ChemInform Abstract: Haloboration of Internal Alkynes with Boronium and Borenium Cations as a Route to Tetrasubstituted Alkenes.

Abstract

Vinyl boronates are extremely useful precursors, especially for the formation of new C–C bonds by cross-coupling and conjugate addition reactions.1, 2 Although alkyne hydroboration is a powerful synthetic route,3, 4 it is not applicable to the synthesis of trisubstituted vinyl boronates. Thus, alternative regio- and stereospecific methods are needed, particularly for subsequent use in the formation of tetrasubstituted alkenes,5–7 as the production of these important biologically active compounds as single isomers by classical methods is challenging.8 One simple approach to trisubstituted vinyl boronates is the functionalization of internal alkynes by metal-catalyzed 1,2-carboboration9–13 and 1,1-carboboration.14 The introduction of two selectively transformable moieties onto an internal alkyne should enable ready access to tetrasubstituted alkenes by successive cross-coupling reactions. Significant progress has been made in this area, particularly in the dimetalation of internal alkynes to provide two nucleophilic sites of distinct reactivity.15–19 The haloboration of internal alkynes is an attractive alternative to dimetalation, as it generates ambivalent synthetic intermediates that contain both a nucleophilic and an electrophilic position.20 These synthetic intermediates are ideally suited for the diversity-oriented synthesis of tetrasubstituted alkenes. To date, the application of alkyne haloboration with boron trihalides (BX3) has been limited to terminal alkynes, and has proved an effective route to produce trisubstituted alkenes with excellent regio- and stereoselectivity.21–25 The haloboration of internal alkynes is unsuccessful with BCl3, and it is either slow25 or produces isomeric mixtures susceptible to B–C bond cleavage when BBr3 is used.21, 26 Recent calculations found that the haloboration of internal alkynes with BCl3 is endothermic, but as the Lewis acidity of BX3 increases (Cl<Br<I), haloboration becomes exothermic, and the energy of the key transition state is also reduced.27 This result suggested that an increase in the electrophilicity at boron beyond that of BX3 would facilitate the haloboration of internal alkynes.