Aluminum derivatives

In order to accomplish the goals of this reviewer in a logical and systematic manner, the first systems to be described will be those of aluminum derivatives that appear to be the most readily discussed in terms of their structures and bonding, followed by the Group II derivatives of beryllium and magnesium, and, finally, by a discussion of the lithium derivatives and of the mixed-metal species, which clearly represent the most difficult systems to describe properly. 

The structures of substantial numbers of simple organoaluminum species have been reported during the past few years. The initial structure determined was that of trimethylaluminum (IV) (68). Subsequently, the structure has been redetermined in the solid state (116) and attempts have even been made to determine the location of the hydrogen atoms to provide details concerning the bonding present within this molecule (54). 

Bond Distances and Angles Found in Carbon-Bridged Organoaluminum Compounds 

Additional studies have been reported on the electron diffraction of both monomeric and dimeric AlMe3 in the gas phase, and a variety of other spectroscopic studies have appeared which all support structure IV (7). Several of the most important parameters for this structure and related carbon-bridged species are collected in Table II. 

The structures of several other symmetrical organoaluminum compounds, which have important implications with regard to electron-deficient bonding, have been determined from X-ray data. The first of these (VI) shows the structure of tricyclopropylaluminum dimer (84, 99). In this compound, the A1—C—A1 bridge is symmetrical and the cyclopropyl rings are oriented so that the p orbital on the bridging carbon atom may interact with the appropriate vacant orbitals on the metal atoms. This interaction appears to increase the stability of the bridge bond as indicated from variable-temperature NMR studies. The 

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Another group of isoprene polymerization catalysts is based on alanes and TiCl. In place of alkyl aluminum, derivatives of AlH (alanes) are used and react with TiCl to produce an active catalyst for the polymerization of isoprene. These systems are unique because no organometaHic compound is involved in producing the active species from TiCl. The substituted alanes are generally complexed with donor molecules of the Lewis base type, and they are Hquids or soHds that are soluble in aromatic solvents. The performance of catalysts prepared from AlHCl20(C2H )2 with TiCl has been reported (101). 

In the foUowiag cases, only those reactions ia which there is no chain growth, or at most dimerisation, are considered (see Olefin polymers). Alkyl titanium haUdes can be prepared from alkyl aluminum derivatives. The ring stmcture imparts regiospecificity to the ensuing carbometalation (216) ... 

A compound index constructed by these principles tells whether a given compound is present. It cannot provide information about compound classes, for example, all aluminum derivatives or all compounds containing phosphorus. 

We note that while tin reagents have often been employed for the organoboron halides/ the use of organostannanes as starting materials can also be applied to the synthesis of heavier group 13 derivatives. In the context of polyfunc-tional Lewis acid chemistry, this type of reaction has been employed for the preparation of ort/ o-phenylene aluminum derivatives. Thus, the reaction of 1,2-bis(trimethylstannyl)benzene 7 with dimethylaluminum chloride, methylaluminum dichloride or aluminum trichloride affords l,2-bis(dimethylaluminum)phenylene 37, l,2-bis(chloro(methyl)aluminum)phenylene 38 and 1,2-bis(dichloroalumi-num)phenylene 39, respectively (Scheme 16). Unfortunately, these compounds could not be crystallized and their identities have been inferred from NMR data only. In the case of 39, the aluminum derivative could not be separated from trimethyltin chloride with which it reportedly forms a polymeric ion pair consisting of trimethylstannyl cations and bis(trichloroaluminate) anions 40. 

While no aluminum derivatives featuring the 1,8-naphthalenediyl backbone have ever been isolated, several gallium species have been successfully prepared. With the... 

The main limitation of the coordination ROP of lactones remains the toxicity of the metal. For instance, aluminum derivatives are suspected to be involved in Alzheimer s disease, and tin(ll) bis-(2-ethyUiexanoate) is cytotoxic. In order to overcome this drawback, many groups have investigated the replacement of tin and aluminum alkoxides by initiators based on less toxic metals such as magnesium [54, 55] and calcium [56, 57] alkoxides. 

These calculations indicate that, for both the aluminum derivatives and for those formed by the Group II metals, one must consider metal-metal bonding interactions particularly through the use of d orbitals, but also take into account repulsion between these centers. A parameter related to these interactions is the metal-metal distance which on comparison with the sum of the metal covalent radii gives an indication of the relative magnitudes of these terms. Also, we must consider the metal-to-bridging atom distance, which must be related to the stability of the bond and should be compared with normal 2-electron bond distances between these same elements. Further, we should consider the electro-... 

