Carbenes formation

The thiepin system 31 is formed quantitatively by ring expansion of the diazoacetate derivative 30 via carbene formation catalyzed by 7r-allylpalladium chloride and its intramolecular insertion[31], The 4-diazomethyl-4//-pyrane 32 is expanded to the oxepine 33 in quantitative yield with the same catalyst[32]. 

In dihalocarbene generation by phase-transfer catalysis the following steps seem to be involved (15) formation of CX anions dynamically anchored at the boundary reversible detachment with the help of the catalyst reversible carbene formation [Q+ CX3 ] [Q + X ] + CX2 addition to olefin. 

Diazirines are in most cases more easily available than linear diazo compounds. Moreover, their decomposition via true carbenes is free of side reactions, whereas linear diazo compounds in presence of H-donors may react by a cationic pathway. Only where reactions of linear diazo compounds are optimized for carbene formation do they give the same products as do decomposing diazirines. 

Organoiron chemistry offers the C-coordination mode, carbene series, ring opening and detelluration. Organoruthenium and especially organoosmium species are sometimes peculiar along with the classical cases of N-coordination and carbene formation, C,N- and S,N-cases can be traced. 

Photopolymerization of MMA was also carried out in the presence of visible light (440 nm) using /3-PCPY as the photoinitiator at 30°C [20]. The initiator and monomer exponent values were calculated as 0.5 and 1.0, respectively, showing ideal kinetics. An average value of kp /kt was 4.07 x 10 L-mol -s . Kinetic data and ESR studies indicated that the overall polymerization takes place by a radical mechanism via triplet carbene formation, which acts as the sources of the initiating radical. 

Polymerization takes place by triplet carbene formation. 

The most convenient tool for the characterisation of NHCs is NMR spectroscopy, in particular C H NMR. As a case study, the carbenes IPr and SIPr, and their corresponding salts IPr HCl and SIPr HCl were chosen. As described above (Scheme 1.2), free carbenes are often obtained by deprotonation of the corresponding salt. The best diagnostic tool to observe the salt deprotonation, and thus indirectly monitor the carbene formation, is H NMR spectroscopy, by means of the disappearance of the characteristic acidic proton resonance. The signal corresponding to the latter (ff) is largely shifted downfield (typically 8-12 ppm) and disappears upon deprotonation (Fig. 1.5). 

The carbene formation can be monitored by C H NMR, as the carbene carbon atom of free NHCs has a signal significantly shifted downfield. Typically, the signal for the O atom is found between 200 and 250 ppm for the free carbene and between 130 and 160 ppm for the corresponding salt... 

While the mechanism in the absence of Eti or HI is still a matter of conjecture, it is unlikely that a hydride mechanism was operable since, whereas we could possibly envision an imidazolium salt donating a hydrogen via carbene formation, there is no corresponding viable source of hydride when using pyridinium and phosphonium salts which are also effective solvents for the process. Therefore, by process of elimination, it was more likely that the process was operating via a nucleophilic process. 

The main synthetic application of the Wolff rearrangement is for the one-carbon homologation of carboxylic acids.242 In this procedure, a diazomethyl ketone is synthesized from an acyl chloride. The rearrangement is then carried out in a nucleophilic solvent that traps the ketene to form a carboxylic acid (in water) or an ester (in alcohols). Silver oxide is often used as a catalyst, since it seems to promote the rearrangement over carbene formation.243... 

The fact that only the vinyl-substituted l,2X5-oxaphosphetanes 22 but not the arylated phosphorus heterocycles 21 undergo photofragmentations is presumably due to the inability of the latter to absorb the light (X > 280 nm) supplied for carbene formation (7- 8) [e.g. 21, Ar = C6H5 e280 200 22b, d e2g0 9000 (in methanol)]I8,20). 

Scheme 6/2.6. Enantioselective domino carbene-formation/1,3-dipolar cycloaddition.

Jr Carbene Formation Reactive Hydrogen Containing Compound Or Insertion with Covalent Bond Formation (Reaction 56)... 

H shift more readily occurs with loss of nitrogen.22 Excited diazirines decay by fluorescence, carbene formation, or 1,2-H(D) migration coupled with N2 loss. C-D bonds are stronger than C-H bonds, so that deuteration retards the latter pathway and therefore RIES, leading to an increase in both fluorescence and carbene formation from 35-d6 22... 

Only little information is available on carbene formation by rearrangement of bridgehead olefins, generated in thermal reactions. A prominent example is the rearrangement of bridgehead olefin 2, obtained as short-lived intermediate from bromosilane 1 by reaction with potassium fluoride in DMSO at 110°C. Carbene... 

Scheme 8.—Proposed Mechanism for Carbene Formation from Photolysis of 5-Deoxy-l,2-0-isopropylidene-/3 L-erythro-pentofuranos-3-ulose (15). Rationalization for the Direction of a-Cleavage.

The problem with carbene formation is that they can displace the phosphine ligands attached to the catalyst and deactivate the catalyst. In general, the active catalyst is a palladium(O) compound and this low oxidation state is best stabilized by very bulky phosphines such as P(lBu)3 mentioned above. 

Scheme 5.6). The reaction can be conducted under basic liquidiliquid conditions but, in order to minimize hydrolysis of the phosphite and to prevent carbene formation, a milder solid (KHCO, liquid system with tetra-/i-butyIammonium bromide is preferred. Under these conditions, yields are almost quantitative [38]. Tetra-bromomethane is generally recommended, in preference to tetrachloromethane, for reactions with weakly basic aryl-amines. 

Ott, L. S. Cline, M. L. Deetlefs, M. et al. Nanoclusters in ionic liquids evidence for A-heterocyclic carbene formation from imidazolium-based ionic liquids detected by H-2 NMR, J. Am. Chem. Soc., 2005, 127(16), 5758-5759 Hamill, N. A., Hardacre, C. McMath, S. E. J. In situ XAES investigation of palladium species present during the Heck reaction in room temperature ionic liquids. Green Chem., 2002, 4(2), 139-142. 

Most recently, Hedrick and co-workers have illustrated the use of alcohol adducts 298 as a sufficient catalyst/initiators for ROP, therefore eliminating the need for external alcohol [153], These adducts undergo carbene formation at room temperature in THF. Additional advantages of these adducts, compared to free NHCs, is that they are not moisture sensitive and they provide the opportunity to synthesize more complex polymers (Eq. 28a). Star polyesters can be generated in one step (Eq. 28b). 

We now turn to reagents which are important in context with the element/halogen exchange Compounds of the type 8 would be interesting reagents for nucleophilic halomethylation but are quite unstable due to carbene formation. Compound 9 would be an attractive synthetic equivalent for the synthons 8. As will be shown... 

A novel route to 2,3-dihydrothiophenes involved a titanocene-promoted carbene formation and subsequenct intramolecular cyclization onto a thiol ester <99SL1029>. Treatment of thioacetal 9 with the low-valent titanium complex 10 gave 2,3-dihydrothiophene 12 by intramolecular olefination of the thiol ester of titanium-carbene intermediate 11. Another metal-mediated cyclization onto the thiophene ring system involved the palladium-catalyzed cyclization of 1,6-diynes <99T485>. For example, treatment of thioether 1,6-diyne 13 with Pdlj in the presence of CO and Oj in methanol followed by treatment with base gave 14. 

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