Unsaturated aldehydes formation

In the presence of a copper salt, aromatic aldehydes, a,/3-unsaturated aldehydes, formates, and formamides react with 38 at or below room temperature to give the corresponding insertion products in good yield.289 This catalytic reaction shows similar stereo- and regiochemistry as does the thermal reaction of formamides. A transmetallation mechanism via an organocopper intermediate has been proposed for the Cu-catalyzed insertion. 

Irradiation (44) of the hydroxycyclobutanone [112] yields the aldehyde [113]. This reaction probably proceeds ip analogy to the unsaturated aldehyde formation from cyclopentanones, as shown. 

The formation of pyridine derivatives from a, P-unsaturated aldehydes and ammonia involves formation of three bonds during the ring synthesis. For example, with an a, P-unsaturated aldehyde, both 2,5-substituted as well as 3,4-substituted pyridines can be obtained, depending on whether a 1,2- (eq. 17) or 1,4-addition (eq. 18) occurs with ammonia. Reactions are performed in the vapor phase with catalysts. 

A reaction of great synthetic value for carbon-carbon bond formation. Nucleophilic addition of an enolate ion to a carbonyl group, followed by dehydration of the p-hydroxy aldehyde, yields an a,p-unsaturated aldehyde. 

Skraup proposed a simple mechanism involving imine formation followed by an acid-mediated cyclization. Unfortunately the observed regioselectivity is not consistent with the proposed mechanism when, for example, electron-rich aniline 4 reacts with a, 3-unsaturated aldehyde 5 to give quinoline 6. ... 

In the presence of catalytic amounts of Pd(0), silicon-substituted vinyloxiranes can rearrange into the corresponding ot-silyl- 3,y-unsaturated aldehydes (Scheme 9.34) [151]. Treatment of 80 with Pd(OAc)2 and P(OPh)3 results in the formation of 7t-allylpalladium complex 81. Bond rotation to give 82, followed by migration of the silyl moiety, affords aldehyde 83, which is trapped in situ to provide the Felkin-Anh product 84. The reaction proceeds with retention of configuration and the ee of the starting material is retained in the product. The size of the silicon substituents is critical for the outcome of the reaction, as is the choice of ligands on palladium. 

Hydroxy(tosyloxy)iodobenzene 2014 reacts with phenyltrimethylsilane 81 in boiling acetonitrile to give diphenyliodonium tosylate 2015 and trimethylsilanol 4 or HMDSO 7 [184, 185]. Likewise, treatment of 2,5-bis(trimethylsilyl)furan 2016 with 2014 in boiling acetonitrile/methanol affords 78% iodonium tosylate 2017 and trimethylsilanol 4 [185]. In the presence of Bp3-OEt2 iodosobenzene oxidizes allyl-trimethylsilanes such as 2018 to unsaturated aldehydes such as 2019 in 63% yield, with formation of iodobenzene and trimethylsilanol 4 [186]. Analogously, vinyltrimethylsilanes such as (Z)-l-trimethylsilyl-2-phenylethylene 2020 afford, via 2021, acetylenes such as phenylacetylene in 61% yield and iodobenzene and trimethylsilanol 4 [187] (Scheme 12.54). 

Nair and co-workers have demonstrated NHC-catalysed formation of spirocyclic diketones 173 from a,P-unsaturated aldehydes 174 and snbstitnted dibenzylidine-cyclopentanones 175. Where chalcones and dibenzylidene cyclohexanones give only cyclopentene products (as a result of P-lactone formation then decarboxylation), cyclopentanones 175 give only the spirocychc diketone prodncts 173 [73]. Of particular note is the formation of an all-carbon quaternary centre and the excellent level of diastereoselectivity observed in the reaction. An asymmetric variant of this reaction has been demonstrated by Bode using chiral imidazolium salt 176, obtaining the desymmetrised product with good diastereo- and enantioselectivity, though in modest yield (Scheme 12.38) [74],... 

Ir catalysts supported on binary oxides of Ti/Si and Nb/Si were prepared and essayed for the hydrogenation of a,P-unsaturated aldehydes reactions. The results of characterization revealed that monolayers of Ti/Si and Nb/Si allow a high metal distribution with a small size crystallite of Ir. The activity test indicates that the catalytic activity of these solids is dependent on the dispersion obtained and acidity of the solids. For molecules with a ring plane such as furfural and ciimamaldehyde, the adsorption mode can iirfluence the obtained products. SMSI effect (evidenced for H2 chemisorption) favors the formation of unsaturated alcohol. 

Scheme 2.18 gives some representative olefination reactions of phosphonate anions. Entry 1 represents a typical preparative procedure. Entry 2 involves formation of a 2,4-dienoate ester using an a, 3-unsaturated aldehyde. Diethyl benzylphosphonate can be used in the Wadsworth-Emmons reaction, as illustrated by Entry 3. Entries 4 to 6 show other anion-stabilizing groups. Intramolecular reactions can be used to prepare cycloalkenes.264... 

Interesting octahydroacridines 2-860 have been prepared by Beifuss and coworkers by combining the condensation step with a rare intramolecular polar [4jt++2 jrj-cydization of a-aryliminium ions 2-859, obtained from anilines 2-857 by reaction with the oo-unsaturated aldehyde 2-858 (Scheme 2.191) [440]. The overall domino process seems to be stereoselective, since the formation of the two di-astereomers 2-860 can be traced to the use of the substrate 2-858 as a diastereom-eric mixture. 

The unsaturated aldehyde in (2.2) very often undergoes intramolecular photocycloaddition with formation of an oxetane (cf, chapter 4.3.6). The... 

An enantioselective organocatalytic 1,3-DC reaction, based on the activation of a,fi-unsaturated aldehydes through the reversible formation of iminium ions with chiral imidazolidinones 100, was described. Good levels of asymmetric induction and diastereocontrol were achieved (up to 94% ee and 94 6 dr) <00JA9874>. 

Stoichiometric ionic hydrogenation of the C=C bond of a,/ -unsaturated ketones by HOTf and [Cp(CO)3WH] results in the formation of -ketone complexes of tungsten [32]. As exemplified in Eq. (17), hydrogenation of methyl vinyl ketone gives a 2-butanone complex of tungsten. The bound ketone is displaced by the triflate counterion, giving the free ketone. Similar reactions were reported for hydrogenation of the C=C bond of a,/ -unsaturated aldehydes. 

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