Dimethylaluminum complex

With the exception of a brief report of a dimethylaluminum complex [5], the coordination chemistry of the monomeric anion in (4) has not been investigated. By contrast, Stahl and co-workers have carried out extensive studies of both main group element and transition-metal complexes of the chelating dianion in the cube (7), which have been summarized in a recent review [9]. A noteworthy feature of the ligand behaviour of this N,N chelating dianion is the additional in-... 

A dimethylaluminum complex [HB(3,5-(CF3)2Pz)3]AlMe2 (Figure 22) of the highly fluorinated tris(pyrazolyl)borate ligand [HB(3,5-(CF3)2Pz3] was obtained from the reaction between the silver adduct [HB(3,5-(GF3)2Pz)3]Ag(THF)... 

The first step is a carbonyl ene reaction, also known in the literature as a Prins reaction.7 A Lewis acid activates formaldehyde (25) for attack on the double bond of 12. This results in zwitterionic intermediate 26, which leads to the ene product 27 in the form of a dimethylaluminum complex through 1,5-migration of a proton. This complex is unstable and spontaneously eliminates methane. Aqueous workup hydrolyzes aluminum alkoxide 28 to alcohol 24. 

Related Reagents. 2-(2-Iodophenyl)ethanethiol 2-(2-Iodo-phenyl)-2-methylpropanethiol 2-(2-Bromophenyl)-2-methylpro-panethiol 2 -Iodobiphenyl-2-thiol Dimethylaluminum Complex. 

Nickel(O) reacts with the olefin to form a nickel(0)-olefin complex, which can also coordinate the alkyl aluminum compound via a multicenter bond between the nickel, the aluminum and the a carbon atom of the trialkylaluminum. In a concerted reaction the aluminum and the hydride are transferred to the olefin. In this mechanistic hypothesis the nickel thus mostly serves as a template to bring the olefin and the aluminum compound into close proximity. No free Al-H or Ni-H species is ever formed in the course of the reaction. The adduct of an amine-stabihzed dimethylaluminum hydride and (cyclododecatriene)nickel, whose structure was determined by X-ray crystallography, was considered to serve as a model for this type of mechanism since it shows the hydride bridging the aluminum and alkene-coordinated nickel center [31]. 

In spite of the modest asymmetric induction it was concluded that at least one of the chiral ligands is coordinated to the nickel in the catalyticaUy active species. An alternative interpretation was given by Wilke and coworkers [29]. They could show that (methylsalicyhdene)dimethylaluminum forms a stable adduct with nickel(O) complexes. It was concluded that the asymmetric induction in Pino s experiment might be attributed to a complex in which the chiral Hgand is complexed to the Lewis acidic aluminum. 

Ene reaction of aldehydes. Aliphatic and aromatic aldehydes are not reactive enophiles however, in the presence of dimethylaluminum chloride, which serves as u mild Lewis acid catalyst and proton scavenger, ene reactions occur in reasonable to high yield. Use of C2HSA1C1 results in complex mixtures of products. This ene reaction is a useful route to homoallylic alcohols.2... 

Prins reaction (cf 10, 186-187). Dimethylaluminum chloride is an effective catalyst for the ene addition of formaldehyde (as trioxane or paraformaldehyde) to mono- and 1,2-disubstituted alkenes.5 When 1.5-2.0 equiv. of the Lewis acid is used, homoallylic alcohols are obtained, usually in high yield. y-Chloro alcohols, formed by cis-addition of -Cl and -CH2OH to the double bond, are sometimes also observed when only 1 equiv. of the Lewis acid is present. The advantage of this reaction over the Prins reaction (using HC1) is that m-dioxanes are not formed as by-products, because formaldehyde no longer functions as a nucleophile when complexed to the Lewis acid. 

Unfortunately, this reaction is a gross oversimplification of the series of reaction steps that occur during the hydrolysis reaction. Hydrolysis has been shown to proceed via the formation of an alkylaluminum-water complex, which subsequently eliminates methane to form a dimethylaluminum hydroxide complex. This rapidly associates to give dimers or larger oligomers in solution. In the case of f-butylalumoxane, some of the intermediate species have been isolated and characterized structurally (73-76). 

Cleavage and resolution of epoxides.1 The aluminum reagent obtained by reaction of (R)-l with diethyl- or dimethylaluminum chloride shows slight reactivity in reactions with epoxides, but the ate complex (2), prepared from 1, (C2H5)2A1C1, and lithium butoxide, converts cyclohexene oxide into the chlorohydrin (3) in 40% ee. 

Recently, Yamamoto et al. (310) have reported that pure methyl-(tris(triphenylphosphine))copper containing only complexed solvent molecules can be obtained by a method which was previously applied to the preparation of alkyl transition metal compounds. Copper(II) acetonylacetonate, dimethylaluminum ethoxide, and triphenylphosphine were reacted in the ratio 1 3 4 in either or toluene at — 10°C. The triphenylphosphine complex was far more stable than previously described (82) and more stable than uncomplexed methylcopper. An aluminum-copper exchange was also postulated (312) to explain the high yield of stereospecifically pure diene from the addition of copper(I) chloride to a vinylalane in THF at 25°-35°C. 

The anionic derivatives of cyclodiphosph(V)azanes, obtained by deprotonation of [RNH(Q)PNR]2 (Q = O, S, Se) with organolithium reagents are versatile ligands, forming unusual metal complexes. These cyclic dimers also react with A1Mc3 to form dimethylaluminum derivatives. ... 

