Borane 1,5-Cyclooctadiene

Borabicyclo [3.3.1] nonane [280-64-8], 9-BBN (13) is the most versatile hydroborating agent among dialkylboranes. It is commercially available or can be conveniendy prepared by the hydroboration of 1,5-cyclooctadiene with borane, followed by thermal isomerization of the mixture of isomeric bicychc boranes initially formed (57,109). 

Addition of alane and borane to alkenes affords a host of alkylated alanes and boranes with various reducing properties (and sometimes bizarre names) diisobutylalane (Dibal-H ) [104], 9-borabicyclo[3.3.1]nonane (9-BBN) (prepared from borane and 1,5-cyclooctadiene) [705], mono- [106,107] and diiso-pinocampheylborane (B-di-3-pinanylborane) (both prepared from borane and optically active a-pinene) [108], isopinocampheyl-9-borabicyclo[3.3.1 Jnonane alias B-3-pinanyl-9-borabicyclo[3.3.1]nonane (3-pinanyl-9-BBN) (prepared from 9-borabicyclo [3.3.1]nonane and a-pinene) [709], NB-Enanthrane prepared from 9-borabicyclo[3.3.1]nonane and nopol benzyl ether) [770] and others. ... 

An especially selective hydroborating reagent is prepared from 1,5-cyclooctadiene and borane. The product is a bicyclic compound of structure 1 (often abbreviated as 9-BBN), in which the residual B-H bond adds to unhindered alkenes with much greater selectivity than is observed with other hydroborating reagents. It is also one of the few boranes that reacts sufficiently slowly with oxygen that it can be manipulated in air. 

A 0.5 M solution of 9-BBN in tetrahydrofuran was purchased from Aldrich Chemical Company, Inc., and was used without additional purification. The preparation2 of the reagent by hydroboration of 1,5-cyclooctadiene with borane/tetrahydrofuran complex is reported. 

Sterically demanding boranes offer enhanced selectivity. One example of a sterically demanding borane (9-BBN) is generated by the double addition of borane to 1,5-cyclooctadiene ... 

Intermediate enolates derived from Michael-type processes can be isolated. For example, enantiomerically pure enolate (5 )-131 can be isolated, as is the case of the almost enantiomerically pure enolate (5 )-131, prepared by 1,4-addition of the phenyl-substituted borane 130 to enone 129, in the presence of a substoichiometric amount of the chiral rhodium mediator [Rh(OMe)(COD)]2-(5 )-BINAP (COD = 1,5-cyclooctadiene, equation 33). Protonation of (5 )-131 with methanol leads to cyclohexanone (5 )-132 in good yield with no loss of enantiomeric purity (equation 34). The protonation is presumably diastereoselective, taking place on the less hindered face of (5)-131, away from the neighbouring phenyl group, as can be inferred from the stereochemical outcome (133)... 

Monochloborane-dimethyl sulfide coexists with small amounts of the borane and dichloroborane complexes, but the bromoborane-dimethyl sulfide complex appears to be almost pure. - These complexes react readily with alkenes at 25 °C and can be used for hydroborations in a variety of solvents. Dialkylha-loboranes are obtained in high yield as their dimethyl sulfide complexes (equation 24), but dimethyl sulfide is readily removed under reduced pressure if required. - The reagents are also useful for cyclic hydroborations of dienes such as cyclooctadiene (equation 25). An alternative approach to dialkylbro-moboranes involves the reaction of dialkyl(methylthio)boranes with bromine. ... 

Another reagent with high regioselectivity is 9-borabicyclo[3.3.1]nonane (9-BBN), which is prepared by hydroboration of 1,5-cyclooctadiene, and has the advantage that it is stable in air. Borane is quite unselective and attacks all sorts of double bonds. Disiamylborane, 9-BBN, and similar molecules are far more selective and preferentially attack less-hindered bonds, so it is often possible to hydroborate one double bond in a molecule and leave others unaffected or to hydroborate one alkene in the presence of a less reactive alkene. For example, 1-pentene can be removed from a mixture of 1- and 2-pentenes, and a cis alkene can be selectively hydro-borated in a mixture of the cis and trans isomers. 

