9-alkyl-9-borabicyclo nonane

The thioboration of terminal alkynes with 9-(alkylthio)-9-borabicyclo[3.3.1]-nonanes (9-RS-9-BBN) proceeds regio- and stereoselectively by catalysis of Pd(Ph,P)4 to produce the 9-[(Z)-2-(alkylthio)-l-alkeny)]-9-BBN derivative 667 in high yields. The protonation of the product 667 with MeOH affords the Markownikov adduct 668 of thiol to 1-alkyne. One-pot synthesis of alkenyl sulfide derivatives 669 via the Pd-catalyzed thioboration-cross-coupling sequence is also possible. Another preparative method for alkenyl sulfides is the Pd-catalyzed cross-coupling of 9-alkyl-9-BBN with l-bromo-l-phe-nylthioethene or 2-bromo-l-phenylthio-l-alkene[534]. 

PALLADIUM(0)-CATALYZED REACTION OF 9-ALKYL-9-BORABICYCLO[3.3.1]NONANE WITH 1-BROMO-1-PHENYLTHIOETHENE 4-(3-CYCLOHEXENYL)-2-PHENYLTH 10-1-BUTENE... 

These ate complexes are prepared by reaction of potassium hydride with a 9-alkyl-9-borabicyclo[3.3. l]nonane (9-R-9-BBN). 

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. ... 

It is clear from these representative results that regioselectivity in hydroboration is controlled by steric effects. As a result, nonsymmetric internal olefins usually yield a mixture of regioisomeric alkylboranes when they react with borane. Several hindered mono- and diakylboranes with sterically demanding alkyl groups, however, have been developed for use in selective hydroboration. Disiamylborane [bis(3-methyl-2-butyl)borane, 34], thexylborane (2,3-dimethyl-2-butylborane, 35), and 9-BBN (9-borabicyclo[3.3.1]nonane) are the most frequently used reagents. Improvements of regioselectivities in hydroboration of both... 

Asymmetric reduction of ketones. Ipc BCl is somewhat superior to B-3-pin-anyl-9-borabicyclo[3.3.1]nonane (12, 397) for enantioselective reduction of alkyl aryl ketones at normal pressures to (S)-alcohols. In general, optical yields are 78-98%. It is also useful for asymmetric reduction of ketones in which one alkyl group is tertiary. Thus 3,3-dimethyl-2-butanone is reduced in 95% ee at 25°.1... 

Two dialkyl boranes arc in common use. The bicyclic 9-borabicyclo[3.3.1] nonane (9-BBN), introduced in Chapter 34 as a reagent for diastereoselcctive aldol reactions, is a stable crystalline solid. This is very unusual for an alkyl borane and makes it a popular reagent. It is made by hydroboration of cyclo-octa-1,5-diene. The second hydroboration is fast because it is intramolecular but the third would be very slow. The regioselectivity of the second hydroboration is under thermodynamic control. 

More recently, Brown and co-workers have demonstrated that six-membered boracyclanes are likewise capable of ring contraction. As an example, the light-induced reaction of bromine with 9-methoxy-9-borabicyclo[3.3.1]nonane (61) in the presence of water affords the boronic acid 62 which is in turn readily oxidized to the alcohol in 65 % overall yield.11S) The structurally related B-alkyl-9-BBN derivatives react under comparable conditions but in the dark to deliver analogous products.116)... 

Midland and co-workers demonstrated that certain p-alkyl-9-borabicyclo [3.3.1] nonanes can reduce benzaldehyde to benzyl alcohols under mild conditions. Several optically active terpenes such as (+)-a-pinene (6.54), (—)-P-pinene (6.55), (—) camphene (6.56) and (+)-3-carene (6.57) have been used to prepare asymmetric (3-alkyl-9-BBN reagents. 

