Preparation of mixed diorganothallium(III) isobutyrates and their replacement reaction with tetramethyltin

Various mixed diorganothallium isobutyrates were prepared. A novel replacement reaction was observed by treating them with tetramethyltin in methanol as follows, where R′ is evidently limited to groups in which the carbon atom bonded to thallium has sp Or sp 2 hybridization. We have briefly reported the preparation of mixed diorganothallium(III) chlorides by the reaction of monoorganothallium diisobutyrates with tetramethyltin or phenylboronic acid, followed by addition of hydrochloric acid. similar reactions are found to proceed smoothly and nearly quantitatively as shown in eqns. (1) and (2). We have also found that monomethylthallium diisobutyrate reacts with other tetraalkyltins. The reaction with either tetraethyl- or tetra-n-propyltin was slow, and the product was contaminated by a dimethylthallium species. It is not yet clear whether this species was formed from the disproportionation of CH 3 TlY 2 or CH 3 RTlY. However, with the exception of these two cases, the alkylation reaction proceeds straightforwardly as shown in (1) and (1′). This is noteworthy in view of the report 3 that phenylthallium dichloride reacts with alkyl Grignard reagents to give a mixture of diphenylthallium halide and the corresponding dialkylthallium halide. Tetramethyltin also reacts with various mixed diorganothallium isobutyrates, causing a novel replacement reaction as shown in eqn. (3) This reaction proceeds smoothly, and methanol is thought to act as a proton source for the formation of the hydrocarbons (R′H). It should be noted that methylethylthallium isobutyrate does not undergo this reaction, i.e. the organic group (R′) is replaced only when the carbon atom bonded to thallium has sp or sp 2 hybridization. Presumably this reaction does not proceed by way of a simple R′-methyl exchange mechanism. In that event the reaction of R(C 6 H 5 )TlY with (CH 3 ) 4 Sn would give the intermediate (CH 3 ) 3 (C 6 H 5 )Sn, but this compound is found to be unreactive toward methanol under the reaction conditions employed. A likely mechanism may involve isobutyryloxy-methyl exchange, and the resulting CH 3 RR′Tl could then react with methanol to lose only the most electronegative R′ group, preferentially forming R′H before the disproportionation of CH 3 RR′Tl takes place. However, such a disproportionation reaction through intermolecular exchange of alkyl groups has been reported to occur very rapidly in mixed tri(methyl, ethyl)thallium and tri(methyl, vinyl)thallium systems4. Alternatively, it is also possible that the methylation by tetramethyltin and the cleavage of the TI-R′ bond take place simultaneously in the sense of a concerted mechanism, without formation of a CH 3 -RRTl intermediate.