Azines shifts 283

Table 1 H Chemical Shifts of the Simple Monocyclic Azines cf. benzene, S 7.24) (b-73NMR>...

Some nitrogen chemical shifts relating to azines are given in Table 8. More detailed information can be found in the review of Witanowski et al. The following trends and observations are noteworthy ... 

Table 8 Nitrogen Chemical Shifts of Typical Azines and their Derivatives ...

A simple preparative route to the previously unknown lH-3-pyrazolines via azomethine imines was developed. Olefins of the type R cH=CHR react with hexafiuoroacetone azine to give azomethine mines, which undergo a sequence of prototropic shifts to form 17/-3-pyrazolines [196, 202], On heating, the latter are transformed into 3-trifluoromethylpyrazoles [196, 206] and, on treatment with bases, into 1,2,5,6-tetrahydropynmidines [206] (equation 45)... 

Table 1 13C chemical shifts (S) of cycl[3.2.2]azine analogues (in CDCI3)... 

Figure 7 1H and 13C NMR chemical shifts of some benzo- and dibenzocycl[3.2.2]azines. 

Three possibilities were considered to account for the curved Arrhenius plots and unusual KIEs (a) the 1,2-H shift might feature a variational transition state due to the low activation energy (4.9 kcal/mol60) and quite negative activation entropy (b) MeCCl could react by two or more competing pathways, each with a different activation energy (e.g., 1,2-H shift and azine formation by reaction with the diazirine precursor) (c) QMT could occur.60 The first possibility was discounted because calculations by Storer and Houk indicated that the 1,2-H shift was adequately described by conventional transition state theory.63 Option (b) was excluded because the Arrhenius curvature persisted after correction of the 1,2-H shift rate constants for the formation of minor side products (azine).60... 

Of course carbene C-H insertion reactions are well known absolute kinetics have been reported for the insertions of ArCCl into isooctane, cyclohexane, and n-hexane,67 and of PhCCl into Si-H, Sn-H, and C-H bonds.68 More recently, detailed studies have appeared of PhCCl insertions into a variety of substrates bearing tertiary C-H bonds, especially adamantane derivatives.69 Nevertheless, because QMT is considered important in the low temperature solution reactions of MeCCl,60,63 and is almost certainly involved in the cryogenic matrix reactions of benzylchlorocarbene,59 its possible intervention in the low temperature solution reactions of the latter is a real possibility. We are therefore faced with two alternative explanations for the Arrhenius curvature exhibited by benzylchlorocarbene in solution at temperatures < 0°C either other classical reactions (besides 1,2-H shift) become competitive (e.g., solvent insertion, azine formation), or QMT becomes significant.7,59,66... 

The possible intervention of classical, competitive reactions in the low temperature solution chemistry of benzylchlorocarbene (10a) requires careful investigation. There are reasons to suspect azine (48) formation Goodman reported minor yields of azine in analogous MeCCl experiments,60 and Liu et al. found 40% of 48 in the photolysis of neat diazirine 9a.65 Perhaps azine formation is also significant at low temperature in hydrocarbon solvents. If so, the intervention of bimolecular azine formation, in competition with the unimolecular carbene 1,2-H shift, could lead to a nonlinear temperature dependence for the disappearance of 10a. Arrhenius curvature could then be explained without invoking QMT. 

Photolytic decomposition of diazirine 9a in methylcyclohexane led to substantial C-H insertion of PI1CH2CCI into the solvent, although azine was a minor product. At 25°C, there were 74% of 1,2-H shift products and 14% of C-H insertion. Insertion increased to 44% at —75°C. Here too, a curved Arrhenius correlation reflected the competition of two classical reactions, not the incursion of QMT.71... 

Calculated and Observed Carbon-13 Chemical Shifts in Azines, Jaffe and Bene CNDO-MO s, Average Excitation Energy (AEE) and Different Excitation Energy... 

TABLE 6.6. Nitrogen rr, tt, and Total o- -I- 77 Net Charges (me) and N NMR Shifts of 4-Substituted Pyridines and Selected Azines... 

Phenazines and their derivatives are known to be biologically significant molecules, particularly in the field of photodynamic therapy. A recent example of the preparation of red-shifted azine dyes potentially useful for photodynamic therapy was reported <1999JHC25>. 

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