Zinc alkoxides

Despite its incorporation of all the reactive components, this model leaves much to be desired. For example, the formation of the zinc alkoxide and zinc sulfonamide... 

Having observed that (i) the zinc carbenoid must be formed prior to addition of the alcohol or sulfonamide and (ii) that the zinc alkoxide should be preformed in order to obtain high selectivity, the importance of the zinc sulfonamide could be assessed (Fig. 3.14). In this sub-set, flask A contains the preformed zinc alkoxide, flask B contains the promoter solution and flask C contains the preformed zinc carbenoid. In sub-protocol Ilia, flasks A and C are combined prior to addi-... 

Although the previous protocol suggests it is not necessary to deprotonate the sulfonamide prior to exposure to the zinc carbenoid, a experimentally simpler procedure can be envisioned wherein the alcohol and promoter are deprotonated in a single flask (Fig. 3.15). In protocol IV, the alcohol and promoter are combined in flask A and are treated with diethylzinc, thus forming the zinc alkoxide and zinc sulfonamide. In sub-protocol IVa, this solution is transferred to flask C which contains the zinc carbenoid. Sub-protocol IVb represents the reversed addition order. Sub-protocol IVa is not only found to be the superior protocol in this sub-set, it is found to out-perform all of the previous protocols Despite the persistence of the induction period, a large rate enhancement over the uncatalyzed process is observed. This considerable rate enhancement also translates to a reduction in the overall reaction time when compared to sub-protocols la and Ilia. Selectivity rises... 

Understanding the importance of the zinc alkoxide, the iodomethylzinc iodide and the zinc sulfonamide allowed Denmark to propose a revised transition state structure xv (Fig. 3.22) [82]. In this picture, the complex, polymetallic aggregate invoked by Rickborn and later by Kobayashi is featured. 

The next step in the calculations involves consideration of the allylic alcohol-carbe-noid complexes (Fig. 3.28). The simple alkoxide is represented by RT3. Coordination of this zinc alkoxide with any number of other molecules can be envisioned. The complexation of ZnCl2 to the oxygen of the alkoxide yields RT4. Due to the Lewis acidic nature of the zinc atom, dimerization of the zinc alkoxide cannot be ruled out. Hence, a simplified dimeric structure is represented in RTS. The remaining structures, RT6 and RT7 (Fig. 3.29), represent alternative zinc chloride complexes of RT3 differing from RT4. Analysis of the energetics of the cyclopropanation from each of these encounter complexes should yield information regarding the structure of the methylene transfer transition state. 

The activation energy for the favored transition state TS4 (22.8 kcal mol ) is still somewhat high. Still, the qualitative predictions of enhanced reactivity of the zinc alkoxide-zinc chloride complexes are in full agreement with contemporary ideas about this reaction and represent a major advance in the theoretical understanding of the cyclopropanation process. 

Zinc carbonate reacts with epoxide to form zinc alkoxide, which in turn reacts with carbon dioxide to regenerate zinc carbonate. The most effective catalyst systems were the reaction products between diethylzinc and polyhydroxy compounds such as water or pdyhydric phenols243,244. This copolymer is interesting as a biodegradable elastomer248. ... 

Magnesium alkoxides (formed by ROH- -Me2Mg —>ROMgMe) have been decomposed thermally, by heating at 195-340°C to give the alkene, CEU, and MgO. Syn elimination is found and an Ei mechanism is likely. Similar decomposition of aluminum and zinc alkoxides has also been accomplished. ... 

We came up with the idea of using a dummy ligand, as shown in Scheme 1.23 [34]. Reaction of dimethylzinc with our chiral modifier (amino-alcohol) 46 provided the methylzinc complex 62, which was subsequently reacted with 1 equiv of MeOH, to form chiral zinc alkoxide 63, generating a total of 2 moles of methane. Addition of lithium acetylide to 63 would generate an ate complex 64. The ate complex 64 should exist in equilibrium with the monomeric zincate 65 and the dimer 66. However, we expected that the monomer ate complex 64 and the mono-... 

Further optimization of this reaction was carried out with TFE as an achiral adduct, since reaction with TFE is much faster than that with neopentyl alcohol. We found that dimethyl- and diethylzinc were equally effective, and the chiral zinc reagent could be prepared by mixing the chiral modifier, the achiral alcohol and dialkylzinc reagent in any order without affecting the conversion and selectivity of the reaction. However, the ratio of chiral to achiral modifier does affect the efficiency of the reaction. Less than 1 equiv of the chiral modifier lowered the ee %. For example with 0.8 equiv of 46 the enantiomeric excess of 53 was only 58.8% but with 1 equiv of 46 it was increased to 95.6%. Reaction temperature has a little effect on the enantiomeric excess. Reactions with zinc alkoxide derived for 46 and TFE gave 53 with 99.2% ee at 0°C and 94.0% ee at 40°C. 

