Coordination of aluminum

Coordination of the aluminum atom of the reducing complex was proposed to take place both to the oxygen atom of the hydroxy group and to the nitrogen atom of the amino group. The asymmetric reduction of enamine perchlorates and ketimines with menthol and bomeol chiral auxiliary reagents (50,51) presumably involves coordination of aluminum to the nitrogen atom of the substrate. 

A new method for the synthesis of 2-substituted, as well as 2,4- and 2,5-disubstituted, cyclopentanones in 53-93% yield has been reported.81 For example, the Lewis acid catalyzed transformation of l-propanoyl-l-(4-tolylsulfanyl)cyclobutane gave 2-ethyl-2-(4-tolylsulf-anyl)cyclopentanone (1) in 93 % yield. The formation of the cyclopentanone is best explained by a mechanism which involves initial coordination of aluminum trichloride to the carbonyl oxygen, followed by ring expansion to form the sulfur-stabilized carbocation. Finally, migration of the ethyl group to the carbocation center regenerates concomitantly the carbonyl function.81... 

X-Ray studies of crown ether complexes of Al reveal some variation in Al—O coordination with ring size. In particular, 15-crown-5 forms (Me3Al)4-crown with Al—O = 200.5 pm, whilst dibenzo-18-crown-6 gives (Me3Al)2-crown with Al—O = 196.7 pm.428 Other facets of crown ether coordination of aluminum are considered in Section 25.1.5.6. 

The influence of calcination and different hydrothermal treatments on the coordination of aluminum in MCM-41 molecular sieves is investigated by 27A1 MAS NMR and FTIR spectroscopy using ammonia as probe molecule. Samples are further characterized by XRD and 29Si MAS NMR spectroscopy. It is found that non-tetrahedral Al species formed during calcination can be reversible transformed into tetrahedral coordinated framework Al by hydrothermal treatment. Re-inserted Al gives rise to Bronsted acidity. 

The TBS protecting group is selectively cleaved in the first step. The released propargylic alcohol is then reduced with Red-AI to an allylic alcohol. Reduction of the propargylic alcohol occurs selectively trims. Since during this reaction a five-membered ring system 41 is created through coordination of aluminum with the alkox-ide, it is assured that aluminum is located m the 3-position relative to the alkoxide. 

Solid AICI3 also contains six-coordinate aluminum. The layered lattice exhibits a rapid increase in electrical conductivity as the melting point (192.4 °C) is approached, at which temperature it rapidly falls to zero. This coincides with an 85% increase in volume, which results Ifom a change in coordination of aluminum from six to four. The melt consists of discrete AlyCle molecnles (11). These molecules predominate in die gas phase up to 400 °C, beyond which they dissociate to trigonal planar AICI3 monomers. 

An alternative model proposed by Northcott and Valenta involves an s-cis conformation of the dienophile as indicated in structure 171 [39]. Shielding of the re face by the iso-propyl group requires a change in the coordination of aluminum to the dienophile oxygen such that the aluminum is coordinated by the oxygen on the same side as the carbon substituent of the aldehyde. This model is consistent with the observation of high induction with dienophiles 159 and 161 in which the conformation is locked s-cis compared with that for dienophile 163 which is locked s-trans (Sch. 17). A combined quantum mechanics-molecular mechanics study has been performed on the... 

The role of sterically demanding ligand systems and their effect on the coordination of aluminum is conveniently illustrated when comparing the phenyl ligand with the mesityl ligand. Trimesitylaluminum, MessAl" (Mes = 1,3,5-trimethylphenyl), is prepared by reaction of dimesityl-mercury with aluminum metal (Equation (7)). 

The difference in the stereoselectivity for (64) and (65) is most easily rationalized on the basis of the transition state geometry, which is determined by the coordination of aluminum to the nitrogen lone pair of electrons. Scheme 4 details the effect that aluminum has on the transition state geometry and, therefore, the course of the overall reaction. 

In alkali fluoroaluminate solid compounds, aluminum is present only in octahedral coordination with fluorine. According to Spearing et al. (1994) and Smith and Van Eck (1999), their Al chemical shifts range between -13 and -1.4 ppm and are typically more shielded than the AlOe octahedrons in oxide compounds. Only a few studies report lower coordination numbers for Al in fluorides. Kohn et al. (1991) have described the Al MAS NMR spectra of glasses of jadeite mixed with cryolite in terms of the 5-fold and 6-fold coordination of aluminum at 22 and -5 ppm, respectively. Herron et al. (1993) reported a Al chemical shift at 49 ppm for the tetrahedral anion AIFJ in a [l,8-bis-(dimethylamino) naphtathalene H ] [AIFJ] saturated solution. 

The most common elements in the earth s crust (after oxygen) are Si and Al, Avith valences of +4 and +3, respectively. The radius ratios of Si and Al coordinated to are 0.29 and 0.36, respectively. Thus, Si can reside only in tetrahedral holes. The ion is also predicted to occupy tetrahedral holes, and does so in many primary minerals, but the radius ratio is sufficiently close to the limiting value of octahedral coordination that 6-coordination of AF" is common. In fact, under the low temperature and pressure conditions that prevail during secondary mineral formation, 6-coordination of aluminum is more common than 4-coordination. Tetrahe-... 

The chemical behavior of vinyl ketone is in sharp contrast to that of acrylonitrile. No polymerization of AN or methacryl-onitrile (MAN) took place with aluminum alkyl in the absence of Lewis bases. In the presence of HMPT, on the other hand, AN or MAN was polymerized to produce colorless polymers as in the case of vinyl ketone. The last named monomer had to be activated by the coordination of aluminum alkyl, but HMPT, a strong base, took away the aluminum alkyl from the coordination site of vinyl ketone monomer. 

From the examination of molecular models, we anticipated that the C methyl group of spiroketals 20, 21, 22 and 23 is large enough to interfere with the coordination of aluminum reagent... 

Notwithstanding the high free surface energy of comndum, which is believed to destabilize fine powders of this polymorph (114,160), the Lewis acid strength is usually reported to be weak (186,207,247,248,250,251). The weak acidity has frequently been attributed to the exclusively octahedral coordination of aluminum ion in the bulk, and the presumed resulting similar configuration of unsaturated cations at the surface. 

Hanada, T.,Aikawa, T.,and Soga, N. (1982) Coordination of Aluminum in Amorphous Sodium Aluminosilicate Films, J. Non-Cryst. Solids, 50, 397-405. 

Further coordination of aluminum alkyl to the anirnis is possible if the coordination number of the central atom is sufficiently large [46] ... 

Continuous aluminate glass fibers are formed in the presence of propane, but not in its absence. A viable mechanism of jet stabilization must therefore explain (1) the function of carbon which enters into the surface or skin of the molten jet, (2) the function of carbides and carbonates which are instantly formed in the molten jet surface, and (3) the increase in tetrahedral from octahedral coordination of aluminum atoms in the surface [51] before the fiber solidifies and secondary overgrowth with carbon can occur. 

Many ene reactions can be catalyzed by Lewis acids, particularly aluminum chloride and ethylaluminum chloride. Coordination of aluminum with the carbonyl group of an enophile increases its electrophilicity, making it possible to carry out ene reactions at room temperatures. Entry 4 illustrates this point. 

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