CHEMCATCHEM MINIREVIEWS tions of boron precursors to unsaturated organic frameworks is becoming increasingly popular. The above mentioned drawbacks of organoborane preparation were overcome after the pioneering discovery of Brown, related to the hydroboration of unsaturated organic compounds, in the middle of the previous century. [25] Moreover, Brown and co-workers achieved unprecedented chiral inductions in classical organic synthesis soon after, by employing chiral hydroborating agents. [26] This methodology continues to attract interest, consequently, many transition metal catalyzed approaches have been developed, which allow different regioselectivity than the uncatalyzed hydroborations. In recent years new catalytic systems have evolved to address regioselectivity and enantioselectivity, common challenges of catalytic hydroboration, as well as issues associated with side reactions such as dehydrogenative borylation and alkene hydrogenation. [27,28] Nevertheless, the need of the substrate scope expansion from simple alkenes to electron deficient conjugated analogues triggered further developments in the field. This challenge was first addressed by Marder et al. [29] who latter on discovered a transition metal catalyzed addition of diboron reagents [30] to vinylarenes and a,bunsaturated olefins. [31] Interestingly, the Pt-catalyzed reaction with diboron reagent on Michael acceptors proceeded as 1,4addition giving the unique b-boryl carbonyl compounds. [31b-c, 32] In contrast, catalytic or uncatalyzed hydroboration with HB(cat) or 9-borabicyclononane provided instead the boron enolate products, which upon treatment with different electrophiles afforded alternative products. [33] The scope of metal catalysts capable of b-borylation of a,b-unsaturated olefins was subsequently extended [34] to Ag, [35] Ni, [36] Zn, [37] Pd, [38] Fe, [39] and Cu I . Furthermore, some examples of transition metal free b-borylations with diboron reagents have also been developed recently (Scheme 2). [40] Notably, among metal catalyzed approaches, Cu I based catalysts appear to be the most widely explored, owing to its alluring cost advantage and solid performance over wide variety of substrates. The story of Cu I -catalyzed b-borylations has begun with concomitant discoveries of Hosomi et al. [41] and Miyaura et al. [42] who reported racemic conjugate additions of B 2 pin 2 to a,b-enones. A stunning progress has been made later on in the direction of asymmetric conjugate b-borylation by groups of Yun, [43] Hoveyda, [44] Fernµndez [45] and others [46] who developed catalytic systems based on Cu I -salts and other transition metals, which enabled the creation of stereogenic centers containing CÀB bond in high efficiency and stereoselectivity (Scheme 2). Importantly, Cu I -catalysis was also successfully used in borylation of alkynes. [47] Nevertheless, the transition metal catalyzed b-borylations must be performed in anhydrous organic solvents. Moreover, Cu I -catalyzed procedures require presence of a strong base and alcohol as p...
