Organolithium compounds indicator

With a difluoroethylene containing hydrogen and chlonne, where both groups can be replaced by lithium, a mixture of two organolithium compounds is formed in a 2 1 ratio, indicating a more facile replacement of chlorine [63] (equation 30)... 

In their papers Rodionov and coworkers described the polymerization of organolithium compounds in terms of the formation of lithium bonds (Scheme 1), analogous to hydrogen bonds, which brought about cyclic or linear association of these compounds in solution . However, the strong association of alkyllithium compounds, persisting even in the vapour phase, indicates that their association takes place through the formation of... 

Early studies (1 ) of the kinetics of polymerization of styrene, isoprene and butadiene in hydrocarbon solvents indicated a half-order rate dependency on growing chain concentration, although there were conflicting data at that time (10, 11) which suggested even lower fractional orders for the dienes. Since the apparent half-order dependency could not be rationalized, as in the case of the polar media, by an ionic dissociation mechanism, some other form of association-dissociation phenomenon offered a possible answer. In view of the known tendency of organolithium compounds to undergo molecular association in non-polar media, the following scheme was proposed by us (l) ... 

This supposition is supported by results for linking reactions of polymeric organolithium compounds which indicate that the steric requirements of a poly(styryl) chain end are larger than those for a poly(dienyl) chain end ( 4,25). Since a larger sensitivity to base steric requirements is exhibited by poly-(isoprenyl)lithium and it is known that the coordination process for poly(styryl)lithium involves coordination to give the unassociated species (eq 1), it is concluded that tetrahydrofuran coordination with poly(isoprenyl)lithium must involve interaction with an associated species (presumably the dimer) to explain the large sensitivity to the steric requirements of the base. 

These results indicate the utility of high-dilution, solution calorimetry for examining and characterizing the nature of base coordination with organolithium compounds. The large, exothermic enthalpies of coordination of bases with polymeric... 

Cation-assisted reactions, on the other hand, are hindered by complexation of the cation [648]. Reduction of carbonyl groups by metal hydrides [654] and the addition of organolithium compounds [655] are examples. This indicates that the coordination between carbonyl group and cation is an important step in these reactions. For appHca-tions of concentrated LiC104/Et20 (LPDE) solutions in cation-catalysed reactions, see Section 5.3.3 and references [803-806]. 

FIG. 8. Structures of dimeric, tetrameric, and hexameric organolithium compounds. Small spheres indicate lithium atoms large spheres represent the carbon atoms of the organic groups. (74)... 

Metalation of Weak Acids. The ability to metalate very weak acids is a property which has been used widely for the direct synthesis of organolithium compounds, for new or improved organic syntheses, for polymer grafting, and for telomerizations involving chain transfer by transmetalation. These major topics are covered in seven chapters of this volume, indicating both the scope and utility of this property of N-chelated organoalkali metal compounds. 

The shifts of some organolithium compounds have been compared to their parent hydrocarbons, and in each case there is a down-field shift of the a-carbon of the lithium derivative. The downfield shift indicates that extensive rehybridization had occurred at the a-carbon atom. However, opposing this downfield shift there is a shielding contribution that is proportional to the negative charge on the a-carbon atoms consequently the shifts to low field in the phenyl-methyl series are variable (Table VIII). 

The rate of RLi additions to methyl isothiocyanate seems to be strongly dependent upon the basicity of RLi [9]. Whereas phenyllithium reacts rapidly at — 90 °C, the addition of the much more weakly basic 2-thiazolyllithium (pK of 1,3-thiazole is about 29) has to be carried out at —30 to —50 °C and the reaction with lithium acetylides RC=CLi (pK acetylenes is 26 or lower) requires 0 to 20 °C (all reactions were carried out at the same 0.5 to 1 mol/1 concentrations). These temperatures indicate the conditions for other organolithium compounds and also the chance of success for other reactions. A rough estimation of the pK values of RH has to be made first, taking into account stabilizing effects of substituents. Side reactions have not yet been observed in the conversions of isothiocyanates with lithium compounds. 

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