Aldimines, olefination

The corresponding iron-catalyzed oligomerization of ethylene was developed by Gibson and coworkers [125]. A combination of an iron precatalyst with MAO (methyl aluminoxane) yields a catalyst that affords ethylene oligomers (>99% linear ot-olefin mixtures). The activity of ketimine iron complexes (R = Me) is higher than that of the aldimine analogs (R = H) and also the a-value of the oligomer is better (Scheme 41). 

Reactions of potassium superoxide solubilized in apolar solvents with crown ethers (see Oxidation reactions, p. 356) are also frequently accompanied by elimination reactions. Thus, in DMSO solution, secondary alkyl bromides only yield olefins when treated with the K02 complex of dicyclohexyl-18-crown-6 (Johnson et al., 1978). Scully and Davis (1978) have studied the elimination of HC1 from N-chloramines with 18-crown-6-solubilized K02, KOH, and KOAc in ether solution (27). High yields of aldimines were obtained with K02,... 

New catalyst design further highlights the utility of the scaffold and functional moieties of the Cinchona alkaloids. his-Cinchona alkaloid derivative 43 was developed by Corey [49] for enantioselective dihydroxylation of olefins with OsO. The catalyst was later employed in the Strecker hydrocyanation of iV-allyl aldimines. The mechanistic logic behind the catalyst for the Strecker reaction presents a chiral ammonium salt of the catalyst 43 (in the presence of a conjugate acid) that would stabilize the aldimine already activated via hydrogen-bonding to the protonated quinuclidine moiety. Nucleophilic attack by cyanide ion to the imine would give an a-amino nitrile product (Scheme 10). 

In previous work, Corey used the free base form of 34 as an effective chiral ligand in the Os04-promoted dihydroxylation of olefins [90]. He later found that ammonium salt 34 catalyzed the addition of HCN to aromatic N-allyl imines (Scheme 5.50) [91]. The U-shaped pocket of the catalyst is essential in fixing the orientation of the hydrogen-bonded activated aldimine via n-n interactions. 

Synthesis of these prolylamide mimics is based on the Peterson olefination between tert-butyl a-fluoro-a-trimethylsilyl acetate and a protected hydroxypentanone. Further introduction of the amino group is rather difficult. This step has been accomplished through conversion of the ester into aldehyde, followed by the formation of the silylated aldimine with LiHMDS, and then the addition of methyl lithium (Figure 7.23). ... 

An interesting example is the hydroiminoacylation reaction, a good alternative to hydroacylation reactions, using aldimines as a synthetic equivalent to aldehydes (Scheme 4) [4]. The rhodium-catalyzed hydroiminoacylation of an olefin with aldimines produced a ketimine which could be further acid-hydrolyzed to give the ketone. The reaction proceeded via the formation of a stable iminoacylrhodi-um(III) hydride (this will be discussed in the mechanism section), production of which is facilitated by initial coordination of the rhodium complex to the pyridine moiety of the aldimine. This hydroiminoacylation procedure opened up the direct... 

Aldehydes of MBH reactions can be replaced by activated aldimines such as N-tosyl, N-mesyl, N-nosyl, N-diphenylphosphinoyl, or N-SES aldimines. The addition of such aldimines to electron-deficient olefins can be mediated by nucleophilic tertiary N or P Lewis bases (Scheme 5.20). 

As in the MBH reactions, / -ICD (44) is also an efficient and remarkably general catalyst in aza-MBH reactions [94, 95]. This catalyst promotes the addition a variety of electron-deficient olefins such as acrylates, enones, and enals with activated aromatic aldimines. Of note, the absolute stereochemistry of the product is generally opposite compared to the analogous MBH reaction with / -ICD catalyst imines gives rise to (S)-enriched adducts, in contrast to aldehydes which afford ( -products [94]. Substitution patterns of the olefin may alter or even invert this trend, however. 

Enantiomeric purities ranging from 20 to 80% have been reported for the acid-promoted asymmetric oxidation of sulfides to sulfoxides by binaphthyl-derived oxaziridines has been described <2007T6232>. A novel amino hydroxylation of olefins has been developed using /ra t-2-phenylsulfonyl-3-phenyloxaziridine 33 <2007JA1866>. The reaction, which is catalyzed by copper(ll) salts, affords good yields of the product. Oxidation of aldimines to oxaziridines using alumina-supported M0O3 catalyst and anhydrous /-butyl hydroperoxide (TBHP) has been described. Yields are excellent. 

First, the enzyme has at least two and probably three active-site basic groups involved in proton transfers to and from substrates, intermediates, and nascent products and all three bases are located on the si face of the substrate-PLP aldimine system as are the protons to be shuffled about, so all the proton transfers are likely to be economically suprafacial. Several pieces of stereochemical evidence suggest that the j5,y-olefinic PLP-p-quinoidal-a-anion (141) can rotate around its C(P)-C(ol) bond and also implicate that the cisoid isomer of this n complex and then the Z-isomer of the nascent aminocrotonate carry 80 % of the reaction flux. Furthermore, a 15% internal retention of the from the Pro-R methylene of ACPC (9) on B2H (85 % exchange with solvent, 15 % internal return) in the active site and the overall 22/78 H /H5 distribution at C(3) of the mono- and dideutero 2-ketobutyrate (138) products at C(3) are also noted. 

In general, the aldimine adducts of natural products can be converted in one step to the corresponding a, /S-unsaturated aldehydes or ketones. They are subjected to a steam distillation in the presence of oxalic acid, in which the final product distils over. Under these conditions the more thermodynamically stabile product is formed, usually the trans olefin. 

In parallel with the directed hydroarylation of olefins, a series of papers described the formation of ketones from heteroarenes, carbon monoxide, and an alkene. Moore first reported the reaction of CO and ethylene with pyridine at the position a to nitrogen to form a ketone (Equation 18.28). Related reactions at the less-hindered C-H bond in the 4-position of an A/-benzyl imidazole were also reported (Equation 18.29). - Reaction of CO and ethylene to form a ketone at the ortho C-H bond of a 2-arylpyridine or an N-Bu aromatic aldimine has also been reported (Equations 18.30 and 18.31). Reaction at an sp C-H bond of an N-2-pyridylpiperazine results in both alkylative carbonylation and dehydrogenation of the piperazine to form an a,p-unsaturated ketone (Equation 18.32). The proposed mechanism of the alkylative carbonylation reaction is shown in Scheme 18.6. This process is believed to occur by oxidative addition of the C-H bond, insertion of CO into the metal-heteroaryl linkage, insertion of olefin into the metal-acyl bond, and reductive elimination to form the new C-H bond in the product. 

Normally, the A-arylimine is obtained by reaction of aldehyde and aniline in acidic condition. Either tetrahydroquinoline or its corresponding substituted quinoline can be generated in the Povarov reaction, depending on the reaction conditions. For instance, DDQ-promoted dehydrogenation, vacuum distillation under acidic condition, oxidation by air or Mn(OAc)3, and Pd/C-catalyzed aromatization of tetrahydroquinoline, provides the corresponding substituted quinolines in good to excellent yield. Since some tetrahydroquinolines are unstable under the reaction conditions, the corresponding substituted quinolines could be isolated as the sole products. Electron-rich olefin, such as vinyl enol ethers, vinyl sulfides, and silyl enol ethers, are widely used as dienophiles in the cycloaddition of A-aryl aldimines to obtain substituted tetrahydroquinolines. To access natural... 

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