Organoboranes for synthesis. 4 : Oxidation of organoboranes with pyridinium chlorochromate. A direct synthesis of aldehydes from terminal alkenes via hydroboration

The oxidation of trialkylboranes containing primary a1kyl groups with pyridinium chlorochromate (PCC) in methylene chloride provides the corresponding aldehydes in good yields. The stoichiometry for the oxidation of alcohols, borate esters and trialkylboranes with PCC has been examined. In view of the poor regioselectivity (only 94% primary alkyl groups) and functional group tolerance observed in the hydroboration with borane (BH₃.THF or BH₃.SMe₂), a more selective hydroborating agent, bis(3-methyl-2-butyl)borane (disiamylborane), was utilized for the preparation of aldehydes from terminal alkenes. However, the formation of 3-methyl-2-butanone as a by-product, and the requirement of six moles of PCC per mole of aldehyde are major disadvantages in this method. This difficulty was circumvented by employing monochloroborane-dimethyl sulfide for hydroboration. This reagent exhibits high regioselectivity (⪢ 99% primary alkyl groups) in the hydroboration of terminal alkenes. Oxidation of the resulting dialkylchloroborane following hydrolysis affords the desired aldehydes in satisfactory yields. Consequently, the hydroboration of terminal alkenes, followed by PCC oxidation, represents a direct convenient method for the transformation of alkenes into the corresponding aldehydes.     

Formylborane Formation with Frustrated Lewis Pair Templates

Boranes R₂BH react with carbon monoxide by forming the respective borane carbonyl compounds R₂BH(CO). The formation of (C₆F₅)₂BH(CO) derived from the Piers borane, HB(C₆F₅)₂, is a typical example. Subsequent CO‐hydroboration does not take place, since the formation of the formylborane is usually endothermic. However, an “η²‐formylborane” was formed by CO‐hydroboration with the Piers borane at vicinal phosphane/borane frustrated Lewis pair (FLP) templates. Subsequent treatment with pyridine liberated the intact formylborane from the FLP framework, and (pyridine)(C₆F₅)₂B? CHO was then isolated as a stable compound. This product underwent typical reactions of carbonyl compounds, such as Wittig olefination.     

Organoboranes—34 : General synthesis of chiral boronic and borinic esters via asymmetric hydroboration-displacement. carbenoidation of chiral borinic esters to acyclic ketones of high enantiomeric purities

Asymmetric hydroboration of appropriate alkenes with diisopinocampheylborane (Ipc₂BH) or monoisopinocampheylborane (IpcBH₂) produces intermediates that readily eliminate α-pinene on treat- ment with acetaldehyde, providing a direct, convenient route to chiral boronic esters of high enantiomeric purities. Mixed chiral trialkylboranes, readily prepared by stepwise hydroboration of appropriate alkenes with IpcBH₂, eliminate α-pinene on treatment with acetaldehyde under very mild conditions. The procedure makes readily available chiral borinic esters of high enantiomeric purities. The synthetic utility of chiral borinic esters is demonstrated by converting them into acyclic ketones including an alarm pheromone of the ant Monica mutica.     

Hydroboration with pyridine borane at room temperature

Treatment of pyridine borane (Py.BH3) with iodine, bromine, or strong acids affords activated Py.BH2X complexes that are capable of hydroborating alkenes at room temperature. Evidence is presented for an unusual hydroboration mechanism involving leaving group displacement. In contrast to THF.BH3, hydroboration with Py.BH2I selectively affords the monoadducts. The crude hydroboration products are converted into synthetically useful potassium alkyltrifluoroborate salts upon treatment with methanolic KHF2.     

Zirconium-Catalyzed Atom-Economical Synthesis of 1,1-Diborylalkanes from Terminal and Internal Alkenes

A general and atom-economical synthesis of 1,1-diborylalkanes from alkenes and a borane without the need for an additional H₂ acceptor is reported for the first time. The key to our success is the use of an earth-abundant zirconium-based catalyst, which allows a balance of self-contradictory reactivities (dehydrogenative boration and hydroboration) to be achieved. Our method avoids using an excess amount of another alkene as an H₂ acceptor, which was required in other reported systems. Furthermore, substrates such as simple long-chain aliphatic alkenes that did not react before also underwent 1,1-diboration in our system. Significantly, the unprecedented 1,1-diboration of internal alkenes enabled the preparation of 1,1-diborylalkanes.     

