Alanes stereoselectivity

Monohydroalumination of terminal propargylsilanes (Chapter 7) with two equivalents of DIBAL proceeds in a stereoselective (9), though not regio-selective, manner to produce an approximately 1 1 mixture of the alkenyl-alanes (1) and (2). Protonolysis of the mixture yields the pure (Z)-allylsilanc... 

Reagents of choice for reduction of epoxides to alcohols are hydrides and complex hydrides. A general rule of regioselectivity is that the nucleophilic complex hydrides such as lithium aluminum hydride approach the oxide from the less hindered side [511, 653], thus giving more substituted alcohols. In contrast, hydrides of electrophilic nature such as alanes (prepared in situ from lithium aluminum hydride and aluminum halides) [653, 654, 655] or boranes, especially in the presence of boron trifluoride, open the ring in the opposite direction and give predominantly less substituted alcohols [656, 657,658]. As far as stereoselectivity is concerned, lithium aluminum hydride yields trans products [511] whereas electrophilic hydrides predominantly cis products... 

Primary and secondary alkyl bromides, iodides, and sulfonates can be reduced to the corresponding alkanes with LiBHEt3 (superhydride) or with lithium aluminum hydride (LiAlH4, other names lithium tetrahydridoaluminate or lithium alanate). If such a reaction occurs at a stereocenter, the reaction proceeds with substantial or often even complete stereoselectivity via backside attack by the hydride transfer reagent. The reduction of alkyl chlorides to alkanes is much easier with superhydride than with LiAlH4. The same is true for sterically hindered halides and sulfonates ... 

Julia S, Guixer J, Masana J, Rocas J, Colonna S, Annuziata R, Molinari HJ (1982) Synthetic enzymes , highly stereoselective epoxidation of chal-cone in a triphasic toluene-walcr-pol y (S )-alan i ne system. J Chem Soc [Perkin 1] 1317... 

Hydroalumination of 1-chloro-l-alkynes. Lithium aluminum hydride adds to 1-chloro-l-alkynes (1) regio- and stereoselectively to form the a-chlorovinyl alanates 2, which are moderately stable at 0°C. On methanolysis they are converted into (E)-l-chloro-l-alkenes (3). They can also be converted into (Z)-l-bromo-l-chloro-l-alkenes (5) and into (Z)-l-chloro-l-iodo-l-alkenes (6). 

Hydroaluminadon. A stereoselective approach to cycloheptenols is based on the cleavage of 8-oxabicyclo[3.2.1]oct-6-enes. The lack of generality for the fragmentation protocol plagued the method. Now a useful procedure consists of Nilcodl -catalyzed hydroalumination and treatment of the alane with i-BUjAlCl. This method is also appropriate for the asymmetric synthesis of cyclohexenols. 

A route to allyl vinyl thioethers with defined E- or Z-geometry at the vinyl double bond has been reported (Scheme 46) the key step is reaction of a vinyl alanate (89) or (90), produced stereoseleCtively, with an allyl thiosulphonate and proceeds with retention of geometry at the alane carbon. The products are potential substrates for thio-Claisen rearrangements. 

Ziegler-Natta-type systems have been used in a variety of organic synthetic procedures, for example the regiospecific and stereoselective carbometallation of alkynyl silanes, Scheme 14, and for the alkylation of various alkynols. Scheme 15. The ethylation and methylation of the olefinic linkage in 3-buten-l-ol has been studied, here the alkenol was incorporated into a TiC and alane system. With Et2AlCl the major products obtained were hexan-l-ol, 3-methylpentan-l-ol, rram-3-hexen-l-ol, and butan-l-ol. EtgAl gave no alkylation products, but... 

The reaction of tosylhydrazones of a -unsaturated enones with sodium boro-hydride in alcoholic solution leads not to reduction, but instead to elimination according to Scheme 35, to provide a synthesis of allyl ethers in good yields. Potassium carbonate or sodium alkoxides can replace borohydride as the base in this sequence. trans-A y ethyl ethers (74) may be synthesized stereoselectively by the addition of aluminium hydrides to 1-alkynes and subsequent reaction of the vinyl alane formed (Scheme 36). This route complements the same authors ... 

The known stereospecific zirconium-catalysed carboalumination of alkynes has been utilized to produce allylic alcohols by conversion of the alkenyl alane adducts into the ate complexes (28) and subsequent reaction with formaldehyde in a stereoselective process (Scheme 12)." Allylic methyl ethers are also available by quenching (28) with chloromethyl methyl ether. 

Influenced by the ZnMe2-assisted addition of terminal aJkynes to nitrones developed by Chavant et al. [119,120], Micouin et al. performed the same type of reaction using AlMes and obtained several propargylic hydroxylamines [121], For a stereoselective variant, Desvergnes, Py, et al. prepared the carbohydrate-derived nitrone 74, but only achieved high diastereoselectivities in the additirm of phenylacetylene (Scheme 14b) [122]. Moreover, the preformed alkynyl alane had to be used because in situ formation from AlMes and the aUcyne predominantly led to methyl addition. From the reaction of stereoisomeric nitrones of the type 74, it was concluded that the diastereoselectivity solely depended on the configuration at C-3. 

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