Nitrogen pivot atom

Recently, Bogatsky et al.34) have reported the synthesis of cryptands incorporating urea moieties in which one of the urea nitrogens serve as a pivot atom. Cyclization of dianion 34 with diamine 18 afforded the macrocycle 35 in 21 % yield, which was converted into the corresponding oxo-cryptand 36 (Scheme 6). 

When the tether is attached to the macroring at carbon, the molecule is said to have a carbon pivot atom. When the side arm is attached as illustrated at the right, the pivot atom is nitrogen. Because the molecular models of these compounds resemble lassos and because the combination of side arm and macroring donors can permit a cation to be roped and tied , we have called such macrocycles lariat ethers . 

The combination of sulfur and nitrogen donor atoms in crown ethers appears to provide an ideal binding site for Hg + as several sensors, mainly electrochemical, which use azathiacrown ethers to target the metal have been reported. The selectivity for mercury over other metals is often astonishingly high. " A 15-crown-5 C-pivot lariat ether incorporating a thioether side chain was found to extract 46% of Hg + from aqueous solution into dichloroethane, although it was also able to remove almost 100% of Ag+ under the same conditions. ... 

II. Rearrangements Involving the Pivotal Nitrogen Atom in the Starting Ring... 

Fig. 18 Top-. Structures of alkali metal complexes of 2.2.2-crypt, 9 (hydrogen atoms omitted for clarity) 139—42. Bottom Torsion angles defined by the two triangles obtained by linking the two sets of three oxygen atoms. The smaller the torsion angle, the higher the distance between the two pivot nitrogen atoms (and the larger the cavity size of the cryptand)...

From these comments, it is evident that the crab-like cyclization reaction is a convenient method for preparation of a great variety of aza-crowns in good yields. Some of the polyaza products included /V-pivot lariat polyaza-crowns, polyaza-crowns and cyclams with one or two unsubstituted ring nitrogen atoms, peraza-cage compounds, and polyaza-crowns containing sulfur atoms. This new method could become the method of choice for preparation of the functionalized polyaza- and peraza-crowns. 

In these flames over 95% of the nitrogen atoms are combusted to Nj. Since Nj is predominantly formed by reactions of NO with NHi, approximately half the nitrogen atoms are cycled through NO while the other half are oxidized no further than NHi. Since the amine radicals contribute to the Nj formation in the rough ratios of 3 1 1 for NH2 NH N, the primary pathway to Nj can be written by the two sequences shown in heavy line. These sequences account for about two thirds of the NO and Nj formation. While this indicates a pivotal role for NHj, neither NH nor N reactions with NO can be eliminated from the reaction scheme. 

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