Ylide elimination mechanism

Cushman and Brown proposed that both the ylide and alkene complexes (II) and (III) were formed by a 3-elimination mechanism as shown in Scheme III [14] ... 

Several other observations suggest that nucleophilic carbene complexes, similarly to, e.g., sulfur ylides, can cyclopropanate acceptor-substituted olefins by an addition-elimination mechanism. If, e.g., acceptor-substituted olefins are added to a mixture of a simple alkene and the metathesis catalyst PhWCl3/AlCl3, the metathesis reaction is quenched and small amounts of acceptor-substituted cyclopropanes can be isolated [34]. 

As well as the Bingel reaction and its modifications some more reactions that involve the addition-elimination mechanism have been discovered. 1,2-Methano-[60]fullerenes are obtainable in good yields by reaction with phosphorus- [44] or sulfur-ylides [45,46] or by fluorine-ion-mediated reaction with silylated nucleophiles [47]. The reaction with ylides requires stabilized sulfur or phosphorus ylides (Scheme 3.9). As well as representing a new route to l,2-methano[60]fullerenes, the synthesis of methanofullerenes with a formyl group at the bridgehead-carbon is possible. This formyl-group can be easily transformed into imines with various aromatic amines. 

The proposed mechanism for formation of 151 is shown in (Scheme 27). Proton abstraction by the hydride base from the activated 2-position of the W-fluoropyridinum triflate yields a highly reactive carbene which undergoes attack by the acetonitrile solvent. The resulting nitrilium ylide eliminates fluoride and subsequently adds the isonitrile with cyclization. Finally, reduction by the hydride reagent and aromatization provide the imidazopyridine 151. The undesired amide 152 is a product of hydrolysis of the intermediate nitrilium compound. 

The chemistry of alkynyliodonium salts has been summarized in several reviews [856, 978, 979]. Reactions of alkynyliodonium salts 724 with nucleophiles proceed via an addition-elimination mechanism involving alkylideneiodonium ylides 725 and alkyUdene carbenes 727 as key intermediates (Scheme 3.290). Depending on the structure of the alkynyliodonium salt 724, the specific reaction conditions and the nucleophile employed, this process can lead to the following products p-functionalized alkenyliodonium salt 726 due to... 

A related approach for the synthesis of spirocyclopenteneoxindoles was developed by Barbas and coworkers. Chiral diphosphines catalyzed the [3+2] cycloaddition between the A-protected methyleneindolin-2-ones 17b and the Morita-Baylis-Hillman (MBH) carbonates 37 [18]. This reaction was initiated by the displacement of the carbonate moiety by the phosphine VI, an addition-elimination mechanism, which was followed by the deprotonation to afford ylide 39. A regioselective nucleophilic addition on 17 by 39, followed by an intramolecular conjugate addition, afforded intermediate 40 that, after elimination of PR3, delivered the corresponding spirocycle 41 (Scheme 10.11). 

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. 

An a hydrogen is obviously necessary in order for the ylide to be formed. This type of mechanism is called a, p elimination, since a P hydrogen is removed by the a carbon. The mechanism has been confirmed by labeling experiments similar to those described at and by isolation of the intermediate ylids. An important... 

Olefination Reactions Involving Phosphonium Ylides. The synthetic potential of phosphonium ylides was developed initially by G. Wittig and his associates at the University of Heidelberg. The reaction of a phosphonium ylide with an aldehyde or ketone introduces a carbon-carbon double bond in place of the carbonyl bond. The mechanism originally proposed involves an addition of the nucleophilic ylide carbon to the carbonyl group to form a dipolar intermediate (a betaine), followed by elimination of a phosphine oxide. The elimination is presumed to occur after formation of a four-membered oxaphosphetane intermediate. An alternative mechanism proposes direct formation of the oxaphosphetane by a cycloaddition reaction.236 There have been several computational studies that find the oxaphosphetane structure to be an intermediate.237 Oxaphosphetane intermediates have been observed by NMR studies at low temperature.238 Betaine intermediates have been observed only under special conditions that retard the cyclization and elimination steps.239... 

The rearrangement of platinacyclobutanes to alkene complexes or ylide complexes is shown to involve an initial 1,3-hydride shift (a-elimina-tion), which may be preceded by skeletal isomerization. This isomerization can be used as a model for the bond shift mechanism of isomerization of alkanes by platinum metal, while the a-elimination also suggests a possible new mechanism for alkene polymerisation. New platinacyclobutanes with -CH2 0SC>2Me substituents undergo solvolysis with ring expansion to platinacyclopentane derivatives, the first examples of metallacyclobutane to metallacyclopentane ring expansion. The mechanism, which may also involve preliminary skeletal isomerization, has been elucidated by use of isotopic labelling and kinetic studies. 

Sulfur ylides behave similarly to phosphorus ylides, but the final products are different. Figure 10-31 shows the mechanism for the prepciration of a sulfur ylide and the reaction of the sulfur ylide with a carbonyl group. Notice that the mechanism for the formation of the sulfur ylide is similar to the formation of a phosphorus ylide. However, the last step in the sulfur ylide mechanism is an internal S, 2 reaction, which eliminates the original thioether (dimethyl sulfide). The reaction of a sulfur ylide with a ketone yields epoxides, whereas the product of a phosphorus ylide with a ketone is an alkene. 

The mechanism proposed involves desilyation of the silver complexed imidate and cycloaddition by the azomethine ylide 35 to give 37 followed by elimination (22). 

Sulfonium compounds undergo elimination similar to that of their ammonium counterparts (7-6 and 7-7) in scope and mechanism. The decomposition by heat of sulfonium hydroxides has been known for many years.218 The ylide reaction was discovered more recently.219 Neither is important synthetically. 

A carbenoid-type mechanism with free or surface-bound species formed by a elimination from methanol promoted by the strong electrostatic field of zeolites was proposed first.433,456,457 Hydrocarbons then can be formed by the polymerization of methyl carbene, or by the insertion of a surface carbene (8) into a C-O bond453-455,458,459 (Scheme 3.2, route a). If surface methoxyl or methyloxonium species are also present, they may participate in methylation of carbene454,455,460,461 depicted here as a surface ylide (9) (Scheme 3.2, route b). A concerted mechanism with simultaneous a elimination and sp3 insertion into methanol or dimethyl ether was also suggested 433,454,457... 

The mechanism of the thiazolium ion-catalyzed conjugate addition reactions5 1s considered to be analogous to the Lapworth mechanism for the cyanide-catalyzed benzoin condensation, the thiazolium ylide playing the role of cyanide. The resulting intermediate carbanion is presumed to be the actual Michael donor. After conjugate addition to the activated olefin, the thiazolium ylide is eliminated to form the product and regenerate the catalyst. 

---