The C-N System

Besides graphite no other solid phases are known in the C-N system at normal pressure. The postulated compound C3N4 [22] was not found as a solid phase until now. N2, CN and (CN)2 are the most important gas species. 

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The formation of a phenanthridine lactone by the persulfate oxidation of 2 -cyanobiphenyl-2-carboxylic acid is interesting in that little is known of the addition of radicals to the C=N system. The mechanism proposed below (Scheme 1) also accounts for the occurrence of fluorenone as a by-product.64... 

Photocycloaddition of 1,1-dichloroethene to the quinolone (110) affords the adduct (111). The triplet excited state of the enones (112) are photoreactive and undergo addition to alkenes to afford reasonable yields of the azetidines (113). - Both electron rich and electron deficient alkenes photochemically add to the enone (114) to afford the cyclobutane adducts (115). Normally the C=N Is unreactive to (2+2)-cycloadditions but the authors believe that in this case the C=N system is activated by the trifluoromethyl group. The azetidine-2-ones (116) can be readily prepared by irradiation of the enones (114) in the presence of ketene. ... 

As for the C/N system, the energies have been corrected for ZPE, properly scaled as described in [61]. Finally, when stndying associative or dissociative steps, the energies have been corrected for the Basis Set Snperposition Error (BSSE) following the connterpoise procedure [71,72]. 

Taylor and Djerassi reason that two different mechanisms are involved alkenes are obtained by initial hydride reduction of the C=N system, whereas alkanes arise by an initial conjugative addition of hydride. Reductions with NaBDsCN show that deuterium in alkenes is attached to the carbon of the original C=0 group. 

Sodium cyanoborohydride is a close relative of sodium borohydride, but is considerably less reactive toward carbonyl groups at neutral pH. The reagent is useful for reducing C=N systems to amines. This reaction occurs rapidly at pH values where the C=N system is protonated. At pH 6-7, NaBHsCN is essentially unreactive toward carbonyl compounds, but readily reduces the protonated imines. This permits preparation of amines from carbonyl compounds without isolation of the intermediate imines. The carbonyl compound and ammonia or an alkylamine are... 

The most recent example is the investigation of the ground state of Bal, the heaviest alkaline earth monohalide. For a long time the hfs in sub-Doppler spectra of the C n - system observed by Johnson et al. could not be Interpreted. A 5-GHz laser scan near the band origin of - x2z+ (0,0) is shown in Fig. 5. In this part of the spectrum... 

In valence bond terms the pyrazine ring may be represented as a resonance hybrid of a number of canonical structures (e.g. 1-4), with charge separated structures such as (3) contributing significantly, as evidenced by the polar character of the C=N bond in a number of reactions. The fusion of one or two benzene rings in quinoxaline (5) and phenazine (6) clearly increases the number of resonance structures which are available to these systems. 

The other C=N systems included in Scheme 8.2 are more stable to aqueous hydrolysis than are the imines. For many of these compounds, the equilibrium constants for formation are high, even in aqueous solution. The additional stability can be attributed to the participation of the atom adjacent to the nitrogen in delocalized bonding. This resonance interaction tends to increase electron density at the sp carbon and reduces its reactivity toward nucleophiles. 

TT-electron delocalization over the O—C—N system. No such delocalization is possible in the N-protonated form. 

Four-membered heterocycles are easily formed via [2-I-2] cycloaddition reac tions [65] These cycloaddmon reactions normally represent multistep processes with dipolar or biradical intermediates The fact that heterocumulenes, like isocyanates, react with electron-deficient C=X systems is well-known [116] Via this route, (1 lactones are formed on addition of ketene derivatives to hexafluoroacetone [117, 118] The presence of a trifluoromethyl group adjacent to the C=N bond in quinoxalines, 1,4-benzoxazin-2-ones, l,2,4-triazm-5-ones, and l,2,4-tnazin-3,5-diones accelerates [2-I-2] photocycloaddition processes with ketenes and allenes [106] to yield the corresponding azetidine derivatives Starting from olefins, fluonnaied oxetanes are formed thermally and photochemically [119, 120] The reaction of 5//-l,2-azaphospholes with fluonnated ketones leads to [2-i-2j cycloadducts [121] (equation 27)... 

The most characteristic feature of the infrared spectra of all 5-mono- and -di-substituted-amino-l,2,3,4-thiatriazoles is a strong band in the 1540-1590 cm range, which without doubt is due to the C=N and N=N stretching vibrations of the heteroaromatic ring system. Various infrared bands between 889 and 1122 cm have been assigned to skeletal vibrations of the thiatriazole ring and a band... 

Under conditions where the rotation about the C-N bond of dimethylformamide is slow relative to the NMR time scale, the two methyl resonances will be separate singlets. Conversely, if the rotation is made to be very fast, the two methyl groups will be chemically equivalent. Their resonance will then appear as a sharp singlet. In between these extremes, kinetic information can be extracted from the line shapes. In most systems the parameter that is changed to go between these limits is the temperature. In some systems, pH or pressure has the same effect. 

The diastereofacial selectivity of this asymmetric [3C+2S] process is explained following a model similar to that described in Sect. 2.6.4.4 for the reaction of chiral alkenylcarbene complexes and 1,3-dienes. Thus, the proposed mechanism that explains the stereochemistry observed assumes a [4+2] cycloaddition reaction between the chromadiene system and the C=N double bond of the imine. The necessary s-cis conformation of the complex makes the imine... 

A remarkable variety of compounds in the Ca-(B,C,N) system has opened a window for research in related fields. With the elements boron, carbon and nitrogen, substance classes such as borocarbides, boronitrides, and carbonitrides can be considered to contain anionic derivatives of binary compounds B4C, BN, and C3N4. Until now, most compounds in these substance classes have been considered to contain alkali, alkaline-earth, or lanthanide elements. Lanthanide borocarbides are known from the work of Bauer [1]. Lanthanide boronitrides represent a younger family of compounds, also assigned as nitridoborates [2] following the nomenclature of oxoborates. 

Fig. 8.15 Concentration simplex of the La-B-C-N system. Combinations include one quaternary system, four ternary systems, and six binary systems.

In contrast, less is known about La-(CNx) compounds. The composition La2(CN2)3 was reported many years ago [43], without any structural information. Solid-state metathesis reactions of lanthanum chloride with Li2(CN2) or Zn(CN2) have recently brought up three series of the lanthanide compounds Ln2(CN2)3 [44], LnCl(CN2) [45], and Ln2Cl(CN2)N [46], Syntheses routes for Ln-(CNx) compounds containing new anions such as [C2N4] are to be developed, as well as for compounds in the La-B-C-N system (Fig. 8.15). 

An attractive alternative to these novel aminoalcohol type modifiers is the use of 1-(1-naphthyl)ethylamine (NEA, Fig. 5) and derivatives thereof as chiral modifiers [45-47]. Trace quantities of (R)- or (S)-l-(l-naphthyl)ethylamine induce up to 82% ee in the hydrogenation of ethyl pyruvate over Pt/alumina. Note that naphthylethylamine is only a precursor of the actual modifier, which is formed in situ by reductive alkylation of NEA with the reactant ethyl pyruvate. This transformation (Fig. 5), which proceeds via imine formation and subsequent reduction of the C=N bond, is highly diastereoselective (d.e. >95%). Reductive alkylation of NEA with different aldehydes or ketones provides easy access to a variety of related modifiers [47]. The enantioselection occurring with the modifiers derived from NEA could be rationalized with the same strategy of molecular modelling as demonstrated for the Pt-cinchona system. 

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