Borane disubstituted

Disubstituted Boranes. Even slight differences in stetic or electronic effects of substituents may have an effect on the hydroboration reaction course. These effects are well demonstrated in disubstituted boranes, and consequentiy a range of synthetically useful reagents has been developed. 

Dibromoborane—dimethyl sulfide is a more convenient reagent. It reacts directly with alkenes and alkynes to give the corresponding alkyl- and alkenyldibromoboranes (120—123). Dibromoborane differentiates between alkenes and alkynes hydroborating internal alkynes preferentially to terminal double and triple bonds (123). Unlike other substituted boranes it is more reactive toward 1,1-disubstituted than monosubstituted alkenes (124). 

Among chiral dialkylboranes, diisopinocampheylborane (8) is the most important and best-studied asymmetric hydroborating agent. It is obtained in both enantiomeric forms from naturally occurring a-pinene. Several procedures for its synthesis have been developed (151—153). The most convenient one, providing product of essentially 100% ee, involves the hydroboration of a-pinene with borane—dimethyl sulfide in tetrahydrofuran (154). Other chiral dialkylboranes derived from terpenes, eg, 2- and 3-carene (155), limonene (156), and longifolene (157,158), can also be prepared by controlled hydroboration. A more tedious approach to chiral dialkylboranes is based on the resolution of racemates. /n j -2,5-Dimethylborolane, which shows excellent enantioselectivity in the hydroboration of all principal classes of prochiral alkenes except 1,1-disubstituted terminal double bonds, has been... 

Borane and sodium in 1-propanol are good reducing agents for all three types of amides. Another reagent that reduces disubstituted amides to amines is trichloro-... 

Alkynes are reactive toward hydroboration reagents. The most useful procedures involve addition of a disubstituted borane to the alkyne, which avoids complications that occur with borane and lead to polymeric structures. Catechol borane is a particularly useful reagent for hydroboration of alkynes.212 Protonolysis of the adduct with acetic acid results in reduction of the alkyne to the corresponding cw-alkene. Oxidative workup with hydrogen peroxide gives ketones via enol intermediates. 

Other disubstituted boranes have also been used for selective hydroboration of alkynes. 9-BBN can be used to hydroborate internal alkynes. Protonolysis can be carried out with methanol and this provides a convenient method for formation of a disubstituted Z-alkene.217... 

Alkoxy groups can be displaced from boron by alkyl- or aryllithium reagents. The reaction of diisopropoxy boranes with an organolithium reagent, for example, provides good yields of unsymmetrically disubstituted isopropoxyboranes.3... 

Prior literature indicated that olefins substituted with chiral sulfoxides could indeed be reduced by hydride or hydrogen with modest stereoselectivity, as summarized in Scheme 5.10. Ogura et al. reported that borane reduction of the unsaturated sulfoxide 42 gave product 43 in 87 13 diastereomer ratio and D20 quench of the borane reduction mixture gave the product 43 deuterated at the a-position to the sulfoxide, consistent with the hydroboration mechanism [10a]. In another paper, Price et al. reported diastereoselective hydrogenation of gem-disubstituted olefin rac-44 to 45 with excellent diastereoselectivity using a rhodium catalyst [10b],... 

The sequential treatment of 1-alkynes with n-BuLi, trialkyl borane and a tellurenyl bromide reaches the (Z)-borosubstituted vinyl telluride which is easily hydrolyzed to the disubstituted vinyl tellurides. The reaction proceeds with high regio- and stereocontrol. ... 

Borane may react sequentially with 3 mol of alkene to form mono-, di-, and trialk-ylboranes. Both the alkene structure and reaction conditions affect product distribution. Trialkylboranes are usually formed from terminal olefins [Eq. (6.57)] and unhindered disubstituted alkenes such as cyclopentene irrespective of the reactant ratio.340 The reaction cannot be stopped at the mono- or dialkylborane stage. In contrast, hindered disubstituted olefins (e.g., cyclohexene) and trisubstituted alkenes are converted mainly to dialkylboranes [Eq. (6.58)]. Careful control of... 

