V. I. Rozenberg

Organomelemental fragmentations : I. Direct and retro fragmentations in organometallic benzyl compounds. Aromatisational metallation

Abstract Organometallic benzyl compounds are known to react with eletrophilic agents via two paths, viz., by direct substitution of the metal by the agent, and by attack of the ortho - and para -positions of the ring. The latter reaction leads to expulsion of the metal and formation of unstable triene intermediates (1-methylene-2,4-cyclohexadienes and 1-methylene-2,5-cyclohexadienes). By attributing these processes to electrophilic fragmentations (referred to as F E ) a new process may be predicted to occur, electrpohilic addition of metal salts to the exo -methylene site of trienes of this type. This process can be regarded as the reverse of the fragmentation reaction. The F E representation proposed here forms the basis for a new organometallic synthesis, aromatisational metallation. It is shown that interaction of 1-methylene-6-ethyl-2,4-cyclohexadiene and 1-methylene-4-ethyl-2,5-cyclohexadiene with mercury dichloride in ether results in the formation of o - and p -ethylbenzylmercury chlorides, respectively.

σ,π-conjugation and 199Hg shielding constants in benzyl derivatives of mercury

Abstract The 199 Hg chemical shifts in a number of benzylmercuric chlorides containing methyl substituents in various positions of the benzene nucleus are studied by the heteronuclear 1 H-{ 199 Hg} double resonance technique. Meta - and para -methyl substituents are shown to have no pronounced action on the shielding whereas ortho -methyl groups each shift the signal by 30 ppm downfield. The observed effect is due to an increase in the population of conformers with the Cue5f8Hg bond lying out of the aromatic cycle plane. The departure from planarity favours conjugation between the electrons of the Cue5f8Hg bond and the π-electronic system of the ring. The J ( 1 Hue5f8 199 Hg) and J ( 13 Cue5f8 199 Hg) coupling constant values obtained in this work confirm the latter conclusion.

Organoelemental fragmentations : III. On the mechanism of isotope enrichment of the aromatic ring in organometallic benzyl compounds when reacted with acids☆☆☆

Abstract The β-effect of a metal is mainly exhibited in decomposition of a metal—carbon bond. Nevertheless, there is a well known case in which it was assumed that the metal—carbon bond was retained in the course of electron delocalisation at the adjacent carbocation site. This reaction is isotopically labelled acid cleavage of benzylmercuric chloride, which results in both electrophilic fragmentation (F E ), leading to the 1-methylene-6-deuteriocyclohexadiene-2,4 intermediate, and in hydrogen/deuterium replacement, leading to isotope enrichment of the aromatic ring of the starting species. However, we found that this process is governed by an electrophilic fragmentation, not an electrophilic aromatic substitution, mechanism. The study of deuterio-demetalation of benzylmercuric chloride with deuterium chloride, containing a mercuric chloride additive, has revealed that the demetalation rate and the isotope enrichment rate are markedly dependent on the mercuric chloride concentration in the system. These results, inexplicable in terms of the Cue5f8Hg hyperconjugation concept (σ—π conjugation), may suggest that the metal—carbon bond conservation in the process under discussion is only apparent. A new two-step mechanism has been formulated for the reaction under study, based on aromatisational metalation of the exo -methylene—cyclohexadiene structures, leading to isotopically labelled benzyl organometallics under the action of metal salts.

Organoelemental Fragmentations : II. A proof of tautomeric arene intermediates in reactions of organometallic benzyl compounds with acids. Aromatisational mercuration of 1-methylene-2,4-cyclohexadiene☆

Abstract Soft Lewis acids of the HgCl 2 type are effective electrophilic traps for tautomeric arenes even in strongly acidic media which catalyse aromatisation of the arenes. Acid cleavage of (PhCH 2 ) 3 SnCl by the action of DCl and HgCl 2 has been studied, and it is shown that the organotin compound is protolysed via a fragmentation mechanism involving a 1-methylene-6-deuterio-2,4-cyclohexadiene intermediate that yields o -D-C 6 H 4 CH 2 HgCl under the action of HgCl 2 .

Generation of free radicals by reaction of some organotin and orga n omer cur y compounds with bromine

1. n nThe dark bromodemetalation of a number of organotin and organomercury compounds in toluene or p-fluorotoluene initiates the free-radical bromination of the solvents in the methyl group, which testifies to the generation of free radicals during bromodemetalation. n n n n n2. n nA halodemetalation mechanism was proposed, which postulates that donor-acceptor complexes between the reactants, in which the organometallic compound functions as the donor and the halogen functions as the acceptor, take part in the reaction. The mechanism makes it possible to interpret the results obtained in the study (generation of radicals), as well as the previously observed generation of carbcations in the given reaction.

Formation of anomalous products in the bromodemercuration of organomercury compounds

1. n nTogether with the bromo derivatives, anomalous products of the involvement of the solvent in the reaction are formed when benzylmercury chloride and the cyclohexyl- and cyclohexylcarbinylmercury bromides are subjected to bromodemercuration in a number of oxygen-containing solvents. n n n n n2. n nThe formation of the anomalous products cannot be explained by the solvolytic transformations of benzyl bromide and the cyclohexyl- and cyclohexylcarbinyl bromides.

Transfer of the reaction center during deuterodemetallization of substituted benzylmercury chlorides

1. n nThe distribution of deuterium in the products of deuterometallization of o-, m-, and p-methyl-, 3,5-dimethyl-, and 2,4,6-trimethylbenzylmercury chloride by reaction with deuterium chloride was investigated. n n n n n2. n nThe presence of deuterium in the benzene nucleus of the products of deuterodemetallization of o-,m-, and p-methyl-and 3,5-demethylbenzylmercury chloride is due mainly to the transfer of the reaction center from theα-carbon atom to the nucleus.

Isotopic exchange of hydrogen in substituted benzylmercury chlorides

1. n nThe authors study the kinetics of splitting and isotopic exchange of substituted benzylmercury chlorides by the action of deuterium chloride in absolute dioxane. n n n n n2. n nIt has been found that the substituents have a symbatic effect on the isotopic-exchange and demetalation velocities owing to the common character of their mechanisms, including transfer of the reaction center from theα-carbon atom to the benzene ring.

Synthesis of organomercury compounds of the benzyl series

We have synthesized several organomercury compounds of the benzyl series, including 2,6-dimethyl-and 2,4,6-trimethylbenzylmercurichloride, which have a steric hindrance, as well as of 2-chloromercurimethylnaphthalene.

Deuterodemetallation of 2-chloromercurimethylnaphthalene

Deuterodemetallation of 2-chloromercurimethylnaphthalene takes place by migration of the reactive centers from the exocyclic carbon atom to the naphthalene nucleus.