Acids reaction with boranes

Reaction of halo sulfonic acid esters with boranes 10-125 Alcoholysis of sulfonic acid derivatives 13-15 Vicarious substitution of aryl nitro compounds... 

Oxidation of trialkyl borates and of trialkoxyboroxines. Trialkyl borates, B(0R)3, prepared by reaction of primary and secondary alcohols with borane-dimethyl sulfide, are oxidized by PCC to aldehydes and ketones in good yield. This indirect oxidation of alcohols does not involve formation of water, which could be detrimental in some cases. Of even greater interest, carboxylic acids can be converted into aldehydes by reaction with borane-dimethyl sulfide to form a tri-alkoxyboroxine followed by oxidation with PCC (equation I). 

Electron-rich catalysts such as 38 are rather better than electron-poor ones. In contrast, those containing electron withdrawing groups, i.e., 39-41, are poor catalysts. A radical redesign of the catalyst structure led Wills and his coworkers to synthesize catalyst 45 derived from (S)-diphenyl prolinol [45]. In this case, upon reaction with borane, a borate ester would be formed and act as the Lewis acid site. 

There is a lot of information on the mercuration of heterocycles in the old literature, but it is seldom used nowadays due to the major disadvantages of toxicity and associated waste management it can be, however, a very useful reaction. The advantages of mercuration are that it can be carried out in hydroxylic and acidic solvents and in the presence of air, and that mercury in the product is easily replaced by ipso-substitution with other electrophiles, such as halogens, and gives boronic acids by reaction with borane. 

Telluroaldehydes have been generated and trapped for the first time by the reaction of benzylidenetriphenylphosphorane with "activated" tellurium (a method analogous to that previously used to prepare selenoaldehydes) (Scheme 22).71 A wide range of reactive ylides have been converted into the adducts (118) by reaction with borane.72 On heating, (118) rearrange to triphenylphosphine-monoalkylborane adducts (119) which undergo the expected hydroboration reactions with alkenes. A new route to phosphaalkenes (121) is available from the reaction of phosphinomethylenetriphenylphosphoranes (120) with Lewis acids.73 In the case of (120, R2=NPr 2) the compounds (121) can be isolated and in one case an X-ray structure was obtained. However, similar reactions of (120, r2=Bu ) lead to the dimers (122). 

In principle, any difunctional chiral compound, such as a diol, diamine, or amino alcohol, can form a cyclic boron derivative by reaction with borane, haloboronic acid or its esters, alkylboronic acids, trihaloboranes or similar compounds. Several syntheses of this type are described in chapters D.1.3.3.3. (together with applications for allylic additions to carbonyl compounds) and D. 1.1.2.1. (with applications of a-haloboronic acids). 

Nitrophenol 141 during alkylation with 2-bromo-2-methylpropionic acid ethyl ester in the presence of cesium carbonate in acetonitrile underwent Smiles rearrangement to afford the corresponding propionamide 142. Further upon reduction of 144, which was obtained from 141 in a three step reaction, with borane dimethyl sulfide complex in THF, a Smiles rearrangement was observed furnishing the propanol 146. ... 

Other methods for the preparation of 6.99 from leucine involved reduction of the acid moiety with borane and then oxidation to an aldehyde with chromium irioxide and pyridine. Another variation added zinc bromide to the enolate condensation reaction,75 which led to greater selectivity for the anti diastereomer. 

ANSWER (d) Hydroxide must be the Lewis base, but where is the empty orbital Where is the Lewis acid Recall the structure of the sp hybridized borane (p. 56). Boron is neutral, but there is an empty 2p orbital on boron, and borane is very definitely a Lewis acid. Reaction with the Lewis base hydroxide looks just like the reaction of the methyl cation with hydroxide (Rg. 2.59) except that the product is negatively charged. 

The desired pyridylamine was obtained in 69 % overall yield by monomethylation of 2-(aminomethyl)pyridine following a literature procedure (Scheme 4.14). First amine 4.48 was converted into formamide 4.49, through reaction with the in situ prepared mixed anhydride of acetic acid and formic acid. Reduction of 4.49 with borane dimethyl sulfide complex produced diamine 4.50. This compound could be used successfully in the Mannich reaction with 4.39, affording crude 4.51 in 92 % yield (Scheme 4.15). Analogous to 4.44, 4.51 also coordinates to copper(II) in water, as indicated by a shift of the UV-absorption maximum from 296 nm to 308 nm. 

B5H9 also acts as a weak Brpnsted acid and, from proton competition reactions with other boranes and borane anions, it has been established that acidity increases with increasing size of the borane cluster and that arachno-boranes are more acidic than nido-horancs ... 

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