Henderson and coworkers studied the reaction of MesAl with a series of aromatic ketones (25, 27, 29) to yield the precipitation of either dimethylaluminum enolates or alkoxides (see equations 9-11). In situ H NMR spectroscopic studies of the reaction between MesAl and acetophenone (29) revealed a complex mixture of products, whereas under the same conditions 2,4,6-trimethylacetophenone (25) reacts cleanly to give the corresponding enolate. The enolate compounds 26 and 28 were isolated and 26 as well as the representative alkoxide 30 were characterized by X-ray crystallography. Both 26 and 30 form dimers with a central AI2O2 core. Ab initio calculations at the HF/6-31G level indicated that both 26 and 30 are the thermodynamic products of the reactions. Equation 12 shows the alkylation and enolization reactions for the ketones 25 and 29 and... 

A simultaneous reduction-oxidation sequence of hydroxy carbonyl substrates in the Meerwein-Ponndorf-Verley reduction can be accomplished by use of a catalytic amount of (2,7-dimethyl-l,8-biphenylenedioxy)bis(dimethylaluminum) (8) [33], This is an efficient hydride transfer from the sec-alcohol moiety to the remote carbonyl group and, because of its insensitivity to other functionalities, should find vast potential in the synthesis of complex polyfunctional molecules, including natural and unnatural products. Thus, treatment of hydroxy aldehyde 18 with 8 (5 mol%) in CH2CI2 at 21 °C for 12 h resulted in formation of hydroxy ketone 19 in 78 % yield. As expected, the use of 25 mol% 8 enhanced the rate and the chemical yield was increased to 92 %. A similar tendency was observed with the cyclohexanone derivative. It should be noted that the present reduction-oxidation sequence is highly chemoselective, and can be utilized in the presence of other functionalities such as esters, amides, rert-alco-hols, nitriles and nitro compounds, as depicted in Sch. 10. 

Organoaluminum compounds are highly oxygenophilic, and hence are capable of forming long-lived monomeric 1 1 complexes with carbonyl substrates. For example, the reaction of benzophenone with McsAl in 1 1 molar ratio gives a yellow, long-lived monomeric 1 1 species which decomposes unimolecularly to dimethylaluminum 1,1-diphenylethoxide after some minutes at 80 °C or many hours at 25 °C [124]. 

This hypothesis has been supported by experiments with a modified bimetallic, two-center system. Initial treatment of bis(dimethylaluminum) derivative 188 with 1 equiv. MeLi generates a new amphiphilic alkylation system 199 which has both electrophilic and nucleophilic centers in one reagent (Sch. 150). This system is found to be much more effective than symmetric 188, and the carbonyl alkylation of aldehydes proceeds even at -78 to -40 °C. A similar process with monoalmninum derivative 190 and its complex with MeLi, however, gave a trace of methylation product 200 (R = Ph) (< 3 % yield), indicating that appropriate internal arrangement of the two metal centers is essential to achieve this remarkable rate enhancement in the new amphiphilic alkylation. 

Scheme 9 outlines the synthesis of a prostanoid intermediate (99) that relies on an intermolecular Nozaki process. It is important to note that unlike the intramolecular case described above, the intermolecular version of this protocol requires an aldehyde as the electrophilic trap however, it is interesting to note that there have been no reports of the addition of Lewis acid activated ketones (presumably, as a preformed complex which would be added via cannula at low temperature) to the preformed aluminum enolate. Finally, in this example, the conversion of enone (96) to adduct (98) is promoted by the less reactive dimethylaluminum phenyl thiolate and not the corresponding ate complex. 

A simultaneous reduction/oxidation sequence of hydroxy carbonyl substrates in the Meerwein-Ponndorf-Verley reduction can be accomplished by use of a catalytic amount of (2,7-dimethyl-l,8-biphenylenedioxy)bis(dimethylaluminum) (49). This represents an efficient hydride transfer from the sec-alcohol moiety to the remote carbonyl group and, due to its insensitivity to other functionalities, should find vast potential in the synthesis of complex polyfunctional molecules including both natural... 

Zirconocene complexes 705 that contain an acetylide ligand bridging between a main group metal (aluminum) and a transition metal (zirconium) are obtained by treatment of dimethyl zirconocene with (alkynyl)dimethylaluminum, (Equation (43)).529 In this reaction, an ( 72-alkyne)zirconocene complex is presumably formed in situ, and it is then trapped by the excess (alkynyl)dimethylaluminum to yield the final product. The molecular structures of the complexes 705 (R = SiMe3, Cy) contain a dimetallabicyclic framework, and one of the bridgehead positions is a planar tetracoordinate carbon center. In these complexes, the -C=CR bridge between zirconium and aluminum can be described as being mainly of /x-(cr-acetylide) character. 

These arrangements are presumably controlled in part by the intramolecular nature of the proton transfer stage of the reaction. While the same stereochemistry is obtained with a variety of Lewis acids, e.g., dimethylaluminum chloride, boron trifluoride-diethyl ether complex, tin(IV) chloride, the opposite preference is observed with bis[4-bromo-2,6-di-/m-buty]phen-oxy](methyl)aluminum (MABR)104 (see Table 6, entries 4-7 and the following example). 

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