Ylidic character

The generally accepted view about the aromaticity of A -phosphinines as summarized in the recent book is that A -phosphinines are not aromatic but they clearly are highly stable substances exhibiting very little ylidic character ." In accordance with the description used for the four-membered ring phos-phetes and diphosphetes, the resonance structures 33a—c are usually considered to describe the electronic structure." ... 

Diazomethylene)phosphoranes 33 (Scheme 8.10), which represent another type of diazocumulenes (12) are easily obtained by the oxidative ylidation of the corresponding phosphanyl(trimethylsilyl)diazomethane with CCI4. The increased stability of these compounds as compared with diazocumulenes (R2C=C=N2) is probably due to the ylidic character of the P=C bond. These diazo compounds exhibit the expected dipolar reactivity toward electron-deficient alkenes, alkynes, phosphaalkenes, and heterocumulenes (12). Thus, 33 reacts with TCNE to form A -pyrazoline 35 (60). Furthermore, 33 could be converted into the phosphonio-borate-substituted diazo compound 34, which underwent subsequent cycloaddition with electron-deficient alkenes (e.g., 34 36) (61). 

Many crystallographic structures of these carbenes and carbenoids has been solved, indicating (1) they are true carbenes with little ylidic character thus the contribution of ionic structures (175b) is relatively small in comparison with (175a) (2) small N-C-N angles at the carbene center (101-102°) in comparison with typical values (108.5-109.7°) for the corresponding angle in imidazolium salts ... 

The sulfoxide derivatives of some thiabenzenes have been prepared these molecules are much more stable than the parents, and may be subjected to a wider range of reactions (Scheme 26) (74JOC3519). It is apparent that the ylidic character of these molecules is reduced vis-a-vis the non-oxygenated species. The lack of reaction with acetyl chloride should be compared with the effect of trimethylsilyl chloride in equation (72) (74JA6119). The mechanism of this reductive dealkylation has not been explored. 

In cyclic sulfoximides such as 3-ethoxy-4,5-dihydro-3-oxo-l-phenyl-and 3,4,5,6-tetrahydro-3,5-dioxo-1 -phenyl-1H-1 A6,2,4-thiadiazine 1 -oxides (46), the C6 proton signals appear at much higher fields [4.35 and 4.9 ppm (2H)] than for aromatic systems [benzene, 7.27 ppm thiophene, 7.2 (C2-H) and 6.95 ppm (C3-H)]. Similarly, the, 3C-NMR signals of the C6 atom appear at higher fields. These facts suggest a degree of ylidic character (77JOC952), which can be demonstrated by the reactivity of such compounds towards electrophiles (see Section III, C,3,f). 

The ylidic character of some cyclosulfoximides (see Section III,B,3 for NMR evidence of ylidic character) can be demonstrated by the ease of... 

In six-membered rings, cr4,).5- 1 can also be a building block (see Scheme 4). The aromaticity of these compounds has been discussed [4,42,125] using different models emphasizing either the delocalization or the ylidic character of the bonding. The stabilization in the six-membered cyclic delocalized system o4,/,5-phosphin-ine 104 (Scheme 56) has also been shown before [42], Furthermore, a condensed... 

Thiabenzene 256 and its benzologues 1-thianaphthalene 257, 2-thianaphthalene 258 and 9-thiaanthracene 259 are also potentially antiaromatic 8ji electron systems provided they are planar. However, they adopt a boat conformation in which the S atom lies above the plane of the C atoms, thereby creating a 671 electron homoaromatic system with ylidic character. Calculations show that the energy barrier to inversion at S increases in the order 259 < 257 < 256 < 258 and the calculated dipole moments indicate the greatest ylide character is found in 259. The relative stability is in the order 259 > 257 > 258 > 256 (Table 19) <2006HAC376>. 

The addition reactions of H2O and MeOH (or EtOH) to azametaUenes and phosphametallenes are completely regioselective with the formation of >M(OR)-E(H)-(M = Si, Ge, E = N, P) compounds, indicating their ylidic character of M +=e 5- Similarly, the addition of hydrogen halides is also regioselective, giving the corresponding halometallanes. 

