Titanium complexes, reaction with pyridines

The homoleptic, bis(trispyrazolylborate)titanium(ll) complex, Tp2Ti (Tp = trispyrazolylborate 203), has been prepared by reaction of 2 equiv. of KTp with TiCl2(TMEDA) (TMEDA = Ar,Ar,Ar, A -tetrarncthylcthylcncdiarninc).122 The dark red, paramagnetic compound has idealized /)v/ symmetry in the solid state and is easily oxidized to the corresponding Ti(m) derivative both electrochemically and chemically with AgPF, 204. Two-electron oxidation to Ti(rv) chalcogenido complexes has been accomplished with pyridine-N-oxide as well as with elemental sulfur and selenium (205, Scheme 31). 

Scheme 350 shows the structures of a number of mono-Cp titanium complexes with more elaborately substituted aryloxo ligands. The compounds are formed by the reaction of Cp TiCl3 with 1 equiv. of the substituted phenol in the presence of an excess of pyridine or by treatment of the lithium phenoxide with Cp TiCl3 some of them have been... 

A small number of enantiomerically pure Lewis acid catalysts have been investigated in an effort to develop a catalytic asymmetric process. Initial work in this area was carried out by Narasaka and coworkers using the titanium complex derived from diol (8.216) in the cycloaddition of electron-deficient oxazolidinones such as (8.217) with ketene dithioacetal (8.218), alkenyl sulfides and alkynyl sulfides. Cyclic alkenes can be used in this reaction and up to 73% ee has been obtained in the [2- -2] cycloaddition ofthioacetylene (8.220) and derivatives with2-methoxycarbonyl-2-cyclopenten-l-one (8.221) usingthe copper catalyst generated with bis-pyridine (8.222). Furthermore, up to 99% ee has been obtained in the [2-1-2] cycloaddition of norbornene with alkynyl esters using rhodium/Hs-BINAP catalysts. This reaction is not restricted to the use of transition metal-based Lewis... 

Teuben et al. reported a related reaction, in which the thermolysis of alkyl-bis(pentamethylcyclopentadienyl)titanium(III) caused a dissociation of alkane with formation of 64. further heating at 150°C resulted in hydrogen dissociation and formation of 70 as a diamagnetic material. Subsequent reaction with acetophenone gave chelate complex 71 in 60% yield, which was fully characterized spectroscopically as well as by an X-ray structure analysis (Scheme 10.24). Interestingly, the authors do not mention the presence of two diasteromers, although the alkyl chain bears an asymmetric carbon atom in addition to the planar chirality of the metallocene. Similar reactions were observed with pyridine derivatives [72]. 

While complexes 1-5 did not promote saturated alkane or fluorocarbon activation, the lessons learned from die synthetic and reactivity studies have led to the synthesis of the tris(triflate) complex 13, which is considerably more reactive due to the labile triflate ligand sphere. A promising example of the enhanced reactivity of 13 is its electrophilic substitution reactions with benzene and pyridine to form titanium-carbon bonds. These "titanations" are formal carbon-hydrogen bond activation processes and should allow access to the many useful organic reactions that titanium-carbon bonds are known to undergo (37,38). 

Titanium imido complexes supported by amidinate ligands form an interesting and well-investigated class of early transition metal amidinato complexes. Metathetical reactions between the readily accessible titanium imide precursors Ti( = NR)Cl2(py)3 with lithium amidinates according to Scheme 84 afforded either terminal or bridging imido complexes depending on the steiic bulk of the amidinate anion. In solution, the mononuclear bis(pyridine) adducts exist in temperature-dependent, dynamic equilibrium with their mono(pyiidine) homologs and free pyridine. 

The addition of a base, typically ammonia, to mixtures of transition metal halides and alcohols allows the synthesis of homoleptic alkoxides and phenoxides for a wide range of metals. Anhydrous ammonia was first used in the preparation of titanium alkoxides where the reaction is forced to completion by the precipitation of ammonium chloride.41 Although useful for the synthesis of simple alkoxides and phenoxides of Si, Ge, Ti, Zr, Hf, V, Nb, Ta and Fe, as well as a number of lanthanides,42-47 the method fails to produce pure /-butoxides of a number of metals.58 Presumably, secondary reactions between HC1 and Bu OH take place. However, mixing MC14(M = Ti, Zr) with the Bu OH in the presence of pyridine followed by addition of ammonia proves successful, giving excellent yields of the M(OBul)4 complexes.59... 

Titanathietane complexes are obtained as brown crystals of high thermal stability when the vinylidene intermediate species reacts with isothiocyanates RNGS by a [2 + 2]-cycloaddition process (Scheme 529). Complexes with the sulfur atom in cr-position bonded to titanium are formed exclusively. The structure for the cyclohexyl derivative has been confirmed by X-ray diffraction. Heating of these titanathietane complexes in the presence of pyridine at 80 °C for 20 min results in isomerization, with the formation of a new titanacyclobutane compound (Scheme 529). The regioselectivity of these reactions was discussed on the basis of Hartree-Fock ab initio calculations.1365... 

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