Boranes with oxygen

An extensive study of the copolymerization of bicyclo[2.2.1]heptadiene with vinyl monomers has been described (170). Radical initiators such as trialkyl-boranes with oxygen were employed and products well into the useful molecular weight range were obtained. On the basis of infrared evidence, all of the bicyclo[2.2.1]heptadiene was stated to be present in the polymers as nortricyclene units. Vinyl chloride copolymers were studied most extensively, but most of the other commercial vinyl monomers were also examined 170). 

The other two R groups then similarly migrate. Retention of configuration is observed in R. Boranes can also be oxidized to borates in good yields with oxygen, ... 

A relatively new method for thermal radical initiation has gained popularity in synthetic applications. A small amount of Et3B is added to the reaction mixture that has not been rigorously deoxygenated." " The borane reacts with oxygen, apparently generating ethyl radicals. A major advantage for this mode of radical initiation is the low reaction temperatures (as low as —78 °C) that can be achieved when stereoselective reactions are desired." ... 

An especially selective hydroborating reagent is prepared from 1,5-cyclooctadiene and borane. The product is a bicyclic compound of structure 1 (often abbreviated as 9-BBN), in which the residual B-H bond adds to unhindered alkenes with much greater selectivity than is observed with other hydroborating reagents. It is also one of the few boranes that reacts sufficiently slowly with oxygen that it can be manipulated in air. 

One important chemistry fallout from the creation of cylindrical names shall arise when the two anchor atoms normally do not have a coordination greater than two — such as is the situation with oxygen. In Chapter 6, after having developed properties of the boranes and the metallocenes, cylindrical nomenclature shall be applied to some supramolecular clusters. 

The boranes react very exothermically with oxygen and were once evaluated as potential fuels for rockets in the U.S. space program. 

The acetal is probably formed by hydride transfer to an intermediate ester. The terf-butyl group apparently stabilizes the second intermediate and consequently changes the course of the reaction. It should be noted that the first cyclic intermediate is stabilized by coordination of the borane with the oxygen of the carbonyl. The results are outstanding when = Me (92-98 %), although selectivity and yield (45-62 %) decrease when R = H (Eq. 48). 

Similar migration of the other two R groups and hydrolysis of the B-0 bonds leads to the alcohol and boric acid. Retention of configuration is observed in R. Boranes can also be oxidized to borates in good yields with oxygen," with sodium perborate NaBOs," " and with trimethylamine oxide, either anhydrous" or in the form of the dibydrate." The reaction with oxygen is free radical in nature." ... 

Earlier in this chapter we discussed the oxidation of boranes with hydrogen peroxide. When borane itself was used, the reaction was quite unambiguous. The trialkylborane 225 is the first product and when it is oxidised any of the three identical alkyl groups may migrate first to oxygen. 

Most dialkylhaloboranes as well as the trialkylboranes are liquid and are oxidized rapidly by atmospheric oxygen. ( Caution. Boranes with high vapor pressures such as chlorodiethylborane are spontaneously flammable.) The B-haloboranes are easily hydrolyzed. Therefore, in all experiments, air and moisture must be excluded by operation under an inert gas (N2, Ar). 

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. ... 

In the reaction of 1-hexene to give 56 as the major product, an essential component is easy to overlook. The ether solvent is important, and the solvent glytne is commonly used. The formal name of glyme is 1,2-dimethoxyethane and it has the structure CH3OCH2CH2OCH3. If borane is simply mixed with 1-hexene with no solvent, no reaction occru-s unless the mixture is heated to about 180-200°C. In ether solvents, however, the reaction occurs rapidly at ambient temperatures (i.e., room temperatru-e). In fact, ethers catalyze the reaction of borane with an alkene and, although the ether structure does not appear in the product, the ether is not simply a solvent. This is probably because the oxygen atom of an ether is a Lewis base (remember the ate complex formed from borane and diethyl ether) and reacts with diborane to form a highly reactive complex that reacts with the alkene. All reactions of alkenes with borane in this chapter will use an ether solvent. 

---