Olefins oxythallation

Monoalkylthallium(III) compounds are unstable (73, 79), and very few examples of this class have been isolated. A number of alkylthallium diacetates have been obtained either from oxythallation of olefins with thallium-(III) acetate (see below) or from exchange reactions such as that shown in Eq. (11) (74, 75). Only four alkylthallium dihalides have been isolated so far, namely a neopentylthallium dihalide (60) [Eq. (12)] and the isomeric 2-, 3-, and 4-pyridiomethylthallium dichlorides (20) [Eq. (13)]. Monoaryl-and monovinylthallium(III) derivatives are considerably more stable than... 

Monoalkylthallium(III) compounds can be prepared easily and rapidly by treatment of olefins with thallium(III) salts, i.e., oxythallation (66). In marked contrast to the analogous oxymercuration reaction (66), however, where treatment of olefins with mercury(II) salts results in formation of stable organomercurials, the monoalkylthallium(III) derivatives obtained from oxythallation are in the vast majority of cases spontaneously unstable, and cannot be isolated under the reaction conditions employed. Oxythallation adducts have been isolated on a number of occasions (61, 71,104,128), but the predominant reaction pathway which has been observed in oxythallation reactions is initial formation of an alkylthallium(III) derivative and subsequent rapid decomposition of this intermediate to give products derived by oxidation of the organic substrate and simultaneous reduction of the thallium from thallium(III) to thallium(I). The ease and rapidity with which these reactions occur have stimulated interest not only in the preparation and properties of monoalkylthallium(III) derivatives, but in the mechanism and stereochemistry of oxythallation, and in the development of specific synthetic organic transformations based on oxidation of unsaturated systems by thallium(III) salts. 

Tl(III) < Pb(IV), and this conclusion has been confirmed recently with reference to the oxythallation of olefins 124) and the cleavage of cyclopropanes 127). It is also predictable that oxidations of unsaturated systems by Tl(III) will exhibit characteristics commonly associated with analogous oxidations by Hg(II) and Pb(IV). There is, however, one important difference between Pb(IV) and Tl(III) redox reactions, namely that in the latter case reduction of the metal ion is believed to proceed only by a direct two-electron transfer mechanism (70). Thallium(II) has been detected by y-irradiation 10), pulse radiolysis 17, 107), and flash photolysis 144a) studies, butis completely unstable with respect to Tl(III) and T1(I) the rate constant for the process 2T1(II) Tl(III) + T1(I), 2.3 x 10 liter mole sec , is in fact close to diffusion control of the reaction 17). 

Oxidation of the steroidal olefin (XXVII) with thallium(III) acetate gives mainly the allylic acetates (XXXI)-(XXXIII) (Scheme 15), again indicating that trans oxythallation is the preferred reaction course (19). Addition of the electrophile takes place from the less-hindered a-side of the molecule to give the thallinium ion (XXVIII), which by loss of a proton from C-4 would give the alkylthallium diacetate (XXIX). Decomposition of this intermediate by a Type 5 process is probably favorable, as it leads to the resonance-stabilized allylic carbonium ion (XXX), from which the observed products can be derived. Evidence in support of the decomposition process shown in Scheme 15 has been obtained from a study of the exchange reaction between frawr-crotylmercuric acetate and thallium(III) acetate in acetic acid (Scheme 16) (142). 

Oxythallation reactions of olefins provide useful synthetic routes to products, eg, glycols, aldehydes, and ketones (31) ... 

In several cases, the oxythallation adducts have been isolated (11,29,35) in nonaqueous solutions, and we have found that even in aqueous solution some intermediates can be isolated if the solution contains excess acetate ion (24). Also stereochemical results (2), as well as the effect of olefin structure on product distribution, are consistent with such an intermediate (21). 

Further evidence for heterolytic decomposition is obtained from the effect of olefin structure on product distribution. Table II shows the ratio of carbonyl to glycol product for three olefins. Listed for comparison is the carbonyl/glycol ratio for the chlorohydrin, which corresponds to the structure of the oxythallation adduct from ethylene and propylene. The effect of structure on the ratio is qualitatively the same for the thallic ion oxidation and the hydrolysis of the corresponding chlorohydrin. Since the product distributions for both are inconsistent with neighboring hydroxyl participation (Reaction 33) the carbonyl/glycol ratio is a measure of the competition between hydride shift vs. water attack in... 

Data for 2-chloro-3-butanol is unavailable, but the low glycol yield for Tl(III) oxidation is understandable on the basis of steric hindrance to Sn2 attack of water on the carbon-thallium bond. With ethylene and propylene, the thallium (III) is attached to a primary carbon while with 2-butene it is attached to a secondary carbon. The situation with 2-butene is somewhat different from that found with the other olefins. In the oxythallation adduct from this olefin. 

In nonaqueous solvents, the oxythallation adduct can actually be isolated in the oxidation of certain olefins, and in cases where it cannot be isolated, the product can be explained by solvolysis of an oxythallation adduct. 

Monoalkylthallium(III)s are formed at RT by treatment of olefins with Tl(III) salts (oxythaiiation). However, these Tl compounds undergo spontaneous decomposition. Although the oxythallation-dethallation has - important application to organic synthesis, few Tl compounds are isolable. The decomposition of the monoalkylthallium(III)s can best be prevented by use of acetate as the anion, even though Tl(III) acetate is slower to react than more ionic salts, such as Tl(III) nitrate. 

Unstrained olefins undergo trans-oxythallation in MeOH to give methoxythallation adducts e.g., 1-hexene reacts with Tl(III) acetate or trifluoroacetate in MeOH to give 2-methoxyhexylthallium diacetate ... 

Oxidative rearrangements, via oxythallation, have been improved in yield and selectivity by the use of thallium(iii) nitrate supported on clay rather than in methanolic solution. Thus, cyclohexene gave an 85% yield of dimethoxymethyl-cyclopentane while 1-tetralone, which normally gives a complex mixture of products, gave a 1 1 mixture of methyl indane-l-carboxylate and 2-methoxytetralone. An efficient, large-scale procedure for the direct cis-dihydroxylation of olefins has been reported. The oxidant is t-butyl hydroperoxide and the catalyst osmium tetroxide, with the reaction conducted under alkaline conditions (E%N OH ), so facilitating a rapid turnover of catalyst via enhanced hydrolysis of the osmate esters. The method appears to be more advantageous for the more substituted olefins than the Hofmann and Miles procedure. 

Measurements of 2o /205 j ijj spin-spin couplings for the organothallium compounds formed in oxythallation reactions provides the first direct evidence for trans addition in the oxythallation of acyclic olefins. Products from the reactions of styrene, u-alkylphenols, propylene, and oct-l-ene were studied. The rate constants for the oxythallation of alkenes shows a reasonable correlation with the first ionization energy of the alkenes. Inductive effects were found to be most important in the oxythallation of RCH=CH2 and R R C=CHa alkenes. ... 

Evidence has been presented for adduct formation in the oxidation of certain olefinic compounds by thallium(m). Alkenols react with the oxidant in a stoicheiometric ratio of 1 1 and two types of behaviour have been described depending on the reducing substrate. In the case of prop-l-en-3-ol, pent-l-en-5-ol, and hex-l-en-6-ol, spectroscopic changes monitored using a stopped-flow apparatus are consistent with two separate reactions (a) an oxythallation,... 

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