Rhodium carbenoids rearrangement

Asymmetric allylic C-H activation of more complex substrates reveals some intrinsic features of the Rh2(S-DOSP)4 donor/acceptor carbenoids [135, 136]. Cyclopropanation of trans-disubstituted or highly substituted alkenes is rarely observed, due to the steric demands of these carbenoids [16]. Therefore, the C-H activation pathway is inherently enhanced at substituted allylic sites and the bulky rhodium carbenoid discriminates between accessible secondary sites for diastereoselective C-H insertion. As a result, the asymmetric allylic C-H activation provides alternative methods for the preparation of chiral molecules traditionally derived from classic C-C bond-forming reactions such as the Michael reaction and the Claisen rearrangement [135, 136]. 

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... 

Rhodium(II) acetate catalyzes C—H insertion, olefin addition, heteroatom-H insertion, and ylide formation of a-diazocarbonyls via a rhodium carbenoid species (144—147). Intramolecular cyclopentane formation via C—H insertion occurs with retention of stereochemistry (143). Chiral rhodium (TT) carboxamides catalyze enantioselective cyclopropanation and intramolecular C—N insertions of CC-diazoketones (148). Other reactions catalyzed by rhodium complexes include double-bond migration (140), hydrogenation of aromatic aldehydes and ketones to hydrocarbons (150), homologation of esters (151), carbonylation of formaldehyde (152) and amines (140), reductive carbonylation of dimethyl ether or methyl acetate to 1,1-diacetoxy ethane (153), decarbonylation of aldehydes (140), water gas shift reaction (69,154), C—C skeletal rearrangements (132,140), oxidation of olefins to ketones (155) and aldehydes (156), and oxidation of substituted anthracenes to anthraquinones (157). Rhodium-catalyzed hydrosilation of olefins, alkynes, carbonyls, alcohols, and imines is facile and may also be accomplished enantioselectively (140). Rhodium complexes are moderately active alkene and alkyne polymerization catalysts (140). In some cases polymer-supported versions of homogeneous rhodium catalysts have improved activity, compared to their homogenous counterparts. This is the case for the conversion of alkenes direcdy to alcohols under oxo conditions by rhodium—amine polymer catalysts... 

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),... 

Rhodium carbenoids react with furan derivatives to generate oxabicyclo[3.2.1] octadienes through the formation and rearrangement of divinyl cyclopropane intermediates. Therefore, treatment of 2,5-dimethylfuran with 33 leads to the endo adduct 34, Eq. 23 [50],... 

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]. 

The cyclopropane intermediate results from the rhodium-carbenoid addition to the less-hindered terminal double bond of the diene. This generates the cyclopropane in which both alkenes are cis related, which allows the subsequent Cope rearrangement of the divinyl cyclopropane (see Section 3.6.1). The conformation of the boat-shaped transition state for the rearrangement must have the methyl and acetate groups trans to one another. See W. R. Cantrell and H. M. L. Davies, J. Org. Chem., 56 (1991), 723. 

Stoltz and May have described a tandem process involving the rhodium-catalyzed Bamford Stevens reaction and the Claisen rearrangement. Rhodium carbenoid 44 undergoes a facile stereoselective 1,2-hydride migration to generate the allyl vinyl ether 45 which rearranges to aldehyde 42 with high diastereoselectivity. ... 

Rainer has studied the sulfonium ylide-mediated thio-Claisen rearrangement. Rhodium carbenoid 443, generated from rhodium acetate and alkenyl diazoacetate 441, reacted with the 2-ethylthioindole 440 to reveal the sulphur ylide 445. Proton shift generated the ketene aminethio acetal 446 which underwent thio-Claisen rearrangement to yield indoline 442 in excellent yield. 

Davies and co-workers" and Davies and Jin" explored the use of rhodium carbenoids for the combined C—H activation/Cope rearrangement (CHCR). This... 

A tandem ylide formation/[2,3]-sigmatropic rearrangement between donor/acceptor rhodium carbenoids and chiral allyl alcohols has been reported as a convergent C-C bond-forming process generating two vicinal stereogenic centres. Any of the four possible stereoisomers can be selectively synthesized by the appropriate combination of the chiral catalyst Rh2(DOSP)4 and the chiral alcohol (Scheme 144). °... 

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]. 

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