Carbene-ylide

The diamagnetic ylide complexes 34 have been obtained from the reaction of electron-deficient complexes [MoH(SR)3(PMePh2)] and alkynes (HC=CTol for the scheme), via the formal insertion of the latter into the Mo-P bond. The structural data show that 34 corresponds to two different resonance-stabilized ylides forms 34a (a-vinyl form) and 34b (carbene ylide form) (Scheme 17) [73]. Concerning the group 7 recent examples of cis ylide rhenium complexes 36 cis-Me-Re-Me) have been reported from the reaction of the corresponding trans cationic alkyne derivatives 35 with PR" via a nucleophilic attack of this phosphine at the alkyne carbon. 

Most recently, we have investigated the use of iterative oxonium ylide [1,2]- or [2,31-shifts as a convenient approach to the polypyran domains often found in the marine polyether ladder toxins (Scheme 18.8) [21]. Initial studies indicated that [l,2]-shifts of O-benzyl oxonium ylides such as 19 a or 19 b were inefficient. Alternative metallocarbene processes including C-H insertion and dimerization were found to predominate in these cases, again suggesting that carbene-ylide equilibration may occur [21b]. On the rationale that concerted [2,3]-shifts of the corresponding O-allyl oxonium ylides might occur more readily, the allyl ethers 19 c, 19 d were then examined. These examples were much more effective, especially in conjunction with the optimized catalyst Cu(tfacac)2 [21a]. However, rhodium(II) triphenylacetate (Rh2(tpa)4) [22] was found to... 

Deprotonation of the salt (235) with triethylamine affords a carbene (ylide), which rearranges to the (unstable) carbodiimide (236) (Scheme 106) <70TL3453>. 

It is perhaps inopportune to elaborate on the nomenclature, but some of the data reported for the tantalum ylide indicate that there may be a fundamental difference between this transition metal compound and the formally related ylides of the Group Yb elements. The most significant discrepancy is found with the 13C NMR shift of the carbene/ylide carbon atoms, which typically is downfield for the Va element, but upheld for the Vb element derivatives. Ylidic carbon atoms may, therefore, possibly bear a much higher negative charge. 

C KIEs experiments were supposed to help distinguishing the rate-limiting formation of an epoxide-carbene ylide complex 52 versus a rate-limiting deoxygenation process. 

Information is also given for excited neutral radicals. Several recent reviews have described the reactions of excited radicals, radical ions, biradicals, carbenes, ylides, and other intermediates. Neutral radicals derived from organic systems... 

The proposed mechanism is an initial As=C cleavage generating a carbene which attacks another ylide molecule. The carbene-ylide intermediate (4) then decomposes, generating the observed olefin (Scheme 6). 

Redox condensation between two carbonyl groups by virtue of intramolecularity can be initiated by an azolecarbene. The more electrophilic formyl group that is attached to an aromatic ring undergoes umpolung by accepting the carbene/ylide then the adduct adds across a proximal carbonyl group. The use of a chiral carbene (e.g., 192) naturally empowers enan-tiomerization of the reaction. ... 

Novikov, M.S. Khlebnikov, A.F. Sidorina, E.S. Kostikov, R.R. A facile tandem carbene-ylide route to 2-fluoropyrrole derivatives. J. Eluorine Chem. 1998, 90, 117-119. 

The ring opening of 2//-azirines to yield vinylnitrenes on thermolysis, or nitrile ylides on photolysis, also leads to pyrrole formation (B-82MI30301). Some examples proceeding via nitrile ylides are shown in Scheme 92. The consequences of attempts to carry out such reactions in an intramolecular fashion depend not only upon the spatial relationship of the double bond and the nitrile ylide, but also upon the substituents of the azirine moiety since these can determine whether the resulting ylide is linear or bent. The HOMO and second LUMO of a bent nitrile ylide bear a strong resemblance to the HOMO and LUMO of a singlet carbene so that 1,1-cycloadditions occur to carbon-carbon double bonds rather than the 1,3-cycloadditions needed for pyrrole formation. The examples in Scheme 93 provide an indication of the sensitivity of these reactions to structural variations. 

Reaction of the keto ylide (588) with cyanazide (589) provided a convenient route to the 1,2,3-triazole-l-carbonitrile (590). On heating, (590) lost N2 readily to form the a-cyanoimino carbene (591) (8lAG(E)ll3). 

