Oxygen carbonyl ylide generation

Carbonyl ylides can be viewed as an adduct between a carbonyl group and a carbene and, in fact, some ylides have been prepared this way (see above). The application of carbonyl ylides to the synthesis of complex natural products has been greatly advanced by the finding that stabilized carbenoids can be generated by the decomposition of ot-diazocarbonyl compounds with copper and rhodium complexes. The metallocarbenoids formed by this method are highly electrophilic on carbon and readily add nucleophiles such as the oxygen of many carbonyl derivatives to form carbonyl ylides. This type of reaction is in fact quite old with the first report being the addition of diazomalonate and benzaldehyde (33,34). 

Imine ylides 7 and carbonyl ylides 8 are not stable but may be generated in situ by pyrolysis of suitably substituted aziridines and oxiranes. The energy of the HOMO, and therefore the nucleophilicity of the parent 18-electron dipoles, decreases from very high to very low across the series 7-18. In the same series, the electrophilicity increases from moderate to high, being consistently higher when the central atom is oxygen. 

As illustrated in Section 4.1.1, the addition of nonstabilized carbenes to the oxygen atom of a carbonyl derivative can lead to the production of carbonyl ylides. However, these methods are not always practical for preparative scale since many side reactions can accompany the decomposition of alkyl diazo and diazirine derivatives. Landgrebe and co-worker (8) extensively studied the thermal decomposition of organomercurials in the presence of carbonyl compounds for the preparative generation of carbonyl ylides (Scheme 4.6). 

The fact that both the cis and trans isomers of diaryl epoxides give cis-ozonide made mandatory the possibility that back electron transfer within the radical ion pair generated a carbonyl ylide in which geometric equilibration can occur before oxygenation (130), although no direct evidence for this process could be obtained spectroscopically (133). 

Interestingly, the Rh(II)-catalyzed reaction of l-(3 -diazo)-acetonyl-2-pyri-done (151) with DMAD was found to give cycloadducts derived from an azo-methine ylide. The initial reaction involves generation of the expected carbonyl ylide dipole by intramolecular cyclization of the keto carbenoid onto the oxygen... 

In addition to the formation and reactions of carbonyl ylides discussed in the previous section, carbenoids also react intramolecularly with ethereal oxygen atoms to generate oxonium intermediates. When the ether is part of a ring as in substrates 63 a-b, the intramolecular addition of rhodium carbenoids produces bicyclic oxonium intermediates, which generated [5.2.1] oxabicycles 64a-b upon rearrangement by a [2,3]-sigmatropic pathway, Eq. 44 [74]. 

Abstract Carbon d ylide dipoles are important intermediates with great application in heterocyclic chemistry. Here, we show how the rhodium-catalyzed a-diazocarbonyl compounds are employed in the generation of carbonyl ylides and their effective use for the synthesis, as well as functionalization, of heterocycles. Herein we discuss recent advancements in this field mainly describing the synthesis and importance of various oxygen-and nitrogen-containing heterocyclic systems and natural products from a-diazocarbonyl compounds. 

Rhodium-generated bicyclic six-membered ring carbonyl ylides from the diazo ketone 158 with p-quinones have also been studied to yield interesting oxygen heterocycles [133]. In line with the five-membered ring carbonyl ylide reactions (see Sect. 2.1), the a-diazocarbonyl compound 158 furnished oxygen-rich heterocyclic systems 161-163 (Scheme 52). 

Carbonyl ylides can be generated by the interaction of a carbene with the oxygen atom of a carbonyl group. This can be readily accomplished by the thermal or photochemical decomposition of diazomethane or transition metal... 

The sulfide-catalyzed enantioselective epoxidation reaction is the most extensively studied transformation in ylide catalysis, and two ylide generation methods (aUcylation/deprotonation and carbene transfer) have been developed. Compared with conventional methods for epoxidation via oxygen transfer to the carbon-carbon double bond, such as the Sharpless epoxidation, Jacobsen-Katsuki epoxidation, and Shi epoxidation, the yhde approach can be regarded as an alkyUdene transfer reaction to carbonyl groups (C=0), providing a different retrosynthetic analysis for the construction of epoxides. In particular, in the synthesis of vinyl epoxides, the ylide route has priority over conventional oxidation methods, since the issue of regjoselectivity in the epoxidation of dienes will not be present [32]. 

Oxygenation of 1,2-dinaphthyloxiranes 24 and 25 by PET was conducted without BP, as both 24 and 25 were able to quench the fluorescence of DCA. Thus, ds-trioxolane 26 was isolated as the sole stereoisomer (Scheme 8). However, the Schaap mechanisms described above, involving a highly stereoselective formation of ds-l,2,4-trioxolanes (Scheme 7 and 8), seem to be inconsistent with the previously described Arnold mechanism (Scheme 2), where two types of radical cations (3 and 4) as well as carbonyl ylides (5 and 6) are generated by the PET oxidation of 1 and 2, respectively. 

In transforming bis-ketone 45 to keto-epoxide 46, the elevated stereoselectivity was believed to be a consequence of tbe molecular shape — tbe sulfur ylide attacked preferentially from tbe convex face of the strongly puckered molecule of 45. Moreover, the pronounced chemoselectivity was attributed to tbe increased electropbilicity of the furanone versus the pyranone carbonyl, as a result of an inductive effect generated by tbe pair of spiroacetal oxygen substituents at tbe furanone a-position. ... 

The reaction mechanism proposed for the LiBr/NEta induced azomethine ylide cycloadditions to a,p-unsaturated carbonyl acceptors is illustrated in Scheme 11.10. The ( , )-ylides, reversibly generated from the imine esters, interact with acceptors under frontier orbital control, and the lithium atom of ylides coordinates with the carbonyl oxygen of the acceptors. Either through a direct cycloaddition (path a) or a sequence of Michael addition-intramolecular cyclization (path b), the cycloadducts are produced with endo- and regioselectivity. Path b is more likely, since in some cases Michael adducts are isolated. 

The oxygen as heteroatom in ethers or carbonyl compounds is weak to moderate Lewis base. Nevertheless, a highly reactive metal carbene complex can interact with the oxygen to generate oxygen ylide. The interaction between ether and metal carbene functional groups is believed to be rather weak as demonstrated by the facts that other metal carbene reactions, such as G-H insertion and cyclopropanation, can proceed in ethereal solvents." These experiments demonstrate that the formation of the metal ylide is much less favored in the equilibrium shown in Equation (1). ... 

Similar to ethers and carbonyl compounds, sulfides or thiocarbonyl groups interact with metal carbene to generate the corresponding ylides. These ylides undergo similar subsequent reactions as their oxygen counterparts (Figure 4). 

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