Covalent ylides

The ylides may be defined as dipolar compounds in which a carbanion is covalently bonded to a positively charged heteroatom. They are represented by the following general formula ... 

The ylides have been classified on the basis of the heteroalom covalently bonded to the carbanion. Accordingly, they can be differentiated into nitrogen ylide (Scheme 2), sulfur ylide Scheme 3, phosphorus ylide Scheme 4, arsenic ylide Scheme 5, antimony ylide (Scheme 6), bismuth ylide (Scheme 7) and thallium ylide (Scheme 8). 

A comparative study on ylide stability as a function of the heteroatom type was carried out by Doering et al. [3,4]. They concluded that the phosphorus and sulfur ylides are the most stable ones. The participation of three-dimensional orbitals in the covalency determines the resonance stabilization of the phosphorus and sulfur ylides [5-8]. The nitrogen ylides are less stable from this point of view. The only stabilization factor involves electrostatic interactions between the two charges localized on adjacent nitrogen and carbon atoms [9]. 

As a recent result an example of C- and 0-covalently bonded tin substituted ylides (respectively 90 and 91) has been reported, the adducts resulting from the reaction of a stabilized yldiide with tin derivatives (RjSnCl, R2SnCl2, or SnCl2) (Scheme 28) [64]. 

Triphenylphosphonium ylide reacts with the silylene complex 93 which has a highly electrophilic silicon center, to give the corresponding cationic adduct 94 [115]. The lengthening of the PC bond indicates a loss of the double bond character of the ylide and corresponds to the formation of a tetrahedral silicon center with four covalent bonds (Scheme 28). 

In 1995, Boyd and co-workers <95TL7971 > covalently linked a porphyrin to fullerene Cgo through a 1,3-dipolar cycloaddition reaction involving the porphyrinic azomethine ylide 28 (Scheme 8). The ylide was generated in situ from befa-formyl-meso-tetraphenylporphyrin 27 and A -methylglycine, and provided the porphyrin-C6o diad 29 in good yield. 

The Ge(TMTAA) complex and the well known Sn(TMTAA) complex undergo facile oxidative addition reactions and reverse ylide formation with Mel and C6F5I because of the reactive M(II) (M = Sn, Ge) lone pair of electrons. In case of the oxidation with Mel it was assumed that, in solution, an ionic-covalent equilibrium exists (equation 48)95. 

Dipolar cycloaddition of azomethine ylides, generated by the condensation of an a-amino acid and an aldehyde, is an efficient method for covalent sidewall functionalisation and has been successfully used to solubilise CNTs in most organic solvents (Tasis et al., 2003 Holzinger et al., 2003). This particular technique has also been utilised to obtain the first example of a bioactive peptide covalently linked to CNTs by the prospect for the potential applications in immunology (Bianco and Prato, 2003 Pantarotto et al., 2003a, b Bianco et al., 2005b). 

Whereas in Wmethyl-4,6-dimethylpyrimidinium ion the covalent addition takes place at C-6, it is assumed that 30 also undergoes covalent hydrazina-tion at C-6. However, the formation of dimer 32 shows the high sensitivity of C-2 in 30 for addition of nucleophiles, and it leads to the daring suggestion that it is the resonance-stabilized ylide 31 that probably is the active species undergoing addition at C-2 (Scheme III. 19). It was calculated (80UP1) that the reactivity at C-2 in the N-ylide 31 is greater than that at C-2 in the Waminopyrimidinium salt 30. 

The use of lithium amides to metalate the a-position of the N-substituent of imines generates 2-azaallyl anions, typically stabilized by two or three aryl groups (Scheme 11.2) (48-62), a process pioneered by Kauffmann in 1970 (49). Although these reactive anionic species may be regarded as N-lithiated azomethine ylides if the lithium metal is covalently bonded to the imine nitrogen, they have consistently been discussed as 2-azaallyl anions. Their cyclization reactions are characterized by their enhanced reactivity toward relatively unactivated alkenes such as ethene, styrenes, stilbenes, acenaphtylene, 1,3-butadienes, diphenylacetylene, and related derivatives. Accordingly, these cycloaddition reactions are called anionic [3+2] cycloadditions. Reactions with the electron-poor alkenes are rare (54,57). Such reactivity makes a striking contrast with that of N-metalated azomethine ylides, which will be discussed below (Section 11.1.4). 

The phosphorus ylide complex [(Ph3PCH2)2Au]2Ag2(C104)4 445 also contains Au-Ag bonds [(2.783(2)/2.760(2) A] unsupported by any covalent bridge within a Au2Ag2 ring, but each silver atom is further bonded to two oxygen atoms from two... 

The overlap of carbon p orbitals with arsenic d orbitals is less effective than with the d orbitals of phosphorus, and so the covalent canonical structure is expected to make less of a contribution to the hybrid structure. This has been confirmed in an X-ray study of 2-acetyl-3,4,5-triphenylcyclopentadienetriphenylarsorane.6 Yamamoto and Schmidbaur7 found (13CNMR) that the bonding in arsenic ylides was probably sp3 (cf. phosphorus, which changes from sp3—>sp2), resulting in arsenic pseudotetrahedral geometry (cf. phosphorus ylides, which are planar). 

