Chlorine exchange

The second preparation for a free iminosilane was found by the Klinge-biel group. It is based on a fluorine/chlorine exchange of lithiated fluoro-silylamines with subsequent thermal LiCl elimination.17... 

After a fluorine/chlorine exchange, a lithiated aminochlorosilane is obtained. Lithium is bound to nitrogen, the strongest Lewis base in this compound. The SiN bond is enlarged (164.2 pm) and the SiNC angle of 138.7° shows the character of the amine.17,18... 

Summary Lithiated aminofluorosilanes can often be regarded as LiF adducts of iminosilanes on account of their structures. They are suitable precursors to iminosilanes after fluorine-chlorine exchange. 

Lithiated aminochlorosilanes are obtained with MejSiCl by a fluorine-chlorine exchange, for example 3 (Figure 2) [2], LiCl is easily eliminated. Monomeric or dimeric iminosilanes are formed [3,4]. 

At 62% yield, the main product of the reaction of 42b with MeLi (molar ratio 1 5.5) was l,2-dichloro-2-methylbicyclo[2.1.1]hexane (66). Again, the most probable mechanism leading to 66 is addition of MeLi to carbene 54 (X=H, X=C1), followed by lithium chlorine exchange of the intermediate tertiary alkyllithium base 67 with the trichloride 42b. An alternative mechanism, addition of LiCl to carbene 54 and methylation of the intermediate carbenoid by MeLi, formed during the reaction from MeLi and 42b, is less probable.24... 

Substantial amounts (24% and 12%, with and without catalyst, respectively) of l,3-dichloro-2-butene arising from bromine-chlorine exchange were formed. h ... 

Compound ClC(0)SSCCl3 is obtained from FC(0)SSCCl3 through fluorine-chlorine exchange with BCI3 C1C(0)SSCF3 and FC(0)SSCF3 are formed as follows ... 

The reaction with fluorocarbonylsulfenyl chloride has to be carried out via the Grignard reagent, since, if SnCl4 is used, a fluorine-chlorine exchange occurs. 

All derivatives, sulfenylated at the nitrogen atom, are stable for a prolonged period of time only at low temperature. 5-Trifluoromethyl-mercaptouracil is an extraordinarily stable compound. In contrast to 5-bromouracil (129), the substituent cannot be exchanged by amines. Nevertheless, a complete fluorine-chlorine exchange can be brought about with the aid of boron trichloride, just as with other aromatic CF3S compounds (170) ... 

Table 22 Dependence of the Oxygen vs. Chlorine Exchange Reaction Rate upon the n Character of the P=0 Bond ...

Hydrogen fluoride can displace chlorines on either side chain or nucleus (especially 2-chloro), and antimony fluoride is specific for all chlorinated methyl groups. Sodium fluoride initially replaces a 4-chloro group (82JFC495). Some of these processes have been subjected to kinetic investigation, which demonstrated that in polar, aprotic solvents fluorine-chlorine exchange is a pseudo first-order, consecutive reaction (87JFC373). 

Of these equations, the first two illustrate SiSi/SiM redistribution, while the last indicates metal-chlorine exchange followed by intramolecular coupling. 

A combination of retention and inversion is in line with the results for the second system. Beltrame et al. (1966) followed the chlorine exchange and the concurrent isomerization of the two isomers of 1-p-anisyl-l-phenyl-2-chloroethylene (105 and 106) with labelled chloride ion (Cl- ) in dimethylformamide. The same steric interactions are... 

Bromine-chlorine exchange with pyrazolotropones is rapid in the case of dibromide 142 [by HCl/methanol, substitution of one bromine atom (66BCJ253)], but exchange proceeds slowly and incompletely with a 5-bromo compound [by acetyl chloride/acetic anhydride (61MI2)]. 

Summary Lithiated aminofluorosilanes can often be regarded as LiF adducts of iminosilanes on account of their structures [1], They are suitable precursors to iminosilanes after fluorine-chlorine exchange followed by elimination of lithium chloride [1,2]. This synthesis shows the following features (a) the method is a general one in the sense that it can be applied to differently substituted aminosilanes (b) the iminosilanes are obtained, either as THF adducts or as free imines in good yield (c) the tedious and sometimes complicated synthesis of aminochlorosilanes as precursors is avoided, since the fluorine-chlorine exchange takes place in one step in the lithium derivative. 

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