Mesomeric form

Enamines have a three-atom tt system and are thus in principle capable of reaction with an electrophile on nitrogen or on carbon as shown by the mesomeric forms (1) and (2),... 

The basicity of the nitrogen is further reduced when Y is an electron-attracting atom such as an oxygen, and the mesomeric form (3) becomes increasingly important. 

A -Methylpyrrole oxidatively adds to [Ru(C=C=C=CH2)(PPli3)2(Cp)][PF6] via its C2 center, the product being the allenylidene species 38 [98JCS(D)467]. Alkyne mesomeric form 39 was postulated to make a significant contribution, which explains well the nature of the deprotonated product 40, obtained from 38 and n-butyllithium. 

The early stages of the reaction of the quaternary salt can be regarded as proceeding in a manner exactly analogous to that by which the isoxazoles themselves are degraded, the j8-oxoketene imine structure (148) being one mesomeric form of a compound which could alternatively be formulated as a nitrilium betaine. However, by contrast with the products from the isoxazoles (i.e., enolates of /3-keto-nitriles), this is electrically neutral and susceptible to further nucleophilic attack. 

In conclusion, with regard to the structure of benzenediazonium compounds with electron donor substituents in the 2- or 4-position, the most recent experimental data, mainly X-ray analyses and 13C and 15N NMR data, are consistent with 4.4 as the dominant mesomeric structure of quinone diazides, as proposed by Lowe-Ma et al. (1988). For benzenediazonium salts with a tertiary amino group in the 4-position the data are consistent with the quinonoid structure 4.20 as the dominant mesomeric form. 

These reactions clearly indicate that the exocyclic carbon of the complexed QM in these systems is nucleophilic in character, in contrast to its electrophilic nature in free o-quinone methides. The Cp Ir metal center stabilizes the mesomeric form in which the exocyclic carbon experiences high electron density (Scheme 3.18).29... 

Pulse radiolysis of N20-saturated aqueous solution of 1,4-cyclohexadiene leads to formation of three radicals, two by addition of either OH or H atoms to give the cyclohexenyl radicals 3 and 4 (equation 12 and 13) and one by abstraction of H atoms (equation 14). The last one, the cyclohexadienyl radical, can exist in two mesomeric forms (5a and 5b). Fessenden and Schuler16 found that the spin density of the cyclohexadienyl radical was highest at the central atom, i.e. form 5a is the predominant one. 

This conclusion is supported by results of detailed study on the decay of hydroxyhexa-dienylperoxyl radicals, formed by addition of OH to benzene, followed by addition of dioxygen molecule. It was found that in the high dose rate of pulse radiolysis, hydro-quinone is the major product whereas catechol was not observed, indicating that only the 1,3-isomer loses HO2" and hence does not lead to dihydroxybenzene. The observation that the yield of 02 is 60% of the yield of the cyclohexadienyl radicals indicates that when dioxygen molecules react with the cyclohexadienyl radical, the radical is 60% trapped in the mesomeric form of 5b, whereas the results from the final products of dimerization in /-radiolysis show that 60% react in the form 5a. 

Conclusions about mesomeric forms arose from X-ray analysis of mesoionic A -[2-(5-methyl-l,3,4-thiadiazolo[3,2- ]pyri-dinio)]acetamidate 28 (Scheme 6). Among the six possible delocalized structures, a large contribution of the 1,3,4-thiadiazolium structures 28a and 28b was observed, rather than pyridinium structures. These two mesomeric forms are in agreement with the high double-bonded character of the S-C6 bond <1996BCJ1769>. 

Betaine 81 or its neutral mesomeric form (Scheme 23) reacted with nitriles affording the thiadiazole 82, and a dipolar addition was suggested <1996H2657>. In a similar reaction, isocyanates and isothiocyanates gave oxo and thiono thiadiazoles 83 (X = 0, S) <1997BMC1275>. 

The product is exclusively carbon monoxide, and good turnover numbers are found in preparative-scale electrolysis. Analysis of the reaction orders in CO2 and AH suggests the mechanism depicted in Scheme 4.6. After generation of the iron(O) complex, the first step in the catalytic reaction is the formation of an adduct with one molecule of CO2. Only one form of the resulting complex is shown in the scheme. Other forms may result from the attack of CO2 on the porphyrin, since all the electronic density is not necessarily concentrated on the iron atom [an iron(I) anion radical and an iron(II) di-anion mesomeric forms may mix to some extent with the form shown in the scheme, in which all the electronic density is located on iron]. Addition of a weak Bronsted acid stabilizes the iron(II) carbene-like structure of the adduct, which then produces the carbon monoxide complex after elimination of a water molecule. The formation of carbon monoxide, which is the only electrolysis product, also appears in the cyclic voltammogram. The anodic peak 2a, corresponding to the reoxidation of iron(II) into iron(III) is indeed shifted toward a more negative value, 2a, as it is when CO is added to the solution. 

