Aluminum alkoxides reaction with alcohols

As indicated in Scheme 27, indoles may be alkylated by their acid-catalyzed reaction with alcohols. Similarly, r-butylation of pyrroles has been effected by the acid-catalyzed reaction with t- butyl acetate (B-77MI30502), and the diarylmethylation of 1-methylpyrrole from the acid-catalyzed reaction with the chromium trichloride complex of the diarylcarbinol has been described (78JA4124). The alkylation of indoles by alcohols in the presence of the aluminum alkoxide and Raney nickel appears to be efficient for the synthesis of 3-substituted indoles, but is less successful in the alkylation of 2-methylindole (79JHC501). The corresponding isopropylation of pyrrole produces 2,5-diisopropylpyrrole and 1-isopropylpyrrolidine, as the major products (79JHC501). 

The aluminum isopropoxide is not only a catalyst but is actually the reducing agent.322 Experiments with aluminum alkoxides derived from alcohols deuter-ated on carbon323 showed that the hydrogen is transferred to the carbonyl group directly from the alkoxide without participation of the solvent thus the reaction can be carried out also in the absence of 2-propanol, by using aluminum isopropoxide in the molten state or in toluene or other hydrocarbon. 

F.H. Oppenauer Oxidation. In oxidation reactions involving both Cr(VI) and DMSO reagents, the alcohol is converted to a complex in which the a-hydrogen of an alcohol can be removed as an acid. A classical and alternative method for the oxidation of alcohols focuses on the reversible reaction between ketones and metal alkoxides, which is especially effective when the metal is aluminuml34 xhe reversibility of the aluminum alkoxide reaction was first demonstrated by Verley 35 Ponndorf 36 for the reaction of a ketone with an aluminum alkoxide, which led to formation of a new aluminum alkoxide and a new ketone. In... 

The alkoxides of some metals of interest, such as aluminum, yttrium, and the rare earths, can be made by reaction with alcohol using a reaction catalyst (e.g., HgCh and Hgh) [16-18] ... 

The strategy of metal alkoxides synthesis is entirely related to the electronegativity of the element concerned. Some electropositive metals, such as alkali metals, alkaline earth metals, and lanthanides, react directly with alcohols. But some less electropositive metals such as magnesium and aluminum require a catalyst (I or HgCy for successful reaction with alcohols. Use of electrochemical synthesis by anodic dissolution of some metals or metalloids (Sc, Y, Ti, Zr, Nb, Ta, Fe, Co, Ni, Cu, Pb, Si, Ge, etc.) in dry alcohol performs a promising procedure because it does not produce any by-products except hydrogen gas. Another applicable method for the synthesis of some alkoxides (B, Si, Ti, Zr, Hf, Nb, Ta, Fe, etc.) is the reaction of their chlorides with alcohols which require a base such as... 

Direct Reaction in Inert Atmosphere (Argon or Nitrogen). Direct reaction with alcohols with evolution of hydrogen gas and formation of metal alkoxides is possible only for the most electropositive metals such as alkali, magnesium and alkaline earth, rare earth metals and aluminum. 

The oxidation of a hydroxyl group by an aluminum alkoxide-catalyzed hydrogen exchange with a receptor carbonyl compound is known as the Oppenauer oxidation. For oxidation of steroidal alcohols the reaction is generally... 

There are also reactions in which hydride is transferred from carbon. The carbon-hydrogen bond has little intrinsic tendency to act as a hydride donor, so especially favorable circumstances are required to promote this reactivity. Frequently these reactions proceed through a cyclic TS in which a new C—H bond is formed simultaneously with the C-H cleavage. Hydride transfer is facilitated by high electron density at the carbon atom. Aluminum alkoxides catalyze transfer of hydride from an alcohol to a ketone. This is generally an equilibrium process and the reaction can be driven to completion if the ketone is removed from the system, by, e.g., distillation, in a process known as the Meerwein-Pondorff-Verley reduction,189 The reverse reaction in which the ketone is used in excess is called the Oppenauer oxidation. 

The polarity of the Al-C bond allows easy derivatization of the five-coordinate aluminum alkyls by alkane elimination (Figure 9). For example, Salen aluminum alkyls LAlMe could be converted to dimeric or polymeric Salen aluminum phosphinates [LAl 02P(H)Ph ] 98 (n = 2 or oo, depending on the Salen ligand backbone)98,99 by reaction with phenyl phosphinic acid, Salen aluminum siloxides LA10SiPh3 by reaction with triphenyl silanol,96 or Salen aluminum alkoxides LAIOR by reaction with an alcohol.100... 

Functional diethyl aluminum alkoxides have been prepared by reaction of AlEt3 with an equimolar amount of the corresponding alcohol (XCH2OH). The... 

