Bicyclo hexa- 1,3,5-triene

Orbital mapping analysis of thermal isomerization of aza- and diaza-bicyclo[2.2.0]hexa-dienes has been carried out (80MI51205). Molecular orbital calculations (MNDO) of a series of bisdehydropyridines showed that the 2-azabicyclo[2.2.0]hexa-l,3,5-triene (125) is the least stable form having a calculated heat of formation of 728.4 kJ mol-1 (77CC539). 

Chelate complexes of the type [PdX2(diene)] (X = Cl, Br) are readily formed by the dienes hexa-1,5-diene (124), bicyclo 2,2,lJhepta-2,5-diene (1, 7) tricyclo[4,2,2,0]-deca-triene and -diene derivatives (10), cyclo-octa-1,5-diene and dicyclopentadiene, but not dipentene (43). These may be converted to complexes of the types [Pd2X2(dieneOR)2] and [PdCl(dieneOR)-(amine)] (R = alkyl) (43), and their properties indicate that they have similar structures to the platinum complexes (XXXI) and (XXXIII). 

Triazines are generally more reactive in [2 + 4] cycloaddition in comparison with 1,2,3-tria-zines. The wide variety of dienophiles can be employed enamines, enaminones, vinyl silyl ethers, vinyl thioethers, cyclic ketene jV,O-acetals, /V-phenylmaleimide, 6-dimethylaminopentafulvene, 2-alkylidene-imidazolidines (cychc ketene aminals), cyclic vinyl ethers, arynes, benzocyclopropene, acetylenes, and alkenes like ethylene, (Z)-but-2-ene, cyclopentene, cyclooctene and bicyclo[2.2.1]hept-2-ene, hexa-1,5-diene, cycloocta-1,5-diene, diallyl ether, cyclododeca-l,5,9-triene,... 

Certain hexa-l,3,5-trienes 2, either employed as such or obtained in situ by photochemical conrotatory opening of respective cyclohexa-1,3-dienes 1, cyclize on ultraviolet irradiation to bicyclo[3.1.0]hex-2-enes 3. ... 

Benzene and its substituted analogues also undergo electrocyclization reactions characteristic of other conjugated triene systems. When 1,2,4-tri-tert-butylbenzene (67) was irradiated in ether solution using a Hanovia Type L lamp and Vycor filter, the corresponding bicyclo[2.2.0]-hexa-2,5-diene (Dewar benzene) 68 was isolated 45>. Nonbonded interactions of the fer/-butyl substituents are partially relieved upon cycliza-... 

The pyrazoline (133) affords only bicyclo[3,l,0]hex-2-ene on thermolysis, but on photolysis this and hexa-l,3,5-triene are obtained. In the vapour phase at pressures below lOTorr the -triene (134) is obtained (8-10%), but at pressures higher than 50Torr the Z-isomer (135) ensues. The formation of triene is thought to proceed in a... 

Goldstein and co-workers have initiated a careful study of the mechanistic possibilities involved in the thermal decomposition of bicyclo[2,2,0]hexene. Analysis of the products, greater than 97 % cyclohexa-1,3-diene, and strict first order kinetics (AH = 32.15 0.09kcalmol" AS = 2.4 + 0.2e.u.) eliminated many pathways including those involving cis-hexa-l,3,5-triene. Use of the deuteriated substrate (269)... 

Earlier predictions that m-benzyne should exist as bicyclo[3,l,0]hexa-l,3,5-triene (413) are further supported by STO-3G calculations and established by dehydrochlorination of (412) (Scheme 63). ° The work adequately eliminates alternative... 

The photolyses of cyclohexa-1,3-diene, cycloocta-l,3,5-triene, and bicyclo[4.2.0]octa-2,4-diene were investigated by matrix-isolation techniques at a relatively early date and certainly before reliable computational methods for IR transitions were available. As an example, both IR and UV-visible spectroscopy were employed to monitor the photoreaction of cyclohexa-l,3-diene at 240-270 nm in Ar matrices at 20 K. It was discovered that the diene photolyzed irreversibly to Z-hexa-l,3,5-triene, which photolyzed more slowly but still irreversibly to -hexa-l,3,5-triene and several other thermally unstable products. The latter could not be identified with certainty, but hexa-l,2,4-triene and exo-2-vinylbicy-clo[1.1.0]butane were proposed as possibilities. 

---