Alkynes pinacolborane addition

The dominant factors reversing the conventional ds-hydroboration to the trans-hydroboration are the use of alkyne in excess of catecholborane or pinacolborane and the presence of more than 1 equiv. of EtsN. The P-hydrogen in the ris-product unexpectedly does not derive from the borane reagents because a deuterium label at the terminal carbon selectively migrates to the P-carbon (Scheme 1-5). A vinylidene complex (17) [45] generated by the oxidative addition of the terminal C-H bond to the catalyst is proposed as a key intermediate of the formal trans-hydroboration. 

As would be expected, catalytic hydroboration is effective for alkynes as well as al-kenes, and prior examples have been reviewed [6]. An interesting development has been the diversion of the normal syn- to the anti-addition pathway for a terminal alkyne, with 99% (catechoborane) and 91% (pinacolborane) respectively (Fig. 2.5) [20]. The new pathway arises when basic alkylphosphines are employed in combination with [Rh(COD)Cl]2 as the catalyst in the presence of Et3N. Current thinking implies that this is driven by the initial addition of the rhodium catalyst into the alkynyl C-H bond, followed by [1,3]-migration of hydride and formal 1,1-addition of B-H to the resulting alkylidene complex. The reaction is general for terminal alkynes. 

New mechanistic studies with [Cp2Ti(CO)2] led to the observation that the tita-nocene bis(borane) complex [Cp2Ti(HBcat)2] (Hbcat = catecholborane) generated in situ is the active catalyst.603 It is highly active in the hydroboration of vinylarenes to afford anti-Markovnikov products exclusively, which is in contrast to that of most Rh(I)-catalyzed vinylarene hydroboration. Catecholborane and pinacolborane hydroborate various terminal alkynes in the presence of Rh(I) or Ir(I) complexes in situ generated from [Rh(COD)Cl2] or [Ir(COD)Cl2] and trialkylphosphines.604 The reaction yields (Z)-l-alkenylboron compounds [Eq. (6.107)] that is, anti addition of the B—H bond occurs, which is opposite to results found in catalyzed or uncatalyzed hydroboration of alkynes ... 

The rate of hydroboration with catecholborane and pinacolborane can be tremendously increased by the addition of transition metal catalysts. Hydroboration of pinacolborane 49 with alkenes 50 <1996JA909> and terminal alkynes 52 <19950M3127> proceeds with high regioselectivity in the presence of catalytic HZrCp2Cl furnishing the terminal boronates 51 and vinylboronates 53, respectively (Scheme 8). 

Addition to multiple bonds. Hydroboration of 1-alkynes with catecholborane (when /-Pr3P and EtjN are also present) gives (Z)-l-alkenylboronates. On the other hand, dehydrogenative coupling between styrenes and pinacolborane is observed. 

The hydroboration of olefins is a classic reaction in organic synthesis. - Dialkylbo-ranes add rapidly to alkenes in the absence of catalyst. However, dialkoxyboranes, such as catecholborane and pinacolborane, add more slowly to olefins and alkynes. Thus, transition metal complexes could catalyze the addition of dialkoxyboranes to olefins and alkynes without interference from the background reaction. The potential to alter chemoselectivity, regioselectivity, enantioselectivity, and diastereoselectivity has led a munber of groups to develop metal-catalyzed versions of hydroboration. " Enantioselective hydroboration would alleviate the need to use boranes containing stoichiometric amounts of chiral substituents to generate optically active alkylboranes. 

The addition of B pin to terminal alkynes has been achieved using nanoporous gold catalysts (Scheme 6.47) [87]. The addition was remarkably selective for incorporation of the pinacolborane fragments on the same side of the resulting alkene. A host of terminal... 

As described earlier, a variety of transition metal catalysts have been used to promote the addition of B-H bonds across alkynes. The development of metal-free approaches has been challenging to design due to the unique reactivity profile afforded by transition metal catalysts. To this end, a metal-free catalyst system has been devised for the addition of pinacolborane to internal alkynes (Scheme 6.48) [91]. Surprisingly, only a carboxylic acid additive was needed to effect the hydroboration reaction. In addition to common alkynes, an alkynylboronate was successfully borylated using the carboxylic acid catalyst. The functional group tolerance was remarkably broad, and a host of alkynes were successfully functionalized. Given the wide substrate scope, availability of the precursors and catalysts, and the metal-free nature of the chemistry, this is a reasonable approach to selective hydroboration reactions. 

SCHEME 6.48 Addition of pinacolborane to alkynes catalyzed by carboxylic acids [91]. 

The nonclassical ruthenium hydride pincer complex [Ru(PNP)(H)2(H2)] (PNP= 1,3-bis(di-t-butyl-phosphinomethyl)pyridine) has been reported to catalyse the addition of pinacolborane to terminal alkynes RC=CH in toluene at room temperature, which gives rise to (Z)-vinylboronates RCH=CHBPin (R = alkyl, aryl). The complex [Ru(PNP)(H)2(HBpin)] (HBpin = pinacolborane), identified on completion of the reaction and also prepared independently, has been proposed as the direct precursor to the catalytic species. ... 

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