Electron deficient species can attack the unshared electron pairs of heteroatoms, to form ylides, such as in the reaction of thietane with bis(methoxycarbonyl)carbene. The S —C ylide rearranges to 2,2-bis(methoxycarbonyl)thiolane (Section 5.14.3.10.1). A"-Ethoxycar-bonylazepine, however, is attacked by dichlorocarbene at the C=C double bonds, with formation of the trans tris-homo compound (Section 5.16.3.7). 

Other non-oxidative procedures have also been used to deaminate aziridines. For example, aziridines react with carbenes to yield ylides which subsequently decompose to the alkene. Dichlorocarbene and ethoxycarbonylcarbene have served as the divalent carbon source. The former gives dichioroisocyanides, e.g. (281), as by-products (72TL3827) and the latter yields imines (72TL4659). This procedure has also been applied to aziridines unsubstituted on the nitrogen atom although the decomposition step, in this case, is not totally stereospecific (72TL3827). 

The reactions of carbenes, which are apparently unique in displaying electrophilic character in strongly basic solutions, include substitution, addition to multiple bonds, and co-ordination with lone pairs of electrons to form unstable ylides. This last reaction is of obvious relevance to a consideration of the reactions of heterocyclic compounds with carbenes and will be summarized. 

The synthesis of aziridines through reactions between nitrenes or nitrenoids and alkenes involves the simultaneous (though often asynchronous vide supra) formation of two new C-N bonds. The most obvious other alternative synthetic analysis would be simultaneous formation of one C-N bond and one C-C bond (Scheme 4.26). Thus, reactions between carbenes or carbene equivalents and imines comprise an increasingly useful method for aziridination. In addition to carbenes and carbenoids, ylides have also been used to effect aziridinations of imines in all classes of this reaction type the mechanism frequently involves a stepwise, addition-elimination process, rather than a synchronous bond-forming event. 

Schrock-type carbenes are nucleophilic alkylidene complexes formed by coordination of strong donor ligands such as alkyl or cyclopentadienyl with no 7T-acceptor ligand to metals in high oxidation states. The nucleophilic carbene complexes show Wittig s ylide-type reactivity and it has been discussed whether the structures may be considered as ylides. A tantalum Schrock-type carbene complex was synthesized by deprotonation of a metal alkyl group [38] (Scheme 7). 

These carbene (or alkylidene) complexes are used for various transformations. Known reactions of these complexes are (a) alkene metathesis, (b) alkene cyclopropanation, (c) carbonyl alkenation, (d) insertion into C-H, N-H and O-H bonds, (e) ylide formation and (f) dimerization. The reactivity of these complexes can be tuned by varying the metal, oxidation state or ligands. Nowadays carbene complexes with cumulated double bonds have also been synthesized and investigated [45-49] as well as carbene cluster compounds, which will not be discussed here [50]. 

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

Sulfur-stabilized ylides underwent photodriven reaction with chromium alkoxy-carbenes to produce 2-acyl vinyl ethers as E/Z mixtures with the E isomer predominating (Table 22) [ 121-123]. The reaction is thought to proceed by nucleophilic attack of the ylide carbon at the chromium carbene carbon followed by elimination of (CO)5CrSMe2. The same reaction occurred thermally, but at a reduced rate. Sulfilimines underwent a similar addition/elimination process to produce imidates or their hydrolysis products (Table 23) [ 124,125]. Again the reaction also proceeded thermally but much more slowly. Less basic sulfilimines having acyl or sulfonyl groups on nitrogen failed to react. 

For a review on reactions of Group 6 metal carbenes with ylides and related dipolar species see Alcaide B, Cassarubios L, Dominguez G, Sierra MA (1998) Curr Org Chem 2 551... 

The insertion of a carbene into a Z-H bond, where Z=C, Si, is generally referred to as an insertion reaction, whereas those occurring from Z=0,N are based on ylide chemistry [75]. These processes are unique to carbene chemistry and are facilitated by dirhodium(II) catalysts in preference to all others [1, 3,4]. The mechanism of this reaction involves simultaneous Z-H bond breaking, Z-car-bene C and carbene C-H bond formation, and the dissociation of the rhodium catalyst from the original carbene center [1]. 

The use of dirhodium(II) catalysts to generate ylides that, in turn, undergo a vast array of chemical transformations is one of the major achievements in metal carbene chemistry [1,103]. Several recent reviews have presented a wealth of information on these transformations [1, 103-106], and recent efforts have been primarily directed to establishing asymmetric induction, which arises when the chiral catalyst remains bound to the intermediate ylide during bond formation (Scheme 11). 

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