Carbonyl deficiency

If the fraction of acid groups which are in the carboxylate form (35%) do not contribute to the carbonyl intensity, a carbonyl deficiency of 0.7 carbonyl groups per polyamic acid repeat unit follows from the model in fig. 3 for PAA. This is close to the 0.6 we found in the analysis of the Cls core level data. 

Whereas the proton transfer does not effect the stochiometry of the final PI when water is eliminated in the imidization reaction (fig. 3F), addition of an excess ODA molecule to polyamic acid could lead to the imine type crosslink formation schematically shown in figure 3G. This would lead to a deficiency of carbonyl oxygen atoms for vapor deposited polyimide and is consistent with our analysis. Mack et al. [16] proposed imine crosslink formation from their Raman spectroscopic studies for vapor deposited polyimides with excess ODA. In accordance with this model we attribute the low binding energy shoulder in the polyimide Nls line (figure 4c) to double bonded nitrogen species. However, the model gives no explanation for the carbonyl deficiency found in spin deposited polyamic acid and polyimide. In this case no excess of ODA is observed and only a very weak shoulder has been reported for the Nls line [4,11]. 

For the cured polyimide, the CF /F ratio was calculated using the integrated peak area results. A value of 0.33 was determined experimentally (vs. the value of 0.33 predicted). Extensions of this approach occurred by calculation of =C=0/N and =C=0/ ratios. Values of 1.08 and 0.32, respectively, were determined experimentally (vs. 2.0 and 0.67 predicted on the basis of structural formula considerations). These results indicated either carbonyl deficiency, an excess of nitrogen or an excess of fluorine in the surface region. 

Hydroxy-THISs react with electron-deficient alkynes to give nonisol-able adducts that extrude carbonyl sulfide, affording pyrroles (23). Compound 16 (X = 0) seems particularly reactive (Scheme 16) (25). The cycloaddition to benzyne yields isoindoles in low- yield. Further cyclo-addition between isoindole and benzyne leads to an iminoanthracene as the main product (Scheme 17). The cycloadducts derived from electron-deficient alkenes are stable (23, 25) unless highly strained. Thus the two adducts, 18a (R = H, R = COOMe) and 18b (R = COOMe, R = H), formed from 7, both extrude furan and COS under the reaction conditions producing the pyrroles (19. R = H or COOMe) (Scheme 18). Similarly, the cycloadduct formed between 16 (X = 0) and dimethylfumarate... 

Electron deficient carbon-carbon double bonds are resistant to attack by the electrophilic reagents of Section 5.05.4.2.2(t), and are usually converted to oxiranes by nucleophilic oxidants. The most widely used of these is the hydroperoxide ion (Scheme 79). Since epoxidation by hydroperoxide ion proceeds through an intermediate ct-carbonyl anion, the reaction of acyclic alkenes is not necessarily stereospecific (Scheme 80) (unlike the case of epoxidation with electrophilic agents (Section 5.05.4.2.2(f)) the stereochemical aspects of this and other epoxidations are reviewed at length in (B-73MI50500)). 

When the carbonyl group is very electron-deficient, thus stabilizing the hemiacetal, a dioxolane can be prepared under basic conditions. 

Perfluorinated carbonyl compounds, especially hexafluoroacetone, are highly electron-deficient species and react vigorously with a wide variety of HX nucleophiles The reaction of these ketones and of most polyfluonnated imines toward nucleophiles can be generahzed by the scheme shown m equation 1... 

The dienophilic character of imines parallels that of carbonyl compounds Consequently, electron deficient imtnes are the most reactive dienophiles of this class, particularly those having C perfluoroalkyl [5, 146, 150, 228], /V-acyl [/2i5 127], or A/-sulfonyl groups [148, 229 230]... 

The LUMO delineates areas which are most electron deficient, hence subjeet to nucleophilie attaek. On the maps above, regions where the absolute value of the LUMO is greatest are indieated in blue , while regions where it is least are indieated in red . As expected, the blue regions are directly over the carbonyl carbon. More interesting, note that the blue spot over the axial face is larger than that over the equatorial faee. This suggests that nueleophilie attack onto the axial face is likely to be more favorable than attack onto the equatorial face, in accord with experimental observation. 

