Al-X bonds

The broken bonds (boldface = dissociated fragment) BDEs (boldface = recommended data reference in parentheses) Methods (reference in References 

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The Ketmedy-Gillham scheme is also dii tly applicable to all t-BuX initiators. Since the X (halogen) becomes attached to the aluminum in the counteranion and since the Al-X bond is relatively strong, the nature of the halogen does not directly affect the identity of the organic products. Again, the rates of the various reactions may be quite different while the nature of the products remains the same. 

The deviation from tetrahedral valence angles at the aluminum atoms in the three species under discussion is adequately explained by the valence-shell electron-pair repulsion theory. The very polar Al-X bonds should require less space than the more covalent Al—C bonds, and the difference should be most pronounced in the fluoride. 

Another target reaction in the reduction terminal end of photoredox cycles has been the reduction of carbon dioxide. Pioneering works have been reported [393-403]. Interesting examples of photochemical fixation of carbon dioxide have also been reported for A1 porphyrins [404-406]. The insertion reaction of CO2 into Al-X bond (X = R, OR, SR, NR2) of Al porphyrin (3) was observed... 

The covalent nature of the aluminum-X bond suggests that polymerization occurs following a coordination process. This hypothesis is reinforced by the exclusive presence of regular head-to-tail enchainments ° and tactidty which is dependent on porphyrin stmcture. ° Indeed the substituents in ortho positions of peripheral phenyl groups affect the reactivity of the Al-X bond polar groups lead to a very high initiation activity, whereas nonpolar substituents result in an opposite effect. 

Indeed the polymerization of 2,3-epoxybutane with aluminum porphyrin revealed a coordinative process with inversion of configuration, which counters the above nondissociative reaction. Aida and Inoue proposed a mechanism involving two porphyrin molecules, one activating the monomer via coordination on the opposite face of the X group while the other proceeds as nucleophilic attack of the activated monomer (see Scheme 30). ° Initiation results from the insertion of the monomer into the Al-X bond leading to an aluminum alkox-ide which becomes the propagating species. Consequently, the synthesized polyether chains possess an X and a hydroxyl group at their extremities. 

Through the effect of the [HF]/[A1] molar ratio, it has been shown that the competition between hydroxyl and fluoride groups has a strong influence in the stabilization of the final compound. To understand the impact of the nature of the Al-X bond (X=OH, F) on the structure, conventional literature data (Table 2.4) are compared to those obtained by Rietveld refinement for the compounds prepared by microwave irradiation (Table 2.5). 

The polymerization of methacrylic esters with aluminum porphyrins (1 X = R SR ) is initiated by the conjugate addition of the Al-X bond of 1 to the monomers, generating Al-enolates (6) as the growing species. For example, the NMR spectrum of the pol3onerization system of tert-butyl ester of perdeuteriated methacrylic acid initiated with 1 (X = CH3) in benzene-d6 clearly shows the ap-... 

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