Titanium reductive elimination with

Reductive elimination from 2-en-l,4-diol derivatives has been used to generate 1,3-dienes. Low-valent titanium generated from TiCl3-LiAlH4 can be used directly with the diols. This reaction has been used successfully to create extended polyene conjugation.305... 

Although the reaction of a titanium carbene complex with an olefin generally affords the olefin metathesis product, in certain cases the intermediate titanacyclobutane may decompose through reductive elimination to give a cyclopropane. A small amount of the cyclopropane derivative is produced by the reaction of titanocene-methylidene with isobutene or ethene in the presence of triethylamine or THF [8], In order to accelerate the reductive elimination from titanacyclobutane to form the cyclopropane, oxidation with iodine is required (Scheme 14.21) [36], The stereochemistry obtained indicates that this reaction proceeds through the formation of y-iodoalkyltitanium species 46 and 47. A subsequent intramolecular SN2 reaction produces the cyclopropane. 

An unusual reductive elimination can ensue from titanacyclobutanes possessing an alkenyl group at the carbon a to the titanium atom. Thus, alkenylcarbene complexes 48, prepared by the desulfurization of (fy-unsaturated thioacetals 49 or l,3-bis(phe-nylthio)propene derivatives 50 with a titanocene(II) reagent, react with terminal olefins to produce alkenylcyclopropanes 51 (Scheme 14.22, Table 14.4) [37]. This facile reductive... 

Reductive elimination of an allylic diol group. A new synthesis of vitamin A involves reduction of the allylic diol 1, prepared in several steps from JJ-ionone, with a low valent titanium formed from TiCl3 and LiAlH, in the ratio 2 1. Thus, the allylic diol group of 1 [either (E) or (Z)] is reduced to an (E,E)-1,3-diene group to form the silyl ether (2) of vitamin A.1 When the primary hydroxyl group is protected as an acetate, the reduction gives a mixture of (E)- and (Z)-2. 

Titanium-catalyzed cyclization/hydrosilylation of 6-hepten-2-one was proposed to occur via / -migratory insertion of the G=G bond into the titanium-carbon bond of the 77 -ketone olefin complex c/iatr-lj to form titanacycle cis-ll] (Scheme 16). cr-Bond metathesis of the Ti-O bond of cis- iij with the Si-H bond of the silane followed by G-H reductive elimination would release the silylated cyclopentanol and regenerate the Ti(0) catalyst. Under stoichiometric conditions, each of the steps that converts the enone to the titanacycle is reversible, leading to selective formation of the more stable m-fused metallacycle." For this reason, the diastereoselective cyclization of 6-hepten-2-one under catalytic conditions was proposed to occur via non-selective, reversible formation of 77 -ketotitanium olefin complexes chair-1) and boat-1), followed by preferential cyclization of chair-1) to form cis-11) (Scheme 16). 

Dienes. Low-valent titanium (TiCl3/LiAlH4 = 2 1) reacts with 2-ene-l,4-diols to give 1,3-dienes by a 1,4-reductive elimination.2 Examples ... 

A number of cycloproparenes have also been prepared from adducts of l-bromo-2-chlorocy-clopropene with furan derivatives by treatment with low-valent titanium. These reactions are regarded as reductive eliminations and are discussed in Section 5.2.2.7.I. 

Low valent titanium reagents have been used in the first low temperature synthesis of the fullerene fragment dimethyl corannulene (53) from the 1,6-bis(bromomethyl)-7,10-bis(l-bromoethyl)fluoranthene (54, see Scheme 24 [107]). Obviously, the high thermodynamic driving forces for the reductive elimination of bromine as well as for the aromatization with 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) in the course of the reaction via the relatively unstrained a s/frans-dimethyltetrahydrocorannulenes enabled the formation of... 

Reductive elimination. All (E) trienes are formed by treatment of 1,6-diben-zoyloxy-2,4-dienes with the low-valent titanium species prepared from TiClj and LiAlH4. This method is applicable to the synthesis of a leukotriene. 

Conjugated polyene fragments occur in a wide variety of naturally occurring organic compounds, such as macrolides, carotenoids, and leukotrienes. The synthesis of all-trans triene 734 is readily accomplished by low-valent titanium-induced reductive elimination of l,6-dibenzoate-2,4-diene 733 [210] (Scheme 97). The requisite cis, c/5-diene geometry is obtained by a stereospecific reduction of diyne 731 with activated zinc—copper couple. 

Synthesis of 3-substituted benzo[fc]furans was realized from a combination of benzotiiazole chloride mediated intramolecular cyclization and low-valent titanium promoted reduction. Thus, the sodium salt of 2-hydroxybenzophenone was first allowed to react with a 1-benzotriazol-l-ylalkyl chloride to give a ketone which was cyclized by LDA. Eventually, A titanium promoted elimination led to the desired 3-phenylbenzo(I>]furan in good yield <01JOC5613>. 

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