As indicated in Section III,A, this does not appear to occur for aluminum derivatives but has recently been revived in discussions... 

An anomalous finding for the aromatic systems is the unusually low-energy exchange path recently reported for the tolyl aluminum derivatives it is accounted for by a difference in exchange mechanism and not by a decrease in the stability of the bridge bond (HO). 

Only a limited number of structural studies have been reported on beryllium compounds. The simple alkyls appear to be polymeric with chain structures as shown in XVI (109). For comparison, the structure of di-(t-butyl)beryllium (XVII) is shown as determined from electron diffraction studies (6). In this case, the compound is a linear monomeric species with a Be—C bond length of 1.699 A. Similarly, dimethylberyl-lium has a Be—C bond distance of 1.70 A in the gas phase (5). Comparison of these beryllium structures with the polymer shows that the Be—C distance in the bridge is considerably greater than that in a normal Be—C single bond, a result similar to that observed for the aluminum derivatives. 

A further point of interest is that in both the dimeric and trimeric species shown, the beryllium atom still has a vacant orbital available which may be used in adduct formation without disruption of the electron-deficient bond. This type of behavior leads to the formation of dimers with four-coordinate beryllium atoms, e.g., structure XX (86). This structure has been determined in the solid state and shows that the phenylethynyl-bridging group is tipped to the side, but to a much smaller extent than observed in the aluminum derivative (112). One cannot be certain whether the distortion in this case is associated with a it - metal interaction or is simply a result of steric crowding, crystal packing, or the formation of the coordination complexes. Certainly some differences must have occurred since both the Be—Be distance and Be—C—Be angle are substantially increased in this compound relative to those observed in the polymer chain. 

XXXVII we also see a bridging group with Al—C distances very close to that observed in other bridged aluminum compounds. The distance between the metal centers in this compound is similar to that observed in the simpler aluminum derivatives but greater than the sum of the covalent radii of the two metals (2.54 A), which may be an indication that Ti—Al interactions do not increase the stability of the bridged system. Structures on Cp2MMe2AlMe2 (M = Y, Er, and Yb) have been recently completed and clearly show stable electron-deficient bonds between the aluminum and the transition metal moiety (XXXVIII) (12). 

Under acidic reaction conditions the formation of isonitriles can compete efficiently with nitrile formation (Scheme 4.87) [377]. Particularly effective reagents for the formation of isonitriles are mixtures of Me3SiCN with Lewis acids such as Zn(II), Pd(II), or Sn(II) salts. Aluminum-derived Lewis acids with Me3SiCN, on the other hand, mediate the conversion of epoxides into nitriles [378, 379]. 

In 1926 Ponndorf3 reported that in independent work he also had evolved this new method of reducing aldehydes. He showed, further, that the reaction can be made more general by employing the aluminum derivatives of the more readily oxidizable secondary alcohols. By the use of aluminum isopropoxide, ketones as well as aldehydes could be reduced satisfactorily, the acetone formed being removed from the equilibrium mixture by slow distillation. 

The reaction involves the transfer of one valence bond of the aluminum atom and one hydrogen atom from the alkoxide to the carbonyl compound. The exact mechanism of this transfer is unknown, although an intermediate aluminum derivative of a hemiacetal (I) has been postulated.1 2- When isopropyl alcohol is the solvent the aluminum iso-... 

The reaction between an aluminum alkoxide and a ketone can be reversed. This is the basis of the Oppenauer oxidation of a secondary alcohol to the ketone.44 8 The aluminum derivative of the alcohol is prepared by mOans of aluminum t-butoxide and is oxidized with a large excess of acetone or cyclohexanone. 

Reduced pressure should not be used when low-boiling ketones are reduced and only a small excess of aluminum isopropoxide is used, since some of the product may be liberated from its aluminum derivative and lost through an exchange with the exoess isopropyl alcohol. 

Finally, work has been done on the exchange of alkyl groups between R2A1X and R2A1Y molecules in the presence of donor base molecules (51, 132, 133). It has been postulated that, depending on the base involved and the nature of the aluminum derivatives, two dominant pathways for exchange are possible. These are the same as those mentioned in Section IV,A,3 and will not be discussed further here. 

The most accessible synthesis of organohydrogermanes was based on the reduction of the corresponding organohalogermanes (R4 nGcX , n = 1-3) with complex hydrides such as LiAlH4173 183 230 237 318 335-342, NaBH4276 343, and LiAlH(OBu-t)3322,344. The less reactive lithium hydride and deuteride have been also recommended for this reduction270,345, and sodium hydride in the presence of boron or aluminum derivatives was also used. 

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