Helmut Quast

The Effects of Substituents on the Degenerate Cope Rearrangement of Semibullvalenes and Barbaralanes

The activation parameters [ΔG# (200 K), ΔH#, and ΔS#] for the degenerate Cope rearrangements of barbaralane (1a) and several semibullvalenes (3a, 7a, 8a), and for those of a number of their 3,7- and 2,6-disubstituted derivatives, have been determined by variable-temperature carbon-13 NMR spectroscopy at 126 and 151 MHz. the disubstituted compounds studied include the barbaralanes 1c, d, e, h, j, k, and 2e, h, j, and the semibullvalenes 3d, e, h, 4e, h, i, l, m, and 5h–8h. Careful attention has been given to the measurement of temperatures. The data for these compound, together with those for related compounds previously reported in the literature, show that conjugating subsituents (e. g. aryl, CH, CO2R) in the 2,6 positions lower the barriers of the degenerate Cope rearrangement, whereas substituents that are electron-withdrawing through the inductive effect (e. g. Cl, CF3, SO2Ar) retard the reaction. Substituents in the 3,7-positons have little effect or are rate-retarding. The presence of 1,5-tri- and tetramethylene bridges in semibullvalenes accelerates the rearrangements, the effect being larger in the case of the former.

Synthesis and degenerate Cope rearrangement of 2,6-diphenylbarbaralane

Abstract The oxygen-sensitive 2,6-diphenylbarbaralanes 15 and 16 are synthesized from bicyclo[3.3.1]nonane-2,6-dione( 9 ) via dibromide 14 in four steps with 46 and 54% overall yield, respectively. Crystalline 2,6-diphenylbarbaralane ( 15 ) is yellow at room temperature and decolourizes reversibly on cooling. Accordingly, the UV spectrum of trans -decalin solutions of 15 changes reversibly in the temperature range 295–450 K. The bromodiphenylbarbaralane 16 is shown to exist as a single valence tautomer. The 13 C chemical shifts of 16 serve as estimates of the analogous shifts in the non-exchanging valence tautomer of 2,6-diphenylbarbaralane ( 15 ) itself. The barrier for the degenerate Cope rearrangement of 15 in [D 2 ]dichloromethane has been determined from 13 C-NMR line width measurements in the temperature range of 169–233 K to be ΔG‡ 200 = 21.6 kJ mol -1 . Hence phenyl substituents at the termini of the allylic parts of barbaralane accelerate the degenerate Cope rearrangement somewhat more than cyano groups.

2,6-dicyano-1,5-dimethylsemibullvalene as a probe for homoaromatic molecules of the Dewar-Hoffman type

Abstract The 2,6-dicyano-1,5-dimethylsemibullvalene (2d) synthesized via the bicyclo[3.3.0]octanedione 7 exists as a classical ground state which is lower in energy by less than 5 kcal/mole than the homoaromatic transition of the Cope rearrangement of 2d.

The photochemistry of 1,4-dihtdro-5h-tetrazole derivatives isolated in low-temperature matrices

Abstract Six 1,4-dihydro-5 H -tetrazole (tetrazoline) derivatives have been photolysed in Ar and N2 matrices at 12 K. The dimethyltetrazolinone ( 1a ) gave as the major product the same diaziridinone obtained previously from solution photolysis, but it also underwent a novel cleavage to MeNCO and, presumably, methyl azide. Iminotetrazolines ( 3a ) and ( 3b ) gave the corresponding carbodiimides ( 5a ) and ( 5b ), and iminodiaziridines ( 4a ) and ( 4b ); while the tetrazolinethiones ( 7a ) and ( 7b ) gave carbodiimides ( 8a ) and ( 8b ), respectively. Photolysis of the vinyl substituted tetrazolinone ( 9 ) proceeded differently in inert ambient-temperature solutions and low-temperature matrices. In the former an imidazolone ( 10 ) was the sole isolable product, while in the latter formation of diaziridinone ( 15 ) competed with an alternative cleavage giving t -butyl isocyanate and vinyl azide. These results are best interpreted on the basis of an intermediate biradical, which, however, could not be detected directly by matrix ir. spectroscopy.