Comparison of zinc alkoxide and zinc hydroxide bond energies has been made. The relative heterolytic bond energies for hydroxide, methoxide, ethoxide, and tert-butoxide were determined from studies of a series of alkoxide exchange equilibria using a four-coordinate monomeric zinc tris(pyrazolyl)borate compound.335... 

Zinc alkoxide and aryloxide complexes have been of particular interest as enzyme models and catalysts. Tetrameric alkyl zinc alkoxides are a common structurally characterized motif.81... 

A series of zinc alkoxide complexes were characterized of the form [RZnOR ]ra where n 2 or 4 (Figure 4). Complexes of the form [Zn3 0(2,6-i-Pr2C6H3) 4R2] were produced by stoichiometry... 

Figure 4 The molecular structure of a tetrameric zinc alkoxide formed from 1-adamantol with (trimethyl-...

Ito and co-workers observed the formation of zinc bound alkyl carbonates on reaction of carbon dioxide with tetraaza macrocycle zinc complexes in alcohol solvents.456 This reversible reaction was studied by NMR and IR, and proceeds by initial attack of a metal-bound alkoxide species. The metal-bound alkyl carbonate species can be converted into dialkyl carbonate. Spectroscopic studies suggested that some complexes showed monodentate alkyl carbonates, and varying the macrocycle gave a bidentate or bridging carbonate. Darensbourg isolated arylcarbonate compounds from zinc alkoxides as a by-product from work on polycarbonate formation catalysis.343... 

The zinc alkoxide of 2-methyl-l-(3-quinolyl)propan-l-ol was used in a catalytic amount to give ee up to 94% in the enantioselective alkylation of quinoline-3-carbaldehyde by diisopropyl-... 

Prasad and Joshi121 presented a conceptually different catalyst system—zinc amides of oxazolidine. Because the addition of dialkylzinc to aldehyde is known to involve a chiral zinc alkoxide with a coordinately unsaturated tricoordinated center, they anticipated that a zinc amide with dicoordinate zinc should be a better Lewis acid. Examining three different zinc species 128-130, zinc amide derived from the corresponding oxazolidine 130 was found to lead to a very fast reaction (4 hours, 0°C) and 100% ee (Scheme 2-50). The reaction proceeds even faster at room temperature (completed within 1 hour) without significant loss of stereoselectivity. This reaction can provide excellent ee for aromatic aldehydes,... 

Figure 5-14 illustrates the transition state in the reaction. The free hydroxyl group is necessary for producing an effective chiral environment, probably through complexation as a zinc alkoxide.118... 

In the epoxidation process (Figure 4.4), the oxygen of the enone s carbonyl function first coordinates with the zinc atom. The ethylperoxy anion then attacks the (3-position, which constitutes a Michael-type addition. The subsequent cyclization gives the epoxy ketone and the zinc alkoxide. 

The essential features of the catalytic cycle are summarized in Figure 12.6. After binding of NAD+ the water molecule is displaced from the zinc atom by the incoming alcohol substrate. Deprotonation of the coordinated alcohol yields a zinc alkoxide intermediate, which then undergoes hydride transfer to NAD+ to give the zinc-bound aldehyde and NADH. A water molecule then displaces the aldehyde to regenerate the original catalytic zinc centre, and finally NADH is released to complete the catalytic cycle. 

Thus, the role of zinc in the dehydrogenation reaction is to promote deprotonation of the alcohol, thereby enhancing hydride transfer from the zinc alkoxide intermediate. Conversely, in the reverse hydrogenation reaction, its role is to enhance the electrophilicity of the carbonyl carbon atom. Alcohol dehydrogenases are exquisitely stereo specific and by binding their substrate via a three-point attachment site (Figure 12.7), they can distinguish between the two-methylene protons of the prochiral ethanol molecule. 

We thought that when i-Pr2Zn was treated with pyrimidine-5-carbaldehyde without adding any chiral substance, extremely slight enantioenrichment would be induced statistically in the initially formed zinc alkoxide of the pyrimidyl aUca-nol, and that the subsequent amplification of chirality by asymmetric autocatalysis would afford the pyrimidyl alkanol with detectable enantioenrichment [Eq. (9.11)]. Indeed, we found that pyrimidyl alkanol with an ee that is above the detection level was formed.Pyrimidine-5-carbaldehyde was reacted with /-Pr2Zn, and the resulting pyrimidyl alkanol was used as an asymmetric autocatalyst for the subsequent asymmetric autocatalysis. The consecutive asymmetric autocatalysis afforded pyrimidyl alkanol of either 5 or 7 configuration with enantiomeric enrichment above the detection level. °... 

Recently, N,N,0-tridentate Schiff-based zinc alkoxide complexes 52a-53e have been developed by our group [76]. All complexes efficiently initiate the polymerization of L-lactide at 25 °C with >90% conversion within 30-240 min, with only one exception, 52c which is inactive. The polymerization was well-controlled (PDI = 1.04—1.09) and showed that the reactivity decreases with an electron-withdrawing... 

---