Ylide-type bond

According to STO-3G calculations, the structural stability of the planar 67r-electron four-membered ring is achieved in thiacyclobutadiene (77T3061), which has a cyclic ylide-type bonding. 

Further chemical fine tuning has been achieved by means of additional ligands with ylide-type bond systems from the iso-electronic series of highly polar P-ylides, P-imines and P-oxides. Examples for HDPEs (high density polyethylenes) are given in Tab. 1.4. 

Tetracyano ethylene oxide, however, which represents a potential 1,3-dipole of the carbonyl ylide type, reacts with diphenyl cyclopropenone to give a cycloadduct of probable structure 415/417263, which may arise from insertion into the cyclopropenone C1(2)/C3 bond. 

The attack of nucleophiles on unsaturated ligands or functional groups bonded to metallic centers, exemplified in Scheme 9 (reaction of metallic carbenes with phosphines or pyridines) or in Scheme 15 (Wittig reaction) can be extended to a wide variety of reagents. Two main groups of reactions can be considered (1) those in which the nucleophile is an ylide and (2) those in which the nucleophile is a phosphine (and less commonly other nucleophiles). Usually these reactions give metallated ylides (type III), that is, species in which the ylide substituents are metallic centers. 

Complexes 75 are remarkably stable at room temperature in the solid state and, when heated, they start to decompose only at about 130 °C (Cr) or 145 °C (W). Such a thermal stability is undoubtedly associated with their strongly dipolar nature, in which six possible ylide-type resonance forms contribute to the bonding (Fig. 12). As expected, analysis of the electronic structure of complex [W (=C=C=C=C=C=C=C(NMe2)2 (CO)5] by DPT methods showed that the LUMO is mostly localized on the odd carbon atoms of the chain, whereas the HOMO is on the even carbons. In accord with these electronic features, it was found that [W =C=C=C=C=C=C=C(NMe2)21(00)5] readily adds dimethylamine across the 05=05 bond, to give the isolable alkenyl-pentatetraenylidene derivative [W =C=C=C=C=C(NMe2)CH=C(NMe2)21(00)5] [69, 70]. 

Topological resonance energy (TRE) values suggest that 346 is considerably less stable than 345. CNDO/2 calculations with inclusion of d-orbital participation for 346 show considerable 7i-bonding between carbon and sulfur of which over half is attributable to pTt-dn overlap the degree of charge separation indicates that canonical resonance forms of type b (carbonyl ylide type) are more important than those of type c (thiocarbonyl ylide type)." ... 

It was soon found that it was possible to form ylides of the C15 ylide type (6), which were resonance-stabilized by conjugated double bonds, from the phosphonium salts, using alcoholates, also in protic solvents, such as alcohols. In addition, they are so retarded in their reactivity that they are no longer hydrolyzed by water... 

The first example of this last class (98), where tetracovalent bonding occurs through valence-shell expansion (for a review of this topic see Salmond308), was obtained in 1961309 and there are still rather few examples known. Discussion has centered upon the nature of the participation by the 3d orbitals and whether the compounds may be regarded as being aromatic or of the ylide type. 

Thiabenzene 1-oxides (e.g., 102) have also figured in the above discussion. Earlier papers313-318 did not rule out the possibility of aromatic conjugation in these compounds, but NMR data are suggestive of ylide character, and bonding of the ylide type has recently been firmly proposed by Hortmann and Harris.313 314 Somewhat related systems are the anions derived from cyclic sulfones such as 103 and 104. Pagani et aZ.319-321 have... 

An ab initio study of structure and bonding in diphosphinylcarbenes indicates that the non-symmetric ylide-type structure (29) is preferred." The configuration and conformation of a number of polyfunctional 1-aminobuta-1,3-dienes (30), prepared by the reaction of dimethyl acetylenedicarboxylate with (Z)-P-enamino-l -phosphazenes, have been studied by n.m.r.. X-ray, and theoretical methods. The P n.m.r. spectra of a number of protonated iminophosphorane-substituted proton sponges, e g. (31), of known crystal structure have been studied in the solid state."... 

Five-membered heterocycles with one nitrogen atom can be prepared from azomethine ylide-type dipoles and alkynes or alkenes. Several solid-supported cycloadditions with maleimide as a dipolarophile have been reported. Trityl resin-bound maleimide captured azomethine ylides that were generated in situ from amino acid methyl esters and aldehydes, and substituted resin-bound pyrrolidines were obtained (Scheme 11.1). Traceless cleavage of the C—N bond between acid-sensitive trityl resin and the IV-unsubstimted cycloadduct was achieved with 50% trifluoroacetic acid. 

