Ylides metal-free

Electron-deficient alkenes undergo cyclopropanation by reaction with a range of metal-free ylides and some of the most recent work in this area has focussed on the development of an organocatalysed cyclopropanation using these species. 

The cyclopropanation of a, (3-unsaturated carbonyls can be achieved using metal-free ylides. This is an attractive strategy avoiding the use of expensive metal-based catalysts and potentially hazardous diazo compounds. Two main approaches to the asymmetric ylide-mediated cyclopropanation have been developed, both utilising enantiopure amines as catalysts. 

The premier example of this process in an ylide transformation designed for [2,3]-sigmatropic rearrangement is reported in Eq. 15 [107]. The threo product 47 is dominant with the use of the chiral Rh2(MEOX)4 catalysts but is the minor product with Rh2(OAc)4. That this process occurs through the metal-stabilized ylide rather than a chiral free ylide was shown from asymmetric induction using allyl iodide and ethyl diazoacetate [107]. Somewhat lower enantioselectivities have been observed in other systems [108]. 

It is interesting to notice that the complex 58 is also able to react with soft donor hgands such as triphenylphosphine resulting in the formation in very mild conditions of the unexpected orthometallated complex 59 (Scheme 23) [89,91, 92]. The ligand here is linked to the metal through both an aromatic and an ylidic carbon. Other transformations are realized from 58 leading, including the compounds of the previous scheme, to four different structures for the bis-ylide (i) C,C-chelate (ii) C,C-orthometallated (iii) C,C-orthometallated and free ylide (iv) C,C,C-terdentate (Scheme 23). 

Assuming a reactive oxonium ylide 147 (or its metalated form) as the central intermediate in the above transformations, the symmetry-allowed [2,3] rearrangement would account for all or part of 148. The symmetry-forbidden [1,2] rearrangement product 150 could result from a dissociative process such as 147 - 149. Both as a radical pair and an ion pair, 149 would be stabilized by the respective substituents recombination would produce both [1,2] and additional [2,3] rearrangement product. Furthermore, the ROH-insertion product 146 could arise from 149. For the allyl halide reactions, the [1,2] pathway was envisaged as occurring via allyl metal complexes (Scheme 24) rather than an ion or radical pair such as 149. The remarkable dependence of the yield of [1,2] product 150 on the allyl acetal substituents seems, however, to justify a metal-free precursor with an allyl cation or allyl radical moiety. 

The q1-coordinated carbene complexes 421 (R = Ph)411 and 422412) are rather stable thermally. As metal-free product of thermal decomposition [421 (R = Ph) 110 °C, 422 PPh3, 105 °C], one finds the formal carbene dimer, tetraphenylethylene, in both cases. Carbene transfer from 422 onto 1,1-diphenylethylene does not occur, however. Among all isolated carbene complexes, 422 may be considered the only connecting link between stoichiometric diazoalkane reactions and catalytic decomposition [except for the somewhat different results with rhodium(III) porphyrins, see above] 422 is obtained from diazodiphenylmethane and [Rh(CO)2Cl]2, which is also known to be an efficient catalyst for cyclopropanation and S-ylide formation with diazoesters 66). 

One of typical reactions of the Fisher-type metal carbene is interaction of the electron-deficient carbenic carbon with a pair of non-bonding electrons contributed by a Lewis base (B ) to generate a metal complex-associated ylide or a free ylide. The ylide intermediate thus generated is usually highly reactive and undergoes further reactions to give stable products (Figure 1). 

The asymmetric catalysis has not been well explored in the reaction of a metal carbene complex-generated ammonium ylide. The ammonium ylide reaction is assumed to proceed through a free ylide rather than a metal... 

The metal-free eyclobutane-1,2-dioxime can be generated by oxidative displacement. It is interesting to note that, unlike ketene dimerization, head-to-head dimerization takes place here. The chromium ketenimine complex 20 is prepared by reaction of the Fischer-type chromium carbene complex with alkyl isocyanides.60 A cyclobutane-1,2,3,4-tetraimine 24 has been reported from the reaction of the ketenimine phosphonium ylide 22.61 Bisimine 23 has been proposed as the intermediate in this transformation. 

