Acetylenes, hydroalumination

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... 

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). 

Conjugate reductions. This combination (usually 1 3 ratio) effects conjugate reduction of a,p-acetylenic ketones or esters to a, 3-enones or unsaturated esters at -50° with moderate (E)-selectivity. The HMPT is believed to function as a ligand to aluminum and thus to promote hydroalumination to give a vinylaluminum intermediate, which can be trapped by an allylic bromide (equation I).1 The re-... 

Kumar, S. S. Roesky, H. W. Hydroalumination reactions on acetylenes and nitriles in the synthesis of carbaalanes and imidoalanes an overview. Dalton Trans. 2004, 3927-3937. 

Telluro(seleno)ketene acetals have been synthesized by the Al/Te exchange reaction, as shown in the following scheme, starting from hydroalumination of acetylenic selenides. ... 

Alkynes are more reactive in hydroalumination than alkenes. Hence, unlike internal alkenes, internal acetylenes readily undergo hydroalumination. Side reactions, however, may occur. Proton-aluminum exchange in terminal alkynes, for instance, leads to substituted products, and geminal dialuminum derivatives are formed as a result of double hydroalumination. In addition, nonsymmetric alkynes usually give mixtures of regioisomers. Appropriate reaction conditions, however, allow selective hydroalumination. Thus, the addition of isoBu2AlH to alkynes is a... 

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... 

The system CpeTiCle/AlClBu is an efficient reagent for hydroalumination of disubstituted acetylenes in a regio-and stereoselective method for the synthesis of -alkenylchloralanes.1169 The reduction carbonylation of Cp2TiCl2 at room temperature by chloroaluminate molten salts or ionic liquids has been reported.1170... 

Terminal alkynes with tetrahydroaluminates lead to substitution of the acidic hydrogen (see 5.3.3.5.2). However, in the presence of a small amount of i-Bu2AlH more than 90 % hydroalumination product is obtained when NaAlH and phenyl ethyne react in diglyme . This is a useful method of trans-hydroalumination of acetylenes, mainly... 

The reaction of dialkylaluminum hydrides with mono- or di-substituted acetylenes provides the best route to alkenyldialkylaluminums . The hydroalumination of acetylene itself gives polymers Monohydroalumination of terminal acetylenes can be conducted either neat or in hydrocarbon solvents (< 50°C). The products are (E)-alkenylalanes as the result of a regio and stereoselective cis addition of Al—H to the C=C bond i- ... 

In donor solvents (EtjO, THF, RjN) the rate of hydroalumination is slower than with hydrocarbons. Nickel compounds catalyze the reaction, but isomerization may take place more readily. The regiospecificity of cis-hydroalumination of 1,2-disubstituted acetylenes with i-BujAlH is inferior to dialkylboranes (see 5.3.2.5.1) ... 

Alumination of terminal acetylenes starting from RjAlH is best carried out in the presence of tertiary amines. This reduces the rate of hydroalumination, and metallation becomes predominant ... 

Metallated acetylenes, obtained by hydrozirconation, hydroalumination or hydroboration, react with elemental iodine to give ( )-iodoalkene. Terminal alkynes 229 (R = octyl or decyl) add hydrogen iodide, generated from the boron triiodide/A,A-diethylaniline complex and acetic acid, in a Markovnikov sense to afford the iodoalkenes 230. cw-Addition of hydrogen iodide, produced in situ from trimethylsilyl chloride and aqueous sodium iodide, to a number of internal alkynes has been reported... 

Terminal acetylenes. The hydroalumination of 1-alkenes followed by treatment with bromopropadiene with catalysis by CuCl results in terminal acetylenes. 

The organometallic derivatives required for the introduction of a 15-deoxy p-chain, not otherwise oxygenated, were obtained most conveniently by hydroalumination of a terminal acetylene with diisobutylaluminum hydride and conversion in situ to the lithio alanate 1 required for conjugate addition. In the absence of other oxy functions in the p-chain this process produces 15-deoxyprostaglandins such as 1 and IM conveniently and in excellent yield, as high as 80-90% for the 11,15-bisdeoxy series (Scheme 18) (25,54). 

Hydrometallations of alkenes and acetylenes such as hydroboration, hydrosila-tion, hydroalumination, hydrostannation and hydrozirconation are useful synthetic reactions. Rhodium-phosphine complexes catalyze hydroboration of substituted styrenes with a catecolborane (Scheme 68). An extensive survey of the phosphine... 

Syntheses via lithium alanates Ethylene from acetylene derivs. via hydroalumination... 

Besides their application at the industrial level (e.g. in the Alfol process or in Ziegler—Natta polymerization catalysts), organoaluminums are the basis of various useful organic reactions. In fact, hydroalumination of acetylenes (a Cp2ZrCl2 catalyzed process when Me3Al is the metallation reagent) is particularly useful for the stereospecific synthesis of prostaglandin precursors. 

Hydroalumination. Many of the reactions reported during this year involve the /ra i -alkenylalanes, obtained by the hydroalumination of acetylenes with dialkyl-aluminium hydrides, and these are summarized in Scheme Treatment... 

Terminal acetylenes can be alkylated to give chain-lengthened internal acetylenes in good yield, either by the rapid addition of trialkylalanes in the presence of nickel(ii) catalysts or by electrochemical addition of organoboranes. Hydroalumination may be used in a convenient conversion of terminal olefins into the corresponding acetylenes. An improved procedure for the synthesis of cyclo-octyne from cyclo-octene by a bromination-dehydrobromination sequence has been reported. ... 

Hydroalumination of mono- and disubstituted acetylenes finds wide use in synthetic organic chemistry for the preparation of practically important trisubstituted... 

Unlike alkenes, disubstituted acetylenes are hydroaluminated under mild conditions by means of fBu2AlH and IBusAl without a catalyst However, often it is impossible to avoid undesired side reactions and the subsequent transformations of vinyl alanes. Hydrolysis of the vinylalanes gives rise to 1,2-disubstituted olefins of Z- or -configuration depending on the namre of the hydroaluminating reagent used [58], the structure of the initial acetylene [59], and the solvent [59]. 

The data presented in the literature indicate that in most cases, catalytic hydroalumination of acetylenes occurs under mild conditions to give alkenyl alanes, which are hydrolyzed to give Z-olefins in high yields. 

Terminal olefins may be converted to the a,p-unsaturated acid bearing one more carbon by hydroalumination followed by treatment with C02 9 Hydro-alumination of disubstituted acetylenes followed by carbonation can be controlled so as to produce isomerically pure cis- or trans-a, 3 -unsaturated acids39. 

---