Sodium reaction with boranes

An atom or a molecule does not have to be positively charged to be an electrophile. Borane (BH3), a neutral molecule, is an electrophile because boron has only six shared electrons in its valence shell. Boron, therefore, readily accepts a pair of electrons in order to complete its octet. Thus, alkenes undergo electrophilic addition reactions with borane serving as the electrophile. When the addition reaction is over, an aqueous solution of sodium hydroxide and hydrogen peroxide is added to the reaction mixture, and the resulting product is an alcohol. The addition of borane to an alkene, followed by reaction with hydroxide ion and hydrogen peroxide, is called hydroboration-oxidation. The overall reaction was first reported by H. C. Brown in 1959. 

The reduction of epoxides withborane is noteworthy as it gives rise to the less substituted alcohol as the major product (7.96), in contrast to reduction with complex hydrides (compare with Scheme 7.71). The reaction is catalysed by small amounts of sodium or lithium borohydride and high yields of the alcohol are obtained. With 1-alkylcycloalkene epoxides, the 2-alkylcycloalkanols produced are entirely cis, and this reaction thus complements the hydroboration-oxidation of cycloalkenes described in Section 5.1, which leads to trans products. Reaction with borane in the presence of boron trifluoride has also been used for the reduction of epoxides and for the conversion of lactones and some esters into ethers. 

The boranes are an extensive series of binary compounds of boron and hydrogen, somewhat analogous to the hydrocarbons. The starting point for borane production is the reaction (in an organic solvent) of sodium borohydride with boron trifluoride ... 

A boron analog - sodium borohydride - was prepared by reaction of sodium hydride with trimethyl borate [84 or with sodium fluoroborate and hydrogen [55], and gives, on treatment with boron trifluoride or aluminum chloride, borane (diborane) [86. Borane is a strong Lewis acid and forms complexes with many Lewis bases. Some of them, such as complexes with dimethyl sulfide, trimethyl amine and others, are sufficiently stable to have been made commercially available. Some others should be handled with precautions. A spontaneous explosion of a molar solution of borane in tetrahydrofuran stored at less than 15° out of direct sunlight has been reported [87]. 

The reaction of 154 with chloroacetonitrile in the presence of potassium /-butoxide afforded 155. The nitrile group was reduced with borane methyl sulfide to the corresponding amine 156, which on treatment with sodium methoxide was transformed into the 1,4-oxazepine 157 (Scheme 23) <1996JOC6060>. 

The oxazolidin-2-ones 53 (R = H=CCH=CH2 or COEt) are obtained in a one-pot reaction of amino alcohol carbamates 52 with sodium hydroxide, followed by allyl bromide or propi-onyl chloride (94TL9533). A modified procedure for the preparation of chiral oxazolidin-2-ones 56 from a-amino acids 54, which avoids the hazardous reduction of the acids with borane and the intermediacy of water-soluble amino alcohols, is treatment of the methyl ester of the amino acid with ethyl chloro-formate to give 55, followed by reduction with sodium borohydride and thermal ring-closure of the resulting carbamate f95SC561). The 2-prop-ynylcarbamates 57 (R = Ts, Ac, Bz, Ph or allyl) cyclize to the methyleneoxazolidinones 58 under the influence of silver cyanate or copper(I) chloride/triethylamine (94BCJ2838). 

Octaborane(12) is a strong Lewis acid forming 1 1 adducts with ethyl ether, trimethyl amine and acetonitrile 223>. With sodium hydride in ether, approximately two thirds of a mole of hydrogen per mole of octaborane(12) is produced but the product is said to be apparently identical with the product of the reaction of sodium amalgam with octa-borane(12) which produces no hydrogen 223>. 

Difluoro-l-(tosyloxy)vinyllithium 586 has been employed as an acyl anion equivalent of the type 587. This anion was prepared by treatment of 2,2,2-trifluoroethyl tosylate with two equiv of LDA at —78 °C860 (Scheme 159). The reaction of intermediate 586 with carbonyl compounds, followed by acid hydrolysis, gave compounds 588 which, after basic treatment with sodium hydroxide, afforded ct-keto acids 589. The reaction of the intermediate 586 with boranes provided the corresponding borates, which have been used in the synthesis of fluorinated molecules861. 

The extremely broad functional group tolerance of the Pd-catalysed N-de protection of Aloe groups was a crucial design feature in a synthesis of the epoxy-quinol core of the Manumycin family of antitumour antibiotics [Scheme 8,80].195 Note the convenient generation of the Pd(0) catalyst in situ from reaction of dichlorobis(triphenylphosphine)palladium(II) with tributylstannane. The use of sodium borohydride and borane dime thy lamine complex is illustrated in Schemes 8.81193 and 8.82194 respectively. 

It should be noted that the reaction of K[BHJ with pyrazole cannot be stopped at the K[H3B(pz)] stage from an incomplete reaction only K[BHJ and K[H2B(pz)2] could be isolated. It has been possible, though, to synthesize the ion [H3B pz-3,5-(CHj) ] by reaction of the borane adduct of 3,5-dimethylpyrazole with sodium hydride, and from it by a carefully controlled reaction with pyrazole in N,N-dimethyl-acetamide to prepare the first example of an asymmetric poly(l-pyrazolyl)borate ligand, i.e., Na[H2B(pz) pz-3,5-(CH3)2 ]... 

Ignition or explosive reaction with metals (e.g., aluminum, antimony powder, bismuth powder, brass, calcium powder, copper, germanium, iron, manganese, potassium, tin, vanadium powder). Reaction with some metals requires moist CI2 or heat. Ignites with diethyl zinc (on contact), polyisobutylene (at 130°), metal acetylides, metal carbides, metal hydrides (e.g., potassium hydride, sodium hydride, copper hydride), metal phosphides (e.g., copper(II) phosphide), methane + oxygen, hydrazine, hydroxylamine, calcium nitride, nonmetals (e.g., boron, active carbon, silicon, phosphoms), nonmetal hydrides (e.g., arsine, phosphine, silane), steel (above 200° or as low as 50° when impurities are present), sulfides (e.g., arsenic disulfide, boron trisulfide, mercuric sulfide), trialkyl boranes. 

The stereoselective synthesis of the title compounds has been achieved. Thexylborane on reaction with 2 molar equivalents of 1-iodo-l-alkyne at 0 °C proceeds to near completion (88% for 1-iodo-l-hexyne) to form fully substituted organoborane (33), which upon treatment with 2 molar equivalents of sodium methoxide at 0 °C readily produces frunx-1,2,3-butatrienes (Eq. 102) The same reaction, however, with either 1-chloro or 1-bromo-l-alkynes is sluggish to form the thexyl-l-halo-l-alkenyl-borane . 

---