Aluminum triisopropoxide

The aldehyde or ketone, when treated with aluminum triisopropoxide in isopropanol as solvent, reacts via a six-membered cyclic transition state 4. The aluminum center of the Lewis-acidic reagent coordinates to the carbonyl oxygen, enhancing the polar character of the carbonyl group, and thus facilitating the hydride transfer from the isopropyl group to the carbonyl carbon center. The intermediate mixed aluminum alkoxide 5 presumably reacts with the solvent isopropanol to yield the product alcohol 3 and regenerated aluminum triisopropoxide 2 the latter thus acts as a catalyst in the overall process ... 

The Mecrwein-Ponndoi f-Verlev reaction involves reduction of a ketone by treatment with an excess of aluminum triisopropoxide. The mechanism of the process is closely related to the Cannizzaro reaction in that a hydride ion acts as a leaving group. Propose a mechanism. 

Without the aluminum triisopropoxide, the reaction does not proceed. Show the structure of the intermediate in which the hydride transfer occurs and use ideas from orbital interaction theory to discuss the factors which enhance the hydride transfer in this reaction. 

Answer. Three factors combine to make this reaction facile (a) activation of the carbonyl group toward nucleophilic addition as a result of coordination to the Lewis acid (aluminum triisopropoxide), as discussed in Chapter 8 (b) activation of the secondary C—H bond as a donor by the presence of the very good X substituent (— —Al, which resembles —O), as discussed in Chapter 4 and (c) opportunity presented by the coordination within the complex shown in Figure B.4,... 

Both the Meerwein-Ponndorf-Verley reaction and the Cannizzaro reaction are hydride transfers in which a carbonyl group is reduced by an alkoxide group, which is oxidized. Note that each aluminum triisopropoxide molecule is capable of reducing three ketone molecules. 

Dialkylzinc initiates homo- and copolymerization of aldehydes such as acetaldehyde 151, 234, 310, 487, 533), formaldehyde 310, 495), butyraldehyde 468), glutardehyde 386), cyanopropionaldehyde 479), chloroacetaldehyde 233, 234, 324, 325, 412, 495, 533), and dichloro-acetaldehyde 325). Aluminum triisopropoxide 485) and phosphorus compounds 339) were proposed as additives for the polymerizations. Polymerization of optically active aldehydes was also reported ). 

Total syntheses of 3-deoxy-D-n o and 3-deoxy-D-arafci o-hexose have been realized via the cross-aldolization of 2,3-0-isopropylidene-D-glyceraldehyde (R)-37 and 1,1-dimethoxyace-tone (O Scheme 67). The key step of the synthesis is the diastereoselective reduction of one of the aldols 171 via boron chelates. Treatment of 171 with triisobutylborane, and then with NaBH4 gives syn-, 3- (173) and awft-l,3-diol 174 in a ratio 95 5. Acidic hydrolysis of 173 provides 3-deoxy-D-n7 o-hexose. If aldol 171 is treated first with an equimolar amount of aluminum triisopropoxide, diol 174 is obtained in 62% yield (together with 15% of 173). Compound 174 is converted then into 3-deoxy-D-arafci o-hexose [307]. 

Kashemirov, V.A., Osipov, V.N., EmeTyanovich, A.M., and Khokhlov, PS., Reaction of triisopropyl phosphoacetate with sulfenyl chlorides upon catalysis by aluminum triisopropoxide, Zh. Obshch. Khim., 62, 1195, 1992 7. Gen. Chem. USSR (Engl. Transl.), 62, 982, 1992. 

Pratt, C., Modified aluminum tri-alkoxide compounds- replacing part of isopropyl alcohol of aluminum triisopropoxide with a higher alcohol, storage stability. US Patent 4,525,307, June 25,1985. 

When a,/3-unsaturated alcohols, which react at relatively low temperatures, are used it is advisable to use a high concentration of hydrogen-acceptor. Cinnamaldehyde has proved particularly valuable as hydrogen-acceptor in the preparation of aliphatic aldehydes, and anisaldehyde in preparation of alicyclic aldehydes. The alcohol to be oxidized is first converted into alkoxide by the calculated amount of aluminum triisopropoxide and is then brought into redox equilibrium with 120 — 200% of the higher-boiling aldehyde. 

Duda, A., and Penczdc, S., Of the Difference of Reactivities of Various Aggregated Fonns of Aluminum Triisopropoxide in Initiating Ring-Opening Polymerizations. 1995, Macromol. lU id Commun., 16 67... 

It is not always easy to deduce the mechanism of a polymerization. In general, no reliable conclusions can be drawn solely from the type of initiator used. Ziegler catalysts, for example, consist of a compound of a transition metal (e.g., TiCU) and a compound of an element from the first through third groups (e.g., AIR3) (for a more detailed discussion, see Chapter 19). They usually induce polyinsertions. The phenyl titanium triisopropoxide/aluminum triisopropoxide system, however, initiates a free radical polymerization of styrene. BF3, together with cocatalysts (see Chapter 18), generally initiates cationic polymerizations, but not in diazomethane, in which the polymerization is started free radically via boron alkyls. The mode of action of the initiators thus depends on the medium as well as on the monomer. Iodine in the form of iodine iodide, I I induces the cationic polymerization of vinyl ether, but in the form of certain complexes DI I (with D = benzene, dioxane, certain monomers), it leads to an anionic polymerization of 1-oxa-4,5-dithiacycloheptane. 

Iniferter polymerizations were also combined with anionic polymerization. The representative example involves the synthesis of PCL-l7-(PMMA-co-PSt)-l7-PCL. ° A polymeric thermal iniferter, PCL-substituted tetraphenylethane, was prepared by anionic polymerization of CL in the presence of aluminum triisopropoxide and benzopinacol. The benzopinacolate groups incorporated into the polymer chain initiated the polymerization of St and MMA via a controlled radical mechanism at 95 °C to yield the desired block copolymers (Scheme 47)... 

---