Carbonyl ylides rearrangements

Many different types of 1,3-dipoles have been described [Ij however, those most commonly formed using transition metal catalysis are the carbonyl ylides and associated mesoionic species such as isomiinchnones. Additional examples include the thiocar-bonyl, azomethine, oxonium, ammonium, and nitrile ylides, which have also been generated using rhodium(II) catalysis [8]. The mechanism of dipole formation most often involves the interaction of an electrophilic metal carbenoid with a heteroatom lone pair. In some cases, however, dipoles can be generated via the rearrangement of a reactive species, such as another dipole [40], or the thermolysis of a three-membered het-erocycHc ring [41]. 

Dipoles can also be generated from rearrangements that take place after the formation of an initial rhodium carbenoid product ]40, 70, 71]. One example of this type of transmutation, also known as a dipole cascade process, involves the formation of an azomethine ylide via the initial formation of a carbonyl ylide [72]. This process was... 

Tetrahydrothiophene-fused Cgg can be generated by its reaction with the thio-carbonyl ylide precursor bis(trimethylsilylmefhyl) sulfoxide 262 [314, 315]. Thermal sila-Pummerer rearrangement leads in situ to the ylide 263, which is readily added to CgQ (Scheme 4.45). 

The carbonyl ylide 1 can undergo an internal cyclization reaction to generate the corresponding epoxide 2, which is in fact an equilibrium process, and epoxides themselves have frequently served as precursors to carbonyl ylides. Other pathways such as concerted rearrangements and internal proton transfers have also been observed to neutralize the charged ylide intermediate and give substituted ethers as represented by 3. Perhaps the best known studies and most synthetically useful... 

Whereas, cii-divinyl epoxides are reactive and well known to undergo thermal Cope rearrangement, the trans isomers are significantly more stable. White and co-worker (24) showed that thermolysis of divinyl epoxides such as 88 could generate the corresponding carbonyl ylide and that it could be intercepted by the addition of an activated acetylene to give the corresponding dihydrofuran 89, albeit in modest yield. 

Friedrichsen and co-workers (133) approached substituted benzotropolones from an aromatic substituted carbonyl ylide with a tethered alkyne as the intramolecular dipolarophUe (Scheme 4.67). Starting from an aromatic anhydride, Friedrichsen was able to make the tethered alkyne via addition of either pentyn-ol or hexyn-ol, then transform the recovered benzoic acid to the a-diazocarbonyl cycloaddition precursor. Addition of rhodium acetate resulted in the tandem formation of cyclic carbonyl ylide followed by cycloaddition of the tethered alkyne producing the tricyclic constrained ether 252. Addition of BF3 OEt2 opened the ether bridge, forming the benzotropylium ion, which subsequently rearranged to form the tricyclic benzotropolone (253). 

Dittami et al. (170,171) was able to generate a carbonyl ylide via photocycliza-tion of an aryl vinyl ether (Scheme 4.85). The photocyclization proceeded through a six-electron rearrangement providing initially the carbonyl ylide, then subsequent to the cyclization, a dipolar cycloaddition takes place with a pendant olefinic tether. 

In the Hodgson s approach, a cyclic carbonyl ylide is trapped by a carbonyl group to afford 6,8-dioxabicyclo[3,2,l] octane 91. This cycloadduct was further converted to alcohol 92, which was subjected to acid-catalyzed rearrangement to give the desired 2,8-dioxabicyclo[3,2,l]octane skeleton 93 (Scheme... 

A promising synthetic transformation is the reaction of carbenoid intermediates with heteroatoms to form ylides that are capable of undergoing further transformations [5,6]. Enantioselective transformations in which the ylide intermediates undergo either 1,2- or 2,3-sigmatropic rearrangement were briefly reviewed in the previous issue (Vol. II, pp. 531-532) and several recent examples have appeared [37]. A major breakthrough has been made in the enantioselective transformation of carbonyl ylides derived from capture of the metal carbenoid intermediates by carbonyl groups. The carbonyl ylides have been ex-... 