The [ 2 + 4]-cycloaddition reaction of aldehydes and ketones with 1,3-dienes is a well-established synthetic procedure for the preparation of dihydropyrans which are attractive substrates for the synthesis of carbohydrates and other natural products [2]. Carbonyl compounds are usually of limited reactivity in cycloaddition reactions with dienes, because only electron-deficient carbonyl groups, as in glyoxy-lates, chloral, ketomalonate, 1,2,3-triketones, and related compounds, react with dienes which have electron-donating groups. The use of Lewis acids as catalysts for cycloaddition reactions of carbonyl compounds has, however, led to a new era for this class of reactions in synthetic organic chemistry. In particular, the application of chiral Lewis acid catalysts has provided new opportunities for enantioselec-tive cycloadditions of carbonyl compounds. 

In the presence of strong bases, carbonyl compounds form enolate ions, which may be employed as nucleophilic reagents to attack alkyl halides or other suitably electron-deficient substrates giving carbon-carbon bonds. (The aldol and Claisen condensations... 

Becanse nitradon of ilkenes v/ith nitronittm s ilts proceeds via carbocadon intermediates, nitradon of electron-deficient ilkenes v/ith nitronittm s ilts is rare Only a few cases are reported The reacdon of ct,fi-ttnsatitrated esters v/ith nitronittm s ilts affords prodticts via highly reacdve ct-carbonyl cadons... 

The Michael addition of nih oalkanes to alkenes substituted with two elecbon-withdrawing groups at the a- and 3-positions provides a new method for the preparation of functionalized alkenes. Although reactions are not new, Ballini and coworkers have used this sbategy in the synthesis of polyfunctionalized unsaturated carbonyl derivatives by Michael addition of nih oalkanes to enediones as shown in Eqs. 7.124-7.126. Success of this type of reaction depends on the base and solvent. They have found that DBU in acetonihile is the method of choice for this puipose. This base-solvent system has been used widely in Michael additions of nitroalkanes to elechon-deficient alkenes (see Section 4.3, which discusses the Michael addition). ... 

The mode of the diastcrcofacial selectivity is completely reversed in the case of reactions with A -methyl A-acyliminium precursors 4176. Now the nitrogen atom of the A-acyliminium ion is not able to chelate with the tin atom and the lower diastereoselectivity is explained by the less rigid nonchelation-controlled transition state 5. An electronic effect, such as n-iz attraction between the electron-deficient carbonyl group of the acyliminium ion and the electron-rich phenyl group of the phcnylthio substituent R, may account for the somewhat higher diastereoselectivity in the case of arylthio substituents R. 

The stability of carbonyls of a metal in the +3 oxidation state is unusual because d7r-p7r back donation is necessary to stabilize the Ir-C bond, generally only one CO group can be bonded to the electron-deficient Ir3+. 

In discussing the elFect of structure on the stabilization of alkyl cations on the basis of the carbonylation-decarbonylation equilibrium constants, it is assumed that—to a first approximation—the stabilization of the alkyloxocarbonium ions does not depend on the structure of the alkyl group. The stabilization of the positive charge in the alkyloxocarbonium ion is mainly due to the resonance RC = 0 <-> RC = 0+, and the elFect of R on this stabilization is only of minor importance. It has been shown by Brouwer (1968a) that even in the case of (tertiary) alkylcarbonium ions, which would be much more sensitive to variation of R attached to the electron-deficient centre, the stabilization is practically independent of the structure of the alkyl groups. Another argument is found in the fact that the equilibrium concentrations of isomeric alkyloxocarbonium ions differ by at most a factor of 2-3 from each other (Section III). Therefore, the value of K provides a quantitative measure of the stabilization of an alkyl cation. In the case of R = t-adamantyl this equilibrium constant is 30 times larger than when R = t-butyl or t-pentyl, which means that the non-planar t-adamantyl ion is RT In 30= 2-1 kcal... 

Radical-based carbonylation procedures can be advantageously mediated by (TMSlsSiH. Examples of three-component coupling reactions are given in Reactions (74) and (75). The cascade proceeds by the addition of an alkyl or vinyl radical onto carbon monoxide with formation of an acyl radical intermediate, which can further react with electron-deficient olefins to lead to the polyfunctionalized compounds. ... 

The per acid first adds to the ketone to give adduct (30) which rearranges via a transition state (31) which is electron-deficient around the former carbonyl group. Consequently,the group which can best supply electrons to combat the deficiency migrates best. It does so with retention as it is a one step reaction in which the chiral centre ( in 30) never becomes detached. 

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