Boranes, reaction with dienes

A hindered borane particularly useful for reaction with dienes ... 

An AMI semiempirical method was used to investigate the Diels-Alder cycloaddition reactions of vinyl sulfenes with buta-1,3-dienes.156 The reactivity and stereoselectivity of vinyl boranes have been reviewed.157 Aromatic methyleneamines undergo reverse-electron-demand Diels-Alder reactions with cyclopentadiene, norbom-ene, and vinyl sulfides.158... 

Reaction of borane with the cyclic diene, cycloocta-1,5-diene, is used to prepare the synthetically useful borane 9-borabicylo[3.3.1]nonane (9-BBN)... 

Hydroboration of conjugated diene, e.g., monohydroboration of 1,3-cyclohexa-diene with disiamylborane95 yields the corresponding allylic boranes. A similar reaction with 9-BBN affords B-2-cyclohexenyl-9-BBN in 95% yield (Eq. 41). 

The dialkenylchloroboranes undergo the usual reactions of vinylic boranes, e.g., protonolysis with acetic acid gives olefins, oxidation with alkaline hydrogen peroxide provides the corresponding carbonyl compounds. However, the most useful reactions of these compounds are their ready conversion to the stereochemically pure (E, Z)-1,3-dienes by the Zweifel reaction with I2-NaOH 37>107.108> and into the symmetrical (E,E)- 1,3-dienes, 09 0), mono-olefins 1U) and 1,4-dienes (Chart 10). 

We now turn to the stereochemistry governed by a ring system, and we shall look at both nucleophilic and electrophilic attack, since usually they have similar stereochemical preferences rather than contrasting preferences. In addition to several reactions that are straightforwardly electrophilic attack, we shall see several which can be described as electrophilic in nature, like the reactions of alkenes with osmium tetroxide, with peracids, with some 1,3-dipoles, and with boranes, and of dienes with dienophiles in Diels-Alder reactions. Some of these reactions are pericyclic, the pericyclic nature of which we shall meet in Chapter 6. For now, it is only their diastereoselectivity that will concern us. 

Cyclic dialkylboranes are prepared by hydroboration of dienes with 1 1 HjB THF. Trifunctional boranes give with difunctional dienes polymeric organo-boranes. Fortunately, these products can be transformed into monomers or dimers by further reaction. Thus, the hydroboration product obtained from 1,3-butadiene is polymeric " . The initial products, which are dumbbell-shaped organoboranes, are cleaved by the remaining borane to give the polymer. 

Chiral (acyloxy)borane 29, easily prepared by mixing the respective monoacylated tartaric acid and phenylboronic acid, catalyzes reactions of simple aldehydes 28 with electron-rich dienes 27 with good to excellent enantiomeric excess80. 

Related techniques have been developed to prepare (Z-,Z)- (Z-, )- and ( -, )- dienes. Hydroboration of diacetylenes followed by protonolysis is a convenient route to (Z-,Z)-dienes, as in the conversion of 89 to 90. The requisite symmetrical diacetylenes are prepared by oxidative coupling with oxygen and cuprous chloride, as in the conversion of 1-cyclohexylethyne (78) to 89. Unsymmetrical conjugated dienes can be prepared by formation of a diacetylene ate complex, prepared from disiamylmethoxyborane by sequential reaction with different acetylides. A similar borane route to unsymmetrical diacetylenes uses dicyclohexyl methyl-thioborane. ... 

A pressure of 70 atmospheres of CO is required since hindered boranes, especially thexylborane, react sluggishly at atmospheric pressure. However, thexylborane reacted with cyclopentene to generate 188, and subsequent reaction with O-acetyl-hept-6-en-l-ol gave the trialkylborane (189). Carbonylation in the presence of water followed by oxidation gave the mixed ketone (190) in 73% yield. 33 In a similar manner, dienes can be transformed into the corresponding cyclic ketones. 34,135... 

The hydroboration of 1,3-dienes has also been reported, and these reactions generate Z-allylic boronic ester products. These reactions have been reported with palladium catalysts and are thought to occur through Tr-allyl intermediates. Reactions with butadiene and isoprene, followed by addition of the product to benzaldehyde, are shown in Equation 16.47. This equation also shows the presumed mechanism that proceeds by generation of a palladium allyl from the combination of diene and palladium hydride formed by oxidative addition of the borane. 

The preparations of lithium and sodium (cyclooctane-1,5-diyl)dihydro-borates(l-) in tetrahydrofuran proceed via isolable, stable etherates. These can be made solvent-free simply by heating under vacuum. 9-Borabicydo(3.3.1 ]-nonane dimer (9-BBN) can easily be prepared from cycloocta-1,5-diene by reaction with tetraethyldiborane(6), tetrahydrofuran-borane or dimethyl sulfide-borane. The synthesis of alkali metal (cyclooctane-1,S-diyl)dihydro-borates is achieved by addition of 9-BBN to a suspension of the alkali metal hydride in tetrahydrofuran. Lithium hydride reacts more slowly than sodium or potassium hydride. The reactions are brought to completion by heating under reflux. 

Several of the new reactions of unsaturated boranes are notable. Vinylboranes can be induced by a Pd catalyst to couple with aryl halides, giving substituted styrenes in high yield,and with vinyl bromides to give E,E-dienes stereo-specifically. " Allylboranes (60) give eryt/iro-alcohols (61) in high stereoselectivity on reaction with RCHO, and S-alkynyl-9-BBN react with RN=C=0 to give (62). ... 

---