Nitrogen-heterocyclic carbene

PALLADIUM-(ACYCLIC DIAMINOCARBENE) SPECIES AS ALTERNATIVE TO PALLADIUM-(NITROGEN HETEROCYCLIC CARBENES) IN CROSS-COUPLING CATALYSIS... 

FIGURE 16.5 Experimental homogeneous catalysts for AB dehydrogenation, (a) (POCOP)lr(H)2 and (b) Ni(NHC>2, where NHC is a nitrogen heterocyclic carbene. 

The amidation of esters and amino alcohols is a less widely investigated area with relatively few literature examples. In 2005, nitrogen-heterocyclic carbenes (NHCs) were reported to be catalytic in this transformation." l,3-Bis(2,4,6-trimethylphenyl)-l,3-dihydro-2i -imidazol-2-ylidene (IMes) was chosen as the most suitable carbene, being readily available, reactive and easy to store. It is required in low catalytic loadings of 5 mol% in THF at 23 °C. These mild conditions are highly desirable so that the reaction is compatible with other functional groups and can potentially be used for enantioselective reactions. Reaction times varied from 1.5-24 hours depending on steric bulk and electrophilicity of the ester. 

Diamino-substituted complexes of type 37 were first obtained by Fischer et al. [12] in two steps via the 1,2-addition-elimination product 34 from di-methylamine and 35 (Scheme 6). The (3-aminoallenylidene)chromium complexes 36, which can be prepared either from 33 [47,48] or directly from 35 [33], can also be transformed to l,3-bis(dialkylamino)-substituted complexes of type 37 (e.g., R2=z Pr) by treatment with dimethylamine in excellent yields [33]. Although the complex 37 is accessible by further reaction of the complex 34 with dimethylamine, and 34 itself stems from the reaction of 35 with dimethylamine, the direct transformation of 33 to 37 could not be achieved [12]. In spite of this, heterocyclic carbene complexes with two nitrogens were obtained by reactions of alkynylcarbene complexes 35 with hydrazine [49] and 1,3-diamines [50]. 

M-heterocyclic carbenes are neutral compounds with a divalent carbon atom located between the two nitrogens. The four types of stable diaminocarbenes used for the synthesis of chiral complexes are listed below (Fig. 2) ... 

The photoelimination of nitrogen from diazo compounds provides a simple and versatile route for the generation of carbenes, and in certain instances, insertion reactions of carbenes can be employed in the synthesis of heterocycles. Carbenes are believed to be involved at least in part in the photochemically induced conversion of N,N-diethyldiazoacetamide (439) into the y-lactam 440 and the /Mactam 441,365 and a similar approach has been successfully employed in the synthesis of a carbapen-2-em366 and of 7-methylcephalosporin analogues.367 Carbene insertion of a different type has been observed on irradiation of the 6-anilino-5-diazouracils 442 to give the indolo[2,3-d]pyrimidines 443.368 Ring contractions in heterocycles... 

Fig. 4. Tripodal N-heterocyclic carbene chelators with mesitylene (mes-carbene, left), carbon (TIME, center), and nitrogen (TIMEN, right) anchoring units.

In addition to the compounds listed above, a number of triazolium [36], thiazolium [37], and benzimidazolin-2-thione [32] derivatives as well as five-membered heterocycles with only one nitrogen atom [38, 39] or two phosphorus atoms [40] within the heterocycle, all of which are suitable for the generation of heterocyclic carbenes, have been described. 

Going one step beyond, the reaction of these n-donor-substituted Group 6 allenylidenes with bifunctional N,N- or W, 5-dinucleophiles opened up a fruitful route for the synthesis of an extensive family of N- or 5-heterocyclic carbenes. Thus, treatment of complex [Cr =C=C=C(NMe2)Ph (CO)5] with benzamidine, guanidine or thioacetamide has been reported to yield the a,(3-unsaturated carbenes 54 (Scheme 16) [62], arising from nitrogen attack at Cy, subsequent HNMe2... 

Deprotonation of the Mes-substituted iminium salt 117, prepared from the silylamidine by addition of (diisopropyl-amino)dichlorophosphine, in the presence of trimethylsilyltrifluoromethanesulfonate, led to the formation of the dimer 13 (Scheme 38) <2004JA10198>. However, treatment of the salt 117, bearing more bulky 2,6-(diisopropyl)-phenyl substituents at the nitrogen atoms, generated the stable four-membered heterocyclic carbene 12. 

N-Heterocyclic carbenes (NHCs) - These are five-membered heterocyles in which a carbene function is stabilized by adjacent nitrogen (or sulfur) atoms on both sides of the carbene. They are often stable solids with sharp melting points and can be recrystallized from hydrocarbon solvents <2000CRV39>. For examples see Sections 24.3.1 and 244.2.5. 

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