Metal aldehyde

The reduction of carboxylic acids or esters requires very powerful reducing agents such as lithium aluminum hydride (LiAlH,) or sodium (Na) metal. Aldehydes and ketones are easier to reduce, so they can use sodium borohy-dride (NaBH,j). Examples of these reductions are shown in Figure 3-13. 

Mikami has also reported a related ene-like process involving glyoxylates and ketone-derived enolsilanes 42 (Eq. 8B2.11) [17]. The enol ether adducts 43 yield the corresponding P-hydroxy ketone upon treatment with mild acid. On the basis of an analysis of the stereo- and regiochemical outcome of the addition reaction Mikami has invoked a monodentate complex between aldehyde and metal, in contrast to the typical transition-state structures involving glyoxylates that are suggested to involve metal/aldehyde chelates. 

Some organics, acetic anhydride, metals, alcohols, wood and its derivatives (see the other tables in this chapter dealing with this compound) Aldehydes, alcohols, unsaturated hydrocarbons Flammable liquids, metals, aldehydes, alcohols, impact, hydrocarbons (unsaturated)... 

Reduction with alkali metals The solvents used for alkali metal reductions include hydrocarbons, ethers and, most commonly, liquid ammonia. Alcohols may also be used, but usually as co-solvents, since they react vigorously with these metals. Aldehydes are not usually reduced in this manner, because they react with ammonia to form unreactive imine condensation products. 

One means of stereoselective cleavage of biaryl lactones [53] is activation of the carbonyl group with a Lewis acid and subsequent attack with a chiral nucleophile. Conversely, activation can be effected with a chiral Lewis acid followed by attack of an achiral nucleophile. Complexation of a biaryl lactone to the chiral fragment [CpRe (NO)(PPh3)j then reduction with K(s-Bu)3BH (K-selectride) and ring opening of the intermediate rhenium lactolate gives the metalated aldehyde (dr = 75 25) which is converted to the alcohol without essential loss of optical purity (Sch. 6) [54]. 

The two most popular methods are the plate incorporation method and the preincubation method. In the plate incorporation method, tester cell suspensions are mixed with an overlay (top) agar and plated immediately onto minimal medium (bottom agar). In the preincubation method, the cell suspension mixture is incubated and then mixed with a top agar before plating onto minimal medium. For both techniques, after 2 or 3 days of incubation, normal sized, revertant colonies are counted and compared to the number of spontaneous revertant colonies on solvent control plates. The Environmental Protection Agency recommends some materials be tested using the preincubation method, namely, classes that include short-chain aliphatic nitrosamines, divalent metals, aldehydes, azo dyes and diazo compounds, pyrollizidine alkyloids, alkyl compounds and nitro compounds. Certain modifications of the methods need to be incorporated for specific types of test articles. 

A nucleophilic attack of the ylide on the solvated metal-aldehyde complex could generate four possible stereoisomeric oxaphosphetanes, i.e., two pairs of enantiomers. This process is likely to occur via a 2-center reaction, where the transition state structure involves the formation of a partial C-C bond and with no P-O bond formation. Since no restriction is placed on the direction of approach of the reactants, all four stereoisomeric transition states can be formed. Ring closure would yield the respective oxaphosphetanes as two racemates. 

Figure 4 Steric interactions between metal, aldehyde and the diene disfavor the endo transition state. Chelation control should prefer the exo transition state in the Diels-Alder cycloaddition pathway...

TiCU also operates through an aldol reaction mechanism. Unlike BF3 OEt2, however, TiCU can form an ordered Zimmerman-Traxler transition state and is also able to chelate a-alkoxy aldehydes. This forces the production of syn pyrones from the reaction with aldehydes that contain groups that are able to chelate to the metal aldehydes that cannot coordinate give trans aldol products. Primary [4 + 2] cycloaddition products (i.e. vinylogous ortho esters) have not been isolated in the TiCU-catalyzed reactions. 

For several experimental and computational investigatioiis on similar reactions, see (a) M. N. Paddon-Row, N. G. Rondan, K. N. Houk, J. Am. Chem. Soc. 1982, 104, 7162—7166. Staggered models for asymmetric induction attack trajectories and conformations of allylic bonds from ab initio transition structures of addition reactions, (b) S. E. Denmark, E. J. Weber, Helv. Chim. Acta 1983, 159, 1655-1660. On the stereochemistry of aUyl-metal-aldehyde condensations, (c) S. E. Denmark, E. J. Weber,... 

The heterometallic ketenyl complexes [0s3W(//-H) G(0)GH2 (G0)3Gp ] (Gp = Gp or Gp ) that possess a pendant Gp W(GO)3 substituent were prepared by the condensation of [Os3(GO)io(NGMe)2] with the metal-aldehyde complex [Gp W(GO)3GH2GHO]. Pyrolysis of these ketenyl complexes in the solid state at 185 °G afforded tetra-nuclear clusters in the case of the Gp derivative, whereas the Gp system gave pentanuclear species.In both cases, G-O bond scission in the ligated ketene fragment was observed. 

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