Design of substituted semibullvalenes suitable for control of the cope rearrangement by two ps IR laser pulses

Abstract A systematic experimental search for substituted semibullvalenes yields systems which have nearly thermoneutral reactants and products ( ΔH 0 ≈ 0.5–1 kJ mol −1 ) separated by moderately low barrier ( ΔE 0 ǂ ≈ 10–30 kJ mol −1 ), e.g. 2,6-dicyanoethylmethylsemibullvalene DEM-SBV (= 7 c ⇌ 7 c ′). Theoretical model simulations suggest that their Cope rearrangement may be controlled by two infrared picosecond (IR ps) laser pulses: The first pump pulse excites the reactants to the transition state, represented by a delocalized wave packet with energy above the potential barrier. The second dump pulse stabilizes the products.

A perimidine-derived non-Kekulé triplet diradical.

6,9-Di(tert-butyl)-1-methyltetrazolo[1,5-a]perimidine (1) has been synthesized from naphthalene in seven steps. The EPR spectra, recorded after irradiation of 1 in a butyronitrile matrix at 77 K (lambda = 351 nm) and in Ar and Xe matrixes at 4.6 K (lambda > or = 345 nm), showed a six-line, high-field signal (Delta m(S) = +/- 1), centered at 3350 G in butyronitrile, along with a half-field signal (Delta m(S) = +/- 2), which is characteristic for triplets. Simulation of the observed EPR spectra gave values for the zero-field splitting parameters of |D/hc|/cm(-1) = 0.0105, |E/hc|/cm(-1) = 0.0014 in butyronitrile and |D/hc|/cm(-1) = 0.0107, |E/hc|/cm(-1) = 0.0016 in Ar. These EPR parameters are consistent with the diradical 5,8-di(tert-butyl)-2-(N-methylimino)perimidine-1,3-diyl ((3)2) as source of the EPR spectra. Linearity of the Curie-Weiss plot and UB3LYP and (14/14)CASPT2 calculations of the singlet-triplet energy difference (DeltaE(ST) approximately 8-10 kcal/mol) indicate that the triplet is the ground state of 2, as predicted for such a nondisjoint diradical.

Competing pathways in the photonitrogenation of a 3,5-dihydro-4H/-1,2,3,-triazol-4-one. A novel route to aziridinones

Abstract The photodenitrogenation of the 3,5-dihydro-4H-1,2,3,-triazol-4-one 6, obtained from 1-azidoadamantane (5) and the lithium enolate of methyl isobutyrate, produced acetone, the isocyanide 10 and the aziridinone 7, which was solvolysed to the α-methoxy amide 8 during photolysis in [D4]methanol.

Crystal and molecular structure and the Cope activation barriers of some dicyano-1,5-dimethylsemibullvalenes

Abstract X-ray structures for 2,6-dicyano- (2), 2,4-dibromo-3,7-dicyano- (3a), and 4-bromo-3,7-dicyano-1,5-dimethylsemibullvalene (3c) are reported together with the NMR determined activation barriers of the degenerate Cope rearrangement of 2, 3a, and 3b and it is shown that distortions in the ground state geometries of semibullvalenes correlate with the barriers for their Cope rearrangement.

Synthesis and structure of 2,4-dibromo-3,7-dicyanosemibullvalene. A substitution pattern retarding the cope rearrangement

Abstract The crystal and molecular structure of 2,4-dibromo-3,7-dicyanosemibulvalene( 3b ) synthesized from the bicyclo [3.3.0]octanedione 4 was determined by X-ray analysis; 3b exhibits a higher activation barrier towards degenerate Cope rearrangement than the parent semibullvalene.

Synthesis of some 3,7-dicyano-1,5-dimethylsemibullvalenes

Abstract The synthesis of 3,7-dicyano-( 2a ), 4-bromo-3,7-dicyano-( 2b ), and 2,6-dibromo-3,7-dicyano-1,5-dimethylsemibullvalene( 2e ) is reported; both 2a and 2e undergo the degenerate Cope rearrangement more slowly than the parent hydrocarbon.