Although boron is more accurately described as a metalloid rather than a metal, this section is concluded by two papers that describe the structures and bonding in several organoboron/organophosphorus compounds that display ylidic character. The X-ray structure of 9-borylanthracene (88) shows that only one of the diisopropylphosphine moieties is bonded to the boron in the solid state. However, H NMR evidence shows that an intramolecular bond-switching process takes place very rapidly in solution. The structures of a series of borabenzene adducts of phosphorus ylides, iminophosphoranes and tertiary phosphines have also been determined. Treatment of l-chloro-3,5-dimethyl-2-(trimethylsilyl)-l,2-dihydroborinine (89) with methylenetriphenylphosphorane (90) produces (triphenylphosphonio)methanide-3,5-dimethylborabenzene (91). However, if the reaction sequence is reversed and (90) is treated with (89), then (trimethylsilyl)(triphenylphosphonio)methanide-3,5-dimethylborabenzene (92) is obtained (Scheme 26). Treatment of an isomeric mixture of l-chloro(trimethyl-silyl)dihydroborinines (93) with N-(triphenylphosphoranylidene)aniline (94) produces iV-(triphenylphosphonio)anilide-borabenzene (95) (Scheme 27). Crystal structures of (91), (92) and (95) show that the P-C or P-N bonds are... 

N-C=C-N fragment which serves as an electron donor and the kinetic stabilization provided by the two bulky adamantanyl substituents. Still, these groups are not mandatory as illustrated by the perdeu-terio derivative 16 which was shown to be a true carbene with negligible ylidic character by X-ray and neutron diffraction studies. [13] A quantitative study of the influence of steri-cally demanding substituents demonstrated that dimesitylcarbene 17 is ca. 160 times more stable than diphenylcarbene at room temperature [14] and for didurylcarbene a still further increase of life-time has been observed because of the buttressing effect of the additional methyl substituents. 

Azasilenes, also called as iminosilanes, are one of the heavier analogs of imines. The difference in the electronegativity between Si (1.8) andN (3.0) gives azasilenes ylidic character (Si +=N ), which makes them susceptible to oligomerization and/or dimerization as in the case with silenes (Si=C). 

The basicities in water and in aqueous surfactant solution of another 15 aryl-Pl(pyrrolidino) [(NC4Hs)3P=N-Ar] and aryl-Pl(dimethylamino) [(NMe2)3P=N Ar] phosphazenes (Ar = mono or di substituted phenyl rings) have been measured and correlated with previous data using acetonitrile and THE. A comparison with other bases such as guanidines, amines and pyridines showed that, in all cases, the phosphazenes were the strongest bases. The results confirmed earlier conclusions about the partly ylidic character of the NP double bond. Density functional theory calculations have shown that the phosphazenes (41) and (42) are very powerful neutral organic superbases at the site marked by an asterisc (the calculated pA a values in acetonitrile were 44.8 and 37.8, respectively). ... 

The first X-ray structure of a thiabenzene derivative, i.e. l-benzoyl-2-methyl-2-thianaphthalene, has been reported. The geometry is similar to that of open-chain benzoyl-stabilized ylides with pyramidal sulphur. Other studies point to the ylidic character of 9,10-diphenyl-10-thia-anthracene . N.m.r. data, including C n.m.r. chemical shifts, have been presented for indole-3-sulphonium ylides and carbonyl-stabilized sulphonium ylides and their Pd" complexes. Infrared spectra of cr-nitrosulphonium ylides have been analysed. Further details have been provided of a study of the pyramidal inversion of unstabilized sulphonium ylides (see Chap. 5, pp. 243 and 251). 

As a result of its dative-coordinate character, formation of a crA B bond is associated withybwia/ charge separation (A -B+ ylidic character ) that is expected to weaken the bonding interaction compared to ordinary covalent interaction. The unusual ylide formal charge pattern is manifested in anomalous ionicity of a coordinate CAm bond compared to the normal covalent-bond ionicity expected from Ha, Sb electronegativity values. 

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