Hydroalumination catalysts

It is noteworthy that the transition metals that serve as hydroaluminating catalysts are also active in establishing the equilibrium between aluminum alkyls and their decomposition products, aluminum, hydrogen and alkene (equation 12). Accordingly, these metals, in addition to hafnium, niobium, vanadium, scandium and lanthanum, have found use as activators for the direct synthesis of aluminum alkyls (equation 12, to the left). Probably most of these metal salts will also be capable of accelerating the hydroalumination reaction. 

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

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... 

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]. 

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. 

Nickel catalysts promote the hydroalumination of alkenes using trialkylalanes R3AI and dialkylalanes such as BU2AIH as the aluminum hydride sources [9, 29, 30, 33]. However, exhaustive studies of the range of substrates capable of hydroalumination with these reagents has not been carried out. Linear terminal alkenes like 1-octene react quantitatively with BU3AI at 0°C within 1-2 h in the presence of catalytic amounts of Ni(COD)2 [30]. Internal double bonds are inert under these conditions, whereas with 1,5-hexadiene cycHzation occurs. 

An advantage of nickel catalysts over other metal systems is that the properties of the active species can easily be tuned by the addition of suitable ligands. For example, the presence of PPhj was shown to have a direct influence on the regiochem-istry of hydroalumination of 1,1-dimethylindene la [33]. While the reaction of BU2AIH with la gave a 4 1 mixture of regioisomeric products 13a/13b after deuterolytic workup, the same reaction carried out in the presence of PPh, yielded 13a and 13b in a ratio of >99 1 (Scheme 2-14). 

In addition to the enhanced rate of hydroalumination reactions in the presence of metal catalysts, tuning of the metal catalyst by the choice of appropriate ligands offers the possibility to influence the regio- and stereochemical outcome of the overall reaction. In particular, the use of chiral ligands has the potential to control the absolute stereochemistry of newly formed stereogenic centers. While asymmetric versions of other hydrometaUation reactions, in particular hydroboration and hydrosi-lylation, are already weU established in organic synthesis, the scope and synthetic utiHty of enantioselective hydroalumination reactions are only just emerging [72]. 

Alkynes are much more reactive toward hydroalumination than alkenes. Hence, they readily react with both dialkylaluminum hydrides and LiAlH4 under mild conditions in the absence of a catalyst [1]. However, it is not always possible to avoid side reactions and subsequent transformation of the vinylalanes formed in this transformation [81, 82]. In addition, ds-trans-isomerization of the metallated C=C bond can take place, thereby reducing the stereoselectivity of the overall reaction [83]. 

The regiochemistry of Al-H addition to unsymmetrically substituted alkynes can be significantly altered by the presence of a catalyst. This was first shown by Eisch and Foxton in the nickel-catalyzed hydroalumination of several disubstituted acetylenes [26, 32]. For example, the product of the uncatalyzed reaction of 1-phenyl-propyne (75) with BujAlH was exclusively ds-[3-methylstyrene (76). Quenching the intermediate organoaluminum compounds with DjO revealed a regioselectivity of 82 18. In the nickel-catalyzed reaction, cis-P-methylstyrene was also the major product (66%), but it was accompanied by 22% of n-propylbenzene (78) and 6% of (E,E)-2,3-dimethyl-l,4-diphenyl-l,3-butadiene (77). The selectivity of Al-H addition was again studied by deuterolytic workup a ratio of 76a 76b = 56 44 was found in this case. Hydroalumination of other unsymmetrical alkynes also showed a decrease in the regioselectivity in the presence of a nickel catalyst (Scheme 2-22). 

Mechanistically, the pre-catalyst Cp2TiCl2 104 is reduced to a [Ti]-H 105 species which is subsequently able to hydrotitanate the triple bond 106. A transmetallation from titanium to aluminum regenerated the [Ti]-H species to generate the (yy )-hydroaluminated compound 107 (Scheme 13). 

