Organotitanium

Although titanium does not form a stable bond with carbon, organotitanium polymers have been formed such as polyalkoxytitanoxanes and polymeric titanate esters (Figure 29.14). 

Reetz, M. T. Organotitanium Reagents in Organic Synthesis. A Simple Means to Adjust Reactivity and Selectivity of Carbanions. 106, 1-53 (1982). 

Most successful approaches involving addition reactions in the presence of chiral additives utilize organolithium, organomagnesium and the recently introduced organotitanium reagents, which are known to coordinate with amines, ethers, metal amides and alkoxides. 

Thus, chiral organotitanium compounds result from the use of (trialkoxy)chlorotitanates derived from chiral alcohols. The transfer of methyl and phenyl groups by chiral titanium... 

M. T. Reetz in Organotitanium Reagents in Organic Synthesis, Springer, Berlin 1986. 

High ee values have also been obtained with organometallics," including organotitanium compounds (methyl, aryl, allylic) in which an optically active ligand is coordinated to the titanium," allylic boron compounds, and organozinc compounds. 

For systems employing BCTD the situation is different. The maximums occur at about 2/3 when employing TEA but no PTC and 4/3 when employing sodium hydroxide but no PTC. Again the maximums are dependent on the nature of the added base and added PTC. Yields vary but are not consistently lower when employing TEA compared with sodium hydroxide. It is possible that TEA may act to influence the transport of one or both of the reactants for systems employing the organotitanium reactant. 

Fluoride ion selective spectrometry was used to determine the stability constants for zinc fluoride complexes in water at 25 °C, giving values j3i[ZnF(aq)]+ = 3.5 0.1 and /l2[ZnF2(-aq)] = 3.8 0.5.643 These results demonstrate that the complexation of fluoride is very weak and in aqueous chemistry no species beyond ZnF+ is of much importance. Organotitanium fluorides have been used as matrices for trapping molecular ZnF2 and MeZnF. 4... 

In general, Ti appears to display the widest range of reactivity among the three members of the Ti triad. The most common oxidation number for all three members is +4. Their complexes in which they exist in the +2 oxidation state have also been implicated in many cases. Furthermore, organotitanium complexes of the +3 oxidation states have been much more widely observed than the corresponding complexes of Zr and Hf.14 The relatively ready accessibility of the +3 oxidative state along with the +4 and +2 oxidation states implies that Ti is more prone to one-electron transfer or radical processes than Zr or Hf, and this indeed has been the case. Undoubtedly, this is one of the main reasons for the versatile reactivity of Ti, which has led to a number of both favorable and unfavorable consequences relative to Zr or Hf. 

Carbometallation Reactions of Organotitanium Compounds 10.06.2.2.1 Controlled monocarbometallation... 

The chemistry of titanium has been reviewed in COMC (1982) and COMC (1995)40 41 as well as in Comprehensive Coordination Chemistry II. 2 Since then, several contributions have covered the coordination chemistry of cyclopenta-dienyltitanium carboxylates and related complexes,43 new titanium imido chemistry,44 the use of titanium(iv) chloride45 and isopropoxide46 in stereoselective synthesis, the preparation and synthetic applications of l, -dicarba-nionic titanium intermediates47 and organotitanium complexes,48 49 and titanium-catalyzed enantioselective... 

Selective vinylation of aldehydes lags far behind allylation as a synthetic method despite importance of chiral allylic alcohols in synthesis. Vinylmetal species are generally much less nucleophilic that their allyl counterparts, and some vinylmetallic species, such as organotitanium, run into stability issues not encountered in alkylmetals.92... 

The organotitanium compounds produced by desulfurization of the diphenyl thioacetals of aldehydes 28 with the titanocene(II) species Cp2Ti[P(OEt)3]2 29 react with carbon—carbon double bonds to form the olefin metathesis-type products. Thioacetals 28 may be transformed into terminal olefins by desulfurization with 29 under an ethene atmosphere (Scheme 14.15) [27]. This reaction is believed to proceed through a titanacyclobutane intermediate, formed by cycloaddition of the titanocene-alkylidene with ethene. 

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