Aluminum compounds alkenylation

Several of the trialkylaluminum and alkylaluminum halides and hydrides mentioned above are commercially available. Alkynyl, alkenyl, cyclopentadienyl, and aryl derivatives are, in general, not commercially available and must be synthesized for laboratory use. Alkynyl derivatives can be prepared by salt metathesis, as in the reaction of Et2AlCl with NaC=CEt to give Et2AlC=CEt. The acidity of terminal alkynes is sufficient for preparation of alkynyl aluminum compounds by alkane or hydrogen elimination upon reaction with a trialkylaluminum or an aluminum hydride (equation 17), respectively. TriaUcynyl aluminum compounds are typically isolated as Lewis base adducts to stabilize them against otherwise facile polymerization. Alkenyl compounds of aluminnm have similarly been prepared. 

It was long believed that the isobutylalane does not give the growth reaction at all 299). This, however, was due to the fact that the reaction vessels employed always contained traces of a transition metal that catalyzed the displacement. With isoprene, isobutene is split off and polymeric alkyl aluminum compounds are formed which still contain some alkenyl groups. These are said to be less sensitive to air and moisture than the trialkyl alanes. The replacement is hastened by Ni as catalyst 61). 

There is ample evidence, however, that certain alkynyl- and alkenyl-aluminum compounds do exhibit inter- or intramolecular interactions between their w-bonds and aluminum centers (62, 87). 

The versatility of lithium aluminum hydride permits synthesis of alkyl, alkenyl, and arylsilanes. Silanes containing functional groups, such as chloro, amino, and alkoxyl in the organic substituents, can also be prepared. Mixed compounds containing both SiCl and SiH cannot be prepared from organopolyhalosilanes using lithium aluminum hydride. Reduction is invariably complete. 

One remarkable derivative of these cube-like clusters was isolated with compound 9, in which one vertex of the Alx cube remained unoccupied [18]. It was obtained by a reaction similar to Eq. (2), however, a slightly better steric shielding was achieved by ethyl groups instead of methyl substituents attached to aluminum [Eq. (5)]. Orange-red crystals of 9 were isolated in 36% yield. It may be described as the product of an incomplete hydroalumination reaction and contains seven aluminum atoms, a hydrogen atom bridging only three metal atoms and two alkenyl groups with C=C double bonds that are located at the open face of the cluster. As... 

ACF is very effective catalyst for other transformations via fluorinated carboca-tions, e.g. alkenylations [12], rearrangements [13], and isomerizations [14]. The advantage of ACF in this type of chemistry could partially come from the exceedingly high affinity of aluminum for fluorine, which enables the facile generation of a carbo-cation by C-F bond cleavage in organofluorine compounds. The authors focus on this aspect of aluminum Lewis acids later in the chapter. 

The substitution of aluminum by oxygen can be effected by various peroxide derivatives, such as di-r-butyl peroxide, benzoyl peroxide and r-butyl perbenzoate. The main reaction is accompanied by telltale free-radical side reactions, such as the formation of RR from R3AI, which become major pathways with aryl and vinyl compounds. Oxidation of alkenyl derivative (56) with r-butyl perbenzoate yields 45% of a mixture of cis and trans ethers (57 equation 44). As of yet there is no generally applicable, highly efficient method for oxidizing vinylaluminum compounds. 

H1 nuclear magnetic resonance measurements have established that, at least for the dialkyl alkynyl alanes, bridge bonds are formed exclusively by the a-carbon atom of the alkynyl group (30). This bond is stronger for the alkynyl than for the alkenyl compounds. In accordance with this, the dialkylalkynyl alanes may be distilled at reduced pressure as dimers without decomposition, whereas the corresponding alkenyl compounds decompose when heated and then undergo further reaction, in which addition of the A1—C bond to the C=C double bond occurs. The resulting aluminum alkyls disproportionate subsequently to trialkylalane and polymeric compounds (253). 

If 1-alkynes are reacted with 2 moles of R2A1H or the dialkyl-l-alkenyl alanes with the addition of a further mole of dialkyl alane, compounds are obtained with two dialkyl aluminum groups on the terminal carbon atom. Likewise 1 mole of dialkylalkynyl alane and 2 moles of dialkyl alanes give tris(l,l,l-dialkylalanyl) alkanes. Deuterolysis of fully hydroaluminated propynylalane, for example, thus gives 1,1,1-trideuteropropane (219) ... 

Activated aromatic compounds can be alkenylated with (Z)-2-bromo-l-(5-nitro-2-thi-enyl)ethylene in the presence of aluminum chloride to form (392) <86CCC il27>. 

Aluminum Reagents. Alkenylalnminnm compounds can also be used as reagents for alkenylation in reactions promoted by palladium. Formation of a 2-alkenylpyridine 216 is shown (Scheme 82). 

Two geranyl substituted flavones, albanins D and E, were isolated from Moms alba and had been assigned as 8-geranylflavones by Takasugi et al. using UV spectrometry (383, 415) since Sherif et al. had reported earlier that 6-alkyl(alkenyl)flavonoids do not exhibit the aluminum-induced shift due to chelation as a result of steric hindrance 399). However, the chemical shifts of the 5-OH of the Moms flavones indicated that the compounds were actually 6-geranylflavones. Synthesis... 

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