N-Heterocyclic Carbene Stabilized 1-Bora-1,3-butadienes.

Deprotonation of [(NHC)(Fmes)B-allyl] + borenium cations (NHC, IMes ( a ) or IMe 2 ( b ); Fmes, 2,4,6-(CF 3 ) 3 C 6 H 2 ) provides an easy entry to the NHC-stabilized 1-bora-1,3-butadienes. They feature a planar s-trans-conformation just like 1,3-butadiene. The 1-borabutadiene 7a undergoes hydroboration reactions; the HB(C 6 F 5 ) 2 hydroboration product is trapped with CO or an isonitrile to give the respective cyclic zwitterionic borenium-borate enolate or enamide products. 1-Borabutadiene 7b undergoes 1,4-chalcogenation with elemental sulfur or selenium, and it gives the six-membered heterocyclic 1,4-addition product with the S═O bond of sulfur dioxide. Compound 7b served as a precursor for the formation of a borylated η 3 -allyl ligand at Ru. 7b formed a Rh complex by reaction with [Rh(ethylene) 2 Cl] 2 . It subsequently underwent an intramolecular C-H activation reaction to a mixture of η 3 -methyl-boraallyl Rh complex isomers.