The intermediate Jt-allyl complex is formally the palladium(II) complex of an allylic anion that can be represented by the two mesomeric forms shown in Scheme 17.2. It is important to note that this is not a fast equilibrium between two cr-allyl complexes but a stable species where palladium is simultaneously bound to both carbon-1 and carbon-3. All eight atoms of the Jt-allyl moiety are almost in the same plane. All three carbon atoms have sp2 character and the rotation between the Cl-C2 and C2-C3 bonds is blocked. As a consequence of the hindered rotation, four dia-stereomeric Jt-allyl complexes are possible. For example, in Scheme 17.2 both R and R are syn to the hydrogen on carbon-2, therefore this complex is called the syn,syn diastereomer. 

Removal of one of the methylene protons generates a carbanionic center, but the corresponding single Lewis formula is a poor description of the electronic structure. More mesomeric forms of 2A may be written to give a more adequate fomu-lation. Alternatively, a circle may be drawn to symbolize the 3c2e n bond in 2A, resulting from overlapping p orbitals perpendicular to the plane of atoms involved. 

We are here picturing the production of real states [mesomeric forms] from unreal states [i.e., the ordinary structural formulas] and not the deformation of real states by some external disturbance, as is the case in most of the physical problems to which perturbation theory is commonly applied.. . . There can be no physical separation. . . between resonance vibrations and other electronic vibrations it follows that the unperturbed structures. .. are only of the nature of intellectual scaffolding, and that the actual state is the mesomeric state. Chemical evidence in support of these ideas is extensive.43... 

Intramolecular charge transfer in 4-nitropyridine N-oxide has been investigated by spectroscopic methods and by comparison with AMI and MNDO semi-empirical methods to obtain the vibrational force field225. The results obtained indicate that protic solvents (water, methanol) favour the mesomeric form 97 which is also favoured in the crystal, by an internal interaction between the nitro and TV-oxide groups226. 

Fig. 6.3. Conformational equilibrium about to, showing its strong preference (ca. 90 kJ moD1) for the tmns-conformation, as stabilized by its mesomeric form with C=N bond character...

A careful analysis based on these experimental results excluded a chain-propagation process [33a]. On account of the 3-position of the methylthio or methoxy substituent in the thiophene or pyrrole rings, three isomeric dimers may be formed. The main reaction path can be deduced from the mesomeric forms of the radical cations (2)". The two most important mesomeric structures are those with the unpaired electron in... 

Fig. 1 Major mesomeric forms of allenylidene complexes of types A and B...

Based on a large number of stoichiometric studies, the main trends of allenylidene reactivity are presently well established [26 0]. They are governed by the electron deficient character of the and Cy carbon atoms in the cumulenic chain, the Cp being a nucleophilic center. This can also be rationalized by considering the mesomeric forms depicted in Fig. 14. However, it should be noted that, as commented on previously, only contribution from the alkynyl resonance form is supported by X-ray diffraction. 

In this chapter, the most efficient synthetic routes, the main stmctural features as well as reactivity patterns of odd-chain metallacumulene complexes bearing 7i-donor substituents, i.e., [M]=C(=C) =CR R ( = 1, 3, 5 R /R = NR2, OR, SR, SeR), are reviewed. In addition, the coordination chemistry of phosphonioace-tylides (R3P C=C ) and tricarbon monoxide (C3O) will also be discussed since these heteroatom-containing 77 -carbon ligands lead to closely related bonding situations, with participation of both neutral cumulenic and zwitterionic alkynyl-type mesomeric forms (Fig. 3). 

Fig. 3 Mesomeric forms of metal complexes derived from phosphonioacetylides and tricarbon monoxide...

Fig. 17. Amide isomerism due to mesomeric forms resulting in (E) and (Z) isomers...

Infrared spectroscopy is the only method that gives direct, diagnostic information about the presence of a diazo group in diazoazoles. The aromatic character common to the diazoazoles and diazocyclopentadiene and the importance of the contributions of the different mesomeric forms to the resonance hybrids in the ground state are reflected in the position of the bands associated with the diazo group in the infrared spectra (Table IV). 

The dipolar structure 1 describes the chemical behavior of thiocarbonyl ylides best, although other mesomeric forms have been used for the representation of the electronic structure of these dipoles. The parent compound, thioformaldehyde (5)-methylide (1), was studied by means of spectroscopic and theoretical methods (2-5), which showed that the molecule possesses a bent allyl-type structure (6). According to theoretical calculations, structures lA and IB have the largest contribution (31.5% each) in the representation of the electronic structure, whereas 1C, which reflects the 1,3-dipolar character, has only a 4.2% contribution (5). 

Figure 9.13—Effect of resonance on carbonyl-containing compounds. Representation of the delocalisation of valence electrons in mesomeric forms of organic compounds. In1 C NMR, the signal corresponding to a carbonyl in an ester is at 165 ppm, whereas it is at 205 ppm for a ketone.

Figure 11.4—The n — a transition in primary amines. This transition corresponds to an increase in the weight of the mesomeric form. Absorption corresponding to this transition disappears when a proton-containing hydrogen halide is added. This effect is due to the formation of a quaternary ammonium salt that mobilises the electron pair of the nitrogen atom, which is necessary for this transition (see formula in brackets).

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