A review about the rearrangement and cycloaddition of carbonyl ylides generated from a-diazo compounds is available <2001CSR50>. Enantioselective intramolecular cyclopropanations of allyl 2-diazo-3-silanyloxybut-3-enoates to yield cyclopropyl 7-butyrolactones have been investigated with a variety of chiral rhodium catalysts. The best results were obtained with Rh2(PTTL)4, where enantioselectivity culminated at 89% ee (Equation 99) <2005TA2007>. 

Dioxolanone 33 is obtained when the unsaturated silyldiazoester 30 is decomposed by Rh2(pfb)4 in the presence of an aldehyde or of acetone (Scheme 11) [21]. The reaction sequence is likely to include formation and (probably reversible) 1,5-cyclization of carbonyl ylide 31, and Cope rearrangement of the allylvinylether 32. In analogy to carbonyl ylide 21, the SiMe3 should occupy the exo-position in 31, thereby bringing the ester carbonyl in a geometry that is favorable to the cyclization step. Again, the choice of catalyst determines the product pattern, since CuOTf catalysis affords not only 33, but also oxirane 22 and the intramolecular cyclopropanation product 34. 

Examples of intramolecular trapping of carbonyl ylide dipoles by alkenes have now been reported.These include, for example, the conversion of the oxirane (172) into the tetrahydrofuran (173). Carbonyl ylides have also been prepared by irradiation of 2,3-bis-(p-methoxyphenyl)oxirane in the presence of dicyanoanthracene as electron-transfer sensitizer direct or triplet-sensitized irradiation, however, leads mainly to rearrangement via carbon-oxygen bond cleavage. In contrast, cyclohexene oxide and styrene oxide, on naphthalene-sensitized irradiation in alcohols, undergo solvolysis via oxide anion-radical intermediates. ... 

Further studies on a-diazo ketones with a second more remote carbonyl group have appeared and formation of a carbonyl ylide and its addition to an added aldehyde yields bicyclic dioxolanes 199 (Scheme 19) <2004TL6485, 2005ARK(xi)146>. A rearrangement is clearly involved in the more complex reaction of a silyl diazo ester to give a dioxolan-4-one (Equation 66) <20020L4631>. 

For clarification of the reaction mechanism, the rearrangements of cis- and trans-phenylvinyloxiranes have been investigated, to avoid the formation of dihydrooxepine. c/s-Dihydrofuran derivatives are formed by conrotational opening of the oxiranes through a carbonyl-ylide intermediate. 

The mechanism for the addition of diazoalkanes to a C=—O double bond is generally written along lines similar to those discussed so far (Scheme 1 X" = N2+). The initial adduct is also the progenitor of the various rearrangement pathways. However, the subject of mechanism is by no means settled, with 1,3-dipolar cycloadditions and carbonyl ylide formation" considered to be prominent alternatives. In general, successful epoxidation of carbonyl compounds improves with increasing electron-poor character of the C—O bond. When the diazoalkane is electron poor, yields of epoxide diminish. 

With the aid of a nitrogen ylide rearrangement, it is possible to transform an allylic secondary amine to an unsaturated carbonyl system, in certain circumstances with transfer of chirality (Scheme 53). ... 

An intramolecular cyclization of a carbenoid to a six-membered carbonyl ylide followed by its rearrangement provides access to both 1,3- and 1,4-oxazine derivatives. Thus, the Cu-mediated reaction of diazo compound 596 (R = Me, C02Me) gives the corresponding 1,3-oxazinone 597 in 48 and 59% yield, respectively (80H1999). 

The interception of the transient rhodium carbenoid formed from diazo compound 601 by a carbonyl oxygen produces carbonyl ylides that, upon dimerization and subsequent rearrangement, give 56% of compound 602 (92JA593),... 

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]. 

Tricyclic dihydropyrrolizines.1 The reaction of N-acyl-2-(l-diazoacctyl)-pyrro-lidines (1) with a catalytic amount of a rhodium II) carboxylatc, particularly rhodium(ll) oclanoatc and a dipolarophilc (DMAD), results in a tricyclic dihydropyrrolizine 3 as the major product. The expected product (2) is formed only in traces (10%). Apparently the intermediate carbonyl ylide a rearranges to the more stable azomcthinc ylide b. 

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