Alane reaction with alkenes

Even under the most favorable conditions (with as low a temperature as possible and a high C2H4 concentration, i.e., pressures of 100 atm or more) there is one chain-breaking reaction with alkene formation for about every one hundred steps in the synthesis. In spite of this, however, the synthesis may be used as it stands for making trialkyl alanes with even-numbered straight chains containing 4-30 carbon atoms. 

Attempts to bring about the reaction of the simplest diene (allene) with dialkyl aluminum hydrides led to no definite organoaluminum compounds 123). Little is known so far about the reaction of alkenes containing conjugated double bonds (butadiene 290), isopropene, etc.) with dialkyl alanes. Diisobutylalane and butadiene gave products whose hydrolysis... 

A marked improvement was effected when it was found that the displacement reaction could be operated with trialkyl alanes and ethylene or suitable 1-alkenes (propene, 1-butene, etc.) even at 300°-350°C, if the components were allowed to react for only a very short time at a moderate pressure of alkene. In spite of the high temperature, no decomposition of the trialkyl alanes to aluminum, hydrogen, and alkenes took place. Moreover, in the high temperature reaction with so short a reaction time there were practically no side reactions, i.e., a-branched alkenes and such compounds were hardly detected (314, 326). 

Bis(diamino)alanes (R2N)2A1H were used for the hydroalumination of terminal and internal alkenes [18, 19]. TiCb and CpjTiCb are suitable catalysts for these reactions, whereas CpjZrCb exhibits low catalytic activity. The hydroaluminations are carried out in benzene or THF soluhon at elevated temperatures (60°C). Internal linear cis- and trans-alkenes are converted into n-alkylalanes via an isomerization process. Cycloalkenes give only moderate yields tri- and tetrasubstituted double bonds are inert. Hydroaluminahon of conjugated dienes like butadiene and 1,3-hexa-diene proceeds with only poor selechvity. The structure of the hydroaluminahon product of 1,5-hexadiene depends on the solvent used. While in benzene cyclization is observed, the reaction carried out in THF yields linear products (Scheme 2-10). 

When secondary alkyl alanes are heated above 100°C, equilibria are established which result in extensive isomerization to the 1-alkyl alanes (40, 94). The isomerization can be catalyzed by Ti(IV) or Zr(IV) salts (9). Since A1H addition to the C=C bond is more selective than for the corresponding organoboranes, it is possible to prepare 1-alkenes from alkenes with C=C bonds in an internal position. Simultaneously, however, the A1—C bond is able to react with the C=C double bond under these conditions (see Section V,C,l,a) i.e., synthetic reactions occur, which lead to considerable loss of the alkene used. With organoboranes this hardly occurs at all. 

Closely related to these results is the fact that trialkyl alanes of the triisobutyl type give the growth reaction only partially with ethylene 53). The concurrent reaction is the displacement to give triethylalane and an alkene of the isobutylene type ... 

Dienes may be transformed into alkenes by partial hydroalumination followed by hydrolysis. The reactions occur particularly unambiguously and often with almost quantitative yields if the dienes contain certain types of C=C double bond (see Section V,B,3) [e.g., 4-vinylcyclohexene — 4-ethylcyclohexene limonene —> l-methyl-4-isopropylcyclohexene 296) 2,6-dimethyl-2,7-octadiene — 2,6-dimethyl-2-octene 227). Cis hydroalumination of alkynes having G=C triple bonds in the middle of the chain leads via the corresponding alkenyl alanes to 1,2-cfs-disubstituted ethylenes 252). 

Preparation. - By Addition to Alkenes. Triethylborane and phenyl-borinic acid have been found to catalyse hydroalumination of alkenes, and reaction of the intermediate alane with atmospheric oxygen efficiently furnished alcohols resulting from anti-Markovnikov hydration.1 Procedures for the preparation of methylborane and dimethylborane and their use in the synthesis of... 

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