Diborane hydroboration

Hydroboration. In chlorohydrocarbon solvents (dichloromethane, chloroform 1,2-dichloroethane, etc), diborane hydroborates unsaturated compounds rapidly at -16°. 

Hydroboration-oxidation (Sections 6 11-6 13) This two step sequence achieves hydration of alkenes in a ste reospecific syn manner with a regiose lectivity opposite to Markovnikov s rule An organoborane is formed by electro philic addition of diborane to an alkene Oxidation of the organoborane inter mediate with hydrogen peroxide com pletes the process Rearrangements do not occur... 

Diborane [19287-45-7] the first hydroborating agent studied, reacts sluggishly with olefins in the gas phase (14,15). In the presence of weak Lewis bases, eg, ethers and sulfides, it undergoes rapid reaction at room temperature or even below 0°C (16—18). The catalytic effect of these compounds on the hydroboration reaction is attributed to the formation of monomeric borane complexes from the borane dimer, eg, borane-tetrahydrofuran [14044-65-6] (1) or borane—dimethyl sulfide [13292-87-0] (2) (19—21). Stronger complexes formed by amines react with olefins at elevated temperatures (22—24). 

The hydroboration step, being very sensitive to steric effects, yields only secondary alkylboranes from trisubstituted double bonds, whereas the less hindered alkylborane is formed predominantly from disubstituted steroidal double bonds. The diborane attack occurs usually towards the a-side and hence results in overall a-hydration of double bonds after alkaline hydrogen peroxide oxidation. ... 

Hydroboration affords an efficient preparation of the 5a-A -system (141, for example) from A" -3-ketones. Reaction with diborane followed by decomposition of the organoboron intermediate with refluxing acetic anhydride gives good yields of olefins. Ketones must be protected, and alcohols are transformed to acetates. A -7-Ketones yield 5oc-A -olefins (for example, 138). 

With this as introduction, let us now look at the individual steps in more detail for the case of hydroboration-oxidation of 1-decene. A boron hydride that is often used is dibomne (B2H6). Diborane adds to 1-decene to give tridecylborane according to the balanced equation ... 

The excess diborane in the hydroboration flask is decomposed by the cautious addition of 20 ml of water. The organoborane is oxidized by the addition of 32 ml of 3 A sodium hydroxide, followed by dropwise addition of 32 ml of 30% hydrogen peroxide... 

Diborane reacts with unhindered olefins to form trialkylboranes (the so-called hydroboration reaction, cf. Chapter 4). In this Chapter, several of the recently discovered carbon-carbon bond forming reactions of trialkylboranes are presented. 

In a 200-ml three-necked flask fitted with a dropping funnel (drying tube) is placed a solution of 13.4 g (0.12 mole) of 1-octene in 35 ml of THF. The flask is flushed with nitrogen and 3.7 ml of a 0.5 M solution of diborane (0.012 mole of hydride) in THF is added to carry out the hydroboration. (See Chapter 4, Section I regarding preparation of diborane in THF.) After 1 hour, 1.8 ml (0.1 mole) of water is added, followed by 4.4 g (0.06 mole) of methyl vinyl ketone, and the mixture is stirred for 1 hour at room temperature. The solvent is removed, and the residue is dissolved in ether, dried, and distilled. 2-Dodecanone has bp 119710 mm, 24571 atm. (The product contains 15 % of 5-methyl-2-undecane.) The reaction sequence can be applied successfully to a variety of olefins including cyclopentene, cyclohexene, and norbornene. 

A mixture of d- and l- hexoses also results from the hydroboration of these 5-enes. Hydroboration results in anti-Markownikoff, cw-hydration of the double bond and the amount of each hexose formed varies according to the nature of the substituent groups. For example, hydroboration (23) of methyl 6-deoxy-a-D-ryZo-hex-5-enopyranose (3) affords methyl a-D-glucopyranoside and methyl / -L-idopyranoside in the ratio of 1 2.5 respectively whereas hydroboration of the fris-trimethylsilyl ether of 3 afforded them in the ratio 1 0.6 respectively. The hydroboration method can be used to achieve specific labelling of hexoses with tritium methyl-/ -L-idopyranoside[5-H3] and methyl a-D-glucopyranoside [5-H3] were thus prepared (23). Similarly, hydroboration of the D-Zt/ro-hex-5-eno derivative (14) with diborane-H3 followed by removal of the isopropyli-dene group, afforded methyl a-D-mannopyranoside [5-H3] and methyl / -L-gulopyranoside [5-H3] in the ratio of 1 2 respectively (23). 

An important stage in the synthesis has been reached. The reaction processes described thus far have proceeded uneventfully and have culminated in the synthesis of compound 9. The stage is now set for an evaluation of the first of two hydroboration reactions. Treatment of 9 with diborane in THF, followed by standard alkaline hydrogen peroxide workup, furnishes an 8 1 stereoisomeric mixture of alcohols in favor of 28 (85 % total yield). On the basis of some important precedent,32 it is presumed that compound 9 preferen-... 

