Chlorine atom migration

Whereas thermolysis of halocyclopropanes in inert solvents occurs via cationic rearrangement, gas-phase pyrolyses of halocyclopropanes, e.g. 1, are believed to proceed by a concerted unimolecular chlorine atom migration and ring opening. From the preparative viewpoint this difference is not significant. 

The isomerization of pentachlorocyclopropane to 1,1,3,3,3-pentachloro-propene, which is also first-order in the range 440—490 K between 10 and 50 Torr, like those of other chlorocyclopropanes, is believed to oceur with concerted chlorine atom migration and no intervention of biradical intermediates. Pyrolysis of trans-l,l-dichloro-2,3-dimethylcyclopropane gives both isomerization to trans-3,4-dichloropent-2-ene and direct dehydrohalo-genation to trans-3-chloropenta-l,3-diene. The elimination is the major reaction whereas, from the cis-isomer, the rearrangement is the predominant reaction, as reported previously. 

In an early work Kandrorand Freidlina473 suggested the occurrence of 1,3 migration of the chlorine atom from sulfur to carbon, viz.,... 

Chlorine would have to lose seven electrons to reach an electron configuration like that of neon. But if it gained one, it would have the same stable electron configuration as argon. So that is what chlorine does. If it meets an atom with a high-energy valence electron, such as sodium, the electron migrates to the chlorine atom and forms a chloride ion ... 

The negatively charged Cl- ions migrate to the positively charged electrode. When a Cl ion comes into contact with the electrode, an electron passes from the ion to the electrode, leaving momentarily an atom of chlorine. The atom immediately combines with another chlorine atom to form a molecule of chlorine which is liberated as gas as depicted below ... 

An ab initio investigation of the transition state for the Lewis acid-associated migration of an alkyl group from boron to an a-dichloro-carbon in a non-racemic boronic ester has been carried out." The calculated transition state has shown that it is important to have the non-participating chlorine atom anti to the metal, e.g. as in (301). The... 

This stereochemical outcome of the Fritsch-Buttenberg-Wiechell rearrangement is well compatible with the crystal structure of the carbenoid 3 (Figure 1, Scheme 4). The aryl moiety trans to the vinylic chlorine atom is bent towards Cl (C1-C2-C9 116.5°). Thus, migration of the fraw -aryl group with simultaneous liberation of lithium chloride becomes evident. The free vinylidene carbene can be ruled out as the intermediate. 

The very high value of Cm for vinyl chloride is attributed to a reaction sequence involving the propagating center XVIII formed by head-to-head addition [Hjertberg and Sorvik, 1983 Llauro-Darricades et al., 1989 Starnes, 1985 Starnes et al., 1983 Tornell, 1988]. Intramolecular migration of a chlorine atom (Eq. 3-114) yields the secondary radical XIX that subsequently transfers the chlorine atom to monomer (Eq. 3-115) to yield poly(vinyl chloride)... 

Bromine trifluoride is used to selectively substitute fluorine for bromine in brominated alkanes and esters. The reactions are carried out by gradual addition of bromine trifluoride to a solution of the substrate in CFC-113 or CFC-112 at 10-20nC. The bromine-fluorine exchange in mono-bromohaloalkanes is nonstereoselective and accompanied, in some eases, by skeletal rearrangements, hydride shifts, and halogen migrations. All three fluorine atoms in bromine trifluoride are involved in the fluorination reaction. Chlorine atoms in the substrate molecules remain intact.109... 

Intramolecular migration of fluorine and chlorine atoms is described in Section 5. The rearrangement of fluorine and chlorine atoms likewise can occur intermolecularly between a number of chlorofluorocarbon molecules. This disproportionation (dismutation) takes place catalyti-cally or thermally. With aluminum trichloride as a catalyst, for example, enrichment of the fluorine atoms takes place in one molecule 1 and enrichment of the chlorine atoms in the other 2. 

It is unclear why in the carbenoid D it is the H atom in cis-position relative to the bromine that undergoes migration, and not the cyclohexenyl residue, which is in trans-position to the bromine. In the Fritsch-Buttenberg-Wiechell rearrangement of the chlorine-containing vinyl carbenoid it is the aryl residue in trans-position to the chlorine atom that undergoes migration ... 

---