Exploring the Limits of Frustrated Lewis Pair Chemistry with Alkynes: Detection of a System that Favors 1,1-Carboboration over Cooperative 1,2-P/B-Addition

The zirconocene complex [{(C6F5)(2)B-(CH2)(3)-Cp}(Cp-PtBu2)ZrCl2] (6; Cp=cyclo-C5H4) was prepared by hydroboration of [(allyl-Cp)(CpPtBu2)ZrCl2] (5) with HB(C6F5)(2) (Piers' borane). It represents a frustrated Lewis pair (FLP) in which both the Lewis acid and the Lewis base were attached at the metallocene framework. Its reaction with 1-pentyne did not result in the 1,2-addition of or deprotonation reaction by the FLP, but rather in the 1,1-carboboration of the triple bond, thereby obtaining a ZIE mixture (1.2:1) of the respective organ-metallic substituted alkenes 7. The analogous reaction of 1-pentyne with the phosphorous-free system [{(C6F5)(2)B-(CH2)(3)-Cp))CpZrCl2] (9) gave the respective 1,1-carboboration products ((Z)-10/(E)-10 approximate to 1.3:1).