Catecholborane

Hydroboration of conjugated dienes proceeds without a catalyst to give 1,2-adducts. However, the less reactive catecholborane reacts with isoprene with catalysis by Pd(PhiP)4, yielding the 1,4-adduct 73[66]. 

Hydrometallation is catalyzed by Pd. Hydroboration of l-buten-2-methyl-3-yne (197) with catecholborane (198) gives the 1,4-adduct 199 with 84% selectivity. The ratio of Pd to phosphine (1 1.5) is important[l 10]. The vinyl sulfide 201 is prepared by a one-pot reaction of the thioalkyne 200 via a Pd-catalyzed hydroborution-coupling sequence using dppf as a ligand[l 11]. 

Several oxygen- and sulfur-substituted boranes have been reported (125—130). 1,3,2-Benzodioxaborole [274-07-7] (catecholborane, CB) (15) is the one best studied. It is commercially available or can be prepared by the reaction of catechol with borane-THF (57,131), or by other procedures (132). The product is a Hquid existing as a monomer, remarkably stable to disproportionation. No... 

Catalytic Asymmetric Hydroboration. The hydroboration of olefins with catecholborane (an achiral hydroborating agent) is cataly2ed by cationic rhodium complexes with enantiomericaHy pure phosphines, eg, [Rh(cod)2]BE4BINAP, where cod is 1,5-cyclooctadiene and BINAP is... 

COREY Enalioselaclive borane reduction Enantioselective reduction ol ketones by borane or catecholborane catalyzed by oxazaborolldine 3... 

Catecholborane halide cleaves benzyl carbamatesi in the presence of ethyl and benzyl esters and TBDMS ethers." ... 

Triple bonds can be monohydroborated to give vinylic boranes, which can be reduced with carboxylic acids to cis alkenes or oxidized and hydrolyzed to aldehydes or ketones. Terminal alkynes give aldehydes by this method, in contrast to the mercuric or acid-catalyzed addition of water discussed at 15-4. However, terminal alkynes give vinylic boranes (and hence aldehydes) only when treated with a hindered borane such as 47, 48, or catecholborane (p. 798)," or with BHBr2—SMe2. The reaction between terminal alkynes and BH3 produces 1,1-... 

When the substrate is an allylic alcohol or amine, the addition is generally anti," though the stereoselectivity can be changed to syn by the use of catecholborane and the rhodium complexes mentioned above. Because the mechanism is different, use of this procedure can result in a change in regioselectivity as well, [e.g., styrene PhCH=CH2 gave PhCH(OH)CH3]. ... 

CONJUGATE REDUCTION OF ,P-UNSATURATED p-TOLUENESULFONYLHYDRAZONES TO ALKENES WITH CATECHOLBORANE 5P-CHOLEST-3-ENE... 

Catecholborane with a purity of 95% was purchased from Aldrich Chemical Company, Inc. 

Various metal complexes catalyze the addition of catecholborane and pinacolbo-rane to aliphatic terminal alkenes (Table 1-2). Neither the borane reagents nor the catalysts alter the high terminal selectivity, but a titanium catalyst does (entry 3). Although Cp2TiMe2 [30] exhibits high terminal selectivity for vinylarenes, aliphatic alkenes afford appreciable amounts of internal products, whereas an analogous Cp 2Sm(THF) [31] allows selective addition of catecholborane to the terminal car-... 

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. 

One most important observation for the mechanistic discussion is the oxidative addition/insertion/reductive elimination processes of the iridium complex (31) (Scheme 1-10) [62]. The oxidative addition of catecholborane yields an octahedral iridium-boryl complex (32) which allows the anti-Markovnikov insertion of alkyne into the H-Ir bond giving a l-alkenyliridium(III) intermediate (34). The electron-... 

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