Preparation borane reagents

Since borane BH3 reacts with only one or two equivalents of a sterically hindered alkene, it is possible to prepare less reactive and more selective borane reagents R2BH and RBH2 respectively. In addition to disiamylborane 8 and thexylbo-rane 10, the 9-borabicyclo[3.3.1]nonane (9-BBN) 14 is an important reagent for hydroboration, since it is stable to air it is prepared by addition of borane 2 to cycloocta-1,5-diene 13 ... 

Exchange with boranes can also be used to prepare alkenylzinc reagents.138... 

In a recent development, a new process of preparing borane-terminated isotactic polypropylene (z -PPs) via an in situ chain-transfer reaction was achieved by a styrene/hydrogen consecutive chain-transfer reagent, which avoids the use of a B—H containing chain-transfer agent.74 This has resulted in the utilization of milder polymerization conditions due to the use of the alkylaluminoxane cocatalyst (MAO) (50) (Fig. 33), which cannot normally be used in the presence of a B—H chain-transferring... 

This borane reagent may be prepared as follows.6 In a dry 500-ml flask equipped with a condenser, a thermometer and a pressure-equalising dropping funnel are placed 80 ml of diglyme, 23.1 g of 2-methylbut-2-ene (0.33 mol) in 20ml of diglyme and 4.7 g of sodium borohydride (0.125 mol). The flask is immersed in an ice bath and 23.5 g of boron trifluoride-etherate (0.165 mol) is added dropwise over a period of 30 minutes. The semi-solid reaction mixture containing 0.165 mol of disiamylborane is permitted to remain an additional 15 hours at 0-5 °C, and then used for hydroboration. Examples of the use of disiamylborane are described in Sections 5.4.3 p. 542 and 5.7.1, p. 588. 

Various polymer-supported hydrides have been applied successfully to reductions of both carbonyl and olefin groups. Rajasree and Devaky13 describe a cross-linked polystyrene-supported ethylenediamine borane reagent for the selective reduction of aldehydes in the presence of ketones (entry 9). This borane reagent is easily prepared and can be recycled after completion of the reaction. This is a practical alternative to standard borane reagents such as diborane, borane-amine, or borane-sulfide complexes. 

Enantioselective aldol reactions.3 4 A related borane reagent, (R,R)-4, prepared by reaction of BBr3 with the N,N-bistosylsulfonamide of (R,R)-1, can effect highly enantioselective aldol reactions of ketones with aldehydes. Thus reaction of... 

In the synthesis of a library of (+)-murisolin (39) and its 15 other stereoisomers, Curran and others used Brown allylation strategy to obtain the four diastere-oisomers of the homoallylic alcohol 4019 (Scheme 3.1p). Thus, the homoallylic alcohol (S,S )-40 was prepared in 95% ee from allylborane reagent l35 and corresponding aldehyde. By using the Mitsunobu reaction, (R,S )-40 was obtained. Similarly, (R,R)-40 was synthesized via the enantiomeric borane reagent d35... 

Preparation. The reagent is prepared by the reaction of (9 )-limonene with thexyl-borane (2, 148 this volume) to give the trialkylborane, This is converted into the borohydride ion (1) by treatment with t-butyllilhium in THF. 

Preparation. The reagent is prepared by the reaction of borane (516 mmole) in THF with a freshly prepared solution of boron trichloride (1032mmole 1, 67-68 2, 34-35 3, 31-32) in THF at 0°. The reagent is stable for several months at 0°. 

Dichloroborane and monochloroborane etherates or their methyl sulfide complexes have been prepared by the reaction of borane and boron trichloride [44]. The hydroboration of alkenes with these borane reagents is, however, usually very slow because of the slow dissociation of the complex. Dichloroborane prepared in pentane from BCI3 and trimethylsilane is unusually highly reactive with alkenes and alkynes hydroboration is instantaneous at -78 °C (Eq. 20) [45]. 

Another Cz symmetric chiral borane reagent has teen utilized by Corey et alP for the construction of syn aldol products in a highly enantioselective manner. When an enolate prepared from 5-phenyl pro-... 

Reduction. The reagent is prepared 6 reagent prepared in situ works equally well, am as borane is cogenerated, owing to the superior... 

This stable lorin of BH , is useful for the preparation of various borane reagents such as ihesylboraiic, disiamylborane, eateeholboranc, and 9-BBN. The reagent is compatible with a wide variety of sr>lvents. ... 

The successful implementation of this strategy is shown in Scheme 1.370. The optically pure stannane 1441 was transmetalated in situ with 1442, preparing a borane reagent that condensed with 1440. After methylation, 1443 was isolated in excellent yield with 10.5 1 diastereoselection. At this point, 1443 was elaborated to incoprorate the pyran moiety (five steps), and the side chain was converted to an aldehyde (two steps). Overall, this reaction sequence to 1444 was accomplished in 46% yield. The skipped triene fragment was then incorporated via the anion of sidfone 1445. Saponification of the pivalate ester then completed the synthesis of 1191a. 

Hawkins et al. observed that the reaction of a, -unsaturated esters with cyclopentadiene in the presence of catalytic IR, 2R)-2-naphthalenyl-cyclohexyldichloroborane leads to the enantio-selective formation of Diels-Alder adducts in high yield and ee (87-97% ee). The borane reagent is prepared via hydroboration of l-(l-cyclohexenyl)naphthalene with dichloroborane to yield the racemic product, which was later resolved via its crystalline complex with (—)-menthone to furnish >99% de after one recrystallization. The menthone-free catalyst was obtained by methanol-ysis, followed by rechlorination with BCI3 (eq 21). 

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