)2 (6) and Ti(13CO)3(dmpe)2. Structural characterization of 6 showed it also to be monomeric, but possessing a monocapped trigonal prismatic geometry. Complexes 4, 5, and 6 may be considered phosphine-substituted derivatives of the as yet unisolated Ti(CO)7, thus representing the only isolable titanium carbonyl complexes where the titanium atom is in the zero oxidation state. 

Demerseman and co-workers have briefly noted that the reduction of CpTiCl3 in THF with either aluminum or magnesium in the presence of CO gave Cp2Ti(CO)2 as the only titanium carbonyl species (27). 

Of special interest is the recently described red zero-valent titanium complex [Ti(CO)2(PF3)(dmpe)2] (dmpe = Me2PCH2CH2PMe2) which is a PF3 derivative of the nonexistent titanium carbonyl complex [Ti(CO)7] (365). A single-crystal X-ray crystallographic study has established the structure shown in Fig. 21 in which the geometry around the metal is approximately a capped trigonal prism. 

The hypothetical titanium heptacarbonyl Ti(CO)7 is as yet unknown. However, Ti(CO)6 has been prepared and characterized by matrix isolation techniques. Ti(CO)6, thus prepared, is found to be unstable above —200 °C Titanium carbonyls Ti(CO)x (x = 1 6) are formed during deposition of laser-ablated titanium atoms with CO during condensation in excess Ne, or on annealing and photolysis of the matrix. Titanium carbonyl complexes see Carbonyl Complexes of the Transition Metals) have also been observed spectroscopically (by IR) as intermediates in the low-temperature reaction of Ti(CH2Ph)4 and its dicyclohexylamine adduct with CO. ... 

The zerovalent cyclopentadienyl see Cymantrene) titanium carbonyl phosphine complexes [CpTi(CO)3(PR3)] and... 

CH3CN), 70 (s, 18C6), 1.0 (s, CH3CN). Although the crystalline product usually requires several minutes to oxidize completely in air, solutions of this substance are extremely air sensitive and moderately light sensitive. The compound is insoluble in hydrocarbons, ethers, and liquid ammonia. Solutions of [Ti(CO)6] in acetonitrile are significantly less stable than those in pyridine and decompose entirely within 48 h at 23°C. In contrast to the isoelectronic [V(CO)6], dichloromethane rapidly decomposes [Ti(CO)6] to unknown products. Also, water, methanol, ethanol, and phenol cause rapid oxidation of [Ti(CO)6] to afford the dimeric species [Ti(CO)4(p-OR)]2, where R = H, Me, Et, or Ph. Other titanium carbonyls obtained from [Ti(CO)6] include [Ti(CO)6(SnR3)] , [Ti(CO)6(Au-... 

The results presented in Section 6.4.3 revealed that the McMurry reactions of ster-ically hindered ketones can in fact be viewed as Wittig-like olefination reactions. While these two reactions were for a long time thought to be mechanistically different, formation of carbenoid species by reduction of ketones with low-valent titanium complexes is not really surprising in view of the highly oxophilic and reducing character of titanium. Carbonyl olefinations by means of titanium carbe-... 

Low valent titanium McMurry carbonyl coupling is believed to go through the vic-diol. vic-diols are smoothly converted to the corresponding olefins under these conditions. JOC 1976, 41, 896... 

Reductive coupling of carbonyl compounds to yield olefins is achieved with titanium (0), which is freshly prepared by reduction of titanium(III) salts with LiAIH4 or with potassium. The removal of two carbonyl oxygen atoms is driven by T1O2 formation- Yields are often excellent even with sensitive or highly hindered olefins. (J.E. McMurry, 1974, 1976A,B). 

F.N. Tebbe (1978 [footnote 20]) and R.R. Schrock (1976) have shown that electrophilic titanium or tantalum ylides can alkylidenate the carbonyl group of esters. Vinyl ethers are obtained in high yields with Tebbe s reagent, p-chlorobis(ri -2,4-cyclopentadien-l-ylXdime-thylaluminum)- 4-methylenetitanium (S.H. Pine, 1980 A.G.M. Barrett, 1989). 

