Ylides types

Schrock-type carbenes are nucleophilic alkylidene complexes formed by coordination of strong donor ligands such as alkyl or cyclopentadienyl with no 7T-acceptor ligand to metals in high oxidation states. The nucleophilic carbene complexes show Wittig s ylide-type reactivity and it has been discussed whether the structures may be considered as ylides. A tantalum Schrock-type carbene complex was synthesized by deprotonation of a metal alkyl group [38] (Scheme 7). 

As in CljSn—P(CMe3)3 the metal-coordinated 39 may be interpreted as an ylide-type compound. Reactions and physicochemical measurements indicate that the phosphane acts as a simple donor130>. 

The reason for the different behavior of dienes like 41 and monoenes 37 or 42 is not yet established. It is hard to believe that simple steric factors should make up for the different orientation of the olefin that approaches a metal carbene intermediate. More likely is stereochemical control by an ylide-type interaction between the halogen atom of the (sterically more flexible) monoenes 37 or 42 and the electrophilic metal carbene. 

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]. 

The MNDO calculations [84JST( 109)277] show the C2 structure (26) to be more stable by 8.3 kcal/mol compared to the planar Z)2A structure. The heteroatoms given in Table VI may be supplemented by atoms of the groups 1, 2, and 17. Halogen atoms can form structures of the cyclic ylide type such as (27) and (28). In this case, a halogen atom formally supplies to the 7r-system three electrons, that is to say, structures (27) and (28) are, respectively, 6- and 87r-electronic. Experimentally, these molecules have... 

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. 

Base-catalyzed exchange proceeds very quickly at C-7, more slowly at C-6, and very slowly at C-2 and C-5. The proton at C-2 of the methiodides exchanges about 1000 times faster than that in the free base (68JOC1087). The results are consistent with an ylide-type intermediate. 

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)." ... 

The process, referred to as methyl alcohol-to-gasoline conversion, involves initial dehydration of methyl alcohol to dimethyl ether, with a subsequent carbenoid-type reaction. It goes probably through an oxonium ylide-type intermediate that is readily methylated by excess methanol (dimethyl ether) producing the crucial Ci —> C2 conversion and then cleaves to ethylene. Once a C precursor is converted into a C2 derivative (i.e., ethylene), the further conversion to higher hydrocarbons of the gasoline range (or to aromatics) follows well-known chemistry. 

Felhammer and co-workers (68-71) (and references cited therein) has shown that metal coordinated a-deprotonated isocyanides (e.g., 127 and 128) are genuine 1,3-dipoles of the nitrile ylide type that react with various dipolarophiles by [3 + 2]... 

A major improvement addressing the issue of practicability and safety by avoidance of the direct use of (potentially) explosive diazo compounds was recently reported by Aggarwal and co-workers [82, 83], The direct addition of diazo compounds was replaced by use of suitable precursors which form the desired diazo compound in situ. The Aggarwal group developed this concept for the corresponding sulfur ylide type epoxidation (see Section 6.8) [82], and successfully extended it to aziridination [83]. Starting from the tosylhydrazone salt 66 the diazo compound is formed in situ under conditions (phase-transfer-catalysis at 40 °C) which were found to be compatible with the sulfur ylide type aziridination [82, 83], The concept of this improved method, for which sulfide 67 (Scheme 5.41) is the most efficient catalyst, is shown in Scheme 5.40. 

Asymmetric sulfur-ylide-type epoxidation is an excellent tool for enantioselective and diastereoselective synthesis of epoxides. By use of Aggarwal-type methodology a broad range of aromatic, enolizable, and base-sensitive aldehydes can be converted into the desired epoxides. In addition to an excellent diastereomeric ratio, the optimized organocatalytic systems of this sulfur-ylide-type epoxidation also... 

In addition to the stable germenes mentioned above, an ylide-type compound 56 was synthesized by the reaction of the nitrogen-substituted germylene 45a with bis(dialkyl-amino)cyclopropenylidene 55 prepared in situ by the treatment of cyclopropenium cation... 

Ylide type non-stabilized ylide semi-stabilized ylide stabilized ylide... 

The simple diastereoselectivity of the formation of oxaphosphetanes depends mainly on which ylide type from Table 11.1 is used. 

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... 

With strong bases it is also possible to remove a proton from the methyl group of triarylmethylphosphonium halides, trimethylsulfonium iodide, or trimethylsulfoxonium iodide (Figure 9.1). Here, too, species are produced that are betaines of the ylide type. 

High selectivity for Z- or -alkenes is obtained depending on the particular circumstances, such as the type of ylide, type of carbonyl compound or reaction conditions. The non-stabilizedylides (R, = H, alkyl) are very reactive and provide the thermodynamically less... 

Base-induced deuterium-protium exchange at the C-5 atom of N-substituted tetrazoles occurs readily via carbanionic type intermediates, e.g. (57). The reaction for 1-substituted tetrazoles had a Hammett p-value of +1.3 in piperidine-MeOD-DMF <69TL3377>. In the acidic pH range 3-5, ylide type species (58 R = H) are also considered to be involved in the exchange arising from initial protonation at N-1 or N-2 followed by abstraction. 

Table 15. Synthesis of Cyclopropanes Using Dimethylaminooxosulfonium Ylides (Type D)...

Table 16. Synthesis of Cyclopropanes Using Dimethylsulfonium Ylides (Type E) Activated by an Anion-Stabilizing...

Diaminocyclopropenethiones 41 also reacted with zwitterionic 2,3-bis(dicyanomethylene)-l-(triethylammonio)cyclopropan-l-ide[42,X = C(CN)2]or2,3-bis(cyanoimino)-l-(triethylam-monio)cyclopropan-l -ide [42, X = N(CN)] to afford sulfur-bridged three-membered-ring compounds 43 of the thiocarbonyl-ylide type. ° ... 

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