Cyanoborohydride and its modified reagents have been used for reductive dehalogenations. Thus, the combination of sodium or tetrabutylammonium cyanoborohydride, sodium or potassium 9-cyano-9-hydro-9-borabicyclo[3.3.1]nonanate [9-BBNCN] (2) or polymeric cyanoborane (3) in HMPA furnishes an efficient and mild system for the reduction of alkyl halides. The reagents are selective in that other functional groups, including ester, carboxylic acid, amide, cyano, alkene, nitro, sulfone, ketone, aldehyde and epoxide, are essentially inert under the reduction conditions thus, the reduction procedure is attractive for synthetic schemes which demand minimum damage to sensitive portions of the molecule. 

Catalytic coupling reactions of alkyl- [121,122] and 1-alkenyl- [123 —125] boronates usually proceed with a base. Cross-coupling between R3B or B-alkyl-9-BBN (BBN = borabicyclo[3.3.1]nonane) with aryl or 1-alkenyl halides, for example. 

Table 1-8. Cross-coupling between triflates and 9-alkyl-9-BBN derivatives (from alkenes and 9-BBN) (BBN = borabicyclo[3.3.1]nonane) (data from [128])...

Cross-coupling between 1-alkenyl- and arylboron compounds with organic electrophiles have found wide application (see Sect. 1.5.1.1) in organic synthesis [125, 132, 269-271]. The value of this methodology is further realized with the use of alkylboranes, such as B-alkyl-9-borabicyclo[3.3.1]nonanes (B-R-9-BBN), which can be conveniently prepared Ijy hydroboration of olefins [272]. The hydroboration proceeds with high stereoselectivity and chemoselectivity. The choice of phosphines in a catalytic system sometimes affects the chemo- and regioselectivity of the hydroboration. Hydroboration of l-(ethylthio)-l-propyne with catecholborane can be satisfactorily carried out with PdClj (dppf) but the regioselectivity is best for the dppe and dppp complexes of Ni [273]. Notably, PPhj complexes perform poorly. 

Reaction of 9-Alkyl-9-borabicyclo[3.3.1]nonane with 1-Bromo-l-... 

Consistently high yields depend on the proper selection of reagents. In general, the best base is the bulky potassium 2,6-di-rerr-butylphenoxide. The best alkylating agent is B-alkyl-9-borabicyclo[3.3.1]nonane, or B-alkyl-9-BBN, available via successive hydroborations of alkenes ... 

That s-alkyl groups do not react under such mild conditions as primary alkyl groups do enables dicyclohexylborane and borabicyclo[3.3.1]nonane to be used in hydro-boration-mercurideboronation sequences, e.g. ... 

An important feature of the Suzuki reaction is that it allows cr/Z Z-alkenyl or alkyl-aryl coupling of fi-a//cy/-9-borabicyclo[3.3.3]nonane derivatives with haloalkenes and haloarenes without concomitant p-hydride elimination. The required organoboranes are obtained by hydroboration of the appropriate alkenes or functionally substituted alkenes with 9-BBN. The coupling reaction tolerates the presence of functional groups in both reaction partners, thus circumventing a requirement of their prior pro-tection. ... 

Photocatalytic carbonylative coupling with 9-alkyl-9-borabicyclo[3.3.1]-nonanes (9-R-9-BBN), however, made it possible to transform alkyl halides to ketones [72]. lodoalkenes or iodoalkynes are thus cyclized to five-membered rings [73]. The oxidative addition of iodoalkyl to palladium(O) proceeds via radicals allowing the ring closure to take place prior to the dual coupling with CO and the alkylboranes. 

Midland, M. M., Tramontane, A., Zderic, S. A. The facile reaction of B-alkyl-9-borabicyclo[3.3.1]nonanes with benzaldehyde. J. Organomet. Chem. 1977, 134, C17-C19. 

Rogic, M. M. Conformational Analysis and the Transition State in Asymmetric Reductions with Boranes Based on (+)-a-Pinene. 1. Benzaldehyde Reduction with Alpine Borane and Other B-Alkyl-9-borabicyclo[3.3.1]nonanes. ASemiempirical Study. J. Org. Chem. 1996, 61, 1341-1346. 

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