The carbonylation of aryl halides under mild conditions in the presence of CsF afford.s the acid fluoride 490 in good yields. Unlike acyl chlorides, acyl fluorides are inert toward Pd(0) catalyst[345]. Benzenesulfonyl chloride (491) undergoes desulfonylation-carbonylation to give the benzoate 492 in the presence of titanium tetralkoxide at 160 °C[346]. 

Chiral Titanium Complexes. Chiral titanium complexes are useful for the enantioselective addition of nucleophiles to carbonyl groups ... 

Among the appHcations of lower valent titanium, the McMurry reaction, which involves the reductive coupling of carbonyl compounds to produce alkenes, is the most weU known. An excellent review of lower valent titanium reactions is available (195). Titanium(II)-based technology is less well known. A titanium(II)-based complex has been used to mediate a stetio- and regio-specific reduction of isolated conjugated triple bonds to the corresponding polyenes (196). 

Titanium(IV) is a powerful but selective Lewis acid which can promote the coupling of allylsilanes with carbonyl compounds and derivatives In the presence of titanium tetrachlonde, benzalacetone reacts with allyltnmethylsilane by 1,4-addition to give 4-PHENYL-6-HEPTEN-2-ONE. Similarly, the enol silyl ether of cyclopentanone is coupled with f-pentyl chloride using titanium tetrachlonde to give 2-(tert-PENTYL)CYCLOPENTANONE, an example of a-tert-alkylation of ketones. 

Triflates of titanium and tin are effective catalysts for various condensations of carbonyl compounds [I2I, 122, 123, 124, 125] Claisen and Dieckmann type condensations between ester functions proceed under mild conditions in the presence of dichlorobis(trifluoromethanesulfonyloxy)titaiiiuin(rV) and a tertiary amine (equations 59 and 60) These highly regio- and stereoselective condensations were used successfully m the synthesis of carbohydrates [122]... 

The yields ranged from 55% for the mixture of enamines formed from morpholine and methylisopropyl ketone to 94% for the enamine formed from dimethylamine and methyl t-butyl ketone. The hindered ketone 2,5-dimethylcyclopentanone could be converted to an enamine, but the more hindered ketone, 2,6-di-t-butylcyclohexanone, was inert. White and Weingarten 43) attribute the effectiveness of titanium tetrachloride in this reaction to its ability to scavenge water and to polarize the carbonyl bond. 

M-C (T bonds are not strong and, as might be expected for metals with so few d electrons, little help is available from synergic n bonding for instance, of the simple carbonyls only Ti(CO)6 has been reported, and that only on the basis of spectroscopic evidence. However, as will be seen on p. 972, the discovery that titanium compounds can be used to... 

Perhaps because of inadequate or non-existent back-bonding (p. 923), the only neutral, binary carbonyl so far reported is Ti(CO)g which has been produced by condensation of titanium metal vapour with CO in a matrix of inert gases at 10-15 K, and identified spectroscopically. By contrast, if MCI4 (M = Ti, Zr) in dimethoxy-ethane is reduced with potassium naphthalenide in the presence of a crown ether (to complex the K+) under an atmosphere of CO, [M(CO)g] salts are produced. These not only involve the metals in the exceptionally low formal oxidation state of —2 but are thermally stable up to 200 and 130°C respectively. However, the majority of their carbonyl compounds are stabilized by n-bonded ligands, usually cyclopentadienyl, as in [M(/j5-C5H5)2(CO)2] (Fig. 21.8). 

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]. 

The initial step of the coupling reaction is the binding of the carbonyl substrate to the titanium surface, and the transfer of an electron to the carbonyl group. The carbonyl group is reduced to a radical species 3, and the titanium is oxidized. Two such ketyl radicals can dimerize to form a pinacolate-like intermediate 4, that is coordinated to titanium. Cleavage of the C—O bonds leads to formation of an alkene 2 and a titanium oxide 5 ... 

The pharmaceutical interest in the tricyclic structure of dibenz[6,/]oxepins with various side chains in position 10(11) stimulated a search for a convenient method for the introduction of functional groups into this position. It has been shown that nucleophilic attack at the carbonyl group in the 10-position of the dibenzoxepin structure renders the system susceptible to water elimination. Formally, the hydroxy group in the enol form is replaced by nucleophiles such as amines or thiols. The Lewis acids boron trifluoride-diethyl ether complex and titanium(IV) chloride have been used as catalysts. 

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