Tetrachloride, titanium rearrangement

As catalysts for the Fries rearrangement reaction are for example used aluminum halides, zinc chloride, titanium tetrachloride, boron trifluoride and trifluoromethanesulfonic acid7... 

Tsou, Magee and Malatesta (39) showed the effect of catalyst ratios on steric control m the polymerization of styrene with alkyllithium and titanium tetrachloride. These authors have shown that the isotactic polymer was produced when the butyllithium to titanium ratio was kept within the limits of 3.0 to 1.75. Outside of this critical range, amorphous polymers were produced. In the discussion of this paper, Friedlander (40) pointed out the cationic nature of the low-lithium-to-titanium-ratio-catalysts which also produced considerable rearrangement of the phenyl groups. Above 2.70 lithium to titanium ratio, an anionic type polymerization set in, which produced atactic polymer. At low ratios cationic catalysis also produced atactic polymer. Tsou and co-workers concluded that crystallinity of the catalyst is not important for steric order in the polymer. 

Protection of phenols by the foregoing methods is complicated by the rapid Friedel-Crafts rearrangement of the nascent rm-butyl ether. By using trifluoro-methanesulfonic add at -78 PC, the rate of /erf-butyl ether formation is fast and the Friedel-Crafts alkylation does not compete [Scheme 4.126].226 Similarly, attempts to deprotect phenol ferf-butyl ethers with trifluoroacetic acid or titanium tetrachloride may give complex mixtures, again as a result of Friedel-Crafts alkylation of the phenol but this side reaction can be suppressed by using a catalytic amount of trifluoromethanesulfonic acid in 2.2,2-trifluoroethanol as solvent at -5 DC. 

Lewis acid-mediated rearrangement of enol ethers affords 1,2-disubstituted cycloaUcanones with high stereoselectivity (Scheme 234). Related reactions of dichloro-substituted enol ethers afford 2,2-dichloro-substituted cycloaUcanones (Scheme 235). When titanium tetrachloride was used as the Lewis acid, a carbonyl-ene Prince-type reaction can take place (Scheme 236). 

The /3-tetraacetate, by etherification of the glycosidic hydroxyl group with methyl iodide and silver oxide, furnished a convenient source for the preparation of methyl tetraacetyl-/3-D-altroside when mutarotation occurred before methylation, considerable methyl tetraacetyl-a-D-altro-side was isolated. The latter was obtained also by rearrangement of the /3-isomer with titanium tetrachloride. These methyl tetraacetylaltro-sides do not contain orthoester linkages, and may be assumed to possess normal pyranoside rings. From the values of - -66.0 and — 61.0°... 

Pacsu and CrameF treated a chloroform solution of hexaacetylmaltose methyl 1,2-orthoacetate with titanium tetrachloride and obtained the normal heptaacetyl-a-maltosyl chloride. In like manner, when dry hydrogen bromide in glacial acetic acid was employed, a rapid formation of the normal heptaacetyl-a-maltosyl bromide resulted. Obviously, the replacement of the methoxyl group of the methyl orthoacetate by a halogen atom was accompanied by a rearrangement of the orthoester... 

A stereochemical control of the Ugi reaction can be effected with carbohydrates as chiral templates (e.g. tetrakis(O-pivaloyl)galactosylamine), which gives rise to easily separable amides. From these a variety of non-natural amino acids can be derived after acidic hydrolysis. The Passerini reaction, related to the Ugi rearrangement, gives a-hydroxyamides. A modification of this reaction using titanium tetrachloride gives a-branched amides in high yields via C-metalated imidoyl chlorides (equation 38). 

Hill, R. K., Khatri, H. N. Titanium tetrachloride catalysis of aza-Claisen rearrangements. Tetrahedron Lett. 1978,4337-4340. 

Fries reaction. Titanium tetrachloride is useful as catalyst for the Fries rearrangement of phenol esters to o- or p-hydroxyketones. For example, p-cresyl acetate is rearranged in high yield in nitrobenzene solution. ... 

Taub et ai at Merck used titanium tetrachloride to effect rearrangement of the phenol ester (1) to the benzophenone (2). In this case use of aluminum chloride led mainly to fragmentation into the components and the best yield of (2) was 5%. 

A procedure has been described for the preparation of 0-hydroxyalkyl ketones via rearrangements of a, -epoxyalkyl silyl ethers with titanium tetrachloride. This procedure has found... 

The aza-Claisen rearrangement can be catalyzed by Lewis acids such as titanium tetrachloride (TiCU). When aldehyde 653 reacted with 3-(A -phenyl)-1 -butene (654) in the presence of TiCU, the initially formed enamine (655) rearranged to 656 and was hydrolyzed to aldehyde 657 in 61% overall yield. ... 

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