Pinacolborane, hydroboration with

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). 

Catalytic hydroboration of vinylic ethers, acetals, and esters with pinacolborane takes place smoothly in the presence of transition metal catalysts. However, a noticeable exception is the catalytic hydroboration of vinyl bromides 59 which do not furnish the expected hydroborated product under these conditions. The reaction of vinyl bromides with pinacolborane initially affords the expected /3-boronoalkylbromide 60. A fast. -elimination ensues to furnish the terminal alkene 61 and 7 -bromopinacolborane 63. The alkene 61 undergoes hydroboration with unreacted pinacolborane to provide the debrominated boronate 62. The intermediate 5-bromopinacolborane 63 cleaves the ethereal C-O bond in the solvent (THF) to provide 4-bromobutyl borate 64 as a side product (Scheme 11) <1996JA909, 2000CSP14505>. 

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). 

Vinylic ethers, acetals, and esters, also undergo catalytic hydroboration with pinacolborane without difficulty. 

However, they are too slow to react with 9-BBN. Catecholborane and pinacolborane react with fluoroolefins in the presence of a transition metal catalyst. However, reactive boranes, such as dichloroborane-methyl sulpfide readily react with mono, and disubstituted terminal perfluoroaUcenes in the presence of BCI3. The latter coordinates to Me2S and releases the free dichloro-borane, which instantaneously hydroborates theperfluoroalkenes. Oxidation under alkaline conditions furnishes the primary alcohol in >98% regioselectivity (eq 8). Alternatively, the free dichloroborane prepared via Matteson s protocol can also be used for the hydroboration of fluoroolefins. 

Scheme 26 1,4-Hydroboration of 1,3-diene derivatives with pinacolborane catalyzed by an iron complex...

For a mechanistic investigation, hydroboration of myrcene with pinacolborane-di was examined. A selective deuteration was observed at the methyl group of the hydroboration product (Scheme 27). 

The differences in the steric effect between catecholborane and pinacolborane, and the valence effect between a cationic or neutral rhodium complex reverse the re-gioselechvity for fluoroalkenes (Scheme 1-4) [26]. The reaction affords one of two possible isomers with excellent regioselectivity by selecting borane and the catalyst appropriately, whereas the uncatalyzed reaction of 9-BBN or SiaiBH failed to yield the hydroboration products because of the low nucleophilicity of fluoroalkenes. The regiochemical preference is consistent with the selectivity that is observed in the hydroboration of styrene. Thus, the internal products are selectively obtained when using a cationic rhodium and small catecholborane while bulky pinacolborane yields terminal products in the presence of a neutral rhodium catalyst. 

More recent work employing diphosphine ligands has focused on both new substrates for hydroboration and also new hydroborating agents. Specifically, Gevorgyan has successfully employed cyclopropenes 56 as substrates, with pinacolboranes 13 as the borane source.20 Impressive enantioselectivities were obtained with a range of diphosphines, for example, with rhodium complexes of NORPHOS (>99% ee), PHANEPHOS (97% ee), BINAP (94% ee), and Tol-BINAP (96% ee), all with near perfect m-selectivity (see Scheme 8). 

The hydroboration of terminal and internal alkenes with pinacolborane can be carried out at room temperature in the presence of an iridium(I) catalyst (3 mol.%) formed by the addition of dppm (2 equiv.) to [Ir(cod)Cl]2 (dppm = Ph2PCH2PPh2), a mixture that presumably furnishes [Ir(cod)(dppm)]Cl as the true catalyst precursor. Hydroboration results in the addition of the boron atom to the terminal carbon of 1-alkenes with more than 99% selectivity [18]. 

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

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. 

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). 

Hydroboration of allenes 65 with pinacolborane in the presence of Pt(DBA)2 and a trialkylphosphine provides either the allyl boronate 66 or the vinyl boronate 67 regioselectively, depending on the stereoelectronic factors of the phosphine employed (Equation 2) <1999CL1069>. Allyl and vinyl boronates are synthetically important because of their ability to undergo nucleophilic addition to carbonyl compounds as well as transition metal-catalyzed cross-coupling. 

The reaction of pinacolborane with styrenes 127 in the presence of bis(chloro-l,5-cyclooctadienylrhodium) at room temperature provides styrenyl pinacol boronate 128 <1999TL2585, 2002BCJ825>. While hydroboration of alkenes is the predominant reaction with phosphine-containing rhodium catalysts such as Wilkinson s catalyst and Rh(PPh3)2COCl, dehydrogenative borylation dominates over hydroboration in the presence of phosphine-free... 

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 of (5) with an excess of pinacolborane at room temperature for 3 days in the absence of solvent followed by oxidation with PDC in the presence of an excess of TMSC1 led to the aldehyde (4h) and the ketone (4i) (75 % and 50 % from (5h) and (5i) respectively after bulb to bulb purification) l2,13, Several other (3-kcto vinylboronates were obtained in good yields via the addition of organozinc or organocuprates to the aldehyde (4h) followed by a PCC oxidation of the corresponding allylic alcohols14. 

Recently, the hydroboration of I -alkynylphosphonates with pinacolborane has been reported. However, the vinylphosphonoboronatcs so obtained are difficult to isolate, and they are immediately subjected to Suzuki coupling reactions with aryl iodides. Similarly, hydrophenylation of diethyl... 

There are a wide variety of hydroborating reagents, including BH3 complexes, pinacolborane, thexyl borane, and catechol borane. The following employs 9-borabicyclo[3.3.1]nonane (9-BBN), which places the boron on the less sterically hindered carbon with high regioselectivity however, completely removing the cyclooctane by-products can be problematic. The alkylborane can be isolated, but is typically used directly in the next reaction, in this case oxidation to the primary alcohol. 

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. 

Caballero A, Sabo-Etienne S (2007) Ruthenium-catalyzed hydroboration and dehydrogenative borylation of linear and cyclic alkenes with pinacolborane. Organometallics 26 1191... 

The hydroboration of alkenes is known to be activated either by pressure or catalysis. Consequently, the combination of these techniques might open the way for the hydroboration of particularly unreactive substrates. Maddaluno el al. recently investigated the hydroboration of some functionalized alkenes, comparing different reagents (catecholborane (CBH) versus pinacolborane (PBH)), and activation by Wilkinson catalyst (RhCl[PPh3]3) and pressure [24]. While bromoalkenes and al-lylamines were found to give the best results with CBH at ambient pressure, 2,3-dihydrofuran (52a, Scheme 7.17) was hydroborated most effectively by PBH in the... 

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