Aluminacyclopropene:Syntheses,Characterization,and Reactivity toward Terminal Alkynes

摘要

Reactions of LAl with ethyne,mono-and disubstituted alkynes,and diyne to aluminacyclopropene LAl[eta~2-C_2(R~1)(R~2)]((L = HC[(CMe)(NAr)]_2,Ar = 2,6-iPr_2C_6H_3);R~1 = R~2 = H,(1);R~1 = H,R~2 = Ph,(2);R~1 = R~2 = Me,(3);R~1 = SiMe_3,R~2 = C ident to CSiMe_3,(4))are reported.Compounds 1 and 2 were obtained in equimolar quantities of the starting materials at low temperature.The amount of C_2H_2 was controlled by removing an excess of C_2H_2 in the range from-78 to-50 deg C.Compound 4 can be alternatively prepared by the substitution reaction of LAl[eta~2-C_2(SiMe_3)_2] with Me_3SiC ident to CC ident to CSiMe_3 or by the reductive coupling reaction of LAlI_2 with potassium in the presence of Me_3SiC ident to CC ident to CSiMe_3.The reaction of LAl with excess C_2H_2 and PhC ident to CH(<1:2)afforded the respective alkenylalkynylaluminum compounds LAl(CH=CH_2)(C ident to CH)(5)and LAl(CH =CHPh)(C ident to CPh)(6).The reaction of LAl(eta~2-C_2Ph_2)with C_2H_2 and PhC ident toCH yielded LAl(CPh=CHPh)(C ident to CH)(7)and LAl(CPh=CHPh)(C ident to CPh)(8),respectively.Rationally,the formation of 5(or 6)may proceed through the corresponding precursor 1(or 2).The theoretical studies based on DFT calculations show that an interaction between the Al(I)center and the C ident to C unit needs almost no activation energy.Within the AlC_2 ring the computational Al-C bond order of ca.1 suggests an Al-C sigma bond and therefore less pi electron delocalization over the AlC_2 ring.The computed Al-eta~2-C_2 bond dissociation energies(155-82.6 kJ/mol)indicate a remarkable reactivity of aluminacyclopropene species.Finally,the ~1H NMR spectroscopy monitored reaction of LAl(eta~2-C_2Ph_2)and PhC ident to CH in toluene-d_8 may reveal an acetylenic hydrogen migration process.