The catalyst efficiency of these hydroalumination varies from a turnover number (TON) of 20-91. It is possible that the catalyst is deactivated by the presence of oxygen and water. Examination of the 31P NMR spectrum of the catalyst indicates that the phosphine monoxide and dioxide are formed in the presence of nickel prior to the addition of the substrate. Rigorous exclusion of oxygen and water is necessary in all these reactions. The enantioselective nickel-catalyzed hydroalumination route to dihydronaphthalenols may prove to be particularly important. Only one other method has been reported for the enantioselective syntheses of these compounds microbial oxidation of dihydronaphthalene by Pseudomonas putida UV4 generates the dihydronaphthalenol in 60% yield and >95% ee.1... 

Negishi reported the zirconium-catalyzed enantioselective carboalumination of alkenes, which consisted of a hydroalumination/alkylalumination tandem process.133-135 This permits the asymmetric syntheses of methyl-substituted alkanols and other derivatives, typically with >90% ee, which represents an increase in ee value by 15% from the previously obtained 70-80%.136-138 The hydroalumination/zirconium-catalyzed enantioselective carboalumination of alkenes was carried out using (—)-bis(neomenthylindenyl)zirconium dichloride as the catalyst (Table 15).133... 

Hydroalumination. Titanocene dichloride is an effective catalyst for hydro-uluminution of alkenes and alkynes with his(dialkylamino)alancs5 and various complex aluminum hydrides. The adducts can be quenched with water or iodine. The reaction is satisfactory for terminal alkenes and internal alkynes, but is not clcun for internal alkenes and terminal alkynes. 

Hydroalumination of allylic alcohols or ethers. llydroalumination of these substrates under usual conditions (9. 276) proceeds poorly because of deoxygenation associated with titanium catalysts.3 The desired reaction, however, can be effected in 50 80" yield with ZrC l4 in the case of alcohols and Cp.ZrCL in the case of cllieis (equations 1 and II). 

In this chapter, recent advances in asymmetric hydrosilylations promoted by chiral transition-metal catalysts will be reviewed, which attained spectacular increase in enantioselectivity in the 1990s [1], After our previous review in the original Catalytic Asymmetric Synthesis, which covered literature through the end of 1992 [2], various chiral Pn, Nn, and P-N type ligands have been developed extensively with great successes. In addition to common rhodium and palladium catalysts, other new chiral transition-metal catalysts, including Ti and Ru complexes, have emerged. This chapter also discusses catalytic hydrometallation reactions other than hydrosily-lation such as hydroboration and hydroalumination. 

ASYMMETRIC HYDROALUMINATION WITH TRANSITION-METAL CATALYSTS... 

Nickel catalysts are well-known to promote hydroalumination of olefins, and the resulting organoaluminum compounds can be converted to the corresponding alcohols through oxidation with molecular oxygen [97J. 

C-E bond formation via hydroalumination, 10, 859 C-E bond formation via hydroboration, 10, 842 olefin cross-metathesis, 11, 195 terminal acetylene silylformylation, 11, 478 Chemspeed automated synthesizer, for high-throughput catalyst preparation, 1, 356 Chini complexes, characteristics, 8, 410 Chiral bisphosphanes, in hydrogenations on DIOP modification, 10, 7... 

Hydroboration proceeds without a catalyst, but hydroboration with less active catecholborane (225) is accelerated by catalysts. Usually 1,2-addition to conjugated dienes takes place, but the Pd-catalysed reaction of catecholborane (225) gives the 1,4-adduct 226. This reaction is not catalysed by Rh complexes [98], Hydroalumination of conjugated dienes catalysed by Cp2TiCl2 affords the allylic aluminium compounds 227 by 1,4-addition. The Pd-catalysed hydrostannation of isoprene with HSnBu3 affords the (Z)-2-alkenylstannane 228 with high regio- and stereoselectivities [99],... 

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