Hydroalumination, of alkynes

In general, transition metal-catalyzed hydroaluminations of alkynes occur in a syn fashion, i.e., both aluminum and hydride are added to the same face of the 7i-bond. Isomerization of the initially formed vinylalane is usually not observed under the mild reaction conditions used for these transformations. 

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). ... 

The hydroalumination of alkynes was first observed by Wilke and Muller in 1955, when they demonstrated that dialkylaluminum hydrides added to disubstituted alkynes stereospecifically in a syn fashion (13b). Subsequently, Zakharkin and coworkers were able to add NaAlH4 to phenylacetylene by employing 5% of Bu 2A1H and diglyme as a solvent, but did not ascertain the stereochemistry of reaction. Some 10 years later, in 1966, Slaugh found that LAH in refluxing THF-diglyme mixtures, hydroalumi-nates such alkynes exclusively in an anti manner (14 equation 6). ... 

The extent and types of interfering reactions during hydroalumination of alkynes were examined in Section 3.11.2.2 (equations 18-24). To a lesser degree two of these reactions are also encountered in A1—H additions to alkenes and cycloalkenes. Analogous to the reductive dimerization of alkynes (equation 23) is the dimerization of a-alkenes by Bu 2AlH, which can be conducted catalytically in hydride to give high yields of dimer (equation 30). ... 

The most extensive compilation of initial relative rates for the hydroalumination of alkynes by Bu 2A1H is presented in Table 4. Normalized for 35 C, such initial rates avoid complications setting in... 

Hydroalumination. The treatment of alkynes with diisobutylaluminum hydride in hydrocarbon solvents results in a aT-addition of the Al-H bond to the triple bond to produce stereodefined alkenylalanes. The hydroalumination of alkynes is more limited in scope than the corresponding hydroboration reaction of alkynes (see Chapter 5) with regard to accommodation of functional groups and regioselectivity. Whereas hydroalumination of 1-alkynes is highly regioselective, placing the aluminum at the terminal position of the triple bond, unsymmetrically substituted alkynes produce mixtures of isomeric alkenylalanes. 

Hydroalumination of alkynes has been well investigated and has provided a very convenient way of synthesizing vinylaluminum species stereoselectively (Scheme 6.131). A general reaction sequence can be used to prepare either or Z alkenes. The pro-... 

Hydro stannylation of terminal alkynes Hydroboration and Hydroalumination of Alkynes Hydroboration of alkynes Hydroalumination of alkynes Hydroalumination ofpropargyl alcohols Hydrosilylation... 

We have already seen examples of cuprates being used in stereochemically controlled conjugate additions to cyclopentenones and similar results can be achieved with ate complexes of vinyl alanes prepared by hydroalumination of alkynes to give e.g. 179 R = hexyl and hence the anti-A-product43 182. 

In 1954, Ziegler and coworkers [11,12] reported that traces of nickel salts dramatically alter the course of the growth reaction of ethylene with trialkylalanes, the Aufbau process. Instead of the low molecular weight polyethylene which was expected, the only product was butene. This observation culminated in Ziegler s discovery of transition metals that were highly effective in polymerizing ethylene, an accomplishment for which he later shared the Nobel Prize. It also opened the door to transition metal catalyzed hydroalumination reactions. In 1968, Eisch and Foxton showed that addition of nickel(II) salts increased the rate of the hydroalumination of alkynes by approximately 100-fold [13]. The active catalyst was believed to be a nickel(O) species. 

Hydroalumination of alkynes. The hydride reacts with alkynes in a hydrocarbon solvent by cis addition to yield a tran.t-vinylalane (1) from 1-alkynes and a cis-vinylalane from disubstituted alkynes. Treatment of the vinylalane with methyl-lithium in ether in a 1 1 ratio to give an ate complex (2) followed by carbonation... 

The hydroalumination of alkynes by i-BujAlH proceeds readily in the presence of BrCu-2 LiCl or CH3Cu-MgClAr-2 LiCl in THF at -20°C. 

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). 

Alkenyl iodides can be generated in situ by hydroalumination of alkynes, followed by iodination, and a,jS-unsaturated esters are prepared by carbonylation without isolation of the iodide. As an example, the propargylic alcohol 10 was aluminated regio- and stereoselectively and converted to the alkenyl iodide 11. The intramolecular carbonylation of 11 afforded the dibutenolide 12 in 81 % yield. The reaction is a key step in the total synthesis of (-l-)-parviflorin [9]. 

A valuable method for the synthesis of isomerically pure a/5-unsaturated nitriles via the hydroalumination of alkynes has been reported by Zweifel, Snow and Whitney [107]. Addition of di-isobutylaluminium hydride to an alkyne gives the trans-vinylalane (33) in 90% yield. Compound (33) does not react with cyanogen at room temperature but gives the vinylalanate (34) with methyl lithium. The latter reacts with cyanogen to given the trans-a,i3-unsaturated nitrile. 

Aluminium and Thallium.—The ( )-alkenyldialkylalanes which are obtained by hydroalumination of alkynes are synthetically useful as agents for transfer of the... 

The hydroalumination of alkynes using DIBAL-H (diisobutylaluminumhydride) is a general method for preparing various alkenylaluminum reagents [26]. Recently, it has been shown that this hydroalumination can be catalyzed with 3% Ni(dppe)Cl2 in THF at 22°C within 2 h. This hydroalumination tolerates some functional groups. Thus, the... 

Another methodology developed to circumvent the formation of Al-acetyUdes and their deleterious effect on conjugate additions is the selective Ni-catalyzed hydroalumination of alkynes [49]. Thus, by varying the Ni-catalysts high levels of a- or p-selectivity can be obtained and more important with very low levels of (<2%) of Al-acetyUde formation (Scheme 7). This methodology has been used to... 

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