Amino-ylide carbenes

The most recent development concerns the heterocyclic (amino)(ylide)carbenes AYC. Such compounds have been known for some years [203] but so far had little impact compared to their diamino stabilized relatives. Both phosphorus ylide (86) and sulfur ylide (87) stabilized AYC ligands have been generated in situ and were stabilized at suitable metal centers (Fig. 27) [204, 205]. The palladium complex 88 with an anionic (amino) [bis(ylide)]carbene is also known [206]. 

First of all, the introduction of an electropositive substituent onto the carbene center increases its nucleophilicity. In particular, the effect of a strong 7t-donating and electropositive ylide substituent is efficient as can be seen in the case of amino (ylide) carbenes and of cyclic vinylidenephosphorane type carbenes. Acyclic structures also increase the HOMO level but they also significantly diminish the S/T energy gap. 

The electron-donating ability of NHCs is determined by the mesomeric and inductive effects of the a-substituents (Section 1.2). While the electronegativity of the a-carbon atom in CAACs is lower than that of nitrogen in NHCs, the sp -hybridized a-carbon atom in CAACs is certainly no n donor. To enhance the 7i-donating ability of carbon atoms in a-position to the carbene center, amino(ylide)carbenes (AYCs) were developed. Similar compounds had been known for some time but up to then had little impact compared to their diamino-stabilized relatives. 

Figure 5.1 Metal complexes comprising the classical imidazol-2-ylidene ligand (A) and representative non-classical carbene ligands (B-N), including normal carbenes (B-E), abnormal carbenes (F-I), remote carbenes (E, G, I), cyclic alkyl(amino)carbenes (J), acyclic carbenes (K, L, M) and amino(ylide)-carbenes (N). Substituted nitrogen centres may be replaced by oxygen or sulfur. The M=C bond representation— while strongly over-emphasizing the differences in the nature of the metal-carbon bond in these non-classical carbene complexes— was used to accentuate normal and abnormal bonding.

Formation of aminocyclopropanes in a carbene cycloaddition to a carbon-carbon double bond containing no nitrogen function requires an amino-substituted carbene as counterpart. Such a type of reaction was observed with special nitrile ylides possessing a suitable double bond within the molecule Thus, nitrile ylides (193) generated from the precursors 191 or 192, underwent intramolecular [2-f-l] or [2-I-3]cycloaddition yielding 194 and 195 respectively (Scheme 4). 195 was easily hydrolyzed to aminocyclo-propane (196). The preferential direction of the cycloaddition was influenced by the nature... 

Recent research on aminocarbenes has led to the development of a very fruitful field. The synthesis of relevant complexes (Scheme 19) such as aminobis(ylide) carbene species (69) [147], cyclic C-amino P-ylides (70) (easily transformed into carbenes) [148] and their conesponding complexes (71) [149], and special ylides (72), which also transform very easily into carbenes by loss of pyridinium group, has been reported. Emphasis has been made on the transformation between ylides and carbenes and on the donor properties of the ylides. From the results obtained the ylides have shown a stronger ct-donor behavior compared with the carbenes. 

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. 

There have been ab initio studies of the bonding for a series of imino and amino phosphorus molecules,15 and for the phosphonium ylides (8).14 The anion character of the ylidic carbon is intermediate between that of olefinic and substituted carbanions. Polarisation of the H3P group has a stabilising effect. The influence of the substituent X on the inversion of the carbanion and also the tendency of the molecule to dissociate to carbene and phosphine is also discussed.14 CNDO/S MO calculations on the... 

Another unique synthetic methodology for the carbene phosphine bidentate ligand is the utilization of destabilized C-amino phosphorus ylides 25 as carbene source [66]. Due to the electrostatic repulsion between the two lone pairs on the nitrogen and on the ylidic carbon, the yUdic bond is very labile. Thus, these C-amino phosphorus ylides readily act as 1,6-bidentate ligand precursors by insertion of the metal fragment between the phosphine and carbene centers. 

In marked contrast, because of the nucleophilicity of the stable amino- and phos-phino-carbenes, numerous examples of carbene-Lewis acid adducts (reverse ylides) (Scheme 8.19) have been reported, whereas a very few examples of normal ylides are known. 

The carbonyl ylide generated from metal carbene can also add to C=0 or C=N bonds. The [2 + 3]-cycloaddition of carbonyl ylide with G=0 bond has been used by Hodgson and co-workers in their study toward the synthesis of zaragozic acid as shown in Scheme n 27a,27d Recently, a three-component reaction approach to syn-a-hydroxy-f3-amino ester based on the trapping of the carbonyl ylide by imine has been reported.The reaction of carbonyl ylide with aldehyde or ketone generally gives l,3-dioxolanes. Hu and co-workers have reported a remarkable chemoselective Rh2(OAc)4-catalyzed reaction of phenyl diazoacetate with a mixture of electron-rich and electron-deficient aryl aldehydes. The Rh(ii) carbene intermediate reacts selectively with electron-rich aldehyde 95 to give a carbonyl ylide, which was chemospecifically trapped by the electron-deficient aldehyde 96 to afford 1,3-dioxolane in a one-pot reaction (Equation (12)). 

More recently, Naidu and West have utilized a ring expansion reaction of spiro azetidinium ylide 167 in the synthesis of pyrrolizidine alkaloids. Spiro azetidinium ylide 167 is generated through a Cu(acac)2-catalyzed intramolecular reaction of a copper carbene complex with a pendant amino moiety. Subsequent [l,2]-shift gives fused bicyclic products 168 and 169 as a diastereomeric mixture. Each diastereomer was further converted to naturally occurring pyrrolizidines ( )-turneforcidine and ( )-platynecine, respectively (Scheme 18). ... 

Cyclic amino-carbenes, in molybdenum carbonyls, 5, 457 Cyclic bis(phosphine) dichlorides, with iron carbonyls, 6, 48 Cyclic carbenes, as gold atom ligands, 2, 289 Cyclic carbometallation, zirconium complexes, 10, 276 Cyclic carbozirconation characteristics, 10, 276 intermolecular reactions, 10, 278 intramolecular reactions, 10, 278 Cyclic dinuclear ylides, and gold , 2, 276 Cyclic 1,2-diols, intramolecular coupling to, 11, 51 Cyclic enones, diastereoselective cuprate additions, 9, 515 Cyclic esters, ring-opening polymerization, via lanthanide catalysis, 4, 145 Cyclic ethers... 

The vast majority of organocatalytic reactions proceeds via covalent formation of the catalyst-substrate adduct to form an activated complex. Amine-based reactions are typical examples, in which amino acids, peptides, alkaloids and synthetic nitrogen-containing molecules are used as chiral catalysts. The main body of reactions includes reactions of the so-called generalized enamine cycle and charge accelerated reactions via the formation of iminium intermediates (see Chapters 2 and 3). Also, Morita-Baylis-Hillman reactions (see Chapter 5), carbene-mediated reactions (see Chapter 9), as well as asymmetric ylide reactions including epoxidation, cyclopropanation, and aziridination (see Chapter 10), and oxidation with the in situ generation of chiral dioxirane or oxaziridine catalysts (see Chapter 12), are typical examples. 

Whereas amino groups are used most frequently as intramolecular Lewis bases (Tables 5-7), stable unsupported intermolecular complexes are typically accomplished by carbene-type donors, such as carbenes, isonitriles and ylides (Table 8). To my knowledge, no unsupported intermolecular adduct of ethers, amines or phosphanes have been structurally characterized so far (except for the more ionic cyclopentadienyl derivatives62). 

---