Acetyl-5//-dibenz[/>,/]azepine-10-car bon itrile (17, R = CN) when treated with sodium borohydride undergoes reduction (73 % yield) at the CIO - Cl 1 double bond without reduction of the acetyl or cyano groups.212 However, hydroboration of 5-acetyl-5//-dibenz[/y/]azepine (17, R = H) with diborane in tetrahydrofuran under standard conditions is accompanied by reduction of the acyl function to yield 5-ethyi-10,l l-dihydrodibenz[6,/]azepin-10-ol (18).72... 

On the other hand, the observed syn preference of 59a is consistent with a study of hydroboration of 59a with diborane by Schueler and Rhodes [127], who obtained a mixture of the monoalcohols (symanti = 74 26) upon oxidative work-up. A similar magnitude of. yyn-preference was found (syn anti = 73 27) in the hydroboration with a bulkier borane, 2,3-dimethyl-2-butylborane (thexyl borane) [127]. This lack of effect of the bulk of the reagent in the hydroboration of 59a is consistent with the idea that the n face of 59a is free from steric bias [127], and that the syn preference of 59a found in dihydroxylation and epoxidation is non-sterically determined [128]. 

The stereochemical outcome of the hydroboration-oxidation of 1, V-bicyclohexenyl depends on the amount of diborane used. When 1.1 equivalent... 

DMSO forms an explosive mixture with nonahydrononaborate(2-) ion and with diborane. It is probable that other boron hydrides and hydroborates behave similarly. 

As mentioned above (Scheme 3), condensation of triallylborane and 3-methoxybut-l-yne led, after treatment with methanol, to 7-(l-methoxymethyl)-3-methoxy-3-borabicyclo[3.3.1]non-6-ene. Hydroboration-isomerization of the latter with a THF solution of diborane gave a THF complex of 2-methyl-1-boraadamantane 15 in 85% yield. Treatment of the latter with (S)-(—)-phenylethylamine gave a mixture of diastereomeric complexes ( ) 57 isolated as white, well-shaped crystals (Scheme 19) <2003MC121, B-2003MI97>. 

Hydroboration can be carried out by using the boron hydride (B2H6) called diborane. 

Borazine originally was obtained by the reaction of ammonia with diborane.1 Mixtures of lithium or sodium tetrahydro-borate with ammonium chloride also have been pyrolyzed to yield this product.2 More recently, the reduction of B,B, B"-trichloroborazine with alkali metal hydroborates has proved to be a convenient laboratory-scale method for the preparation of this compound.3-7 The procedure described herein is a variation of the last method as reported by Dahl and Schaeffer.7 This method is effective for the synthesis of iV-substituted alkyl- and arylborazines, i.e., compounds of the formula (HBNR)3 where R is CH3, C2H6, C6H , C6H5, p-C6H4CH3, or p-C6H4OCH3.5... 

Hydroboration, the addition of a boron-hydrogen bond across an unsaturated moiety, was first discovered by H. C. Brown in 1956. Usually, the reaction does not require a catalyst, and the borane reagent, most commonly diborane (B2H6) or a borane adduct (BH3-THF), reacts rapidly at room temperature to afford, after oxidation, the /AMarkovnikov alkene hydration product. However, when the boron of the hydroborating agent is bonded to heteroatoms which lower the electron deficiency, as is the case in catecholborane (1,3,2-benzodioxaborole) 1 (Scheme 1), elevated temperatures are needed for hydroboration to occur.4 5... 

The hydroboration of alkynes gives predominantly cis isomer, as in the following example. Diborane adds as BH3 to alkynes in a cis isomer and forms vinyl boranes which react with acetic acid at low temperature to give cis-alkenes. 

The doso-carboranes with a C2 B3 skeleton are of particular interest because their structure and bonding have been described as classical and non-classical. Koster et al. [49] reported the preparation of pentaalkyl-1,5-dicarba-doso-pentaboranes by hydroboration of dialkyl(l-alkynyl)boranes with tetraalkyl-diboranes(6) (R2BH)2. By using a large excess of (Et2BH)2 as a hydride bath the route to pentaethyl-1,5 -d ic ar ba- doso-pe n tabo ra n e(5) was found to proceed via the 1-carba-aradwo-pentaborane(lO) [50] (see also Section 3.2.2.4 [33]). 

Hydroboration of alkenes in non-ethereal solvent has been reported using diborane generated in situ from a quaternary ammonium borohydride and bromoethane (see Section 11.5). Almost quantitative yields of the alcohols are reported [e.g. 1 ]. As an alternative to the haloalkane, trimethylsilyl chloride has also been used in conjunction with the ammonium borohydride [2]. Reduction of the alkene to the alkane also occurs as a side reaction (<20%) and diphenylethyne is converted into 1,2-diphenylethanol (70%), via the intermediate /ra 5-stilbene. 

Phase-transfer generated diborane has been used for the hydroboration of alkenes and their conversion into alcohols [1,2] and the procedure has also been employed for the cleavage of formamido compounds to yield the amines [3]. Cyclododecan-1,3- and 1,4-diones have been obtained in a 3 1 ratio and overall yield of 59% via... 

Reduction and hydroboration reactions using in situ generated diborane... 

The stereochemical outcome of the hydroboration-oxidation of l,l -bicyclohexenyl depends on the amount of diborane used in the hydroboration. When 1.1 equiv is used, the product is a 3 1 mixture of C and D. When 2.1 equiv is used, C is formed nearly exclusively. Offer an explanation of these results. 

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