Alkynes hydroborations, pinacolborane

Scheme 4.S8. Zirconium- and rhodium-catalyzed hydroborations of alkenes and alkynes with pinacolborane.

Hydroboration. Pinacolborane is a stable, easily prepared and stored hydroborating agent. Unlike catecholborane which requires harsh reaction conditions for hydroboration of alkenes (100 °C) and alkynes (70 °C), hydroboration with pinacolborane proceeds under mild conditions furnishing the boronates. JCnochel and co-workers observed an excellent level of regioselectivity for hydroboration of alkynes with pinacolborane at room temperature (eq 2). Alkenes, however, react slowly with pinacolborane and often require heating for 2-3 days to furnish the terminal pinacolboronates as the major regioisomer (>98%) (eq 3). 

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. 

Schwartz s reagent (1) is an excellent catalyst for pinacolborane hydroboration of alkynes (Scheme 8-32) [234, 235]. 

A formal trans-hydroboration of terminal alkynes with catecholborane and pinacolborane to yield cis-l-alkenylboronates has also been carried out in the presence of Rh(I) and Ir(I). The dominant factors for reversing the conventional cis-hydroboration to the trans-hydroboration were the use of NEts and the use of bulk phosphines such as P Pr3 and PCy3, and an excess of alkyne in front of the boron reagent [46]. 

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 ... 

Pinacolborane (PBH, 537) sluggishly hydroborates alkynes and alkenes. Hydro-boration of alkyne with PBH is catalyzed by hydrozirconocene chloride (HCp2ZrCl) [206], CpNi(Ph3P)Cl and Rh(CO)(Ph3P)2Cl [207] at room temperature. Hydrobora-tion of 4-octene with PBH at room temperature gives either terminal or internal boranes 538 or 539 regioselectively, depending on the catalyst used [207]. PBH is more stable than CBH, and easier to handle. 

Pinacolborane 49 is a highly stable hydroborating agent. It can be easily prepared and stored without decomposition. Pinacolborane 49 reacts with alkenes and alkynes under relatively milder conditions unlike catecholborane 38. Alkenes 50 react slower than alkynes and usually undergo hydroboration in 2-3 days at 50 °C furnishing the terminal pinacol boronates 51 as the major regioisomer (>98%). Hydroboration of terminal alkynes 52 with pinacolborane proceeds at room temperature with an excellent level of regioselectivity to yield the terminal vinyl boronates 53 (Scheme 7). 

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). 

Alkenylboronic acids and esters have been prepared by thermal or catalyzed hydroboration of 1-alkynes with catecholborane (HBcat), pinacolborane (HBpin), or dihaloboranes 41-43, followed by hydrolysis to boronic acids or alcoholysis to boronic esters. A convenient alternative to improve chemo- and regioselectivity is the hydroboration of alkynes with dialkylboranes. For selective removal of dummy groups, the oxidation of two cyclohexyl groups was conduced by treatment of l-alkenyl(dicyclohexyl)borane intermediates with Me3N-0 (Equation (7)).116 The... 

Hydroboration. Synthesis of of fi-alkenylpinacolatoborons is achieved via reaction of l-alkynes with the new complex obtained from admixture of BH3 Me2S with (CgF5)3B, followed by treatment with pinacolborane. 

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. 

Tucker, C. E. Davidson, J. Knochel, P. Mild and Stereoselective Hydroborations of Functionalized Alkynes and Alkenes Using Pinacolborane. J. Org. Chem. 1992,57,3482-3485. 

The hydroboration of alkenes and alkynes to highly chemo-, regio-, and stereoselective alkyl and vinyl boronic esters with pinacolborane is catalysed by bis(imino)pyridine Fe(II) complex with activating agent tolylMgBr in THF. Preliminary mechanistic experiments suggest that an Fe(I) catalyst may be formed under the reaction conditions. ... 

Catalytic hydroboration of alkenes and alkynes can be achieved by the reaction of dialkoxyboranes such as pinacolborane 55 and catecholborane 56 in the presence of transition metal catalysts such as CpjZrHCl, Rh(PPh3)3Cl, or Rh(PPh3)2COCl, (Scheme 28.14). This reaction is of particular significance because it readily provides alkyl/vinylboronates directly in one step for further utility in Suzuki-Miyaura cross-coupling reaction. 

Ruthenium hydride pincer complex [Ru(PNP)(H)2(H2)j [PNP=l,3-bis(di-terf-butyl-phosphinomethyl)pyridine] and its borane analog [Ru(PNP)(H)2(HBpin)] (HBpin=pinacolborane) catalyze the hydroboration of terminal alkynes to give selectively Z-vinylboronates in high yields (Scheme 32) [146]. Mechanistic studies... 

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. 

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