However, certain phosphonium ylides, such as those with an electron-withdrawing substituent in the alkylidene moiety, are relatively unreac-tive toward certain substrates such as ketones (22, 77, 95). This led us to consider whether arsonium ylides might be preferable to phosphonium ylides in certain reactions (48, 94). The overlap of the p orbitals of carbon with d orbitals of arsenic is less effective than with d orbitals of phosphorus. Therefore the covalent canonical form (la) should make a smaller contribution to the overall structure of arsonium ylides than to that of the corresponding phosphonium ylides. 

On going down the periodic table (from P to As, Sb, and Bi), the absorption maxima are greatly shifted toward longer wavelengths. The shift is consistent with a decreasing contribution from the covalent form or the increasing contribution by the dipolar form of the ylides with increasing atomic number of the heteroatom. 

The systematic naming of these substances is cumbersome, but they have come to be known as ylides. The genesis of this name may seem obscure, but it is an attempt to reconcile the presence of a C-X a bond, which is covalent and nonpolar as in alky/ derivatives, as well as an ionic bond as in metal h Udes. Hence, the combination yl-ide.2... 

Ferrocene is composed of a pair of 6-7r-electron carbon arrays and a 6-d-electron iron(II) atom. Ferrocene-fullerene donor-acceptor dyads carry all the requisites for electron-transfer phenomena. However, data for the formation of ferrocene-fullerene hybrids are not abundant. Some such dyads have already been synthesized following the methodology of 1,3-dipolar cycloaddition of the appropriate azome-thine ylides to C60, with either variable-spacing building blocks or a rigid-bridge all-cj-bonded framework, in order to tune the redox properties of the system [40,234, 248-251]. Another novel dyad that contained two covalently bound ferrocene units was recently synthesized via cyclopropanation of the fullerene core [252]. 

The vast majority of organocatalytic reactions proceeds via covalent formation of the catalyst-substrate adduct to form an activated complex. Amine-based reactions are typical examples, in which amino acids, peptides, alkaloids and synthetic nitrogen-containing molecules are used as chiral catalysts. The main body of reactions includes reactions of the so-called generalized enamine cycle and charge accelerated reactions via the formation of iminium intermediates (see Chapters 2 and 3). Also, Morita-Baylis-Hillman reactions (see Chapter 5), carbene-mediated reactions (see Chapter 9), as well as asymmetric ylide reactions including epoxidation, cyclopropanation, and aziridination (see Chapter 10), and oxidation with the in situ generation of chiral dioxirane or oxaziridine catalysts (see Chapter 12), are typical examples. 

At 100 °C pentamethylarsorane (/) is quantitatively decomposed to trimethyl-arsine, methane and a little ethylene, thus supporting the assumption of intermediate formation of ylide 148 ethane is only formed in traces131). With water and acids tetramethylarsonium salts are produced m) whereas with alcohols, hydroxylamines and oximes covalent pentacoordinate arsoranes 149 are obtained in which one methyl group has been replaced by the respective electronegative group Y 132,133). 

It is possible to remove a proton from the methyl group of a trialkylmethylammonium halide with strong bases. Thereby, a betaine (see Section 4.7.4) is produced with the structure R3N+—CH2. A betaine in which the positive and the negative charges are located on adjacent atoms as in R3N+—CH2 is called an ylide. The yl part of the name ylide refers to the covalent bond in the substructure N+—CH2. The ide part indicates that it also contains an ionic bond. When one wants to distinguish the ylide R3N+—CH2 from other ylides, it is called an ammonium ylide or an N ylide. 

A different effect is observed when a C=C double bond is attached directly to the sulfonium center (vinylic system see Fig. 2k and Fig. 3c). Nucleophihc addition prevails because an ylide intermediate generated after addition at the adjacent carbon atom is stabilized by resonance involving d-orbitals of the sulfur atom. Consequently, the whole cofactor is covalently attached to the target molecule and irreversibly inhibits the enzyme (39). 

In the reviewed period, there appeared a number publications utilizing C6o-fullerene to make Jt-exTTFs. The synthesis of the first highly conjugated TTF analogues 944 covalently attached to Ceo-fullerene was carried out by 1,3-dipolar cycloaddition of the appropriate azomethine ylides to Cso-fullerene (Scheme 143) <1997JOC5690>. 

N-Metallated azomethine ylides 140 of ester-stabilized types are tautomeric to the metal ester enolates (141) of chelate-stabilized types. The only structural difference is which heteroatom between the imine nitrogen and the ester carbonyl oxygen is connected with the metal (M) by a covalent bond. The difference in chemical properties expected for the ylidic forms 140 and enolate forms 141 is not yet clear. 

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