Titanium complexes bromides

A synthetically useful diastereoselectivity (90% dc) was observed with the addition of methyl-magnesium bromide to a-epoxy aldehyde 25 in the presence of titanium(IV) chloride60. After treatment of the crude product with sodium hydride, the yy -epoxy alcohol 26 was obtained in 40% yield. The yyn-product corresponds to a chelation-controlled attack of 25 by the nucleophile. Isolation of compound 28, however, reveals that the addition reaction proceeds via a regioselective ring-opening of the epoxide, which affords the titanium-complexed chloro-hydrin 27. Chelation-controlled attack of 27 by the nucleophile leads to the -syn-diastereomer 28, which is converted to the epoxy alcohol 26 by treatment with sodium hydride. 

The titanium complex 4, prepared from (A./ )-2,3-0-isopropylidenc-1,1.4,4-tetraphenyl-1,2,3,4-butanetetrol7,113 and chlorotriisopropoxytitanium or tetraisopropoxytitanium, is treated with 2-propenylmagnesium bromide. The resulting titanate affords, with benzaldehyde, ( —)-(5)-l-phenyl-3-butenol. Several further attempts, which do not include allylation, have also been reported113, as have examples using the dichloride114. 

The first synthetically useful reaction of titanium complexes of type 4, leading to the formation of two new carbon—carbon bonds, was developed by Kulinkovich et al. [55]. They found that treatment of a carboxylic acid ester with a mixture of one equivalent of titanium tetraisopropoxide and an excess of ethylmagnesium bromide at —78 to —40 °C affords 1-alkylcyclopropanols 9 in good to excellent yields (Scheme 11.2) [55,56], This efficient transformation can also be carried out with sub-stoichiometric amounts of Ti(OiPr)4 (5—10 mol%) [57,58]. In this case, an ethereal solution of two equivalents of EtMgBr is added at room temperature to a solution containing the ester and Ti(OiPr)4. Selected examples of this transformation are presented in Table 11.1 (for more examples, see ref. [26a]). 

Tire early-late transition metal complex of Raymond et al. is interesting in that it requires both metal atoms to form the basic C3 structure (ideally D i, may form). Titanium complexation to three catechol units leads to a tridentate ligand, and, only when palladium bromide is added, trans coordination to palladium gives the agglomerate 31 [78]. 

METHYLENATION Alkyidimesitylboranes. p,-Chlorobis(cyclopentadienyl)(dimethyl-aluminum)-p.-methylene-titanium. Methylene bromide-Zinc-Titanium(IV) chloride. Organomolybdenum reagents. Titanocene methylene-Zn halide complex. N,N,P-Tri-methyl-P-phenylphosphinothioic amide. Trimethylstannylmethyllithium. 

The mixed cyclopentadienyl-cycloheptatrienyl ( 5-7 ) titanium complex Cp( -C7H7)Ti has been prepared in 33 -40% yield by the reduction of CpTiCb with isopropylmag-nesium bromide or Mg in the presence of cycloheptatriene. Reduction of Cp TiCb in THF with Mg in the presence of cycloheptatriene gives Cp ( -C7H7)Ti in 68% yield. Titanium 5-7 complexes exhibit sandwich structures see Sandwich Compound) with the five-membered and seven-membered rings nearly parallel to each other. 

Titanium, tetrakis(trimethysilyl)oxy-, 334 Titanium complexes alloy hydrides, 353 amino adds, 342 antimony, 345 arsenic, 345 bromides, 357 chlorides, 355, 356 fluorides, 354 Group IV derivatives, 352 halides, 354 electron spectra, 358 hexamethylphosphoramide, 335 iodides, 357... 

Titanium complexes have been shown to be active for the synthesis of cyclic carbonates or either di- or trithiocarbonates from epoxides and either carbon dioxide or carbon disulfide (Scheme 5.6). Titanium-catalysed synthesis of cyclic carbonates has been recently reviewed by North and coworkers. Titanium-salen complexes find application as catalysts, in combination with tetrabutylammonium bromide or tributylamine, for the synthesis of di- or trithiocarbonates from epoxides and carbon disulfide. It is worth highlighting that the catalyst loading can be reduced to 0.5 mol%, although 1 mol% of catalyst was required in order to achieve quantitative yields. The catalyst system showed a preference for dithiocarbonate formation for most of the epoxides studied. 

Excellent chelation control was observed using tributyl(2-propenyl)stannane and a-benzyloxy-cyclohexaneacetaldehyde with magnesium bromide or titanium(IV) chloride, whereas useful Cram selectivity was observed for boron trifluoride-diethyl ether complex induced reactions of the corresponding ferr-butyldimethylsilyl ether89. 

For a-benzyloxycyclohexaneacelaldehyde and 2-butenylstannanes, good chelation control was observed using zinc iodide and titanium(IV) chloride, but only weak synjanti selectivity. Better syn/anti selectivity was found using boron trifluoride-diethyl ether complex, but weak chelation control. Magnesium bromide gave excellent chelation control and acceptable syn/anli selectivity90. 

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