Photochemical reactions of tetrachloro-1,4-benzoquinone (chloranil) with tricyclo[4.1.0.0(2,7)]heptane (Moore's hydrocarbon) and bicyclo[4.1.0]hept-2-ene (2-norcarene).

Solutions of chloranil (CA) in benzene were irradiated in the presence of Moore's hydrocarbon (MH) and 2-norcarene (NC). These reactions brought about four common products, namely 2,3,5,6-tetrachlorohydroquinone (TCH) and three 1 : 1 cycloadducts, whose C7H10 subunits were reorganised in comparison to the skeletons of MH and NC. As the fifth product, a norcar-3-en-2-yl ether of TCH was formed in the case of NC, whereas MH gave rise to a substance having the structure of the diastereomeric bis(endo-2-norcaryl) ethers of TCH. A control experiment demonstrated that this substance is also produced from MH and TCH without irradiation. In view of the known addition of acids onto MH to give norcaranes substituted in position 2 and the known acid-catalysed isomerisation of MH to NC, it seems obvious that TCH was the only genuine product of the photoreaction of CA with MH. Being an acid, TCH then not only took up two equivalents of MH furnishing the bisethers referred to but also catalysed the rearrangement of MH to NC, which served as a substrate for excited CA to yield the three 1 : 1 cycloadducts mentioned.

Photocycloadditions of tetrachloro-1,4-benzoquinone (chloranil) onto cyclobutene and cyclopropene. Expected and unexpected products.

Solutions of chloranil (CA) in chlorobenzene were irradiated in the presence of cyclobutene and cyclopropene. Cyclobutene gave rise to two conventional 1 : 2 cycloadducts onto the dichloroethene subunits of CA and an α,ß-unsaturated α,γ-dichloro-γ-lactone. Heating of the crude product in methanol converted the lactone into an α,ß-unsaturated methyl γ-oxocarboxylate (25% yield) and a large amount of the major 1 : 2 cycloadduct, which contains chlorocyclobutane entities, into a cyclopropylcarbinyl chloride derivative (24% yield). An entirely new product type was the result in the case of cyclopropene. After treatment of the crude product with methanol a tetracyclic acetal containing a cyclopentanone and a dihydropyran subunit was isolated in 36% yield. Apparently, CA had taken up two molecules of cyclopropene. One of the resulting cyclopropane entities must have undergone a rearrangement en route to the final product.

1-Phenyl-1,2-cyclohexadiene: generation, interception by activated olefins, dimerisation and trimerisation.

Four possible precursors of 1-phenyl-1,2-cyclohexadiene (2) were examined, namely, 6,6-dibromo-1-phenylbicyclo[3.1.0]hexane, (1alpha,5alpha,6alpha)-6-bromo-6-fluoro-1-phenylbicyclo[3.1.0]hexane, 1-bromo-2-phenylcyclohexene and 1-bromo-6-phenylcyclohexene. All four compounds could be converted into 2, as demonstrated by the products of the interception of 2 with activated olefins. Styrene, 1,1-diphenylethene, indene, furan and 2,5-dimethylfuran were employed as such. Whereas the first three gave [2+2] cycloadducts of 2, the last two provided one [4+2] cycloadduct each. To create the [2+2] cycloadducts, the pi bond of 2 that is more remote from the phenyl group reacted, whereas the pi bond of 2 conjugated with the phenyl group exclusively produced the [4+2] cycloadducts. The generation of 2 in the absence of a trapping reagent brought about relatively good yields of a dimer or a trimer of 2 depending on the mode of the liberation of 2. Being derivatives of triphenylene, the dimer as well as the trimer have unusual structures, thereby indicating that a phenyl group is participating in the formation of these compounds. The most surprising structure of the trimer was elucidated by X-ray crystal diffraction. As to the mechanisms, diradical intermediates are proposed both for the cycloadditions and for the dimerisation. The initial steps of the latter seem to proceed also in the trimerisation.

1-Phenyl-1,2-cyclohexadiene: astoundingly high enantioselectivities on generation in a Doering-Moore-Skattebøl reaction and interception by activated olefins.