The synthesis of aziridines through reactions between nitrenes or nitrenoids and alkenes involves the simultaneous (though often asynchronous vide supra) formation of two new C-N bonds. The most obvious other alternative synthetic analysis would be simultaneous formation of one C-N bond and one C-C bond (Scheme 4.26). Thus, reactions between carbenes or carbene equivalents and imines comprise an increasingly useful method for aziridination. In addition to carbenes and carbenoids, ylides have also been used to effect aziridinations of imines in all classes of this reaction type the mechanism frequently involves a stepwise, addition-elimination process, rather than a synchronous bond-forming event. 

A second example of step-growth polycondensations with formation of the ole-finic double-bond are Wittig- and Wittig-Horner-type condensations. The Wittig-type polycondensations involve AA/BB-type reactions of aromatic bisal-dehydes with bisphosphonium ylides [99,100] with formation of PPV derivatives (75) and lead to products of only moderate molecular weight (DP 10-20). 

The 1,3-dipolar cycloaddition reactions to unsaturated carbon-carbon bonds have been known for quite some time and have become an important part of strategies for organic synthesis of many compounds (Smith and March, 2007). The 1,3-dipolar compounds that participate in this reaction include many of those that can be drawn having charged resonance hybrid structures, such as azides, diazoalkanes, nitriles, azomethine ylides, and aziridines, among others. The heterocyclic ring structures formed as the result of this reaction typically are triazoline, triazole, or pyrrolidine derivatives. In all cases, the product is a 5-membered heterocycle that contains components of both reactants and occurs with a reduction in the total bond unsaturation. In addition, this type of cycloaddition reaction can be done using carbon-carbon double bonds or triple bonds (alkynes). 

It is commonly accepted5,6,19 that unstable betaines IV (X = C) are intermediates of the cyclopropanation of olefins with the polar C=C bond by phosphorus ylides. However, only one compound of this type, viz., Me3P( + )-CH2-CMe2-C5H4( ) (1), synthesized in the reaction of dimethyl-fulvene with methylenetrimethylphosphorane, was isolated and characterized by multinuclear NMR spectroscopy.20... 

The reaction of 34 with triethylethylidenephosphorane is more complex. According to the multinuclear NMR data, the reaction occurs at the 1 2 ratio of the reactants. The Sn-O bond is cleaved to give phosphonium phenoxide (38) and stannylene (37) in which the tin atom is also bound to the ylide carbon atom of phosphorane (Scheme 17).61 Metallation reactions of this type are well known.61,75... 

As mentioned above (see Scheme 1), three main directions of the decomposition of intermediates that formed are possible when phosphorus and arsenic ylides react with compounds bearing C=X bonds 5,6,19,63,64,88 (i) elimination of R3E15=X to form olefins (Wittig type reaction) (ii) retro-Wittig type decomposition and (iii) elimination of R3E15 and formation of three-membered cycles (Corey-Chaykovsky type reaction). According to the data of Erker and coworkers,12,13,51 under kinetic control, the reaction of phosphorus ylides with thiocarbonyl compounds also affords phosphines and thiiranes, whose further transformations lead to olefins and R3PS under thermodynamic control. 

The addition of trimethyltinlithium to triphenylvinylphosphonium bromide (Scheme 3) produced a new type of metallo-ylide (7), which on addition to cyclohexanone gave (8), containing a very nucleophilic double bond, which in the... 

Polyenes are most often synthesized by cross-coupling reactions between unsaturated systems. Typically these reactions require an activated carbon, often in the form of an organometallic reagent. Enolates and phosphonium ylides, Wittig-type reagents, are also commonly employed in carbon-carbon bond formation. Pericyclic rearrangements also result in the generation of new carbon-carbon bonds and will be treated separately. 

Metallic groups as in case (c) lead to electrophilic or even carbocation-like carbene complexes. Typical examples are Fischer-type carbene complexes [e.g. (CO)5Cr=C(Ph)OMe] and the highly reactive carbene complexes resulting from the reaction of rhodium(II) and palladium(II) carboxylates with diazoalkanes. Also platinum ylides [1,2], resulting from the reaction of diazoalkanes with platinum(Il) complexes, have a strong Pt-C o bond but only a weak Pt-C 7t bond. In situation (d) the interaction between the metal and the carbene is very weak, and highly reactive complexes showing carbene-like behavior result. Similar to uncomplexed carbenes. 

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