Of N-heterocycles

Studies on covalent hydration of N-heterocycles (67AG(E)919,76AHC(20)117) have revealed the diagnostic value of alkyl substituents in structural assignments due to their steric hindrance effects in addition reactions. C-Methyl substituents are therefore also considered as molecular probes to solve fine-structural problems in the pteridine field. The derivatives... 

Derivatives of N-heterocycles with reactive substituents as dyes for animal fibers and synthetic polyamides 97CLY149. 

Alkyl(alkoxy)silylalk yl-derivatives of N-heterocycles 99UK318. 

Complexes of N-heterocycles with boron compounds 99JOM(581)129. 

Free-radical cyclization in the synthesis of N-heterocycles 97T17543. 

Synthesis of N-heterocycles using ureas and related compounds 98UK333. 

Synthesis of N-heterocycles with C—N bond formation catalyzed by transition metal catalysts 97SL749. 

Transition metal-catalyzed heterocyclizations with formation of N-heterocycles 98AG(E)2046. 

Unsaturated germanates and stannates in the synthesis of N-heterocycles by [2-1-3] cycloaddition 98KGS1155. 

Dihydropyridines not only are intermediates for the synthesis of pyridines, but also are themselves an important class of N-heterocycles an example is the coenzyme NADH. Studies on the function of NADH led to increased interest in the synthesis of dihydropyridines as model compounds. Aryl-substituted dihy-dropyridines have been shown to be physiologically active as calcium antagonists. Some derivatives have found application in the therapy of high blood pressure and angina pectoris. For that reason the synthesis of 1,4-dihydropyridines has been the subject of intensive research and industrial use. The Hantzsch synthesis has thus become an important reaction. 

The surprising stability of N-heterocyclic carbenes was of interest to organometallic chemists who started to explore the metal complexes of these new ligands. The first examples of this class had been synthesized as early as 1968 by Wanzlick [9] and Ofele [10], only 4 years after the first Fischer-type carbene complex was synthesized [2,3] and 6 years before the first report of a Schrock-type carbene complex [11]. Once the N-heterocyclic ligands are attached to a metal they show a completely different reaction pattern compared to the electrophilic Fischer- and nucleophilic Schrock-type carbene complexes. 

In the synthesis of N heterocycles, this technique also overcomes competitive retro Michael addition that lowers the yield of 1,4-adduct in the Michael addition of amines to nitro olefins. Thus, a toluene solution of nitro olefin 159 was treated with allylamine 160, EtgN, and TMSCI under nitrogen at ambient tern-... 

Nolan SP, Viciu MS (2005) The Use of N-Heterocyclic Carbenes as Ligands in Palladium Mediated Catalysis. 14 241-278... 

Pyykkd, P. and Runeberg, N. (2006) Comparative Theoretical Study of N-Heterocyclic Carbenes and Other Ligands Bound to Au. Chemistry - An Asian Journal, 1, 623-628. 

The third chapter Recent Developments in the Chemistry of N-Heterocyclic Phosphines presents a survey on five- and six-membered phosphorus-nitrogen heterocyclic compounds whose rings combine a phosphazene or phosphazane unit... 

Linke, S., Kurz,., Lipinski, D., and Gau, W., Annealation reactions of N-heterocycles to condensed pyridones with bridgehead nitrogen, Ann. Chem., 542, 1980. 

The 5-dig-mode of cyclization has been applied in the synthesis of N-heterocycles. For example, treatment of the /i-allenyl dithiosemicarbazide 37 with Bu3SnH and AIBN in hot benzene furnishes the substituted 3H-pyrrole 38 in 41% yield and the isomeric heterocycle 39 in 30% yield (Scheme 11.13) [68], Iminyl radical 40 is formed via Bu3Sn addition to the thiocarbonyl group of the radical precursor 37 and fragmentation of the adduct (not shown). Nitrogen-centered radical 40 adds 5-dig-selectively to provide substituted allyl radical 41. The latter intermediate is trapped by Bu3SnH to furnish preferentially product 38 with an endocydic double bond. 

The iminium salt photoaddition reaction has been applied to the synthesis of N-heterocyclic systems (equation 70)126,127. 

W. C. SCHATTENMANN, M. SpiEGLER, et at, Angew. Chem. 1998, 110, 2631-2633 Angew. Chem. Int. Ed. 1998, 37, 2490-2493 (b) Highly Active Ruthenium Catalysts for Olefin Metathesis The Synergy of N-Heterocyclic Carbenes and Coordi-natively Labile Ligands, T. Weskamp,... 

The catalytic activity of Cp Ir(III) complexes in the Oppenauer-type oxidation of alcohols was considerably enhanced by the introduction of N-heterocyclic carbene ligands. Here, high turnover numbers (TONs) of up to 950 were achieved in the oxidation of secondary alcohols [40]. 

The cyclizahon of amino alcohols should be an attractive method for the synthesis of N-heterocyclic compounds, mainly because they can be obtained in a single step and without the generation of wasteful byproducts. Carbon-nitrogen bond formation catalyzed by Cp lr complexes has been extended to the synthesis of N-hetero-cyclic compounds by the cyclization of amino alcohols. 

Singh and Han [60] have reported the preparation of another dihydropyrrole by using a N-Cbz derivative obtained by decarboxylative amidation (Scheme 9.27, results line 1) as starting material. N-Alkylation with allyl bromide followed by RCM (Grubbs II catalyst) furnished the dihydropyrrole in excellent yield (95%). Lee et al. have similarly transformed the amination product (Table 9.2, entry 21) into a variety of N-heterocycles [54]. 

In spite of the large number of N-heterocyclic carbenes known to date, new NHC ligands are regularly introduced. A selection of most recent developments is presented in this section. 

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