Considerable effort has been carried out by different groups in the preparation of amphiphihc block copolymers based on polyfethylene oxide) PEO and an ahphatic polyester. A common approach relies upon the use of preformed co- hydroxy PEO as macroinitiator precursors [51, 70]. Actually, the anionic ROP of ethylene oxide is readily initiated by alcohol molecules activated by potassium hydroxide in catalytic amounts. The equimolar reaction of the PEO hydroxy end group (s) with triethyl aluminum yields a macroinitiator that, according to the coordination-insertion mechanism previously discussed (see Sect. 2.1), is highly active in the eCL and LA polymerization. This strategy allows one to prepare di- or triblock copolymers depending on the functionality of the PEO macroinitiator (Scheme 13a,b). Diblock copolymers have also been successfully prepared by sequential addition of the cyclic ether (EO) and lactone monomers using tetraphenylporphynato aluminum alkoxides or chloride as the initiator [69]. 

Hydrogen transfer reactions from an alcohol to a ketone (typically acetone) to produce a carbonyl compound (the so-caUed Oppenauer-type oxidation ) can be performed under mild and low-toxicity conditions, and with high selectivity when compared to conventional methods for oxidation using chromium and manganese reagents. While the traditional Oppenauer oxidation using aluminum alkoxide is accompanied by various side reactions, several transition-metal-catalyzed Oppenauer-type oxidations have been reported recently [27-29]. However, most of these are limited to the oxidation of secondary alcohols to ketones. 

A wide range of aluminum alkoxides can easily be synthesized by the reaction of alcohols with triethylaluminum. These alcohols can even be substituted by compatible functional groups such as bromides, olefins, and tertiary amines (Fig. 10) [20, 21]. An alternative route towards aluminum alkoxides relies on the reaction of the alcohols with aluminum isopropoxide in toluene. Isopropanol (iPr) formed during this reaction is withdrawn by the distillation of the azeotrope made up of toluene and isopropanol [20, 21]. 

Carbamates have been prepared by heating ethyl carbamate with a higherboiling alcohol in the presence or absence of catalysts [31-33], Aluminum iso-propoxide has been reported [34] to be an excellent catalyst for the interchange reaction between ethyl carbamate and benzyl alcohol. The interchange reaction is also effective for /V-alkyl carbamates as well as unsubstituted carbamates [35]. This catalyst is effective in preparing mono- and dicarbamates in excellent yields from primary and secondary alcohols and diols. Other effective catalysts are dibutyltin dilaurate [36], dibutyltin oxide [37], sulfuric acid or p-toluenesulfonic acid [31], and sodium metal (reacts with alcohols to give the alkoxide catalyst) [33]. 

Instead of the displacement reaction, aluminum alkyls may be oxidized with dry air to produce aluminum alkoxides that, after hydrolysis with dilute sulfuric acid or water, yield linear primary alcohols with an even number of carbon atoms (fatty... 

Although formally involving the reaction with a C—H bond, metal alkoxides will react with /1-diketones and / -keto esters to form six-membered chelates and alcohols. Hence the acetyl-acetonate (acac) derivatives of aluminum can be obtained (equation 63).238... 

It is noteworthy that solubility of the formed metal alkoxide is in many cases even more important for the reaction than the values of the metal standard electrode potentials or the mobility of protons of the alcohol. The following examples illustrate this statement. Despite higher acidity of MeOH in comparison with EtOH(pK= 15.5 and 16, respectively), the insoluble Ca(OMe)2 is formed very slowly in comparison with the soluble derivatives Ca(OEt)2 or Mg(OMe)2 (both latter compounds crystallize from solutions as solvates) [1646]. Aluminum readily reacts with PrOH with the formation of the highly soluble Al(OPrf)3 even in the absence of the catalyst (pK ROH = 18, E°AP7Al,ld = -1.66 V). On the other hand, polymeric Al(OMe)3 and Al(OEt)3 are formed only on prolonged refluxing of the metal with alcohols in xylene (140°C) in the presence of HgCl2 and I2 [1301]. 

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. 

Aluminum alkoxides can be prepared by reaction of alcohols with the metal in the presence of catalytic amounts of HgCl2 or I2 (equation 37). As mentioned previously, they can also be obtained by treatment of AICI3 with sodium alkoxides. Furthermore, the alkoxides may be interconverted by a transesterification reaction (equation 38). 

Thirty-three grams (0.19 mol) of aluminum ethoxide is vacuum-distilled into a 250-ml. round-bottomed flask, and 80 g. (0.6 mol) of vacuum-distilled ethyl acetoacetate (59.8° at 8 mm.) is added. Other aluminum alkoxides cannot be substituted successfully for aluminum ethoxide in this synthesis since some substitution of the alkyl group of the alk-oxide, for ethyl, appears to occur in the resulting aluminum acetoacetate derivative. The flask is fitted with a reflux condenser, and the mixture is heated gently with an oil bath the temperature is raised gradually from 85 to 140° over a period of 4 hours. The aluminum ethoxide dissolves slowly as the reaction proceeds, and the ethyl alcohol formed is distilled off at 170° and 25 mm. (A considerably lower temperature can be used if a better vacuum source is available.) At the end of 4 hours the product is cooled to room... 

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