Aldehydes methylene activation

The imide nitrogen atom was also most reactive to a variety of electrophilic species (hydrogen halides, pseudohalogens, and alkyl halides) in the parent Rimidophosphazenes, R(C—NH)-N=PPh3. With t-butyl hypochlorite the /V-chloro-derivatives, R(C=NCl)-N=PPh3, were obtained. R/ -Vinyl-phenylphosphazenes have been prepared by condensation of aldehydes with active methylene compounds ... 

Tellurium tetrachloride is an efficient catalyst in the Knoevenagel reaction of non-enoUz-able aldehydes with active methylene compounds. ... 

However, all the broad spectrum of products formed via the reaction of salicylic aldehydes with methylene-active nitriles cannot be shown on a one simple scheme. In the presence of ammonium acetate different chromenes 165 and 166 can be isolated, depending on the order of addition of reagents, the amount of catalyst, and temperature (77S871) (Scheme 60). 

Quantitative Knoevenagel condensations of aldehydes with active methylene compounds are most desirable due to the frequent use of the electron-poor alkenes that arise [107]. But previous techniques use catalysts and produce dangerous wastes even if highly energy-consuming microwave irradiation upon polar solid supports is additionally used. 

Keywords aldehyde, ketone, active methylene compound, Knoevenagel condensation, basic alumina, microwave irradiation, olefin... 

Dibutyl telluride 59 reacts with iodomethyl triphenylphosphonium iodide to give triphenylmethylidene phosphor-ane, which reacts with aldehydes leading to the methylenation products in good yields.119 The same reagent 59 also assists the reaction of dibromomalonates 60 with aldehydes and activated olefins affording alkylidene malonates 61 and cyclopropanes 62 derivatives, respectively (Scheme 30). 

Saturated 1,4-dicarbonyl compounds give 1,4-dihydro-pyridazines or -pyridazinones, etc., which are easily oxidized. 1,2-Diketone monohydrazones 124 and esters 125 containing a reactive CH2 group give 3-pyridazinones 126 (Scheme 64) . A popular modification of this approach is the Ugi four-component condensation of diarylethane-l,2-dione monohydrazones 127 with isocyanides, aldehydes, and methylene active acids leading to the corresponding substituted pyridazin-3(2//)-ones 128 (Scheme 65) The intermediate Ugi condensation products have never been observed because of their tendency to cyclize <2003S691>. 

In one pot and under aqueous conditions, a proline-catalysed Knoevenagel -cyclocondensation sequence involving an aromatic aldehyde, an activated methylene compound and dimedone results in the rapid formation of fused 477-pyrans (Scheme 2) <06SL263>. 

BiX mediates the Mukaiyama-Michael addition of a./i-urisaluraled carbonyl compounds (Scheme 14.87) [171, 172 b, c]. The BiCb-catalyzed Michael addition of 1,3-dicarbonyl compounds to methyl vinyl ketone and benzal acetophenone proceeds efficiently under microwave irradiation [179]. The Knovenagel condensation of aldehydes with active methylene compounds can also be promoted by BiCl3 (Scheme 14.88) [180]. 

Primary amines anchored to silica have been utilized as catalysts for Knoevenagel condensation reactions. For example, silica gel functionalized with a propylamino moiety [SiO2-(CH2)3-NH2] was employed in the reaction of different methylene active compounds with aldehydes and ketones for the production of electron-poor olefins 73 (Scheme 3.21). ... 

A wide variety of reaction conditions have been proposed for the reaction of starch with aldehydes. Early studies entailed use of alkaline solutions, perhaps in order to gelatinize starch.1276 Because of the known behavior of aldehydes in alkaline media (Cannizzaro disproportionation and aldol condensations of aldehydes with active active a-methylene groups), this reaction has no simple outcome. However, a process of acetalation in alkaline solutions was subsequendy patented.1293 An exothermic reaction was observed within 3 4 min using an excess of formaldehyde. It was also reported that a,(3-unsaturated aldehydes reacted readily in slightly basic media.1294... 

Unsaturated 8-lactones are biologically active structural elements in many natural products. The synthesis of this heterocyclic moiety can be achieved by a Knoevenagel condensation of 3-hydroxy aldehydes with active methylene in the presence of TiCU/pyridine or under standard conditions. ° " ... 

The importance of pyrimido[l,2-a]pyrimidines is well recognized by biological chemists. A one-pot access to these structures was realized by condensation of 2-aminopyrimidine, aromatic aldehydes and active methylene reagents under the action of microwave irradiation [116],... 

