Dicyclohexylborane hydroboration

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. ... 

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... 

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. 

An important aspect of the metal catalyzed hydroboration reaction is its ability to selectively reduce certain functionalities within a molecule. For instance, a key step in the synthesis of a tripeptide derivative containing the Phe-Arg hydroxyethy-lene dipeptide iosostere is the selective rhodium-catalyzed hydroboration of a lactone. The use of disiamylborane, 9-H-BBN, dicyclohexylborane, and (.9)-alpmeborane, however, gave only low to variable yields of the alcohol due to competitive reduction of the y-lactone to the hemiacetal (equation 8). In another example, hydroboration of the diene illustrated in equation (9) with HBcat and RhCl(PPh3)3 gave exclusive formation of the terminal alcohol derived from reaction of the less substituted alkene. Interestingly, uncatalyzed reactions failed to hydroborate this substrate selectively. ... 

Hydroboration of allenes. With few exceptions, the 9-BBN hydroboration of allenes affords B-allylic-9-BBN derivatives. In contrast, the reactions of allenes with disiamylborane or dicyclohexylborane afiord predominantly vinylic boranes. With unsymmetrical allenes, 9-BBN binds to the less substituted carbon atom. Allene itself affords a 1,3-dibora derivative. The B-allylic-9-BBN derivatives are useful reagents for the allylic boration of carbonyl compounds. ... 

This ease of migration of fluorine from carbon to boron has inhibited the development of hydroboration techniques in fluorinated systems. However, when the carbon-fluorine bond is sufficiently remote from the boron, then hydroboration works well and Markov-nikov or anti-Markovnikov additions may be obtained, depending on the hydroborating system dicyclohexylborane, (Chx)2BH, is less electrophilic but sterically more demanding than dihaloboranes (Figure 10.25). 

Conjugated diynes can be reduced to cis enynes by the well-known hydroboration-protonolysis sequence . cw-5-Dodecen-7-yne (215) is obtained from the diyne 213 in 76% yield as shown in equation (23). The orientation of addition of the disiamyl-borane was established by using CH3CO2D in the second step. Addition of a second mole of disiamylborane to 214 is very slow, but reduction to the cis,cis diene can be accomplished by using dicyclohexylborane instead . 

BBN-H is readily prepared (Section 3.10.2.1, equation 8) and is commercially available. It shows considerable stability, even in air for limited periods, and is therefore a very convenient hydroborating agent.Unlike di-primary-alkylboranes it is not prone to disproportionation, but it is substantially less hindered than other di-5-alkylboranes such as dicyclohexylborane and disiamylborane. Thus, it hydroborates hindered alkenes such as 2,3-dimethyl-2-butene slowly. It is less sensitive to steric factors and more sensitive to electronic factors than disiamylborane. Thus, it shows relatively little ability to discriminate between ( )/(Z) pairs but readily discriminates between 4-methoxystyrene and 4-(trifluoro-methyl)styrene. ... 

Disiamylborane and dicyclohexylborane, in contrast to 9-BBN-H, reduce the triple bond of enynes in preference to the double bond. Hydroboration of functional alkynes is also possible e.g. equations 38 and 39 see also ref. 97). 

Except for a few very hindered double bonds, virtually all alkenes undergo hydroboration. Extreme crowding may cause isomerization of the initial organoborane. The presence of a functional group may interfere with hydroboration (see Table 4.3). In such cases, it is often necessary to use a sterically hindered organoborane. For example, dicyclohexylborane (ChXjBH) is an excellent reagent for the selective hydroboration of 1-alkenes in the presence of RCHO and R2C=0 functionality. ... 

Borabicyclo[3.3.1.]nonane (9-BBN) is available by hydroboration of 1,5-cyclooctadiene with one equivalent of BH3," or commercially either as a crystalline dimer or as a THF solution. The reagent is thermally more stable than dicyclohexylborane. It is frequently used as an anchor group in organoborane reactions, allowing an efficient utilization of valuable alkenes. 

With sterically hindered dialkylboranes, such as dicyclohexylborane (Chx2BH) and disiamylborane (Sia2BH), the stoichiometric hydroborations of both terminal and internal alkynes stops at the monoaddition stage. However, the stoichiometric reaction of 1-alkynes with 9-BBN affords mainly the 1,1-diborylalkanes. ... 

Thus, chemoselective hydroborations of the triple bond in various enynes have been achieved. A similar chemoselectivity is predicted for hydroborations with the slightly less hindered dicyclohexylborane. - ... 

Hydroboration of alkynylsilanes with dicyclohexylborane proceeds in a stereo-and regioselective manner, placing the boron at the silicon-bearing carbon. Oxidation of the resultant alkenylborane with excess alkaline hydrogen peroxide produces, via an acyl silane intermediate (see Section 7.9), the carboxylic acids in greater than 80% yields. The alkynylsilanes may be generated in situ as illustrated by the one-pot transformation below. 

A )-and (Z)-alkenylboranes, or the corresponding boronic esters or potassium tri-fluoroborates, are conveniently available via hydroboration of terminal alkynes or 1-halo-1-alkynes using either disiamylborane, dicyclohexylborane, dibromoborane dimethyl sulfide, or catecholborane as hydroborating agents. ... 

Dicyclohexylborane is comparable to disiamylborane (1, 57-59,2,29) in effecting selective hydroboration of dienes1 and of olefins containing reactive functional groups near the double bond (e.g., ethyl vinylacetate and allyl benzoate ). 

The reagent is used in a new sequence for the preparation of alkylmercuric halides via hydroboration-mercuration.3 A terminal olefin is converted into an organoborane by reaction with dicyclohexylborane, and the organobqrane is treated with mercuric acetate. An alkyl mercuric acetate is formed, which is then converted into the more easily handled alkylmercuric halide by reaction with a sodium halide and water. 

HYDROBORATION Bis-3-methyl-2-butyl-borane. Diborane. Dicyclohexylborane. Trimethylamine oxide. 

For trisubstituted olefins and certain hindered disubstituted olefins, e.g., cyclohexene and longifolene, hydroboration proceeds rapidly only to the dialkylborane stage. By careful control of conditions, the dialkylboranes can be obtained. In this way, certain important hydroborating agents, such as bis-3-niethyl-2-butylborane, dicyclohexylborane , dilongifolylborane " and diisopinocampheylborane , are synthesized ... 

A limitation of SiajBH is the low thermal stability of both the reagent and the organoboranes derived from it. Dicyclohexylborane showing similar hydroboration characteristics often substitutes for SiajBH. In applications requiring thermal treatment... 

Dimesitylborane is highly selective in the hydroboration of substituted alkynes and can be used for the syntheses of alkenyldimesitylboranes . Dicyclohexylborane and Sia2BH are applied in the synthesis of cis-alkenylboranes not available by direct... 

An additional hydrogenation-type procedure is hydroboration of 1-TMS-alkynes, e.g. (97b), by means of dicyclohexylborane (DCB) in TMF and, consecutively, acidolysis by means of acetic acid/H202, to yield Z-l-TMS-alk-l-ene (107) (equation 53)73. 

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