Amino carbene

Alkoxy(carbene)iron(0) and amino(carbene)iron(0) complexes usually react with alkynes to give rj4-pyrone iron complexes and furans, respectively [54]. Nevertheless the chemoselective formation of naphthols was reported for alkoxy(carbene)iron(0) complexes with the electron-poor alkyne dimethyl... 

Merlic developed a new variation of the thermally induced benzannulation reaction. The dienylcarbene complex 132 was reacted with isonitrile to give an orf/zo-alkoxyaniline derivative 135 [76] (Scheme 56). This annulation product is regiocomplementary to those reported from photochemical reaction of chromium dienyl(amino)carbene complexes. The metathesis of the isocyanide with the dienylcarbene complex 132 generates a chromium-complexed di-enylketenimine intermediate 133 which undergoes electrocyclisation. Final tau-tomerisation and demetalation afford the orf/zo-alkoxyaniline 135. 

The 1,3,4-oxadiazole 113 is formed from the azo compound 112 by the action of triphenylphosphine <96SL652>. A general synthesis of 1,3.4-oxadiazolines consists in boiling an acylhydrazone with an acid anhydride (e.g., Scheme 18) <95JHC1647>. 2-Alkoxy-2-amino-l,3,4-oxadiazolines are sources of alkoxy(amino)carbenes the spiro compound 114, for instance, decomposes in boiling benzene to nitrogen, acetone and the carbene 115, which was trapped as the phenyl ether 116 in the presence of phenol <96JA4214>. 

Fig. 3.18 Catalysts 46-48 bearing cyclic (alkyl)(amino)carbenes...

Reaction of unsaturated chromium and tungsten carbene complexes 407 with piperidazine provides amino-carbene complexes 408 and 409. Although various solvents such as CH2C12 benzene, ether, and THF can be used in the reaction, the yields of the desired products 410 are 32-59%, and considerable amounts of by-products 409 are formed. The carbene complexes 408 are rather stable and can be stored in a refrigerator. Oxidation of both complexes with iodoso-benzene affords oxo derivative 410 in 70% (M = Cr) and 41% (M =W) yields (Scheme 64) <1994CL777>. 

Bertrand et al. demonstrated with the stable cyclic (aIkyl)(amino)carbenes (CAACs) 16 that only one nitrogen atom is sufficient for the stabilization of the carbene center. The resonance of the carbene carbon atom in carbenes of type 16 is... 

In addition to complexes of type 44 with a C2 coordinated NHC ligand, complexes of type 45 with abnormal C4 or C5 bound carbene ligands have recently been described (Fig. 16) [143, 144]. The carbene carbon atom in these complexes is stabilized by only one nitrogen atom. A similar situation has been observed for the cyclic (alkyl)(amino)carbene (type 16, Fig. 6) [38, 39]. 

NHCs possess a lone pair of electrons and an accessible vacant orbital. This combination resembles the situation in many transition metal centers and could thus mimic their chemical behavior. While cyclic diaminocarbenes are inert towards hydrogen, it has been demonstrated that cyclic (aIkyl)(amino)carbenes of type 16... 

Fig. 6) can activate hydrogen under mild conditions [219]. In contrast to transition metals, that act as electrophiles towards hydrogen, the (alkyl)(amino)carbenes mainly behave as nucleophiles initially creating a hydride like species, which then attacks the positively polarized carbene carbon atom. 

Other substituent, which does not participate in the stabilization, can be considered as a spectator substituent (Scheme 3) [34]. This stabilization system is particularly efficient with an amino group which is not only a 71-donating substituent but also a a-electron withdrawing group. This stabilization mode provides a large choice of substituents allowing an easy functionalization of the corresponding stable mono-aminocarbenes. In fact, several types of acyclic and cyclic amino carbenes have been already synthesized [8, 9]. 

This is probably due to the high electron donating character of the acyclic bis (amino)carbene as compared to the NHCs, and to its structural flexibility which could liberate the reaction site on the metal center. 

Otherwise, while the reactions of chromium mono(amino)allenylidene complexes 5 with dimethylamine afforded the aIkenyl(amino)carbene derivatives 49 by means of the expected N-H addition across the C (=Cp double bond (Scheme 13)... 