Highly thermally stable, three-dimensional, spongelike mesoporous Ce Zri 02 nanocrystallines acted as acid-base bifunctional solid solutions for the Knoevenagel condensation of aldehydes with active methylene compounds [44]. This catalyst can be recycled at least twice for the reaction of benzaldehyde with malononitrile. 

In both cases, the IL-coated material exhibited smaller pore size, surface area, and pore volume compared to the starting materials SBA-15 and MCM-41. The as-prepared catalysts have been tested in the Knoevenagel reaction between various aldehydes and activated methylene compounds and showed superior performance... 

Aldol-type condensation of an aromatic aldehyde with activated methylarene or phenylacetic acid is a useful reaction for preparing stilbene derivatives. Starting from para-substituted toluenes or para-substituted aromatic aldehydes, one can obtain 4,4 -disubstituted stilbenes. This reaction is relatively simple but has low yield. As an example, condensation of 2,4-dinitrotoluene and 4-nitrophenylacetic acid with aromatic aldehyde was studied [26]. The reaction involves carbanion addition to the carbonyl group. The carbanion is formed by the extraction of proton from the active methylene group of 2,4-dinitrotoluene by the base (usually, piperidine). The carbanion then adds to carbon atoms of the carbonyl group of the aldehyde. The reaction will therefore be facilitated by the ease of both the formation of the... 

Knoevenagel condensation is the addition of a nucleophile from active methylene compound to a carbonyl group followed by dehydration to form a P-conjugated enone. The Knoevenagel condensation between different aldehydes (15), including various aliphatic, aromatic, and heterocyclic aldehydes with active methylene compounds (119) in the presence of nano-ZnO under solvent-free conditions (Scheme 9.37) has been reported (Hosseini-Sarvari et al. 2008). Most of the aldehydes investigated reacted smoothly to afford the corresponding products in excellent yields (90%-98%) in a reaction time of 5 min to 3 h. 

Knoevenagel condensation between aromatic aldehydes and active methylene compounds has also been induced by Lewis acid ionic liquids leading to the generation of electrophilic alkenes as major products. Extension of this methodology to o-hydroxybenzaldehydes resulted in the formation of coumarins (Harjani et al. 2002). 

M. Vilches-Herrera, I. Knepper, N. de Souza, A. Villinger, V. Y. Sosnovskikh, V. O. Iaroshenko, ACS Comb. Sci. 2012, 14,434—441. One-pot, three-component synthesis of 7-azain-dole derivatives from V-substituted 2-amino-4-cyanopyr-roles, various aldehydes, and active methylene compounds. 

Hu Y, Wei P, Huang H, Le ZG, Chen ZC (2005) Organic reactions in ionic liquids ionic liquids ethylammonium nitrate promoted knoevenagel condensation of aromatic aldehydes with active methylene compounds. Synth Commun 23(35) 2955-2960... 

Kassaee et al. [88] used zinc oxide NPs as an effective and reusable catalyst for one-pot, four-component conpling of aldehydes, dimedone, active methylene compounds, and ammonium acetate to produce polyhydroquinoline derivatives under solvent-free conditions at room temperatnre (Scheme 5.37). 

In general, strongly emitting molecules contain extensive conjugated 8 -systems and emission in the NIR can be achieved from the condensation products from aromatic aldehydes and active methylenes [40]. However, relatively small fluorophores may give useful results. Thus, bis(alkenyl)-dipiperidino benzenes have been found to emit visible light in solution (quantum yield 0.09-0.66) and, more strongly, in the solid state (0.43-0.98) as a thin film or incorporated in a polystyrene films (0.43-1.00, Scheme 9.15) [41]. 

Keywords Aldehydes, dimedone, active methylene compounds, ammonium acetate, solvent-free, catalyst-free, room temperature, grinding, one-pot multicomponent synthesis, Hantzsch condensation, polyhydroquinolines... 

Glycerol was found to be a very efficient solvent for another catalyst-free three-component reaction furnishing fused pyrans 105, 106, and 161-164 (Scheme 97) [141]. The 2-aminopyran ring was annulated to a variety of scaffolds including 1,3-cyclohexanones, a- and 3-naphthols, 4-hydroxy-6-methyl-2-pyrone, and barbituric acid using the reaction with aromatic aldehydes and methylene active nitriles. It was reported that glycerol recovered from the model reaction (105, Ar = Ph, R = CN, = Me) was reused 20 times in this reaction without... 

---