Lithium halide complexes

The reactive intermediates under some conditions may be the carbenoid a-haloalkyllithium compounds or carbene-lithium halide complexes.158 In the case of the trichloromethyllithium to dichlorocarbene conversion, the equilibrium lies heavily to the side of trichloromethyllithium at — 100°C.159 The addition reaction with alkenes seems to involve dichlorocarbene, however, since the pattern of reactivity toward different alkenes is identical to that observed for the free carbene in the gas phase.160... 

Lithium halide complexes with ethylenediamine, en, LiXen2, provide a nice illustration of both chelation and bridging by this ligand (26), resulting in a tetrahedral environment for the cation with Li—N, 2.07 A. 

Other studies have shown that rapid exchange occurs at room temperature between an alkyllithium and a lithium halide in ether or tetrahydrofuran solutions (88, 146). This exchange can be stopped at low temperature with the formation of mixed alkyllithium-lithium halide complexes. Further studies have shown that when these systems are enriched with 13C, 7Li-13C coupling can be observed at low temperatures (38). While this clearly shows the interaction which occurs between the metal and carbon atoms,... 

Structures of some lithium halide complexes (a) [fBu2Si(F)]2N Li 2THF, (b) [LiCl-HMPAL,... 

Shirakawa, E., Yamasaki, K., Hiyama, T. Cross-coupling reaction of organostannanes with aryl halides catalyzed by nickel-triphenylphosphine or nickel-lithium halide complex. Synthesis-Stuttgart 99R, 1544-1549. 

Although these carbenoids are usually discussed in relation to insertion reactions, some of them undergo polymerization and other reactions which are similar to those of the ylid. Thus, in ylid chemistry the (CH3) 3N+ group may be considered, as a pseudo-halogen. Although it has not been shown that the ylid reacts by an insertion reaction, it is possible that the conditions under which insertion can occur have not been realized. If the ylid is considered as a carbenoid, its polymerization reactions may proceed via a lithium halide complex. Alternatively, the complex may rearrange to the bromomethyllithium which may be the reactive intermediate. 

Path 5 has not been confirmed in the ylid, but products which can be attributed to this path are noted in the decomposition of the fluor-enylid and in alkoxy-substituted quaternary ammonium salts. Although ethylene and polymethylene are observed in the decomposition of the ylid, it has been suggested that these products are formed via a stepwise alkylation reaction. However, the ylid may be regarded as a carbenoid, with the N+(CH3)3 group behaving as a pseudohalogen, which reacts either as the free ylid or as the lithium halide complex. 

Reversal correlates with the presence of lithium ion and also with the involvement of betaine species. These two risk factors are interrelated because lithium halides rapidly cleave oxaphosphetane 31 or 32 (Scheme 8) at — 70°C resulting in the reversible formation of the betaine lithium halide complexes 40 or 41, respectively (18b). Donor solvents shift the equilibrium toward the oxaphosphetane by coordinating the lithium halides and thereby promote stereospecific decomposition to the alkenes. If the solvent is not an effective lithium coordinating agent, then 40 and 41 decompose slowly, and the risk of... 

The exact formulation of the reactive intermediate in a-elimination reactions using organolithium compounds as bases has been difficult. Apart from the free carbene, various carbenoids are possible, including the a-haloorganolithium formed on metalation, and carbene-lithium halide complexes of various degrees of association. In the case of the dichlorocarbene-trichloromethyllithium equilibrium, the... 

The stereoselectivity of Wittig olefination reactions is variable and depends strongly on the nature of the ylide— whether it is a stabilized ylide or an unstabilized one—and on how the ylide is prepared—whether it exists as a salt-free solution of the ylide or as an ylide-lithium halide complex. 

The reaction of nonstabilized ylides with aldehydes can be induced to yield trans 3 kenes with high stereoselectivity by a procedure known as the Schlosser modification of the Wittig reaction." In this procedure, the ylide is generated as a lithium halide complex and allowed to react with an aldehyde at low temperature, presumably forming a mixture of diastereomeric betaine-lithium halide complexes. At the temperatures under which the addition is carried out, fragmentation to an alkene and triphenylphosphine oxide does not occur. This complex is then treated... 

The polymethylene formation is frequently attributed to a "polymerization" of the carbene (methylene) resulting from C-N bond scission. Such a spontaneous a-elimination seems to occur indeed with trimethylamine butoxymethanide and phenoxymethanide. Unlike methylene itself, the corresponding alkoxy- and aryoxy-carbenes can be trapped in the presence of cyclohexene as the [l+2]-cycloadducts. However, the unsubstituted methylene should be energetically too unfavorable to be accessible in this way. Moreover, the alleged "polymerization" would have to start with a dimerization to ethylene and this would hardly produce a trimethylene. A more plausible fate of such a A-ylide/lithium halide complex is to undergo a direct carbenoid substitution... 

---