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Carbenes from ylides

Closely related with the synthesis of ylides from carbenes is the use of ylides as carbene transfer reagents (CTR), that is processes in which the ylide is cleaved homolytically, liberating the nucleophile and the carbene, which could remain both coordinated to the metal or not (Scheme 10). Diphosphirane (34) can be obtained from the diphosphene by reaction with sulfur ylide Me2S(0)=CH2, which behave as a carrier of the CH2 unit [95]. Recent work of Milstein et al. shows that sulfur ylides decompose in the presence of Rh derivatives with vacant coordination sites affording Rh(l)-carbene complexes [96, 97]. Complexes (35-37) can be obtained from... [Pg.24]

Generation of iminium ylides from carbenes and carbenoids and their synthetic application 05UK183. [Pg.19]

Aziridine Formation with Chalcogen-Ylides from Carbene Sources... [Pg.257]

In pentane, the distribution of 1,3-insertion product 25 to 1,2-Me shift product 26 is 91 9. Upon addition of 2-methyl-1-butene, the yield of 25 smoothly decreases (to 19% with 4 M alkene), but the yield of 26 is unaffected 1 Moreover, correlation of addn/l,3-CH insertion (to 25) for 18 is nicely linear. The simplest interpretation is that 25 comes directly from carbene 18, whereas the 1,2-Me shift product 26 comes from the excited diazirine.27 Interestingly, thermolysis of 24 at 79°C produces 90% of 25 and 10% of 26, but now the yields of both products smoothly decrease in the presence of an alkene. In thermolysis the (electronically) excited diazirine is unavailable, both 25 and 26 stem from the carbene, and their formation is suppressed by the alkene s interception of the carbene. A pyridine ylide kinetic study gave the 1,3-CH insertion rate constant (18 - 25) as 9.3 x 10s s"1.27-47... [Pg.64]

Ylides from R N2. This reagent is more effective than bis(acetylacetonate)-copper(II) (5, 244) for generation of carbenes from diazo compounds.2 The decomposition proceeds at a lower temperature, even at room temperature. The mild conditions are particularly useful in the preparation of heat-sensitive ylides, such as those of antimony, bismuth, and tellurium. [Pg.46]

Fig. 4.8. Formation and Stevens rearrangement of ammonium ylides from acceptor-substituted carbene complexes. Fig. 4.8. Formation and Stevens rearrangement of ammonium ylides from acceptor-substituted carbene complexes.
Fig. 4.9. Formation and transformations of azomethine ylides from imines and electrophilic carbene complexes. Fig. 4.9. Formation and transformations of azomethine ylides from imines and electrophilic carbene complexes.
Table 4.17. Generation and rearrangement of azomethine ylides from acceptor-substituted carbene complexes and imines. Table 4.17. Generation and rearrangement of azomethine ylides from acceptor-substituted carbene complexes and imines.
Fig. 4.11. Generation and transformations of oxonium ylides from electrophilic carbene complexes. Fig. 4.11. Generation and transformations of oxonium ylides from electrophilic carbene complexes.
Fig. 4.13. Formation and reactions of carbonyl ylides from carbonyl compounds and electrophilic carbene complexes. Fig. 4.13. Formation and reactions of carbonyl ylides from carbonyl compounds and electrophilic carbene complexes.
Ab initio and RRKM calculations indicate that the reactions of C, CH, and (H2C ) with acetylene occur with no barrier." Laser flash photolysis of the cyclopropanes (69) and (70) was used to generate the corresponding dihalocarbenes. The absolute rate constant for the formation of a pyridine ylide from Br2C was (4-11) x 10 lmoP s. The rates of additions of these carbenes to alkenes were measured by competition with pyridine ylide formation and the reactivity of BrClC was found to resemble that of Br2C rather than CI2C . [Pg.262]

Scheme 9 Ylide complexes obtained from carbene derivatives... Scheme 9 Ylide complexes obtained from carbene derivatives...
Recent research on aminocarbenes has led to the development of a very fruitful field. The synthesis of relevant complexes (Scheme 19) such as aminobis(yhde) carbene species (69) [147], cyclic C-amino P-ylides (70) (easily transformed into carbenes) [148] and their corresponding complexes (71) [149], and special ylides (72), which also transform very easily into carbenes by loss of pyridinium group, has been reported. Emphasis has been made on the transformation between ylides and carbenes and on the donor properties of the ylides. From the results obtained the ylides have shown a stronger a-donor behavior compared with the carbenes. [Pg.31]