Borane-catalyzed hydroboration of substituted alkenes by lithium borohydride or sodium borohydride

In the presence of a catalytic amount of BH₃·Me₂S, TiCl₄ or Me₃SiCl, LiBH₄ or NaBH₄ are capable of hydroborating alkenes by following the unusual order of decreasing reactivity: tetramethylethylene > 1-methylcyclohexene > cyclohexene; the key step of the catalytic cycle is the exchange reaction between LiBH₄ and the mono- or dialkylboranes resulting from hydroboration of the more substituted alkenes with BH₃.     

Zirconium‐Catalyzed Atom‐Economical Synthesis of 1,1‐Diborylalkanes from Terminal and Internal Alkenes

A general and atom‐economical synthesis of 1,1‐diborylalkanes from alkenes and a borane without the need for an additional H₂ acceptor is reported for the first time. The key to our success is the use of an earth‐abundant zirconium‐based catalyst, which allows a balance of self‐contradictory reactivities (dehydrogenative boration and hydroboration) to be achieved. Our method avoids using an excess amount of another alkene as an H₂ acceptor, which was required in other reported systems. Furthermore, substrates such as simple long‐chain aliphatic alkenes that did not react before also underwent 1,1‐diboration in our system. Significantly, the unprecedented 1,1‐diboration of internal alkenes enabled the preparation of 1,1‐diborylalkanes.     

Organoboranes for synthesis. 9. Rapid reaction of organoboranes with iodine under the influence of base. a convenient procedure for the conversion of alkenes into iodides via hydroboration

The reaction of organoborane with iodine is strongly accelerated by sodium hydroxide. Organoboranes derived from terminal alkenes react with the utilization of approximately two of the three alkyl groups attached to boron, providing a maximum of 67% yield of alkyl iodide. Thus, hydroboration-iodination of 1-decene gives a 60% yield ofn-decyl iodide. Secondary alkyl groups, derived from internal alkenes, react more sluggishly and only one of the three alkyl groups attached to boron is converted to the iodide. Thus, the procedure applied to 2-butene provides a 30% yield of 2-butyl iodide. The use of disiamylboranebis-(3-methyl-2-butylborane, Sia₂BH) as hydroborating agent increases the yield of iodides from terminal alkenes since the primary alkyl groups react in preference to the secondary siamyl groups. Consequently, hydroboration of 1-decene with Sia₂BH, followed by iodination gives a 95% yield ofn-decyl iodide. The use of methanolic sodium methoxide in place of sodium hydroxide provides alkyl iodides in considerably higher yields. The combination of hydroboration with iodination in the presence of a base provides a convenient method for theanti-Markovnikov hydroiodination of alkenes. The base-induced iodination of organoboranes proceeds with the inversion of configuration at the reaction center, as shown by the formation ofendo-2iodonorbomane from tri-exo-norbomylborane.     

Boron Lewis Acid‐Catalyzed Hydroboration of Alkenes with Pinacolborane: BArF3 Does What B(C6F5)3 Cannot Do!

The transition‐metal‐free hydroboration of various alkenes with pinacolborane (HBpin) initiated by tris[3,5‐bis(trifluoromethyl)phenyl]borane (BAr^F₃) is reported. The choice of the boron Lewis acid is crucial as the more prominent boron Lewis acid tris(pentafluorophenyl)borane (B(C₆F₅)₃) is reluctant to react. Unlike B(C₆F₅)₃, BAr^F₃ is found to engage in substituent redistribution with HBpin, resulting in the formation of Ar^FBpin and the electron‐deficient diboranes [H₂BAr^F]₂ and [(Ar^F)(H)B(μ‐H)₂BAr^F₂]. These in situ‐generated hydroboranes undergo regioselective hydroboration of styrene derivatives as well as aliphatic alkenes with cis diastereoselectivity. Another ligand metathesis of these adducts with HBpin subsequently affords the corresponding HBpin‐derived anti‐Markovnikov adducts. The reactive hydroboranes are regenerated in this step, thereby closing the catalytic cycle.     

The hydroboration of propargyl bromide. Simple one-Pot three-component routes to (Z)-1-bromoalk-1-en-4-ols and to anti-homoallylic alcohols

The hydroboration of propargyl bromide with dialkylboranes takes place regioselectively to give 3-bromoprop-1-en-1-yl dialkylboranes 13 which, upon quaternization with bromide ion, undergo a series of transformations into a number of allylic boron species. By a suitable choice of the experimental conditions it is possible to trap the reaction intermediates with aldehydes and to steer the process toward either the synthesis of (Z)-1-bromoalk-1-en-4-ols 6 or anti-homoallylic alcohols 8. Two one-pot three-component processes were developed based on a sequence of four reactions; preparation of dialkylborane and hydroboration of propargyl bromide are the first steps. Then, quaternization with TEBABr may be carried out either in the presence of the aldehyde when (Z)-1-bromoalk-1-en-4-ols 6 are requested, or in the absence of the aldehyde in order to allow the formation of gamma-substituted allyl borane 18 which, successively, adds to the aldehyde affording anti-homoallylic alcohols 8.     

