Carbon monoxide with boranes

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

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. 

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. 

B2H6 -I- 2N(C2H5)3 —> 2(C2H5)3N BH3 B2H6 -I- (CH3)2S —> 2(CH3)2S BH3 Boranes react with carbon monoxide forming more than one adduct ... 

Many transition metal alkyls react with carbon monoxide to give acyl compounds. In all these cases the acyl derivatives can be detected at least by infrared methods and in most cases isolated. Molybdenum, manganese, rhenium, iron, cobalt, rhodium, nickel, palladium, and platinum alkyls, Grignard reagents, and boranes, all react with carbon monoxide, and one can explain the products from these on the basis of carbon monoxide inserting into the metal alkyl. 

The complexes of borane (BH3) itself with amines, phosphines, dialkyl sulfides and carbon monoxide, the first representatives prepared only in die 1930s were thoroughly discussed in several books and reviews.2,9,25-36 Therefore, in this section, only a brief summary will be given outlining the most basic aspects of these compounds, whereas complexes of substituted boranes will be discussed in more detail. 

Alkenylboranes also react with carbon monoxide in the primes of j ladium chloride and sodium acetate in methanol to yield oc, -unsaturated otrboxylic acid esters while retaining the original configuration of the double bonds of 1-alkenyl-boranes (Eq. 109)... 

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... 

The compound borane-carbonyl occupies a special position in any discussion of molecules derived from boron hydrides with boron-carbon bonds. Carbon monoxide reacts with diborane (20 atm/room temperature) to give an adduct of the borane group which does not rearrange or lose hydrogen (39) The adduct OC BH3 does, however, dissociate into carbon monoxide and diborane at ordinary temperatures. No carbonyls of other Group III acceptor molecules have been prepared so far 133), and it has been suggested 27, 58) that borane-carbonyl owes its existence to the ability of the hydrogen atoms of borane to transfer electrons to a vacant p,-orbital in carbon monoxide. 

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. 

Monomeric borane has a very short lifetime, but it is a strong Lewis acid and may be stabilized by forming complexes with Lewis bases. BH3 resembles a transition metal atom in a low oxidation state, in the sense that it can form complexes with, say, carbon monoxide and phosphorous trifluoride having negligible basicity. It has been suggested that the complexes of BH3 with... 

In addition to carbon monoxide, sodium cyanide or chlorodifluoromethane can serve as the one carbon fragment in conversion of trialkylboranes to trialkylcar-binols. In the former case, the borane is first treated with sodium cyanide, and the resulting adduct is then treated with trifluoroacetic anhydride. With chlorodifluoromethane, the other reagent required is a potassium t-alkoxide. An oxidation step (hydrogen peroxide) then yields the product. Formulate a mechanism for each system. 

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. ... 

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