Hydroaluminations

Additioaal uses for higher olefias iaclude the productioa of epoxides for subsequeat coaversioa iato surface-active ageats, alkylatioa of benzene to produce drag-flow reducers, alkylation of phenol to produce antioxidants, oligomeriza tion to produce synthetic waxes (qv), and the production of linear mercaptans for use in agricultural chemicals and polymer stabilizers. Aluminum alkyls can be produced from a-olefias either by direct hydroalumination or by transalkylation. In addition, a number of heavy olefin streams and olefin or paraffin streams have been sulfated or sulfonated and used in the leather (qv) iadustry. 

The retrosynthetic elimination of olefinic stereocenters (E or Z) was illustrated above by the conversion 147 => 148 under substrate spatial control. It is also possible to remove olefinic stereocenters under transform mechanism control. Examples of such processes are the retrosynthetie generation of acetylenes from olefins by transforms such as trans-hydroalumination (LiAlH4), ci5-hydroboration (R2BH), or ci -carbometallation... 

Catalysts Prepared from Ni Hydroaluminate, Bull. Soc. Chim. Fr. (1969) 2963-2971 CONOCO Library Transl. TR 71-15. 

Internal propargylsilanes undergo hydroalumination much less successfully. However, hydroboration proceeds without difficulty ... 

The DPE reduction is used as a test reaction to characterize the materials and optimize the preparation conditions of the catalyst. Since hydroaluminations can also be used for the synthesis of carboxylic acids, deuterated products, or vinyl halides via quenching with CO2, D2O or Br2 [44], the method is also a valuable organic synthesis tool. However, as compared with molecular catalysts like Cp2TiCl2 that are known to catalyze hydroaluminations [44], the titanium nitride materials described here are solid catalysts and can be separated by centrifugation. Moreover, they can be reused several times, which is an advantage as compared to molecular catalysts. 

Identification and characterization of the intermediates was only recently realized by Uhl who reported the structure of several hydroalumination products [46]. In the case of DPE hydroaluminations, structural analyses or NMR investigations have not been carried out. We have therefore separated the intermediates from the catalyst and measured NMR spectra after various reaction times. Identification of the intermediates and assignment of the Hnes to particular structural fragments is difficult in that case, since the spectra show complicated multiplets which indicate oligomers. However, an important result from NMR data is that neither the lines of DPE nor signals of any of the stilbenes can be recognized in the spectra. Erom that observation, we conclude that an intermediate is formed in the course of the reaction, probably a hydroalumination product... 

The AAA of allylphosphates by vinyl aluminium reagents generated in situ by hydroalumination of terminal aUcynes with DIBAL-H in situ has recently been reported (Scheme 2.24). 

A variety of other addition reactions occurring regioselectively are also known. These include hydrocyanation, hydroalumination, hydrosilylation, and hydrozirconation. 

In contrast to the related organoboranes, which are mostly used in the addition to non-polar carbon-carbon multiple bonds, aluminum hydrides have found their widest use in organic synthesis in the addition reaction to polar carbon-carbon and carbon-heteroatom multiple bonds including carbonyl, nitrile and imino groups as well as their a,(J-unsaturated analogs. Although these reduction reactions are also sometimes referred as hydroalumination reactions in the Hterature, they are outside the scope of this review. 

In order to get reliable information about the yield of organoaluminum compounds formed in a transition metal-catalyzed hydroalumination reaction it is essential to derivatize the organometallic products by quenching the reaction mixture with electrophiles like D2O, O2 or halogens. It is often observed that hydrogenation... 

Early attempts by Asinger to enlarge the scope of hydroalumination by the use of transition metal catalysts included the conversion of mixtures of isomeric linear alkenes into linear alcohols by hydroalumination with BU3AI or BU2AIH at temperatures as high as 110°C and subsequent oxidation of the formed organoaluminum compounds [12]. Simple transition metal salts were used as catalysts, including tita-nium(IV) and zirconium(IV) chlorides and oxochlorides. The role of the transition metal in these reactions is likely limited to the isomerization of internal alkenes to terminal ones since no catalyst is required for the hydroalumination of a terminal alkene under these reaction conditions. 

In 1976, Sato reported the hydroalumination of terminal alkenes with LiAlH4 in the presence of ZrCh [13]. For example, 1-hexene was quantitatively converted into n-hexane at room temperature after hydrolytic workup, whereas no reaction occurred in the absence of a catalyst Halogenation of the reaction mixtures indicated that these reactions in fact proceed through organoaluminum intermediates. Later, TiCh was found to be an even more active catalyst [14, 15]. 

The hydroalumination of terminal alkenes by LiAlH4 catalyzed by titanocene and zirconocene dichloride, CpjTiCh and CpjZrCh, respectively, has been reported by Isagawa [16] and Sato [14]. Again, the titartium compound proved to be more active... 

Based on these observations the authors propose the following mechanism for the nickel-catalyzed hydroalumination (Scheme 2-4) During the catalysis process... 

T kCyclooctenyl)(cyclooctadiene)cobalt was also reported to catalyze the hydroalumination of terminal alkenes by BujAl [30]. It should be noted that organoboranes catalyze the addihon of CfAlH to alkenes [39-41] and aUenes [42, 43]. 

Tab. 2-1 Hydroalumination-iodinolysis ofl-decene with BUS3AI in the presence of late transition metal compexes ...

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). 

Hydroalumination of terminal alkenes using EtjAl as the hydride source must be carried out with titanium catalysts [24], since zirconium compounds lead to the formation of alumacyclopentanes [60, 61] (Scheme 2-11) and carbometallated products [62]. Suitable substrates for hydroalumination include styrene, allylnaphthalene and vinylsilanes. Only one of the ethyl groups in EtjAl takes part in these reactions, allowing the synthesis of diethylalkylalanes, which are difficult to obtain by other methods. 

The hydroalumination of alkenes with BujAlCl catalyzed by Cp2ZrCl2 produces higher dialkylaluminum chlorides, which cannot be prepared by non-catalytic hydroalumination (Scheme 2-12) [63-65]. Terminal alkenes, internal linear alkenes and cycloalkenes can serve as substrates at reaction temperatures increasing in this order. 1,5-Dienes react to give cyclized products. 

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