Kinetics and mechanism of hydroboration of oct-1-and-4-ene by dimeric dialkylboranes

The kinetics and mechanism of hydroboration of oct-1-and-4-ene with a series of dimeric dialkylboranes was investigated. The kinetic results showed that the hydroboration of terminal olefins proceeds via a three-halves-order mechanism, first-order with respect to the olefin and one-half-order with respect to the dimer. Using dicyclohexylborane, diisopinocamphenylborane, and 3,6-dimethylborepane the observed rate constants for the hydroboration of oct-4-ene were approximately 6 times smaller than those for oct-1-ene. Supporting computations showed that both steric and electronic effects influence the rate of hydroboration of both internal and terminal olefins. A model computational study of the isomerization of oct-4-ene with di(prop-2-yl)borane showed that formation of the terminal hydroborated complex is thermodynamically favored over the internal complex.     

1,1‐Hydroboration and a Borane Adduct of Diphenyldiazomethane: A Potential Prelude to FLP‐N2 Chemistry

Diphenyldiazomethane reacts with HB(C₆F₅)₂ and B(C₆F₅)₃, resulting in 1,1‐hydroboration and adduct formation, respectively. The hydroboration proceeds via a concerted reaction involving initial formation of the Lewis adduct Ph₂CN₂BH(C₆F₅)₂. The highly sensitive adduct Ph₂CN₂(B(C₆F₅)₃) liberates N₂ and generates Ph₂CB(C₆F₅)₃. DFT computations reveal that formation of Ph₂CN₂B(C₆F₅)₃ from carbene, N₂, and borane is thermodynamically favourable, suggesting steric frustration could preclude carbene–borane adduct formation and affect FLP‐N₂ capture.     

Asymmetric Catalysis with Chiral Frustrated Lewis Pairs

The chemistry of frustrated Lewis pairs (FLPs) provides the most important approach for the metal‐free hydrogenation and hydrosilylations. Great progress has been achieved in this area for the past decade. Some promising results have also been obtained. This perspective article mainly focuses on the recent advances for the synthesis of chiral Lewis acidic boranes in category of three protocols, 1) hydroboration of chiral internal alkenes with Piers’ borane HB(C₆F₅)₂; 2) in situ hydroboration of chiral alkenes or alkynes without any purification; 3) and substitution reaction of (C₆F₅)_nBCl_3–n with chiral organometallic reagents, as well as their applications in the metal‐free asymmetric hydrogenations and hydrosilylations.

     

Hydroboration d'amines insaturees—II: Régio et stéréosélectivité de l'hydroboration-alkylation d'amines acétyléniques

The hydroboration reaction of acetylenic amines R¹R²NCH(R³)CCR⁴ was studied. We report the first results of a study of the reactivity of dialkylborane R₂BH towards these amines which allow us to propose a new method for the synthesis of differently substituted β-ethylenic amines. The regioselectivity and the stereoselectivity of this reaction are examined and allow us to set out the possibility of a trans hydroboration.     

Regioselective hydroborations of methylvinylchlorosilanes

The regioselectivity of the hydroboration of the methylchlorovinylsilanes, Cl_nMe_3?nSiCHCH₂ (n= 0 ? 3), has been investigated using BH₃←THF, 9-BBN, disiamyl- and dicyclohexylborane. Methylation of the adducts with methylmagnesium bromide is complicated by formation of tetraalkylboronates. Alkaline hydrogen peroxide oxidation of the boronates gives reasonable yields of the corresponding α- and β-trimethylsilylethanols forn= 0 and 1. Forn= 2 and 3, conversion of the adducts to the corresponding α- and β- deuteroethylsilanes by treatment with excess sodium methoxide in methanol-0-d provides a more effective means of derivatization. Addition of the alkenes,n= 2 or 3, to excess BH₃←THF givesca. 90% of the α-boro-organo-chlorosilanes. For all of the alkenes, the dialkylboranes giveca. 80% of the β-boron adducts.     

