Boranes, reaction with carbon monoxide

The BH3 molecule is not stable as a separate entity. This molecule can be stabilized by combining it with another molecule that can donate a pair of electrons (indicated as ) to the boron atom to complete the octet (see Chapter 9). For example, the reaction between pyridine and B2H6 produces C5H5N BH3. Another stable adduct is carbonyl borane, OC BH3 in which a pair of electrons is donated from carbon monoxide, which stabilizes borane. In CO, the carbon atom has a negative formal charge, so it is the "electron-rich" end of the molecule. Because the stable compound is B2H6 rather than BH3, the bonding in that molecule should be explained. 

Several alternative procedures have been developed in which other reagents replace carbon monoxide as the migration terminus.11 The most generally applicable of these methods involves the use of cyanide ion and trifluoroacetic anhydride (TFAA). In this reaction the borane initially forms an adduct with cyanide ion. The migration is induced by N-acylation of the cyano group by TFAA. Oxidation and hydrolysis then give a ketone. 

Not everyone wants to use carbon monoxide and a more convenient, though equally deadly, one-carbon reagent is cyanide ion. This does not require the high pressures often needed to make CO react.39 Cyanide forms stable ate complexes 247 with boranes and we shall use two different alkenes to illustrate this possibility. Thexyl borane 227 is essential for reaction with two different alkenes and it is better to use the more hindered alkene first. These precautions lead to a more selective reaction with the first alkene. 

Reaction with trialkylboranes. Carbon monoxide added to a solution of a trialkyl-borane formed in situ in diglyme solution reacts at 100° at atmospheric pressure the resulting organoboron intermediate is oxidized by hydrogen peroxide to a tri-alkylcarbinol in excellent yield.1 /-Carbinols with highly branched alkyl groups... 

Diborane is not unique among the boron hydrides in its ability to form a carbonyl. High pressure reaction of either pentaborane-f / or tetra-borane-10 with carbon monoxide forms a substance which behaves like borane-carbonyl in its manner of decomposition. The formula of this polyborane-carbonyl has recently been established as OC B Hg (mp, — 114.5° bp, 59.6°) 41a). It reacts with trimethylamine without release of carbon monoxide. 

The conversion of acetylenes into olefinic esters by use of addition reactions has been illustrated by the following two examples, (i) 1-Alkenyl boranes, which are readily prepared by the hydroboration of alkynes, are converted into a,fi-unsaturated carboxylic esters in good yield by reaction with carbon monoxide in the presence of palladium chloride and sodium acetate in methanol the process is carried out at atmospheric pressure and occurs with retention of configuration with respect to the alkenyl borane. (ii) Carboxylic acids add to acetylenes in the presence of silver carbonate to provide a novel synthesis of enol esters, which are formed in an 8 2 mixture of isomers. ... 

Borane-hydrogen peroxide sodium hydroxide Reaction of carbon monoxide with hydroborated ethylene derivs. 

Homologation of olefins by four carbon atoms is achieved by reaction of the derived borane (265) with buta-1,3-diene monoxide in the presence of catalytic amounts of oxygen. Trialkylboranes are also converted into the 4,4-dialkyl-c/j-but-2-ene-l,4-diols (266) in good yields by addition to a-lithiofuran and oxidation of the intermediate boroxarocyclohexenes (267) with hydrogen peroxide. ... 

---