Hydroalumination of alkenes

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. 

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. 

Hydroalummation of Functional Croups 63 2.4.1.2 Enantioselective Hydroalumination of Alkenes... 

Some hydrometalation reactions have been shown to be catalyzed by zirconocene. For instance, CpiZrCf-catalyzed hydroaluminations of alkenes [238] and alkynes [239] with BU3AI have been observed (Scheme 8-34). With alkyl-substituted internal alkynes the process is complicated by double bond migration, and with terminal alkynes double hydrometalation is observed. The reaction with "PrjAl and Cp2ZrCl2 gives simultaneously hydrometalation and C-H activation. Cp2ZrCl2/ BuIi-cat-alyzed hydrosilation of acyclic alkenes [64, 240] was also reported to involve hydrogen transfer via hydrozirconation. 

Negishi reported the hydrogen transfer hydroalumination of alkenes with (/-Bu AKTIBA) and catalytic amounts of palladium and other late transition metal complexes.125 Although uncatalyzed hydroaluminations of alkenes with di-and trialkylalanes at elevated temperatures have long been known, their scope and limitations as well as their synthetic utility have not been extensively explored. 

Hydroalumination (cj8, 506-507). This zirconium compound can catalyze hydroalumination of alkenes with trialkylalanes containing /5-hydrogens, such as tri-isobutylalane and lri-/-butylalane.5... 

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... 

Hydroalumination of alkenes. Hydroalumination of alkynes is a well-known reaction, but hydroalumination of alkenes has been achieved only recently under catalysis by TiCU or ZrCU, (8, 288). As expected hydroalumination affords a convenient, high-yield route to primary alkanes (by hydrolysis), terminal primary alcohols (by oxygenation), and primary alkyl halides (reaction with halogens, N-halosuccinimides, or CuXa). ... 

This hydrozirconation/transmetallation sequence provides a route to net hydroalumination of alkenes and alkynes the direct reaction is often difficult to achieve. The use of catalytic amounts of Zr will be discussed in Section 3.9.4.2... 

The vast majority of work here is on hydroalumination of alkenes. Hydroboration (Volume 8, Chapter 3.10) generally proceeds perfectly well without any requirement for catalyst. One reason for employing a catalyst might be to allow the use of boron hydrides such as or NaBHi, which are somewhat... 

Hydroalumination of alkenes by LAH, catalyzed by Ti and Zr chloride complexes, was demonstrated by two Japanese groups (equation 58).Subsequently this reaction has been extended to other sources of A1—H and other catalysts. Further information on catalyzed hydroalumination may be found in Chapter 3.11 of this volume as well as in an earlier review.Hydrogallation by NaGaH4/[Cp2MCl2] (M = Ti, Zr) has been briefly examined and appears quite similar,little if anything is known about the synthetic utility of organogallium reagents. 

Only with reagents that attack all available carbon-aluminum bonds (H2O, D2O, O2, X2, SO2, etc.) will the hydroalumination of alkenic substrates and the derivatization of such adducts prove useful. That two of three C—Al bonds of adduct (23) are expended in the formation of side products, which have to be separated, may be an acceptable price to pay for the convenient, high-yielding generation of a desired specifically deuteriated reduction product, (24) or (25), ° primary alcohol (26), primary halide (27) or sulfinic acid. Of course the isobutylated side products should, in each case, be easily separable (Scheme 6). 

The hydroalumination of alkenes, either by direct addition of R2AIH or by aluminum hydride transfer from triisobutylaluminum (Scheme 3), proceeds readily and without catalysis with terminal alkenes, whether or not there is a chain branch in the -position (Table 1). 

Since the facial selectivity of the process is controlled by the chiral keto ester and the observed absolute diastereomeric excess of each addition reaction is determined by steric factors within the orga-noaluminate, there is no apparent need to prepare chiral alkoxytrialkylduminates. Thus, there should be very good stereodiscrimination for the additions of tetraalkylaluminates to chiral keto ester substrates (equation 9). It is advantageous to utilize tetraalkylaluminates since they are readily prepared by the titanium-catalyzed hydroalumination of alkenes (equation lO). ... 

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... 

Hydroalumination of alkenes and alkynes has been reviewed. From kinetic data it is concluded that diisobutylaluminium hydride, which is present as its trimer, initially dissociates to the monomer. In attacking the unsaturated bond, the monomer acts as an electrophile. This electrophilic character appears, for instance, from the high rate ratio of addition to PhC=CNMe2 and PhC=CPh, namely 19,000 1. 

Hindered t-alkyl halides with lithium triethylborohydride in the presence of chromous chloride furnish highly crowded coupled products in good yield (equation 111) . Alkanes obtained by hydroalumination of alkenes with LAH in the presence of titanium tetrachloride can be dimerized in the presence of cupric acetate to furnish alkanes (equation 112) ... 

The nickel-catalyzed hydroalumination of alkenes has been extensively investigated in a variety of contexts. The interaction of aluminum hydrides with nickel(0) and the mechanism of alkene hydroalumination has been studied in detail by Wilke, Eisch, and Zweifel. The most extensive synthetic applications from Lautens involve the hydroalumination of oxabicyclic alkenes followed by ring opening to produce substituted cyclohexenes and cycloheptenes (Scheme 74). - An as5mimetric variant emplo5dng 2,2 -bis(diphenylphosphino)-l,l -binaphthyl (BINAP) is also quite general for various oxabicyclic starting materials. 

Other Methods. Some immobilized Ti" species, supported on silica or polystyrene [e.g. (68)], have been examined as catalysts for the hydroalumination of alkenes with LiAlH4 (cf. 3, 152). They show more selectivity than soluble Ti" species for example mono-hydroalumination-bromination of dienes (69 n = 1, 2, or 4) (Scheme 35) can be accomplished more easily. 

Instead of MesAl, DIBAH-derived alkylaluminums via hydroalumination of alkenes were also applicable (Equation 60) [68]. 1-Decene is converted into n-decylaluminum (142) and subjected to ZACA reaction in the presence of isobuty-laluminum oxide (IBAO) to afford the corresponding optically active compound (143). 

In a related sequence, a new method for conversion of the tetra-alkyl-aluminate adducts (63), from titanium-catalysed hydroalumination of alkenes, into chlorides and bromides involves treatment with the Cu halides (Scheme... 

Scheme 4 Zr-Catalyzed hydroalumination of alkenes with liAlH4...

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