Oxygen reaction with boranes

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." ... 

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." ... 

The regioselectivity comes from the first step. The boron s empty p orbital bonds to the more nucleophilic end of the alkene and hydride is transferred to give a borane. Reaction with alkaline H2O2 leads to migration of an alkyl group from boron to oxygen and eventually to the alcohol. 

There is a second effect. In the transition state in which the stronger Lewis acid complexes the carbonyl oxygen, the carbonyl group is a better electrophile. Therefore, it becomes a better hydride acceptor for Brown s chloroborane than in the hydride transfer from Alpine-Borane. Reductions with Alpine-Borane can actually be so slow that decomposition of this reagent into a-pincnc and 9-BBN takes place as a competing side reaction. The presence of this 9-BBN is problematic because it reduces the carbonyl compound competitively and of course without enantiocontrol. 

Crotyl silanes offer the possibility of diastereoselectivity in reactions with aldehydes in the same way as the corresponding boranes. The mechanism is completely different because crotyl trialkylsilanes react via an open transition state as the silicon is not Lewis acidic enough to bind the carbonyl oxygen of the electrophile. Instead, the aldehyde has to be activated by an additional Lewis acid or by conversion into a reactive oxonium ion by one of the methods... 

Ignition or explosive reaction with metals (e.g., aluminum, antimony powder, bismuth powder, brass, calcium powder, copper, germanium, iron, manganese, potassium, tin, vanadium powder). Reaction with some metals requires moist CI2 or heat. Ignites with diethyl zinc (on contact), polyisobutylene (at 130°), metal acetylides, metal carbides, metal hydrides (e.g., potassium hydride, sodium hydride, copper hydride), metal phosphides (e.g., copper(II) phosphide), methane + oxygen, hydrazine, hydroxylamine, calcium nitride, nonmetals (e.g., boron, active carbon, silicon, phosphoms), nonmetal hydrides (e.g., arsine, phosphine, silane), steel (above 200° or as low as 50° when impurities are present), sulfides (e.g., arsenic disulfide, boron trisulfide, mercuric sulfide), trialkyl boranes. 

Tris(methylseleno)borane and tris(phenylseleno)borane react with terminal alkynes in the presence of oxygen or in the presence of AIBN as radical initiator [97,98]. The mechanism of this reaction is not completely elucidated, however, the generation of phenylselenyl radical intermediates is assumed by the authors. When applied to enynes, this reaction furnish cyclic compounds. An approach to the frameworks of kainoids is depicted in Eq. (39). Treatment of the enyne... 

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