Sp2 nitrogen

The sp2 nitrogen in a pyridine ring can activate a C-H bond of 2-arylpyridines (Equation (13)). The alkylation of the 2-arylpyridine is satisfactorily catalyzed by rhodium catalysts such as [RhCl(coe)2]2/PCy3 and [RhCl(coe)2]2/PPh3.13 13a... 

The carbonylation of imidazole derivatives with several olefins takes place in high yields with the aid of an Ru3(CO)i2 catalyst.112,112a The carbonylation occurs exclusively at the a-position to the sp2 nitrogen (Equation (85)). A wide range of olefins can be utilized in this reaction, and a variety of functional groups are compatible under the reaction conditions. The (/i-H)triruthenium clusters such as 12 are proposed as a key species in this carbonylation reaction. Other five-membered A-heteroaromatic compounds, such as pyrazoles, oxazoles, and thiazoles, can be used for the carbonylation reactions, where the carbonylation takes place at the a-C-H bond to the sp2 nitrogen. 

The carbonylation of a C-H bond at the -position to the sp2 nitrogen proceeds with the aid of Ru3(CO)i2 as catalyst (Equation (91)).119 The reactivity appears to be sensitive to the polarity of the solvent. The choice of DMA as the solvent is crucial for the reaction to proceed efficiently. 

The aminoboranes containing NR2 groups do not form neutral complexes because the B—N bond possesses a higher n order than B—O and B—S bonds. In contrast, the pyrazol-l-yl and pyrrol-l-yl boranes with sp2 nitrogens bonded to the boron do give stable amine complexes... 

Other examples of nickel complexes with homologous tridentate ligands containing sp2 nitrogen donors are reported in Table 51. Some cationic bis chelates [NiL2]2+ are changed to... 

This substance is the strongest electrically neutral organonitrogen base known. The basic nitrogen is the imino (sp2) nitrogen, which on protonation forms a particularly stable conjugate acid in which the three NH2 groups become identical because of electron delocalization ... 

In principle, the sp2 nitrogen atom of the C=N double bond of N-substituted imines (or of imidines) is a better nucleophile (and a better base193) than the sp3 nitrogen atom... 

A chiral Ru hydride 23 is formed and it is assumed that the hydrogen transfer occurs via metal-ligand bifunctional catalysis. The N-H linkage may stabilize a transition state 24 by formation of a hydrogen bond to the nitrogen atom. Stereochemistry is determined by formal discrimination of the enantiofaces at the sp2 nitrogen atom of the cyclic imine. 

In addition to five- and six-membered N-heteroaromatics, other N-heterocyclic compounds have been found to serve as substrates for C-H carbonylation reactions. Benzoimidazole 13 reacted in the same way with CO and olefins in the presence of Ru3(CO)12, but carbonylation occurred at the C-H bond / to the sp2 nitrogen to give 14 (Table 1). The same relationship between reactivity and the pK, values of conjugate acids of heterocycles was also observed in the /8 carbonylation [4],... 

The powerful ability of trigonal (sp2) nitrogen to coordinate to lithium229 means compounds of general structure 267 may be ortholithiated, provided addition to C=N can be avoided. 

Murai et al. [28] found that the reaction of a,/J-unsaturated imines with CO results in a [4+1] cycloaddition to give unsaturated y-lactams (Eq. 12). For the reaction of imines which contain a /1-hydrogen, the initially produced /J,y-un-saturated y-lactams are isomerized to the stabler a,/J-unsaturated isomers. This success can be attributed to the facile coordination of the sp2 nitrogen of the substrates to a ruthenium center that assembles the substrates to the ruthenium complex. 

Carbonylation at a C-H bond fi to the sp2 ring nitrogen can also be achieved by a Ru3(CO)12 catalyst. The Ru3(CO)12-catalyzed reaction of 1,2-dimethyl-benzimidazole with an alkene and CO provides the corresponding /J-acylated product in high yield with complete site-selectivity [42] (Eq. 25). A tri-nuclear ruthenium cluster 19 is proposed as the key catalytic species. A similar basicity-dependent reactivity of substrates as described in the a-carbony-lation was observed in the case of the carbonylation at C-H bond to the sp2 nitrogen. 

Some other directing groups which involve the sp2 nitrogen can also function as a directing group in place of the pyridine ring. The reaction of aromatic imines with CO and ethylene in the presence of Ru3(CO)12 did not stop at the carbonylation step, but rather indenone derivatives were the final products and were formed via an intramolecular aldol-type reaction of the expected carbonylation products in situ (Eq. 27) [44]. The treatment of the reaction mixture with silica gel selectively afforded indenones in good yields. 

Imines have a planar trigonal framework of an sp2 carbon atom and an sp2 nitrogen atom. Each uses one sp2 orbital to form a c bond to the other atom and a p orbital to form a n bond to the other atom. The carbon uses two sp2 orbitals and the nitrogen one to form c bonds to the substituents (here the general R3). There is one lone pair of electrons on the nitrogen atom. 

Even more basic is guanidine, piCan 13.6, nearly as strong a base as NaOH On protonation, the positive charge can be delocalized over three nitrogen atoms to give a very stable cation. All three nitrogen lone pairs cooperate to donate electrons but protonation occurs, as before, on the sp2 nitrogen atom. 

Cyclic analogues of hydrazones [151], 2-pyrazolines, show both protonation and alkylation on N-l, as has already been discussed on page 326. The sp2 nitrogen (which distinguishes these systems from enamines) does not appear to play any direct part. 

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