Triisopropoxides

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. 

The combination of metal tuning and double stereodifferentiation helps to prepare chelation and nonchelation products in the imine series7. In the case of an alkoxy substituent adjacent to the aldimino, the chelation product 10 is predominantly obtained with allylmagnesium chloride, chloromagnesium allyltriethylaluminate or allylzinc bromide, while the use of allyl-boronates or allyltitanium triisopropoxide, which lack the requisite Lewis acidity for chelation, gives 11 with good Cram selectivity. 

The best titanium mediator appears to be methyltitanium triisopropoxide, yet good yields are also obtained with titanium tetraisopropoxide and chlorotitanium triisopropoxide. The methyl group on titanium serves as a dummy alkyl ligand which is eliminated as methane after hydride transfer from the... 

In contrast to allyl magnesium chloride and allyl titanous triisopropoxide, allyl tita-nous trisdiethylamide reacts cleanly with imidazolides to give j ,y-unsaturated ketones practically free of concomitant carbinols. Furthermore, the products can be obtained free of isomers (a,/ -unsaturated ketones) if they are purified by distillation rather than chromatography. 

Table 11.4. 1,2-Disubstituted cyclopropanols 22 from carboxylic acid esters 8 and 2-substituted ethyl-magnesium halides in the presence of titanium tetraisopropoxide or chlorotitanium triisopropoxide. Entry Starting Product Conditions Yield Ref. Ester R1 R3 [mol% (%) R2 Ti(OR)4] (d. r. Z/Eb) ...

Improved yields of cyclopropylamines 47 could be obtained by using methyltitanium triisopropoxide (53) instead of titanium tetraisopropoxide [108], as well as by adding the Grignard reagent to the mixture of the amide and the titanium reagent at ambient rather than low temperature (Schemes 11.15 and 11.16, and Table 11.9) [67,69]. In principle, methyltitanium triisopropoxide requires only one equivalent of the alkylmagnesium halide to generate a dialkyltitanium diisopropoxide intermediate 55, and in this particular case P-hydride elimination can only occur at the non-methyl substituent so that methane... 

Table 11.9. /V,/V-Dialkylcyclopropylamines 47 from /V,/V-dialkylcarboxamides 44 and ethyl- as well as substituted ethylmagnesium bromides 27 in the presence of methyltitanium triisopropoxide.

Scheme 11.16. Preparation of N,N-dialkylcyclo-propylamines 47 in the presence of methyltita-nium triisopropoxide. For details, see Table 11.9.

Few X-ray diffraction data are available for the organotin alkoxides. Trimethyltin methoxide in the crystal is a five-coordinate linear polymer 47,380 and both tin atoms in the methanol solvate, CH2(SnPh2OMe)2-MeOH, are five coordinate, one by intramolecular coordination, and the other by solvation 48.381 Dimethyltin dimethoxide 49,382 dimethyltin diphenoxide 50,383 and isopropyltin triisopropoxide 51384 are five-coordinate oxygen-bridged dimers. All these compounds are monomers in solution. 

It was found that aryltitanium triisopropoxides 52 (ArTi(0 Pr)3) also participate in asymmetric conjugate addition to a,yS-unsaturated ketones in an aprotic solvent [38] (Scheme 3.20). The addition of 52 to 2-cyclohexenone la was complete within 1 h in the presence of 3 mol% of [Rh(OH)((S)-BINAP)]2 in THF at 20°C to give high yields of the titanium enolates 53 as conjugate addition products. The enantioselectivity is very high, 99.5, 99.0, and 99.8% enantiomeric excess, for Ar = Ph, 4-FC6H4, and... 

MeOC6H4, respectively. The titanium enolates were converted into silyl enol ethers 54 by treatment with chlorotrimethylsilane and lithium isopropoxide. Additionally, cyclic enones lb and Ic, and linear enones Id and le, are also good substrates for the asymmetric conjugate addition of phenyltitanium triisopropoxide, giving the corresponding arylation products with over 97% enantioselectivity. 

The reaction of the racemic lactol, 5-(ten-butyldiphenylsilyloxymethyl)tetrahydro-2-furanol, with methyltitanium triisopropoxide yielded two racemic nucleophilic addition products in a 12 1 ratio syn vs. anti configuration had to be determined104 (see p 469). 

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 samarium iodide and ytterbium triisopropoxide catalyse the ring opening45 of epoxides by TMS-azide in the latter case, due to an acidic work-up, the products are isolated as vicinal azido alcohols. 

Full details on this ring opening46 under titanium tetraisopropoxide or aluminium triisopropoxide catalysis have been published. Using chirally modified titanium catalysts, cyclohexene oxide provides47 fraws-2-azidocyclohexanol in up to 63% ee. 

Selective cleavage of oxiranes.1 This reagent is more effective than allyltitanium triisopropoxide for selective cleavages of oxiranes at the more-substituted carbon atom. 

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