Dicyclohexylborane

Dicyclohexylborane [1568-65-6], Chx2BH (12) is prepared in quantitative yield by the same method. It is a white soHd, sparingly soluble in ether or tetrahydrofuran. For most purposes isolation is not necessary and it can be used as a slurry in these solvents. Its stetic requirements are slightiy lower as compared to Sia2BH (57,106—109). Much better thermal stabiUty allows its use at higher temperatures. 

A synthetically useful virtue of enol triflates is that they are amenable to palladium-catalyzed carbon-carbon bond-forming reactions under mild conditions. When a solution of enol triflate 21 and tetrakis(triphenylphosphine)palladium(o) in benzene is treated with a mixture of terminal alkyne 17, n-propylamine, and cuprous iodide,17 intermediate 22 is formed in 76-84% yield. Although a partial hydrogenation of the alkyne in 22 could conceivably secure the formation of the cis C1-C2 olefin, a chemoselective hydrobora-tion/protonation sequence was found to be a much more reliable and suitable alternative. Thus, sequential hydroboration of the alkyne 22 with dicyclohexylborane, protonolysis, oxidative workup, and hydrolysis of the oxabicyclo[2.2.2]octyl ester protecting group gives dienic carboxylic acid 15 in a yield of 86% from 22. 

The use of reductive alkylation conditions has been employed to access tricycles from the azide 353 <2002S242> (Equation 95). Hydroboration of the alkene double bond with dicyclohexylborane followed by reaction with the azide and subsequent elimination of nitrogen and cyclization gave the linear tricyclic diketopiperazine 354 and 355 as a mixture of diastereoisomers. 

The synthesis of the rare amino acid 3-hydroxy-4-methylproline (8)3 involves an aldol reaction of the oxazoiidinone 5 with methacrolein to provide the a-bromo-0-hydroxy adduct 6. Azide displacement and removal of the chiral auxiliary gives 7. On treatment with dicyclohexylborane, 7 undergoes hydroboration-cycloalkyl-ation to provide, after hydrolysis, the methyl ester hydrochloride (8) of (2S,3S,4S)-3-hydroxy-4-methylproline in >97% de. This cycloalkylation should be a useful route to cyclic amino acids as well as pyrrolidines. 

In the hydroboration of terminal alkenes, carrying a ketone or aldehyde group, with a variety of borane reagents, dicyclohexylborane has been identified as the most efficient reagent. Analogous hydroboration of alkynyl ketones and alkynyl aldehydes with dicyclohexylborane yields the corresponding olefinic carbonyl compounds after protonation, or dicarbonyl compounds after oxidation. ... 

Reduction of 2-methylcyclohexanone gave the following percentage proportions of the less stable m-2-methylcyclohexanol (with axial hydroxyl) with lithium aluminum hydride 25%, with diborane in tetrahydrofuran 26%, with di-sec-amylborane in tetrahydrofuran 79%, with dicyclohexylborane in diglyme 94%, and with diisopinocampheylborane in diglyme 94% (the balance to 100% being the trans isomer) [837. ... 

Care must be taken to avoid loss of product that is easily taken up in the syringe. The checkers found it more convenient to perform the reaction in a tared, round-bottomed Schlenk flask. Solvent was then removed from the solid dicyclohexylborane by filtration using a positive nitrogen flow. 

Dicyclohexylborane bumps during the drying step, and the checkers found that it is advantageous to use a vacuum adapter containing a glass frit and an auxiliary cold trap to prevent contamination of the vacuum line with the product. 

The reaction of dicyclohexylborane and trifluoromethanesulfonic acid is highly exothermic. On one occasion, the checkers cooled the reaction in an ice bath during the addition period, with no effect on product yield. The submitter reports that he once experienced a sudden vigorous reaction under cooling conditions, probably due to accumulation of unreacted triflic acid. It thus appears safer to add the acid at room temperature, slowly, so that it reacts immediately. 

