Aluminum aryloxides

Aluminum-alkynyl covalent bonds, characteristics, 9, 249-250 Aluminum-aluminum bonds in A1(I) compounds, 9, 261 in Al(II) compounds, 9, 260 Aluminum aryloxides, reactivity, 9, 254—255 Aluminum(III)ates, in organic group-selective transfers, 9, 279 Aluminum(I)-boron bonds, characteristics, 9, 263 Aluminum(III)-boron exchange, process, 9, 266 Aluminum-calix[4]arene catalyst, for alternating epoxide-CC>2 co-polymerization, 11, 617... 

Although dealing primarily with Al—O bond interaction in four-coordinate aluminum aryloxide systems, Barron (156) tried to define a normal Al—O bond (157) as a considerably polar covalent a bond with an additional ir interaction between the Op and Al-X a antibonding orbitals, weakening the latter in derivatives such as AlR (OAr)3 (L)x (n =0, 1, or 2 and = 0 or 1). 

ATPH (Scheme 2-5). Aluminum tris(2,6-di-/er/-butyI-4-methylphenoxide) (ATD) [15, 16] was first reported by Barron and co-workers. They exploited a two-step synthesis of ATD starting from LiAlH4 as an aluminum source. Apparently the structural modification of aluminum aryloxides that leads to numerous variants offers real advantages over conventional Lewis acids regarding preparation and handling. 

Despite steric hindrance, all monomeric aryloxide compounds reported to date have readily formed Lewis acid-base complexes, in which the aluminum is four-coordinate [106, 109, 110]. For their intrinsic attractive features, sterically hindered three-coordinate aluminum aryloxides have been developed and subsequently used as Lewis acid catalysts for stereo-, regio-, and chemo-selective carbon-carbon bond-forming reactions [111]. Compared with classical Lewis acids, these aluminum reagents coordinate strongly with various oxygen-containing substrates, and this coordination is affected by the steric environment of then-ligands. 

Metal-Oxygen Bond Lengths, M—O—C Angles, and Torsion Angles for Three-Coordinate Aluminum and Gallium Alkoxides and Aryloxides... 

Anwander et al. isolated and characterized the products which are obtained from the reaction of various highly substituted neodymium aryloxides with trimethyl aluminum. A reaction scheme is put forward which accounts for the whole variety of species isolated [185,234]. 

Scheme 85 Synthesis of heteroscorpionate aluminum alkyl and aryloxide complexes.

It is only with sufficiently sterically bulky ligands, such as 2,6-di-tertbutyl-4-methylphenolate (BHT), that monomeric A1 species are formed Me2Al(BHT) and MeAl(BHT)2 (12 and 13, Fig. 6) [106, 107]. The isolation of these monomeric compounds is undoubtedly attributable to the steric hindrance of the aryloxide precluding dimerization via bridging by the aryloxide moieties. For these compounds, the short Al-O distances and large Al-O-C bond angles observed in the solid state were proposed to arise from n-interaction between the vacant p orbital on aluminum and the lone pairs on the aryloxide oxygens (structure 14, Fig. 6). 

Although such a bonding scheme is compatible with the commonly accepted concept (i.e., the presence of any form of ft-bonding to a group 13 element would require a trigonal planar coordinatively xmsaturated metal center), the relative importance of jt-donation from the aryloxide to aluminum in this group of compounds has not achieved a consensus [106-109]. 

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