Induction of Chirotopicity in the Self-Assembly Process of Heterocuprates:  Structural and Stereochemical Aspects of Neutral CuLi2BrAryl2 Aggregates

Self-assembling reactions of the ortho-diamine chelated aryllithium dimers of type [Li(C∧N∧N‘)]2 with 1 equiv of CuBr resulted in the selective formation of hetero cuprates 1, 2, 3a, and 3b, respectively, which all have [CuLi2Br(C∧N∧N‘)2] stoichiometry (C∧N∧N‘ = Ar, [C6H4(CH2N(Me)CH2CH2NMe2)-2]- (1); Naph, [1-C10H6(CH2N(Me)CH2CH2NMe2)-2]- (2); (R)-Ar, (R)-[C6H4(CH(Me)N(Me)CH2CH2NMe2)-2]- (3a); or rac-Ar, rac-[C6H4(CH(Me)N(Me)CH2CH2NMe2)-2]- (3b)). The structure of 1 and 2 in the solid state comprises [(C∧N∧N‘)2Cu]- and [Li2Br]+ fragments which are joined by intramolecular N,N‘-chelation of the ortho-diamine ligand to lithium, thus forming an, overall neutral, 2:1 cuprate species. The coordinated benzylic nitrogen centers (NMe) in these trinuclear CuLi2 aggregates have a stable configuration, R N or S N. In fact, 1 and 2 exist in the solid state as R N R N and S N S N diastereoisomers, respectively. Accordingly, the halocuprates 1 and 2 can serve as model complexes for investigation of the structure of corresponding 2:1 cyanocuprates in solution. Reactions of both 1 and 2 with excess CuBr results in the formation of mixed (C∧N∧N‘)Cu−CuBr aggregates of formula [Cu3Br(C∧N∧N‘)2] and [Cu4Br2(C∧N∧N‘)2], respectively. Interestingly, multinuclear NMR spectra of 3a and 3b are similar and show only one resonance pattern for the aryl ligands, which resembles that of 1 and 2 in solution. From this it follows that 3a possesses either a R C S N,R C S N or a R C R N,R C R N configuration. Consequently, cuprate 3b is formed as either the S C R N,S C R N/R C S N,R C S N or the R C R N,R C R N/S C S N,S C S N enantiomeric pair. The formation of cuprates 3a and 3b provides excellent examples of highly specific enantioselective recognition during the self-assembly of aryllithium and -cuprate species containing an ortho-diamine chelating C∧N∧N‘-type of ligand.