Electron-withdrawing moiety

This reaction is reported to proceed at a rapid rate, with over 25% conversion in less than 0.001 s [3]. It can also proceed at very low temperatures, as in the middle of winter. Most primary substituted urea linkages, referred to as urea bonds, are more thermally stable than urethane bonds, by 20-30°C, but not in all cases. Polyamines based on aromatic amines are normally somewhat slower, especially if there are additional electron withdrawing moieties on the aromatic ring, such as chlorine or ester linkages [4]. Use of aliphatic isocyanates, such as methylene bis-4,4 -(cyclohexylisocyanate) (HnMDI), in place of MDI, has been shown to slow the gelation rate to about 60 s, with an amine chain extender present. Sterically hindered secondary amine-terminated polyols, in conjunction with certain aliphatic isocyanates, are reported to have slower gelation times, in some cases as long as 24 h [4]. 

An unusual method for the preparation of 3-(trifluoromethyl)-4-aryl-furazans 49a,b in 47-77% yield has been reported (99H627) (Scheme 19). Thus, dehydration of l,l,l-trifluoroalkane-2,3-dione dioximes 48a,b was accomplished on heating with silica gel. If, as in 48b, Ar was an electron-withdrawing moiety, the conversion proceeded more smoothly. The dehydration of the same dioximes using traditional methods failed. 

Poly(pyrazabole)s containing an electron withdrawing moiety in the main chain were prepared the same way (scheme 39).69 The coupling reaction between... 

OMe Electron Releasing Moiety —N02 Electron Withdrawing Moiety... 

Unfortunately, these aza-ethers showed limited solubility in the polar solvents that are typically preferred in nonaqueous electrolytes, and the electrochemical stability window of the LiCl-based electrolytes is not sufficient at the 4.0 V operation range required by the current state-of-the-art cathode materials. They were also found to be unstable with LiPFe. Hence, the significance of these aza-ether compounds in practical applications is rather limited, although their synthesis successfully proved that the concept of the anion receptor is achievable by means of substituting an appropriate core atom with strong electron-withdrawing moieties. 

S.3.2.3. Lithium Borates with Nonaromatic Ligands. The presence of aromatic ligands in Barthel s salts was believed to be responsible for the high melting points and basicity of the borate anions, which in turn translate into moderate or poor solubilities and ion conductivities as well as low anodic stabilities. To avoid use of these bulky aromatic substituents, Xu and Angell synthesized a series of borate anions that are chelated by various alkyl-based bidentate ligands, which serve as electron-withdrawing moieties by the presence of fluorine or carbonyl functionalities. Table 13 lists the... 

While a more detailed review of two-photon 3D microfabrication can be found in another chapter of this issue, this chapter will concentrate on our work on characterization and apphcation of commercial photoinitiators. The development of better 2PA photoinitiators would be expected to facilitate 3D photopolymerization technologies. Efficient 2PA compoimds based on phenylethenyl constructs bearing electron-donating and/or electron-withdrawing moieties have been reported [105-107]. Among these are electron-rich derivatives that have been found to undergo a presumed two-... 

In general, the hydrolysis of amides requires conditions that are not compatible with the survival of several functional groups [10]. If the amide nitrogen is linked to an electron-withdrawing moiety, alkaline hydrolysis of the amide group is easier. Following this observation Martens and co-workers used 4-methoxy-2-nitrophenyl isocyanide 14 (or 2-methoxy-4-nitrophenyl isocyanide) as a convertible isocyanide for the preparation of Peptide Nucleic Acid (PNA) monomers [8a] (for another example, see Ref. [11]). The Ugi-4CR between isocyanide 14, carboxymethyl nucleo-... 

If the R-group in the alkyne is a strongly electron-withdrawing moiety such as a carbonyl function, regioselectivity may be reversed, leading to the problem of E/Z-selectivity (Scheme 2). Irrespective of the type of alkyne, the one-step atom-economical process would need to be catalyzed because a simple thermal process does not provide the products. 

Azo compounds like esters or imides of azo dicarboxylic acid act as reactive dienophiles in normal electron demand hetero Diels-Alder reactions due to the strong activation caused by two electron-withdrawing moieties. In the last years, considerable attention has focused on alkyl and phenyl derivatives of 1,2,4-tria-zoline-3,5-diones since their cycloadditions to chiral dienes proceed with often excellent facial selectivities. Thus, when reacting an oxapropellane derived diene with N -methyltriazolinedione, Paquette et al. obtained the cycloadduct as single diastereomer, but both maleic anhydride and N-phenyl maleimide were distinctly less reactive and turned out to undergo cycloadditions with poor selectivities [289]. 

In the previously described reactions, the basicity of anion radicals and their reactions with proton donors was emphasized. In the absence of viable proton donors or even in their presence, if the anion radical is relatively stable, radical coupling may be the dominant reaction. Thus even in aqueous solution, the anion radicals of alkenes substituted with strongly electron withdrawing moieties may undergo coupling in preference to protonation. The synthesis of adiponitrile from acrylonitrile (Scheme 71) is an outstanding example [118]. 

Modification of the aldoxime ether group by introduction of an electron withdrawing moiety led to the finding (186) that the Z-N-methoxyimidoyl nitrile group serves as a stable methyl ester bioisostere. An example of this molecular modification, compound (148),... 

Despite several obvious ineonsistencies, the results indicate the importance of electron withdrawing groups at positions 7 and 2. Another study by Borea employingthe Free-Wilson method was based on the inhibition of [ HJdiazepam binding by a set of 39 benzodiazepines (112). In addition to corroborating the positive effect of electron withdrawing moieties at positions 7 and 2, the results of... 

TP chromophore 52 bears a triazine as electron-withdrawing moiety, which is also known as a strong acceptor [449, 450], This is concluded from electrochemical measurements [451]. 8 is comparable to the TPA of 41f, 49a, and 49b bearing strong electron-withdrawing groups. 

To take advantage of MLCT transition directionality, Coe et al. have designed new bipyridine ligands functionalised by electron-withdrawing moieties (Figure Due to the absence of electron-donating... 

Presence of an electron-withdrawing moiety at C-6 is an absoluble necessity for the diuretic activity. A few important and vital observations are as enumerated below ... 

The stepwise (room temperature/thermal) cycloreversion sequence failed to provide furans from acetylene derivatives activated by just a single electron withdrawing moiety (e.g. propiolate esters). These derivatives, less reactive than DMAD, did not undergo cycloadditions to the immobilised dipoles at an appreciable rate at room temperature. 

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