The addition compounds (I) are insoluble in diethyl ether, and the slurries obtained are quite stable. In more strongly solvating media, such as tetrahydrofuran or dimethoxyethane, the compounds are soluble but show rapid decomposition, with trimethylamine and polymethylene as the main products. These experiments indicate (9, 40) that when the lithium salt is trapped by donor solvent molecules, the free ylide quickly undergoes decomposition (40). No free trialkylammonium ylide has yet been prepared, even under very mild conditions (35). On the other hand, it has been shown, that the tetramethylammonium cation can even be metalated twice by organolithium reagents (102) to afford dimethyl-ammonium bismethylides ... 

Formation of sulphur ylide complexes of Pt(Pd) of type [PtX2(R2S)Sy] (X = Cl, Br or I R = Me, Et Sy = Me(Ph)SCHC(0)C6H4Cl-p) has been reported.105 The v(CO) band of the metal complexes occurs at higher frequencies than in the free ylide, and in addition spin-spin coupling between the ylide methine proton and the 195Pt nucleus is observed, which is indicative of the Pt—C bond (47). 

On the basis of these results, a mechanism (Scheme 8.10) involving the intermediacy of a silver-carbene 54 was proposed in which the insertion product arises from the formation of the halonium ylide 55, followed by a 1,2 shift (55 —> 26, or 51 or 52). Alternatively, if the substrate and thus the halonium ylide 56 contain a (3-hydrogen, this could be removed by an intramolecular deprotonation with concomitant loss of halide resulting in formation of the olefin 57 and the a-haloacetate 53. At this stage, no independent evidence has been obtained to support this pathway thus this mechanism is purely speculative (see text below). Indeed, although the pathway has been depicted as involving metal-free intermediates, it is quite likely that this is not the case, but this awaits independent experimental verification. 

Important work concerning the question of the intermediacy of metal-bound ylides was published in 2001 (Scheme 98) [232]. It was found that the diastereoselectivity of the reaction was independent of the catalyst used, but markedly influenced by the size of the ester groups. (With R = CH(fPr)2 and Rh2(S-PTPI)4 the anti-398 syn-398 ratio was 94 6.) From these results it was deduced that after the enantiotopos differentiating yHde formation, the metal dissociates off and the absolute topicity of attack of the reactive sites on each other is controlled by the factors discussed for the free yHde pathway, as illustrated in Scheme 95. 

Scheme 99 More evidence for metal-catalyzed Type M rearrangements via free ylides...

Phosphorus ylides are metal-free carbon nucleophiles that provide irreversible reactions and have been shown to react with support-bound aldehydes. DoUe et al. have reported such a transformation using 4 equiv. of the Wittig reagent in THF [382]. 

Maulide et al. developed a unified method for the direct transfer of ylides and the metal-free arylation of carbonyl compounds. This was applied in the synthesis of pyrrole 76 from 75 upon treatment with Martin s sulfurane in toluene at room temperature. The dearomatization of both indole and pyrrole could be effected with a variety of electron-donating or -withdrawing groups in good to excellent yields (13JA7312). 

D) or free oxonium ylide E triggers [1,2]-hydrogen shift to form the product F with regeneration of the metal catalyst. They calculated that, for the Rh (Il)-catalyzed O-H bond insertion reaction, the product F was formed through the metal-free oxonium ylide E. For Cu (I)-catalyzed O-H bond insertion reaction, product F tended to be formed via oxonium ylide coordinated by monovalent copper ion. 

A transition-metal-free approach for the efficient synthesis of benzofurans was developed by Liang, Li, and coworkers by the addition of arynes to iodonium ylides (Scheme 34) [57]. The reaction proceeds via the addition of iodonium ylide 79 to arynes generating the alkoxy ylide 80, which undergoes intramolecular cyclization followed by release of iodobenzene affords the product. CsF was used for the generation of arynes and the reaction proceeds under mild conditions, and using this method, the synthesis of various of functionalized benzofurans can be achieved. 

The silylene precursor LSi L=CH(C=CH2)(CMe)(NAr)2, Ar = 2,6-jPr2C6H3 possesses a ylide-like (zwitterionic) electronic structure and exhibits an electron-rich butadiene moiety in the backbone that can be utilized for the metal-free activation of E—H bonds or the addition of Lewis acids. By virtue of this ability and the basic 5-donation by the silicon atom the arsasilene functionality can be obtained by a straightforward reaction procedure. 

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