HYDROXY CARBOXYLIC ACIDS Dicyclohexylborane a -HY DROXY-a, [J-ENONES m-Chloropcrbcnzoic acid y-HYDROXY-a,B-ENONES Potassium peroxomonosulfate. a HYDROXY ESTERS Tris(phenylthio)methyllithium. a-HYDROXY-p-KETO ESTERS ... 

Dicobalt octacarbonyl, in Pauson—Khand reaction homogeneous catalysis, 11, 340 metal-coupled promoters, 11, 339 non-oxidative promoter-assisted, 11, 338 oxidative promoter-assisted, 11, 337 physical promoters, 11, 339 solid-supported promoters, 11, 339 Dicobalt triple-decker sandwiches, preparation, 3, 14 (+)-Dictamnol, via [5+2]-cycloadditions, 10, 613-614 Dicyclohexylborane, for alkene hydroboration, 9, 150... 

Treatment of allenylsilane 2 with dicyclohexylborane at -15 °C for 6 h followed by addition of hexanal and then 2-aminoethanol gave a 96 4 mixture of hydroxyallylsilanes 3a and 3b, whereas treatment of 2 with dicyclohexylborane at 50 °C for 7 h followed by the same subsequent steps gave a 9 91 mixture of 3a and 3b. Explain. 

RCHiCN. These primary nitriles can be obtained in 70-98% yield by hy-droboration of a terminal alkene with dicyclohexylborane followed by addition of an excess of CuCN and Cu(OAc)2 and a catalytic amount of Cu(acac)2, which is not essential but which markedly improves the yield. The overall process effects antt-Markownikoff hydrocyanation of 1-alkenes.1... 

Tris(glycosyl)boranes and dicyclohexylboranes are iodinated with Nal/Chloramine T or brominated with NaBr/Chloramine T to yield iodinated and brominated sugars, respectively946. BC13 is used for the selective chlorination of carbohydrates947. 

The synthesis of 9-alkenyl-9-BBN via hydroboration of terminal alkynes with 9-BBN suffered from the formation of 1,1-diborylalkanes via dihydroboration along with desired monohydroboration products. Alternatively, selective monohydroboration of terminal alkynes with dicyclohexylborane was followed by transmetallation with 9-MeO-9-BBN (Equation (193)).710 Treatment of the hydroboration intermediates with DIBAL-H in the presence of a borane-trapping reagent such as 1-hexene gave 1-alkenylaluminum compounds with complete retention of the (E)-stereochemistry (Equation (194)).711... 

Double bonds with different geometry behave in a unique way. For example, the monohydroboration of caryophyllene (9) with dicyclohexylborane followed by oxidation brought about the participation of the (E)-trisubstitutcd double bond in preference to the exocyclic double bond during hydroboration to give the corresponding alcohol (10) (Eq. 10)50t. However, under similar conditions, isocaryophyllene (11), which has a (Z)-endocyclic double bond, affords a mixture of unsaturated alcohols... 

The mildness of these reagents tolerates the presence of various functional groups such as ester, ether, halogen, and nitrile. The stereospecific cis nature of hydroboration gives exclusively the tram alkenylboranes, often also in high regioisomeric purity (Eq. 53). On the other hand, highly pure (Z)-l-alkenyl-dialkylboranes are prepared without any difficulty via the monohydroboration of 1-halo-1-alkynes with disiamyl-borane or dicyclohexylborane, followed by treatment with t-butyllithium (Eq. 55)106). 

It is to be pointed out that vinylboranes synthesized via hydroboration with di-siamylborane and dicyclohexylborane are less satisfactory and afforded l93) byproducts in addition to the desired frarw-alkenylmercuric acetate. 

As the second addition of disiamylborane is very solw, the dihydroboration can be carried out with sterically less hindered dicyclohexylborane. Protonolysis of the intermediate organoboranes with acetic acid affords the corresponding cis, m-dienes (Eq. 132). 

---