Optimized reaction conditions call for the use of Wilkinson s catalyst in conjunction with the organocatalyst 2-amino-3-picoline (60) and a Br0nsted add. Jun and coworkers have demonstrated the effectiveness of this catalyst mixture for a number of reactions induding hydroacylation and C—H bond fundionalization [25]. Whereas, in most cases, the Lewis basic pyridyl nitrogen of the cocatalyst ads to dired the insertion of rhodium into a bond of interest, in this case the opposite is true - the pyridyl nitrogen direds the attack of cocatalyst onto an organorhodium spedes (Scheme 9.11). Hydroamination of the vinylidene complex 61 by 3-amino-2-picoline gives the chelated amino-carbene complex 62, which is in equilibrium with a-bound hydrido-rhodium tautomers 63 and 64. 

The relatively sensitive acylamino chromium complexes (e.g., 43) can be prepared in situ from stable amino carbene complexes (e.g., 42) as shown for the generation of miinchnone 44 and conversion to pyrrole 45 with DMAD (Scheme 10.5) (21). 

Up to 2000, the number and variety of stable carbenes have been limited by the perceived necessity for two strongly interacting substituents. The preparation of stable or persistent (aryl)- or (alkyl)-(phosphino)carbenes as well as (aryl)(amino)-carbenes demonstrates that a single electron-active substituent allows the spectroscopic characterization of singlet carbenes under standard laboratory conditions. It has been shown that an amino substituent is more efficient than a phosphino substituent to stabilize a carbene center and that the steric bulk of the spectator substituent can be decreased even to the size of a methyl group in the phosphino series, so that a broad range of observable carbenes will be readily available. 

To obtain amino acids, amino carbenes (X = NH2 in Fig. 6) must be used, and if chirality is required, the asymmetry of the new a-carbon center must be controlled. Higedus and co-workers have achieved good stereochemical control by using a chiral oxazolidine chromium carbene. 

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 second example shows that such a reaction need not always lead to an uncharged system. If, instead of thiocarbene complexes, amino-carbene complexes are treated with hydrogen halides, products of saltlike character are isolable (66). One finds the halogen at the metal and the hydrogen at the removed carbene ligand, and one obtains iminium halo-genopentacarbonylmetalates ... 

A complementary access to alkoxy- and aminocarbene complexes ( Semmelhack-Hegedus route ) involves the addition of the pentacarbonylchromate dianion 18 (obtained from the reduction of hexacarbonylchromium with C8K) to carboxylic acid chlorides and amides [27] (Scheme 10). While alkylation of acyl chromate 19 leads to alkoxycarbene complexes 12, addition of chromate dianion 18 to carboxylic amides generates the tetrahedral intermediates 20, which are deoxygenated by trimethylsilyl chloride to give amino carbene complexes 14. 

Scheme 10. Semmelhack-Hegedus route to alkoxy and amino carbene complexes.

Scheme 12. Acylamino carbene complexes as amino carbene complex ...

A complementary access to extended indole and carbazole systems is based on the thermal intramolecular benzannulation of ortfio-al kyn ylan i 1 i nocarbene complexes or on a photo-induced benzannulation of phenylpyrrolylcarbene complexes. The first example involves an intramolecular access to the carbazole skeleton. Refluxing a solution of orflio-alkyn yl phen yl amino carbene complex 109 in acetonitrile gave a 63 % yield of benzocarbazole 110. Less strongly coordinating solvents (TBME, THF, or di-n-butyl ether) or other substituents less bulky than 2,4,6-trimethylphenyl (for example, phenyl or 4-methylphenyl) led to a considerably reduced yield (Scheme 39) [83]. 

Stable cyclic (alkyl)(amino)carbenes (CAAC) have been developed by Bertrand et al. and can be readily prepared in a few steps starting from simple imines 16 (Fig. 4, Scheme 9) [64,65,82-84], A special feature of these 5-membered ring carbenes is their stabibzation by the help of a quarternary carbon next to the carbene. 

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