Dienes hydroalumination

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). 

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. 

Tab. 2-2 Hydroalumination of a,co-dienes with BU3AI catalyzed by (PPh3)2PdCl2...

The results suggest that once hydroalumination has occurred at one end of the diene, the course of the reaction at the other double bond is significantly affected by the alkylaluminum group, possibly due to chelation in which a palladium-chloride-aluminum bond is thought to be important. 

Many of the characteristic features of hydroalanation of alkenes (reactivities, selectivities) are very similar to those of hydrosilylation. Terminal alkenes react readily in hydroalumination, whereas internal alkenes are much less reactive. Aluminum usually adds selectively to the terminal carbon. Hydroalumination of styrene, however, leads to a mixture of regioisomers.392 When hydroalumination of alkenes is followed by protolysis, saturated hydrocarbons are formed that is, net hydrogenation of the carbon-carbon double bond may be achieved. The difference in reactivity of different double bonds allows selective hydroalumination of the less hindered bond in dienes 393... 

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],... 

Dienes.1 (E)-l-Alkenylalanes, readily obtained by hydroalumination of 1-al-kynes,2 couple with vinyl iodides in the presence of this Pd(0) catalyst and zinc chloride to give 1,3-dienes with retention of the configuration. Alkenylzinc derivatives may be the actual reactants. Nickel catalysts are less stereoselective than palladium-phosphine catalysts. 

A variety of conjugated dienes can be synthesized stereospecifically via the hydroalumination of al-kynes. For example, the first-formed adduct from an alkyne can insert a second mole of alkyne and upon hydrolysis yield the ( , )-1,3-alkadiene (equation 23). Alternatively, the first-formed adduct can directly oxidatively dimerize to the ( , )-1,3-diene (equation 64). ... 

Generation of stereo- and regio-defined alkenylmetals via hydrometallation or carbometallation of alkynes followed by cross-coupling (Scheme 1-11) is a synthetically attractive notion for preparing arylated alkenes, conjugated dienes, and conjugated enynes. Its feasibility was demonstrated in 1976 [14,15] in the prototypical examples shown in Schemes 1-4 and 1-5. In these processes, hydroalumination was employed for generating the required alkenylmetals. 

Like hydroalumination and hydrozirconation, hydroboration of alkynes also provides a convenient and Stereospecific route to alkenyl metal reagents. However, initial attempts to achieve palladium-catalyzed cross-coupling of alkenylboranes with alkenyl halides were unsuccessful, due to the poor carbanionic character of these reagents. Later, Suzuki discovered that the desired transformation could be effected in the presence of an alkoxide or hydroxide base weaker bases, such as sodium acetate or triethylamine, were not generally effective. The reaction is suitable for the preparation of ( , )-, ( ,Z)- and (Z,Z)-dienes. Since reactions of alkenylboronates are higher yielding than those of alkenylboranes, the recent availability of (Z)-l-alkenylboronates " substantially improves the Suzuki method for the preparation of (Z)-alkenes. An extension of the methodology to the synthesis of trisubstituted alkenes has also been reported. " ... 

The reactivity of the alkenes toward LiAlH -TiCl decreases in the series CH2=CHR > CH2=CR2 > RCH=CHR. Thus dienes can be hydroaluminated selectively at the terminal double bond , e.g. ... 

Selective partial hydroalumination of dienes and trienes or their mixtures is possible because of the differences in the rate of A1H addition to various sorts of C=C double bonds. Further transformations of the alkenyl alanes open the way to some interesting possibilities in preparative work. For example, by partial hydroalumination of 4-vinylcyclohexene or limo-nene, wherein the A1—H bond reacts with the terminal double bond, followed by air oxidation, it is possible to obtain cyclohexenylethanol or 18-terpineol in good yield 312). A further example from terpene chemistry is the preparation of citronellol 225). 

Complete hydroalumination of dienes with two separate C=C double bonds of the same type should give aluminum compounds of the type R2Al(CH2)nAlR2, which can then react with ethylene (cf. Section V,C,1) and thus be transformed into long-chain bifunctional compounds. 

Intramolecular A1C addition to the C=C double bond is also observed to a small extent (ca. 1%) in the hydroalumination of 1,6-heptadiene (88). Such C—C coupling hinders the synthesis of long-chain bifunctional compounds from low molecular dienes through organoaluminum intermediates. In contrast to this, 1,7-octadiene gives only the bis-hydroalumination products. With 1,7-octadiene (which may be prepared by pyrolysis of cyclo-octene) (226) as the starting material, a route is available to the long-chain bifunctional bis-hydroalumination products and their derivatives (see Section V,C,l,a). 

Among the cyclodienes, the reaction of cyclo-l,5-octadiene (258) with (C2Hb)2A1H has been studied in detail (255). With 2 moles of diethylalane and 1 mole of diene a bis-hydroalumination product is first formed. This, on prolonged heating and subsequent hydrolysis, gives the bicyclic pentalane in high yield ... 

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). 

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