Thallium alkoxides

A variant of the Williamson ether synthesis uses thallium alkoxides. The higher reactivity of these can be of advantage in the synthesis of ethers from diols, triols and hydroxy carboxylic acids, as well as from secondary and tertiary alcohols on the other hand however thallium compounds are highly toxic. 

Ether synthesis. A variation of the Williamson reaction involves conversion of an alcohol to the thallium(I) alkoxide followed by alkylation. In practice the method is not useful for simple alcohols because the thallium alkoxide is very sparingly soluble in C6H6 or CH3CN. The method is useful however for substrates containing an additional oxygen function such as OH, OR, COOR, and CONR2 (listed in the order of effectiveness). It is particularly useful for alkylation of chiral hydroxyl groups, since it is free from racemizalion. 

Te tram eric alkyl beryllium, zinc and cadmium alkoxides are formed by controlled alcoholysis of the metal dialkyls,16U62 while cubanoid thallium alkoxides are obtained by reaction of the metal with refluxing ethanol, and alcohol metathesis.163 Reaction (4) affords a tin phosphide cubane in low yield 164... 

Mixed-metal alkoxide complexes of thallium are also known. For example, Sn(/u-t-BuO)3Tl has both Sn(II) and T1(I) ions." The thallium site is unreactive as a donor for metal carbonyls. However, as indicated earlier, the indium(I) site of the indium analogue shows Lewis-base character. The Sn(IV)/Tl(I) mixed alkoxide [Sn(EtO)eTl2] exists as a one-dimensional polymer. This adduct reacts quantitatively with SnCl2 to form the homoleptic, mixed-valent [Sn2(OEt)6]n. Thallium-titanium double alkoxides have been synthesized using thallium alkoxide as one of the starting materials. ... 

To date two structures of organoindium alkoxides have been reported, [In(/t-OEt)Bu 2 and [In (/u,-OCH2SiMe3)(CH2SiMe3)2]2- Even less structural data is available on thallium alkoxides. The tetranuclear [Tl4(/Z3-OMe)4] was shown to have a distorted cubane TI4O4 framework (Fig. 4.6) although the Tl-O bond distances were not determined. Recently the remarkable chain copolymer [WCl2(PMe2Ph)4,... 

BetMIC is characterized by two key properties first, the good leaving ability of the benzotriazole (Bet) moiety and, second, the high acidity of the methylene protons. BetMIC is reactive with ketones to afford 4-ethoxy-2-oxazolines (94), which can be hydrolyzed to synthetically useful a-hydroxyaldehydes (95). It must be noted that unlike TosMIC, BetMIC forms 94/96 without the presence of highly toxic thallium alkoxides. The reaction works well in the absence of ethanol and provides better yields when using aliphatic ketones. With aromatic ketones, for example, ben-zophenone, partial conversion to oxazoline occurs probably due to steric effects. Reaction of BetMIC with aldehydes typically generates oxazoles 97 (Scheme 7.24) [43,44]. 

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