Borabicyclo[3.3.1]nonane (9-BBN) has been prepared by the thermal redistribution of 9-n-propyl-9-BBN, and the hydroboration of 1,5-cyclooctadlene with borane-tetrahydrofuran complex followed by thermal isomerization of the mixture of dialkylboranes at BS C. Solutions of 9-BBN have been prepared from the hydroboration of 1,5-cyclooctad ene with borane-methyl sulfide in solvents other than THF.6 The present procedure involves the cyclic hydroboration of 1,5-cyclooctadiene with borane-methyl sulfide in 1,2-dimethoxyethane.7 Distillative removal of the dimethyl sulfide in this special solvent system provides a medium that gives high purity, large needles of crystalline 9-BBN dimer in excellent yield. The material can be handled in air for brief periods without measurable decomposition. 

Finally, reaction of borane with 1,5-cyclooctadiene gives a product where two of the B-H units have reacted with the two C=C units across the ring to give a bicyclic product (see Chapter 9, Section 9.7), 62. The name of this dialkylborane is 9-borabicyclo[3.3.1]nonane, which is often abbreviated 9-BBN or drawn as shown later. Nomenclature rules for bicyclic systems such as this are described in Chapter 9, Section 9.7. A. There are a total of nine atoms (nonane) but C9 has been replaced with boron (9-bora). All three of these boranes react as Lewis acids with an added alkene. 

However, the direct and convenient synthesis of (9-BBN)2 has been reported by Knights and Brown [2], and this development opened the door for its application in hydroboration [2-5]. The synthesis involves the cyclic hydroboration of 1,5-cyclooctadiene with a borane-tetrahydrofuran (THF) complex [2, 3] in a 1 1 ratio, followed by refluxing the mixture at 65 °C, thus producing a solution containing (9-BBN)2 in ca. 90% yield. 

In fact, the borane adds to 1,5-cyclooctadiene to afford 1,4- and 1,5-isomers in a 30 70 mixture. With simple thermodynamic considerations, it is apparent that the 1,4-isomer, which has a seven-membered ring fused to a five-membered ring, is less stable. Consequently, the 1,4-addition product is easily isomerized to the 1,5-isomer at 65 C (Eq. 3.2). This process affords a microcrystalline product with a melting point (m.p.) of 142 C. This material is further purified by vacuum sublimation, with an increase in m.p. to 152-155 °C [5]. 

These studies also reveal that the hydroboration of 1,5-cyclooctadiene using borane-dimethylsulfide to prepare a solution of 9-BBN can be carried out in solvents other than THF [9]. 

The calculated energy barrier starting with the mono-hydroborated D8 1,5-cyclooctadiene was 8.25 kcal/mol, which is easily overcome in room temperature, and therefore, a rapid reaction is expected. Note, however, that to determine the actual activation energies, the relative energy of the reactant states also needs to be accounted for, as B-H-B bridges are generally more stable than the interacting intramolecular n-bond-borane adduct [17]. We have not calculated this here, but estimate it to be below 7 kcal/... 

For many purposes, borane does a great job in terms of both reactivity (high) and selectivity (not always so high). However, there are processes where a less reactive and more selective borane will be preferred, and numerous compounds are available. If borane is reacted with 2,3-dimethyl-2-butene, only a single addition takes place, because of the steric hindrance that would result from a further addition. The product is termed thexylborane (11.27). Similarly, disiamylborane (SiajBH, 11.28) is prepared from 2-methyl-2-butene. The number of times the borane reacts is clearly a function of steric hindrance. 9-Borabicyclo[3.3.1]nonane (9-BBNH, 11.29) is prepared from 1,5-cyclooctadiene, and catecholborane (HBcat, 11.30) from catechol (1,2-benzenediol) and borane. All of these have been used to improve selectivity for specific reactions—in general, the more hindered the borane, the more selective the reaction. 

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