Generation of Carbonyl Ylides from NonstabiUzed Carbenes... [Pg.253]

Formation of Oxygen Ylide from Metal Carbene Complexes and Subsequent Reactions 152... [Pg.151]

Formation of Nitrogen Ylide from Metal Carbene Complex and Subsequent Reactions 168... [Pg.151]

Figure 3 Generation of carbonyl ylide from metal carbene. Figure 3 Generation of carbonyl ylide from metal carbene.
For a monograph, see Johnson Ylid Chemistry, Academic Press New York. 1966. For reviews, sec Morris. Surv. Prog. Chem. 1983, 10, 189-257 Hudson Chem. Br. 1971, 7, 287-294 Lowe Chem. Ind. (London) 1970, 1070-1079. For a review on the formation of ylides from the reaction of carbenes and carbenoids with heteroatom lone pairs, see Padwa Hornbuckle Chem. Rev. 1991, 91, 263-309. [Pg.39]

Apart from this feature there are many similarities between ylides and carbene complexes, primarily among the structural criteria. The carbene carbon may be, but not necessarily, in a planar configuration, and the M—C bonding indicates some multiple bonding character just as in most of the ylides. On the other hand, carbene transfer reactions have been observed with ylides [e.g., Eq. (36)3, indicating that the carbene complex formalism can, indeed, be successfully applied with ylides. There is hope, therefore, for a fruitful symbiosis of ylide and carbene complex chemistry, which may soon become complementary as more data become available from this new area of transition metal chemistry. [Pg.240]

Kende and co-workers have reported the formation of a nitrile ylide intermediate from carbenes and methyl acrylonitrile. Thermolysis of p-diazooxide 256 in methyl acrylonitrile as solvent gave spirocyclic product 259 in 48% yield.140 The formation of 259 was interpreted in terms of the generation of nitrile ylide 258 followed by 1,3-dipolar cycloaddition across the C=C bond of a second molecule of methylacry-lonitrile. The regiochemistry of the cycloaddition is consistent with FMO theory. [Pg.152]

Stoffiegen, S.A., Heying, M., and Jenks, W.S. (2007) S,C-Sulfonium ylides from thiophenes potential carbene precursors. Journal of the American Chemical Society, 129 (51), 15746-15747. [Pg.132]

Kende and coworkers have reported on the formation of a nitrile ylide intermediate from carbenes and methyl acrylonitrile. Thermolysis of p-diazooxide... [Pg.146]

The barriers just described were calculated with 1-methylorotic acid (11) as a reference point to model the uncatalyzed reaction in solution. However, the computed free-energy barriers for decarboxylation of zwitterions 4b and 6b are 8.4 and 7.6 kcal mol-1, respectively. This difference of 0.8 kcal mol-1 is significantly smaller than the 6 kcal mol-1 difference calculated by Lee and Houk for the 2-protonation and 4-protonation pathways. This discrepancy arises from an internal hydrogen bond (12) between the Nl-H and the carboxylate that artificially stabilizes the 02-protonated zwitterion 4a, and renders its corresponding decarboxylation barrier too high. When the Nl-H is replaced by a methyl, the hydrogen bond is removed, and the ylide and carbene mechanisms become closer in energy nonetheless, 4-protonation is still favored. [Pg.192]


See other pages where Carbenes from ylides is mentioned: [Pg.131]    [Pg.109]    [Pg.301]    [Pg.131]    [Pg.109]    [Pg.301]    [Pg.879]    [Pg.879]    [Pg.274]    [Pg.160]    [Pg.406]    [Pg.164]    [Pg.133]    [Pg.357]    [Pg.183]    [Pg.247]    [Pg.168]   
See also in sourсe #XX -- [ Pg.936 , Pg.937 , Pg.938 , Pg.939 , Pg.940 ]

See also in sourсe #XX -- [ Pg.551 , Pg.552 ]




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Carbene from ylides

Carbene from ylides

Carbene-ylide

From carbenes

Sulfur ylides, from metal carbene complexes

Ylide carbonyl from carbenes

Ylide oxonium from carbenes

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