Thallium substituents

Thallation is almost exclusively ortho to certain substituents like —OOOH, COOCH3, and —CH2OCH3 (even though some of these are normally meta-directing), and this is attributed to prior complexing of the electrophile with the substituent thallium is held at just the right distance for easy intramolecular delivery to the ortho position. For example ... 

Thallium(III) acetate reacts with alkenes to give 1,2-diol derivatives (see p. 128) while thallium(III) nitrate leads mostly to rearranged carbonyl compounds via organothallium compounds (E.C. Taylor, 1970, 1976 R.J. Ouelette, 1973 W. Rotermund, 1975 R. Criegee, 1979). Very useful reactions in complex syntheses have been those with olefins and ketones (see p. 136) containing conjugated aromatic substituents, e.g. porphyrins (G. W. Kenner, 1973 K.M. Smith, 1975). 

It has been reported that photolysis of arylthallium ditrifluoroacetates in benzene suspension results in replacement of the thallium substituent by a phenyl group (i.e., phenylation) to give unsymmetrical biphenyls in excellent yield (152) this reaction is summarized in Section III,C. An analogous reaction occurs upon photolysis of diarylthallium trifluoroacetates in benzene suspension unsymmetrical biphenyls are formed in comparable yield (40-95%). The mechanism of this conversion is undoubtedly similar to that... 

In summary, then, the orientation of electrophilic thallation can be controlled by an appropriate manipulation of reaction conditions. Under conditions of kinetic control, ortho substitution results when chelation of the electrophilic reagent (TTFA in the studies described above) with the directing substituent permits intramolecular delivery of the electrophile, and para substitution results when such capabilities are absent this latter result is an expression of the very large steric requirements of the bulky thallium electrophile. Under conditions of thermodynamic control, however, meta substitution is observed. 

From the point of view of the synthetic organic chemist, the importance of aromatic thallation, and the remarkable degree of orientation control which can be exercised over this process, lies in the ease with which the resulting ArTlXj compounds can be converted into substituted aromatic derivatives in which the new substituent group has entered the ring at the position to which thallium was originally attached. Syntheses of phenols, nitroso compounds, biaryls, aromatic nitriles, thiophenols, and deuterated aromatic compounds have all been achieved these results are summarized briefly below. 

The versatility of ArTlXj compounds as intermediates for the synthesis of substituted aromatic compounds has been substantially extended by the observation that the aryl-thallium bond is extremely labile photochemically. The resulting aryl radical can then be captured by appropriate reagents (see below) to give substituted aromatic compounds. A remarkable feature of these photochemical conversions of ArTlXj compounds to substituted aromatics is that, as before, the new substituent always enters the ring at the position to which thallium was originally attached. 

In the case of the cyclohexene adduct (XIV), the most favorable conformation should be that in which the bulky thallium substituent is equatorial ... 

Aryltrimethylsilanes has been found to be a useful complement to direct thallation in the preparation of arylthallium(III) intermediates. The thallium(III) replaces the silyl substituent and the scope of the reaction is expanded to include some EWGs, such as trifluoromethyl. How does the silyl group function in these systems ... 

This section will focus on homonuclear neutral or anionic clusters of the elements aluminum, gallium, indium, and thallium, which have an equal number of cluster atoms and substituents. Thus, they may clearly be distinguished from the metalloid clusters described below, which in some cases have structures closely related to the allotropes of the elements and in which the number of the cluster atoms exceeds the number of substituents. The compounds described here possess only a single non-centered shell of metal atoms. With few exceptions, their structures resemble those of the well-known deltahedral boron compounds such as B4(CMe3)4 [30], B9CI9 [31] or [B H ]2 [32]. The oxidation numbers of the elements in these... 

The only thallium compound to be discussed here is the Tlf,Cl2[Si(CMe3)3]6 cluster 61. This remarkable compound was formed by the reaction of thallium) 111) chloride with NaSi(CMe3)3 [Eq. (28)] and precipitated in the form of black crystals in 21% yield, when a solution in toluene was stored at —25 °C for six months [92], Solutions of 61 in benzene decompose slowly at room temperature by the formation of ClSi(CMe3)3 and a black, not identified precipitate. The structure of 61 consists of two four-membered T13C1 heterocycles, which are connected by one Tl-Tl and two Tl-Cl bonds. A monomeric TI3CI heterocycle was isolated as a byproduct in which one thallium atom was bonded to two Si(CMe3)3 substituents. 

Thallium(III) substituents (see 36) exert even larger effects on a-caibon atoms than mercury does (141) (see 37 and 38) this has been ascribed to displacement of electrons in the carbon-metal a-bond toward the metal atom, due to its increased nuclear charge. 

The majority of transformations carried out at ring substituents of bicyclic azetidinones were described in Section 5.12.3.3.2 for azetidinones related to cephalosporins and penicillins. Nucleophilic displacement reactions of 7-halo-l-azabicyclo[4.2.0]octan-8-ones with thallium phthalimide and hydrazine have been reported (76ACS(B)318). 

The higher acidity of pyrroles and indoles bearing electron-withdrawing substituents at the a- or /3-positions permits their alkylation under mildly basic conditions, but although the thallium salt of 2-formylpyrrole is Af-alkylated, the corresponding alkylation of the thallium salts of ethyl pyrrole-2-carboxylate yields a complex mixture of products resulting from iV-alkylation and transesterification (B-77MI30502). N-Alkylation of pyrrolyl and indolyl esters is most conveniently effected under phase-transfer conditions. 

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