Hydroborations 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). 

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 proposed mechanism for Fe-catalyzed 1,4-hydroboration is shown in Scheme 28. The FeCl2 is initially reduced by magnesium and then the 1,3-diene coordinates to the iron center (I II). The oxidative addition of the B-D bond of pinacolborane-tfi to II yields the iron hydride complex III. This species III undergoes a migratory insertion of the coordinated 1,3-diene into either the Fe-B bond to produce 7i-allyl hydride complex IV or the Fe-D bond to produce 7i-allyl boryl complex V. The ti-c rearrangement takes place (IV VI, V VII). Subsequently, reductive elimination to give the C-D bond from VI or to give the C-B bond from VII yields the deuterated hydroboration product and reinstalls an intermediate II to complete the catalytic cycle. However, up to date it has not been possible to confirm which pathway is correct. 

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. 

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]. 

Catecholborane and pinacolborane, in which the boron has two oxygen substituents, are much less reactive hydroborating reagents than alkyl or haloboranes because the boron electron deficiency is attenuated by the oxygen atoms. Nevertheless, they are useful reagents for certain applications.161 The reactivity of catecholborane has been found to be substantially enhanced by addition of 10-20% of N,N-dimethylacetamide to CH2C12.162... 

Catecholborane and pinacolborane are especially useful in hydroborations catalyzed by transition metals.163 Wilkinson s catalyst Rh(PPh3)3Cl is among those used frequently.164 The general mechanism for catalysis is believed to be similar to that for homogeneous hydrogenation and involves oxidative addition of the borane to the metal, generating a metal hydride.165... 

Migration of boron to terminal positions is observed under much milder conditions in the presence of transition metal catalysts. For example, hydroboration of 2-methyl-3-hexene by pinacolborane in the presence of Rh(PPh3)3Cl leads to the terminal boronate ester. 

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. 

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). 

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