With 9-BBN-THF

Both aldehydes and ketones consume 1 equiv of 9-BBN (3-5% excess) for complete reduction. 

Cyclic and bicyclic ketones such as cyclohexanone, 2-methylcyclohexanone, 4-tert-butylcyclohexanone, and norcamphor undergo complete reduction in 30-60 min. Hindered ketones, like diisopropyl ketone and camphor, need 6-12 

The reduction of monocyclic ketones with disiamylborane and di-3-pinanyl-borane takes place from the less hindered side to yield predominantly the less stable of the two possible isomers. In contrast, 9-BBN exerts little influence on the direction taken by the reduction. With 3-methyl- and 4-tert-butylcyclohex-anones, the products are predominantly the more stable of the two possible isomers. Reduction of bicyclic ketones such as norcamphor and camphor proceeds with preferential attack of the 9-BBN from the less hindered side, yielding the less stable of the two possible isomers (Eq. 25.1). 

The stereochemical outcome after the reduction are summarized in Table 25.1 [1]. 

Anthraquinone reacts with 2 equiv of 9-BBN to give cleanly 9,10-dihydro-9,10-anthracenediol (Eq. 25.2). However, p-benzoquinone gives unsatisfactory results. 

---

Hydroboration.1 Hydroborations with this reagent in CH2C12 or THF (heterogeneous) are relatively slow (5-12 hours at 25°), but application of ultrasound permits complete reaction in 1-2 hours. Ultrasound also increases the rate of hydroboration with catecholborane or with 9-BBN (THF or neat). 

To a reaction flask charged with 9-BBN (24.4 g, 200 mmol) in THF (400 mL) was added 4-tosyloxybut-l-ync (22.4 g, 100 mmol). The resulting solution was allowed to stir for 3 h at 25 °C. Then 1.7 M MeLi in Et,0 (118 mL. 200 mmol) was added dropwise over 30 min and the solution allowed to stir for 6 h. II20 (25 mL) was added and the solution treated with K2C03. The organic phase was separated, dried (MgS04), and distilled to give the product yield I2.6g (65%) bp 113 C/lOTorr. 

In an analogous case (7.6.) the 9-borabicyclo[3.3.1]nonane (BBN) unit was introduced onto the quinoline core through lithium-halogen exchange and the quenching of the intermediate with 9-BBN-OMe. The quinolylborane coupled with 3-bromopyridine under the earlier mentioned conditions (THF, , TBAB) to give the pyridylquinoline in acceptable yield.8... 

Sodium 9-cyano-9-hydrido-9-borabicydo 3.3.1 nonane (9BBN-NaCN) (1). Mol. wt. 171.03, tacky noncrystalline solid. The reagent is prepared by reaction of NaCN with 9-BBN in THF. 

Trost first introduced the di-fe/7-butylsilylene derivative as a means for protecting 1,2- and 1,3-diols during a synthesis of PiUaromycinone derivatives.213 Di-ferf-butylsilylene derivatives are not as robust as isopropylidene or benzylidene acetals and their use is best reserved for systems requiring deprotection under very mild conditions. Di-isopropylsiiylene derivatives are occasionally used but they usually only survive in highly crowded environments.214 Di-feri-butylsily-lene derivatives survive hydroboration with 9-BBN, mild oxidation (e g the Dess-Martin, ozone), Lewis acids such as trifluoroborane e the rate and titanium tetrachloride, mild acids (pyridinium p-toluenesulfonate). camphorsulfonic acid, strong bases such as feri-butyllithium (THF, -50 °C), DDQ, and sodium meth-oxide in methanol at 0 C — conditions used to cleave acetate esters. 

The reagent is prepared by reaction of 2,3-dimeth 1-2-butanol with 9-BBN and then with potassium hydride. The borohydride is stable for more than a year in THF solution. 

Furthermore, the products of reactions with 9-BBN-H seem to be less prone to isomerization than those derived using Iwrane-THF. For example, 1-methylcyclooctene is readily converted into 9-(trans-2-methylcyclooctyl)-9-BBN (equation 30), whereas its reaction with borane-THF produces a complex mixture. 

The rates of reaction of alkenes with disiamylborane vary over a much wider range than those with bo-rane-THF, and alkynes react more rapidly than comparable alkenes. The great selectivity shown allows ready distinction between different alkenes, or between two different double bonds in a nonconjugated diene e.g. equation 36). Selective monohydroboration of conjugated dienes is more troublesome. However, selective monohydroboration of some conjugated dienes, such as 1,3-cyclohexadiene, can be achieved with disiamylborane, as is also the case with 9-BBN-H. 

B. Techniques. The synthesis and purification of a, u)-di (tejrt.-chloro ) poly isobutylene C1-PIB-C1 (formula I in Scheme I) and the preparation of ct, io-di ( isobut-enyl)polyisobutylene (formula II in Scheme I) by quantitative regioselective dehydrochlorination of Cl-PIB-C1 have been described [1,3]. Details of the synthesis of a, u-di (hydroxy)polyisobutylenes HO-CH2-PIB-CH2-OH (formula IV in Scheme I) by hydroborations of ct,w-di-( isobutenyl)polyisobutylene with 9-BBN or BH3 THF, have been given elsewhere [4]. 

These dimers are less reactive or unreactive in catalytic reactions of interest, and thus some substitution on the cyclopentadienyl ligands is necessary to prevent dimer formation through steric hindrance to association. Pentamethylcy-clopentadienyl (Cp ) ligands are useful for this, and lead to catalytic systems that are highly reactive and yet exquisitely selective in the insertion of mono-substituted alkenes. One such precatalyst system is the yttrium complex Cp 2YMe THF. Although Lewis bases normally depress catalytic activity because they compete for empty coordination sites on the catalyst, in this precatalyst the single THF of solvation appears to catalyze the hydrometallation process [Eq. (5)] [15] in the same manner that Lewis bases catalyze the hydroboration of olefins with 9-BBN [16] ... 

Hydroboration of olefins with 9-BBN is normally complete after 1 hr. in refluxing THF or after 8 hr. in refluxing benzene or hexane. The regioselectivity in olefin hydroborations is higher with 9-BBN than with disiamylborane or dicy clohexylborane. ... 

Lithium B-isopinocampheyiT9-borabicycio[3.3.1]nonyl hydride (1). Mol. wt. 266.21. This trialkyIborohydride is prepared by hydroboration of (+)-a-pinene with 9-BBN followed by reaction with t-butyllithium in THF (-78°). 

Table 4.11 Relative rates between mono- and dihydroboration of alkynes with (9-BBN)2 in THF...

However, the instability of most of the hydroborating agents and their derivatives has limited their use in hydroboration and their subsequent synthetic applications. On the other hand, both 9-BBN and B-R-9-BBN compounds exhibit remarkable thermal stabilities. The hydroboration with 9-BBN is carried out at 25 °C or at a higher temperature to achieve essentially complete hydroboration of alkenes of widely different structural types [4] as compared with disiamylbo-rane, where the reaction is normally carried out at 0 °C. In general, the hydroboration with 9-BBN is carried out in THF as the solvent, but carbon tetrachloride, benzene, and hexane are also employed for the reaction. 

Trialkylboranes (B-R-9-BBN) derived from hydroboration of alkenes with 9-BBN are remarkably stable to thermal isomerization [6]. The alkylborane, derived from (Z)-hex-3-ene by hydroboration with 9-BBN, requires the heating at 150 °C for 163 h to attain the equilibrium distribution of boron along the hexyl chain. At the same temperature the isomerization of the alkylborane derived from (Z)-hex-3-ene using borane-THF is complete in 1 h. 

---