Titanium complexes dicarbonyl

Bis(cyclopentadienyl)titanium(II) dicarbonyl complexes, preparation and reactivity, 4, 250 Bis(cyclopentadienyl)titanium(II) dinitrogen complexes, preparation and reactivity, 4, 250 Bis(cyclopentadienyl)titanium halides ligand metathesis reactions, 4, 537 olefin polymerization, 4, 538 organic reactions, 4, 540 properties, 4, 530 reductions, 4, 532 synthesis, 4, 510... 

The Lewis acid-catalyzed conjugate addition of silyl enol ethers to a,y3-unsaturated carbonyl derivatives, the Mukaiyaraa Michael reaction, is known to be a mild, versatile method for carbon-cabon bond formation. Although the development of catalytic asymmetric variants of this process provides access to optically active 1,5-dicarbonyl synthons, few such applications have yet been reported [108], Mukiyama demonstrated asymmetric catalysis with BINOL-Ti oxide prepared from (/-Pr0)2Ti=0 and BINOL and obtained a 1,4-adduct in high % ee (Sch. 43) [109]. The enantioselectiv-ity was highly dependent on the ester substituent of the silyl enol ether employed. Thus the reaction of cyclopentenone with the sterically hindered silyl enol ether derived from 5-diphenylmethyl ethanethioate proceeds highly enantioselectively. Sco-lastico also reported that reactions promoted by TADDOL-derived titanium complexes gave the syn product exclusively, although with only moderate enantioselectiv-ity (Sch. 44) [110]. 

Bis(cyclopentadienyl)titanium(n) Dicarbonyl, Isonitrile, and Phosphine Complexes 250... 

Compounds containing CO or NO are poorly established, presumably because of the lack of re-bonding electron density on titanium. The dicarbonyl (/25-C5H5)2Ti(CO)226 is reasonably stable like [Ci0H10Ti]2 it undergoes, with loss of CO, a number of oxidative addition (Chapter 24) reactions. The lower valence states for titanium of 0 and — 1 are known only in the bipyridine complexes such as TTbipy3.27... 

Another example of a titanium complex is the binuclear titanocene complex 18 which is obtained by the insertion of titanocene dicarbonyl into the S-S bond of a fused ring compound 17, involving elimination of CO (Scheme 5)... 

In order to obtain high selectivities in the reduction of linear carbonyl compounds, very often chelating reagents are added to fix the substrate in a specific conformation. Titanium complexes are known to be versatile reagents in such a reduction process, especially with dicarbonyl compounds [41]. Thus, the stereoinduction in the reduction of diketones alternates with the distance of the two carbonyl groups from each other [42]. The 1,2-induction in the reduction of diacetyl or benzil 91 is independent of the addition of titanium tetrachloride and predominantly yields the meso products 92 (displayed in Scheme 3.20). 

Preparation of Bis(p-indenyl)dicarbonyl Complexes of Titanium and Zirconium... 

Two other functionally substituted 17-cyclopentadienyl titanium dicarbonyl complexes prepared by Rausch and co-workers include the vinyl Cp compound (i7-C5H4CH=CH2)CpTi(CO)2 (81) and the carbomethoxy Cp compound (rj-C5H4C02Me)CpTi(C0)2 (82). Both were synthesized via the aluminum-induced reductive carbonylation of the corresponding dichloride derivatives. 

Because of the low oxidation state of the metal [M(II)] in the group 4B metallocene dicarbonyl compounds, all of them, perhaps with the exception of (17—C5Me5)2Ti(CO)2 (27), are very air sensitive and decompose rapidly on exposure to air, forming a yellow solid for the titanium compounds and cream-colored solids for the zirconium and hafnium analogs. While the dicarbonyl 27 is indeed air sensitive, its decomposition appears qualitatively to be much slower relative to the other related complexes. 

The proposed catalytic mechanism for intramolecular McMurry reaction begins with the reduction of TiCl3 by zinc metal to generate the activated titanium species A-19. Reductive cyclization of the dicarbonyl substrate forms the McMurry coupling product, along with titanium oxide complex B-15. To close the catalytic cycle, the oxide complex B-15 is converted to TiCl3 by Me3SiCl (Scheme 63).8d,8e... 

Binding energy, pentacarbonyliron, 6, 3 Binuclear complexes bis-Cp titanium halides, 4, 522 with Ni-M and Ni-C cr-bonds heterometallic clusters, 8, 115 homometallic clusters, 8, 111 Binuclear dicarbonyl(cyclopentadienyl)hydridoiron complexes, with rand C5 ligands, 6, 178 Binuclear iridium hydrides, characteristics, 7, 410 Binuclear monoindenyl complexes, with Ti(IV), 4, 397 Binuclear nickel(I) carbonyl complexes, characteristics, 8, 13 Binuclear osmium compounds, with hydrocarbon bridges without M-M bonds, 6, 619... 

With regard to PK-type reactions, Buchwald studied titanium species as efficient catalysts in the PKR and in PK-like reactions with cyanides. Following preliminary results with [Cp2Ti(PMe3)2] and [Cp2TiCl2] [79-81], they reported a more practical procedure which improved the TON using commercial titanocene dicarbonyl (33) [82,83]. This complex is able to catalyze the... 

With C02, the titanium center of the dicarbonyl complex promotes the disproportionation of C02 giving CO and a tetranuclear carbonato complex of titanium(III) (117). 

Table I summarizes the application of various low-valent titanium metallocenes as catalysts for olefin hydrogenation. Compounds 10 and 37 are very effective hydrogenation catalysts for C2H4 and cyclohexene. Since different researchers have used widely varying conditions, we can only estimate that the polystyrene-supported (7j-C8H8)2Ti (142) is comparable in activity to compounds 10 and 37. When one recalls that 37 was prepared by a formal oxidation of the Ti centers in 10, it is remarkable that 37 is as good a catalyst as 10. Solutions prepared by reaction of l-methyl-17-allylbiscyclopentadienyltitanium (54) with H2 do appear to be more active hydrogenation (126) catalysts than 10 and 37. The dicarbonyl complex, (17-CsH5)2Ti(CO)2 (39), has been shown to be a catalyst for the hydrogenation of acetylene at —50 atm of H2 (143). It does not catalyze the hydrogenation of simple olefins. However, Floriani and Fachinetti discovered that if...

As previously mentioned, 1-alkynyltrialkylborates (18) have become increasingly important in the formation of carbon-carbon bonds via attack of electrophiles. However, such complexes cannot react with simple Qc,P-unsaturated carbonyl compounds such as methyl vinyl ketone, because of their weak electrophilicity. Recently it was ascertained that ,P-unsaturated carbonyl compounds react with 18 via a Michael-type reaction in the presence of titanium tetrachloride, and the usual alkaline hydrogen peroxide oxidation leads to the synthesis of 5-dicarbonyl compounds... 

Reductive amination. The low-valent titanium species derived from TiCl, Li, and MojSiCl absorbs atmospheric nitrogen at ambient temperature to form a complex which is capable of elaborating nitrogen azacycles such as pyrroles from 1,4-dicarbonyl compounds,... 

Substituted-titanocene dicarbonyl complexes, ( ]S-CsRBHs B)2Ti(CO)2, have also been used as isolable precursors to highly reactive titanium sandwiches, ( ]S-CsRBHs B)2Ti. For example, reaction of a,/3-unsaturated ketones with ( -CsHs TFCO 68 results in reductive coupling of the organic substrate to form (77S-CsHs)2Ti(0C(R1)CHCH(R2)CH(R2)CHC(R1)0) 69 with liberation of carbon monoxide (Scheme 12).47 48... 

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