Selective Metal‐free HB(C6F5)2 Catalyzed Allene Cyclotrimerization: Formation of 1,3,5‐Trimethylenecyclohexane and Its Tris‐hydroboration Product

Allene is cyclotrimerized under metal‐free conditions with the borane HB(C₆F₅)₂ catalyst to selectively give 1,3,5‐trimethylenecyclohexane ( 3 a ). Three‐fold hydroboration of the 1,3,5‐cyclotrimer with Piers’ borane gives the all‐cis 1,3,5‐CH₂B(C₆F₅)₂ substituted cyclohexane product 14 .     

Molecular addition compounds. 17. Borane and chloroborane adducts with organic sulfides for hydroboration

The following sulfides have been examined as borane carriers in comparison with dimethyl sulfide and 1,4-oxathiane: tert-butyl methyl sulfide, isoamyl methyl sulfide, ethyl isoamyl sulfide, tert-butyl isoamyl sulfide, diisoamyl sulfide, tetrahydrothiophene, tetrahydro-thiopyran, thioanisole, 3-ethylthiotetrahydrofuran, bis(3-tetrahydrofuryl) sulfide, and bis(2-methoxyethyl) sulfide. Their complexing ability toward borane increases in the following order: thioanisole < ether-sulfides < dialkyl sulfides < dimethyl sulfide. Borane adducts of the sulfides are liquids above 0 degrees C. The thioanisole adduct loses diborane at room temperature. The reactivity of the adducts toward 1-octene increases in the reversed order of the complexing ability of the sulfides. Diisoamyl sulfide has a mild, ethereal, agreeable aroma, its synthesis is economical and the borane adduct, 4.2 M in BH3, is stable over prolonged periods at room temperature. The sulfide can be recovered from hydroboration-oxidation products by distillation. Consequently, diisoamyl sulfide is a new promising borane carrier. Bis(2-methoxyethyl) sulfide, easily synthesized from the low cost thiodiethanol, is three times more soluble in water than 1,4-oxathiane. Its borane adduct is 6.0 M in BH3 and can substitute for more expensive borane-1,4-oxathiane in hydroboration reactions. Applications of these new borane adducts in the synthesis of mono- and dichloroborane adducts was also studied. The equilibrium ratios observed for the new chloroborane adducts were similar to that observed for dimethyl sulfide adducts. However, the hydroboration of 1-octene with these new chloroborane adducts are much faster than the corresponding adducts of dimethyl sulfide, which are currently used extensively.     

Bis(1-Methyl-ortho-Carboranyl)Borane

The Lewis superacid, bis(1-methyl-ortho-carboranyl)borane, is rapidly accessed in two steps. It is a very effective hydroboration reagent capable of B−H addition to alkenes, alkynes, and cyclopropanes. To date, this is the first identified Lewis superacidic secondary borane and most reactive neutral hydroboration reagent.     

A Convenient Deoxygenation of Sulfoxides with CO(II) and NaBH4

Complex species, presumably transition metal hydrides, prepared from NaBH₄ and transition metal salts have been used in the reduction of nitro, cyano, halo and amido functional groups.¹ We recently reported that Co (II) and NaBH₄ in ethanol reduced alkenes and alkynes in high yields, and that the regent displayed remarkable steric selectivity in the reduction of alkenes: mono>di>tri- and tetra-substituted alkenes.² We now report that the same reagent could be conveniently employed for the efficient reduction of sulfoxides to sulfides. A number of conditions have previously been reported to deoxygenate sulfoxides to sulfides in varying yields: for example, SO⁹ ₃, etc.     

An Alkene‐Promoted Borane‐Catalyzed Highly Stereoselective Hydrogenation of Alkynes to Give Z‐ and E‐Alkenes

The stereoselective hydrogenation of alkynes to alkenes is an extremely useful transformation in synthetic chemistry. Despite numerous reports for the synthesis of Z‐alkenes, the hydrogenation of alkynes to give E‐alkenes is still not well resolved. In particular, selective preparation of both Z‐ and E‐alkenes by the same catalytic hydrogenation system using molecular H₂ has rarely been reported. In this paper, a novel strategy of using simple alkenes as promoters for the HB(C₆F₅)₂‐catalyzed metal‐free hydrogenation of alkynes was adopted. Significantly, both Z‐ and E‐alkenes can be furnished by hydrogenation with molecular H₂ in high yields with excellent stereoselectivities. Further experimental and theoretical mechanistic studies suggest that interactions between H and F atoms of the alkene promoter, borane intermediate, and H₂ play an essential role in promoting the hydrogenolysis reaction.