Propargylic C(sp³)–H Bond Activation for Preparing η³-Propargyl/Allenyl Complexes of Yttrium

Abstract

Propargylic C(sp 3 )—H bond activation of 1-substituted-1-propynes, such as 1-trimethylsilyl-1-propyne, 2-hexyne, and 1-phenyl-1-propyne, was achieved by treatment with an alkylyttrium complex 8 bearing an ene-diamido ligand to give the corresponding (η 3 -propargyl/allenyl)yttrium complexes 7a – c . A unique delocalized η 3 -propargyl/allenyl structure of these three complexes was revealed by NMR spectroscopy and X-ray single crystal analyses. To elucidate the reactivity of the η 3 -propargyl/allenyl unit of complexes 7a – c, we conducted two reactions with N -methylaniline and N, N ′-dicyclohexylcarbodiimine. For protonation by N -methylaniline, we found that the product distribution of monosubstituted internal alkynes and allenes depended on the substituent on the η 3 -propargyl/allenyl moiety: 7a and 7b afforded the corresponding internal alkynes as the major products, whereas the major protonation product of 7c was phenylallene. For the insertion of N, N ′-dicyclohexylcarbodiimine, complex 7a selectively yielded η 3 -{ N, N ′-dicyclohexyl-2-(3-trimethylsilylpropargyl)amidinate}yttrium 12a, while complex 7c produced η 3 -{ N, N ′-dicyclohexyl-2-(1-phenylallenyl)amidinate}yttrium complex 13c, though complex 7b gave a mixture of η 3 -{ N, N ′-dicyclohexyl-2-(3-normalpropylpropargyl)amidinate}yttrium complex 12b and η 3 -{ N, N ′-dicyclohexyl-2-(1-normalpropylallenyl)amidinate}yttrium 13b in an 83:17 ratio. On the basis of the product distributions in these two-types of reactions, (η 3 -propargyl/allenyl)yttrium complexes were shifted into preferentially favorable η 1 -allenyl species or η 1 -propargyl species depending on the substituents prior to the reaction with electrophiles via a four-membered cyclic mechanism.