The N-N bond of polystyrene-bound hydrazines, which are prepared by reaction of organolithium compounds with resin-bound hydrazones [457], can be cleaved by treatment with borane to yield a-branched, primary amines (Entry 9, Table 3.23). An additional example of reductive cleavage to yield amines is shown in Entry 10 (Table 3.23), in which a resin-bound a,a-disubstituted nitroacetic ester undergoes decarboxylation and reduction to the primary amine upon treatment with lithium aluminum hydride. 

Fig. 10.24. Asymmetric carbonyl group reduction with Alpine-Borane (preparation Figure 3.27 for the "parachute-like" notation of the 9-BBN part of this reagent see Figure 3.21). The hydrogen atom that is in the cis-position to the boron atom (which applies to both ft- and /T-H) and that after removal of the reducing agent leaves behind a tri- instead of a disubstituted C=C double bond (which applies to ft-, but not / -H) is transferred as a hydride equivalent. In regard to the reduction product depicted in the top row, the designation S of the configuration relates to the aryl-substituted and R to the Rtert-substituted propargylic alcohol.

The amino groups of ovomucoid, lysozyme, and ovotransferrin were alkylated extensively (40-100%) with various carbonyl reagents in the presence of sodium borohydride. Monosubstitution was observed with acetone, cyclopentanone, cyclohexanone, and benzaldehyde, while 20-50% disubstitution was observed with 1-butanal and nearly 100% disubstitution was observed with formaldehyde. The methylated and isopropylated derivatives of all three proteins were soluble and retained almost full biochemical activities. Recently amine boranes have been shown to be possible alternative reducing agents for reductive alkylation... 

B-Alkenyl-9-borabicyclo(3,3,l)nonane (B-alkenyl-9-BBN) adds across the carbonyl group of a simple aldehyde to afford in good yields, the corresponding allylic alcohol S2). The reaction proceeds with complete retention of vinylic borane stereochemistry resulting in the /raw-disubstituted olefin linkage in the final product. The reac-... 

Better results (80 % ee) have been reported by Mikami, Nakai and co-workers [3c] for the addition of crotyl silane also catalyzed by complex 1. Yamamoto and co-workers [3b] used chiral acyl-oxy boranes to catalyze the Sakurai-Hosomi-reac-tion. While an excellent 96 % ee was obtained for the addition of 2,3 -disubstituted allyl groups, the conversion with parent allyl silane was low (46 %) and the asymmetric induction mediocre (55 % ee). Gauthier and Carreira [5] then made a big leap forward by using the difluorotita-nium-binaphthol complex 3. The catalyst 3 is prepared in situ via the TiF4-binaphthol adduct 4 and formal HF elimination mediated by allyl silane 5. The addition of 5 to aldehydes 6 ( 7) catalyzed by 10 % of 3 proceeds with 61 - 94 % < e and good yields (69-93 %), the best results being observed for aldehydes with tertiary alkyl residues (Scheme 1). 

However, the utility of this Zweifel synthesis was limited in the past by the limited availability of dialkylboranes, because direct hydroboration leads cleanly to the formation of dialkylboranes only in the case of relatively hindered alkenes such as 2-methyl-2-butene and cyclohexene. More generally, the hydroboration fails to stop at the R2BH stages. Recent developments have provided a general preparation of a variety of dialkylboranes via the hydridation of dialkylhalo-boranes. Thus, dialkylvinylboranes prepared via the hydridation of dialkylhalo-boranes in the presence of an alkyne, react with iodine under basic conditions to produce disubstituted alkenes (Eq. 58) and trisubstituted alkenes (Eq. 59) of established stereochemistry. These results indicate a mechanism analogous to that... 

A very important feature of the monoalkylmonoisopinocampheyl boranes obtained from prostereogenic alkenes is their tendency to crystallize from the reaction mixture. This crystallization process is an efficient resolution technique providing crystalline material of high optical purity. Utilizing this technique, chiral alcohols of essentially 100% ee can be prepared by the hydroboration of prostereogenic, A -disubstituted and trisubstituted alkenes with monoisopinocampheylborane71, 73. 

Hydroboration of acetylenic selenides with 9-BBN led to the regio- and stereoselective formation of a-selanylalkenyl boranes which were then converted into Z-a, -disubstituted vinyl selenides by cross-coupling reaction with aryl bromides [80] (Scheme 58). With unsubstituted acetylenic selenides, an inversion of regioselectivity during the hydrozirconation was observed [81,82]. 

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