Titanium tetrachloride exchange reactions

An exceptionally mild procedure for the cross-condensation of aldimines and enolsilanes has been described (eq. [67]) (80). This titanium tetrachloride-mediated reaction is predicated on the previous analogies provided by Mukaiyama for related aldol condensations (73a). Depending on aldimine structure and reaction time, either -lactams or their penultimate amino esters may be isolated from the reaction. The authors postulate that these reactions are proceeding via titanium enolates derived from ligand exchange by... 

Titanium tetrachloride is a very effective catalyst for the addition of LiAlH4 or alane to the olefinic double bond. The mechanism of this reaction involves intermediate transition metal hydrides, as in the case of reaction of NaBPU and Co11-salts. The hydrotitanation of the double bonds is probably followed by a rapid metal exchange reaction (Scheme 3)94. 

These are usually reactions of anhydrous transition and B metal halides with dry alkali metal salts such as the sulphides, nitrides, phosphides, arsenides etc. to give exchange of anions. They tend to be very exothermic with higher valence halides and are frequently initiated by mild warming or grinding. Metathesis is described as a controlled explosion. Mixtures considered in the specific reference above include lithium nitride with tantalum pentachloride, titanium tetrachloride and vanadium tetrachloride, also barium nitride with manganese II iodide, the last reaction photographically illustrated. 

Titanium tetrachloride promoted a three-component cyclization involving a carbonyl compound, an allylsilane, and an acetal, as shown in Eq. (69) [188]. The reaction was initiated by allyl addition to the aldehyde this was followed by acetal exchange and intramolecular Friedel-Crafts-type alkylation of the olefinic moiety. 

The exchange reactions between titanium tetrachloride and titanium/silicon alkoxides at low temperatures lead to the deposition of less soluble chloride... 

The reaction of an alkylalumlnum compound with titanium tetrachloride produces at room temperatures an almost Instantaneous precipitation of lower valence titanium chloride (B-TiClg) and evolution of gaseous hydrocarbons (36). This process (Reactions 5 and 6) represents a ligand exchange reaction of an alkyl and a chlorine group followed by decomposition of the unstable o-bonded alkyl titanium trichloride. These reactions are typical for numerous Ziegler-type catalysts ... 

Exchange reactions between two different metal chlorides and alkoxides have been employed for the synthesis of a wide variety of homo- and heteroleptic metal alkoxides. For example, Bradley and Hill" observed that a mixture of excess titanium tetrachloride and silicon dichloride diethoxide at O C deposited a crystalline product which was characterized as titanium trichloride monoethoxide ... 

Scheme 10.68. A representation of the reaction of enol exchange with 1-ethoxycyclohexene and 3-(trimethylsilyl)-l-propanol in the presence of p-toluenesulfonic acid catalyst followed by a Claisen-type rearrangement. Subsequent ring closure under the influence of titanium tetrachloride is also represented (see, e.g., Trost, B. M. Dong, G. Vance, J. A. J. Am. Chem. Soc., 2007,129,4540). HOTs = toluenesulfonic acid.

The reactions of ligand exchange have frequently been applied to synthesize chelates with the coordination unit containing N,0-donor centers. The complexes of titanium [280], thorium, and uranium [281] tetrachlorides with fe(salicylidene)alkylene-amines, obtained in THF solution, are evidently formed through THF complexes of the type MX (THF). An example of such syntheses is the transformation (3.107) [280] ... 

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