Dewar pyridine

The steric and dipole-dipole effects of the CF3 group on valence isomerization in the Dewar pyridine-azaprismane-pyridine system have been studied. These reveal themselves in the high stability, compared to the pyridine, of the valence isomer arising out of the large activation energy for rearomatization. The transformation of a 1-Dewar to 2-Dewar pyridine was observed (89T3115). 

Kinetic studies on the quaternization reaction of pentamethylpyridine (154A, Scheme 64) and its deuterated congener suggest that there may be an equilibium between this aromatic pyridine and its valence isomer, pentamethyl-Dewar-pyridine (154B). It is known that such valence isomers are stabilized by steric factors such as encountered in pentamethylpyridine.194195 On determining conductometrically the initial rate of quaternization with methyl iodide for polymethylpyridines in acetone, it was found that all but one followed clean bimolecular kinetics. Pen-... 

Irradiation of the six-membered heterocycles can, as mentioned above, lead to valence bond isomerization. Pyridine gives only one of the two possible Dewar pyridines, namely (19 equation 8). Derivatives of the alternative Dewar pyridine have been prepared as outlined in equation (9) and compound (20) in turn is converted on further irradiation into the azaprismane derivative (21). 

Pyridine and some of its derivatives have been photolyzed under various conditions in the quest for Dewar pyridines and azaprismanes, amongst other products. This quest has proven successful (76MI20503). Irradiation of pyridine itself in n-butane at -15 °C produces Dewar pyridine that can be observed spectrophotometrically and intercepted by sodium borohydride and water (Scheme 219). When pyridine is photolyzed in a matrix, hydrogen cyanide and acetylene are formed (equation 184). The same products have been obtained from the vapour phase photolysis of pyridine. On vapour phase photolysis, alkylpyridines isomerize for example, 2-picoline gives a mixture of 3- and 4-picolines. Azaprismanes (288) have been suggested as the intermediates in this process (equation 185). 

Certain polysubstituted pyridines yield isolable Dewar pyridines, as illustrated in equation (2) (see CHEC 2.05). 

Irradiation of pyridine itself gives Dewar pyridine, observable spectroscopically, which in water is hydrolytically ring-opened to form H2N(CH = CH)2CHO, but in a matrix fragments to cyclobutadiene and HCN. 

Photolysis of fully substituted tricyclic Dewar pyridines carrying 2,3,4-triObutyl) substituents (7) produces azaprismanes (8), which, on standing or further irradiation, give the pyridine derivatives (9) (89T3115). 

The cycloaddition potential of compound (47) is also demonstrated by its reaction with alkynes leading to isolable 1-Dewar pyridines (50). The latter were rearranged thermally or by acid catalysis into pyridines (49). The photochemical rearrangement of (50) led to azaprismane derivative (51) (89T3115). 

The photo-isomerization of certain pyridines to Dewar pyridines is described in Section 3.2.1.2.2, and the formation of bridged ring 6H-1,2-dihydro-3-pyridones in Section 3.2.1.10.4. 

Heating Dewar pyridine 2 in hexafluorobenzene aL 170 C for 21 hours gives 13% conversion into pyridine l,30 which corresponds to a half-life of 104 hours. Azaprismane 3 is 71 % consumed in 2 hours at 170 C and yields Dewar pyridine 2 (26%), pyridine 1 (30%), and an unidentified component (15%) which rearranges to pyridine 1 on further heating.30... 

Irradiation of perfluoro(4-ethyl-2,6-diisopropyl-3,5-dimethylpyridine) (4) at 254 nm as a solution in 1,1.2-trichlorotrifluoroethane gives quantitative conversion into a mixture containing Dewar pyridine 5 (minor component) and azaprismanes 6 and 7 (1 l).31... 

Dewar pyridines 14-18 revert to the corresponding pyridines 11-13 when heated at 160°C.32 Simple bond-energy summation indicates that the 2-aza Dewar pyridine structure should be energetically favored over the 1-aza structure the predominance of 1-aza structures for per-fluoro(pentaalkylpyridines) (vide supra) is explained by the higher steric congestion of the pen-tasubstituted 2-aza structures.32... 

Pyridine/ Dewar pyridine valence isomerizations are also observed for pentasubstituted pyridines 1933 and 20.34... 

Pyridine 20 has been prepared by the silver- or acid-catalyzed isomerization of 1-aza-bicyclo[2.2.0]hexadiene 22, which in turn was obtained in a [2 + 2]-cycloaddition reaction of tris(to7-butyl)azete with hexafluorobut-2-yne. Irradiation of Dewar pyridine 22 gives a high yield of azaprismane 23, which isomerizes thermally to rearranged pyridine 24. Finally, pyridine 24 yields 1-aza Dewar pyridine 25 photolytically.34... 

Cycloaddition of iminium salts 650 onto azete 627 afforded the Dewar pyridines 651 in good yields (Equation 248) <2002S497>. 

The studies on Dewar pyridines started recently. Molecular orbital calculations show that the 2,5-bonded Dewar pyridine is more stable than the 1,4-bonded one.84-85... 

---