Boranes, trialkyl

In organic chemistry, LiH serves as a condensation agent. In the presence of trialkyl boranes very powerful reducing agents, LiBHR, which are soluble in THF, are obtained. These materials reduce aUphatic haUdes and in some cases highly stereospecific reductions can be accompHshed. 

Boronic acids RB(OH)2 were first made over a century ago by the unlikely route of slow partial oxidation of the spontaneously flammable trialkyl boranes followed by hydrolysis of the ester so formed (E. Frankland, 1862) ... 

Hydroboration preferentially gives the 2,5-tmns isomer resulting from the attack of a borane from the less hindered side of a molecule, and hydroboration does not proceed to the trialkyl borane stage. Further elaboration of these alcohols afforded (3-hydroxypiperidine alkaloids such as racemic pseudoconhydrine 120, racemic A -methyl pseudoconhydrine 121, and (—)-5-hydroxysedamine 122. 

Carbonsauren spalten dagegen, wahrscheinlich infolge eines Koordinationskomple-xes mit der Carbonyl-Gruppe, Trialkyl-borane viel leichter13,14 ... 

Ammoniak und prim. Amine spalten Trialkyl-borane nur bei hohen Temp, zu Koh-lenwasserstoffen und Amino-boranen1. Zur Protonolyse von Trialkyl-boranen ist Platin(II)-acetat ebenfalls geeignet2. 

Acrylsaure- bzw. 2-Methyl-acrylsaure-athylester kann nur in ungeteilter Zelle mittels Trialkyl-boran reduktiv an Platin in Acrylnitril/Tetrabutylammoniumjodid alkyliert wer-... 

Migration cannot occur past a quaternary carbon, however, since the required elimination is blocked. At equilibrium the major trialkyl borane is the least-substituted terminal isomer that is accessible, since this isomer minimizes unfavorable steric interactions. 

Reactions of 1-boraadamantane with alkynylsilicon and alkynylgermanium derivatives, in contrast to other trialkyl-boranes, proceed at room temperature to give 4-methylene-3-borahomoadamantane derivatives 89 and 90 quantitatively (Scheme 33) <2001JOM(620)51>. 

Whilst Lewis acids like SbCls or A1C13 form stable adducts with diphenyl cyclopropenone, from which the ketone can be regenerated unchanged208, trialkyl boranes effect a remarkable ring expansion to 2-phenyl indenone derivatives 309 containing an additional residue in the 3-position215. ... 

Similar compounds can be prepared from organic thiocyanates and trialkyl-boranes 33> as illustrated in Eq, (37). 

In the late 1960s, methods were developed for the synthesis of alkylated ketones, esters, and amides via the reaction of trialkyl-boranes with a-diazocarbonyl compounds (50,51), halogen-substituted enolates (52), and sulfur ylids (53) (eqs. [33]-[35]). Only one study has addressed the stereochemical aspects of these reactions in detail. Masamune (54) reported that diazoketones 56 (Ri = CH3, CH2Ph, Ph), upon reaction with tributylborane, afford almost exclusively the ( )-enolate, in qualitative agreement with an earlier report by Pasto (55). It was also found that E) - (Z)-enolate isomerization could be accomplished with a catalytic amount of lithium phenoxide (CgHg, 16 hr, 22°C) (54). 

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. ... 

Trialkyl-borane werden durch Elektrolyse in Nitro-methan mit Tetraalkyl-ammonium-halogeniden als Leitsalz in gutcn Ausbeutcn zu hohercn Nitro-alkanen umgesetzt. So fiihrt die Elektrolyse von Tricyclopentyl-boran zu Cyclopentyl-nitro-methan (150% bez. auf das eingesetzte Organoboran, 50%)1 ... 

The alkylation of activated halogen compounds is one of several reactions of trialkyl-boranes developed by H. C. Brown1560 (see also 5-12, 5-19, 8-24 to 8-28, etc ). These compounds are extremely versatile and can be used for the preparation of many types of compounds. In this reaction, for example, an alkene (through the BR3 prepared from it) can be coupled to a ketone, a nitrile, a carboxylic ester, or a sulfonyl derivative. Note that this is still another indirect way to alkylate a ketone (see 0-95) or a carboxylic acid (see 0-96), and provides an additional alternative to the malonic ester and acetoacetic ester syntheses (0-94). 

Litkiam trialkylborohydrides. The reaction of r-butyllithium with trialkyl-boranes provides a convenient route to lithium trialkylborohydrides.- ... 

You may have noticed that only one of the three alkyl groups of a trialkyl-borane is converted to an aldehyde by the carbonylation-reduction-oxidation sequence. To ensure that carbonylation takes the desired course without wasting the starting alkene, hydroboration is achieved conveniently with a hindered borane, such as 9-BBN, 12. With 12, only the least-hindered alkyl group rearranges in the carbonylation step ... 

When 26 reacts with c/s-2-butene in CH3OCH2CH2OCH2CH2OCH3 as solvent, a trialkyl borane is produced. Oxidation of this product with H202 yields isopinocampheol, 27, and (—)-2-butanol in 76% enantiomeric purity. Write equations for these reactions and account for the observed asymmetric synthesis. 

The resulting trialkylboroxines disproportionate reversibly into trialkyl-borane and boric oxide upon heating above 200° C (166). The method was later improved based on the equilibrium of Eq. (82)... 

Answer Since XV-d is not a methyl ketone only procedure XV-4 which requires an a-halo ketone and a trialkyl borane is available. 

One of the simplest classes of nucleophiles that attacks borane is that of alkenes. The result, described as hydroboration, is an overall addition of borane across the double bond. Unlike most electrophilic additions to alkenes that occur in a stepwise manner via charged intermediates (Chapter 20), this addition is concerted so that both new bonds are formed more or less at the same time. The result is a new borane in which one of the hydrogen atoms has been replaced by an alkane. This monoalkyl borane (RBH2) is now able to undergo addition with another molecule of the alkene to produce a dialkyl borane (R2BH) which in turn undergoes further reaction to produce a trialkyl borane (R3B). All these boranes have a vacant p orbital and are flat so that repeated attack to produce the trialkyl borane is easy and normal if an excess of alkene is present. 

Hydroboration-oxidation is normally done via the trialkyl borane... 

So far we have shown all reactions taking place on the monoalkyl borane. In fact, these compounds are unstable and most hydroborations actually occur via the trialkyl borane. Three molecules of alkene add to the boron atom three oxidations and three migrations transfer three alkyl groups (R = 2-methylcyclopentyl) from boron to oxygen to give the relatively stable trialkyl borate B(OR)3, which is hydrolysed to give the products. 

If we have a mixed trialkyl borane, you may be concerned about which of the alkyl groups migrates—the usual answer is that they all do Oxidation proceeds until the borane is fully oxidized to the corresponding borate, which then breaks down to give the alcohols. 

Sequential treatment of an ethynylalkanol acetate with butyllithium and a trialkyl-borane produces an allenic borane (23), which when protonated with acetic acid forms an allene, while upon treatment with water affords an acetylene derivative, presumably via a cyclic process (Eq. 51) A novel synthesis of homopropargylic... 

---