The resolution of (1alpha,5alpha,6alpha)-6-bromo-6-fluoro-1-phenylbicyclo[3.1.0]hexane (rac-5) provided the enantiomerically pure precursors (-)-5 and (+)-5 of 1-phenyl-1,2-cyclohexadiene. On treatment of (-)-5 with methyllithium in the presence of 2,5-dimethylfuran, the pure (-)-enantiomer of the [4+2] cycloadduct of 2,5-dimethylfuran onto 1-phenyl-1,2-cyclohexadiene was obtained exclusively. From this result, it is concluded that pure (M)-1-phenyl-1,2-cyclohexadiene ((M)-7) emerged from (-)-5 and was enantiospecifically intercepted to give the product. In the case of indene as trap for (M)-7, the (-)- and the (+)-enantiomer of the [2+2] cycloadduct were formed in the ratio of 95:5. Highly surprising, remarkable enantioselectivities were also observed, when (M)-7 was trapped with styrene to furnish two diastereomeric [2+2] cycloadducts. Hence, the achiral conformation of the diradical conceivable as intermediate cannot play a decisive part. The enantioselective generation of (M)- and (P)-7 by the beta-elimination route was tested as well. Accordingly, 1-bromo-2-phenylcyclohexene was exposed to the potassium salt of (-)-menthol in the presence of 2,5-dimethylfuran, and the enantiomeric [4+2] cycloadducts of the latter onto (M)- and (P)-7 were produced in the ratio of 55:45.

Stable five-membered-ring allenes with second-row elements only: not allenes, but zwitterions.

A pyrazole derivative, considered as the stable five-membered cyclic allene 1 a by Dyker, Bertrand, et al., is reinterpreted to be a zwitterion 1 b. It is shown that the bonding characteristics typical for an allene are not possible for structure 1 a. Ar = 2,6-dimethylphenyl.

Unsubstituted bicyclo[1.1.0]but-2-ylcarbinyl cations.

A synthesis for the unsubstituted bicyclo[1.1.0]but-2-ylmethanols (endo- and exo-9) from 1,3-butadiene has been developed. Solvolyses of their sulfonates 10 and 11 took entirely different courses, as the endo compound 10 gave rise exclusively to rearranged products such as cyclopent-3-en-1-ol (14), while the exo compound 11 underwent only the substitution of the tosylate group with complete retention of the exo-bicyclo[1.1.0]but-2-ylmethyl skeleton. Under solvolytic conditions, 10 reacted at very similar rates to the corresponding monocyclic substrate, that is, cyclopropylcarbinyl mesylate (19); in contrast, 11 reacted only three times as fast as n-butyl tosylate and about 1000-fold slower than 10. The nature of the bicyclo[1.1.0]but-2-ylcarbinyl cations has been probed by quantum chemical calculations. Whereas, the exo isomer (exo-18) corresponds to a local energy minimum, the endo isomer is only a transition state [endo-18(TS)] for an automerization of the nonclassical cyclopent-3-en-1-yl cation (13) and converts into 13 by a Wagner-Meerwein rearrangement. The most favorable isomerization of exo-18 also leads to 13 but via a transition state resembling the 2-vinylcycloprop-1-yl cation [25(TS)]. On the introduction of methyl groups at positions 1 and 3 of exo-18, the cation is no longer an energy minimum and it becomes a transition state [27(TS)] for an automerization of the nonclassical 1,3-dimethylcyclopent-3-en-1-yl cation (28). The large effect of the methyl substitution rationalizes the puzzling results of the previous product and rate studies, which utilized various substituted derivatives of bicyclo[1.1.0]but-2-ylcarbinyl sulfonates as substrates.

Computational assessment of the electronic structure of 1-azacyclohexa-2,3,5-triene (3 delta 2-1H-pyridine) and its benzo derivative (3 delta 2-1H-quinoline) as well as generation and interception of 1-methyl-3 delta 2-1H-quinoline.

Treatment of a solution of 3-bromo-1-methyl-1,2-dihydroquinoline (9) and [18]crown-6 in furan or styrene with KOtBu followed by hydrolysis afforded a mixture of 1-methyl-1,2-dihydroquinoline (10) and 1-methyl-2-quinolone (11). If the reaction was performed in [D(8)]THF and the mixture was immediately analysed by NMR spectroscopy, 2-tert-butoxy-1-methyl-1,2-dihydroquinoline (17) was shown to be the precursor of 10 and 11. The structure of 17 is evidence for the title cycloallene 7, which arises from 9 by beta elimination of hydrogen bromide and is trapped by KOtBu to give 17 so fast that cycloadditions of 7 with furan or styrene cannot compete. Since this reactivity is unusual compared to the large majority of the known six-membered cyclic allenes, we performed quantum-chemical calculations on 8, which is the parent compound of 7, and the corresponding isopyridine 6 to assess the electronic nature of these species. The ground state of 6 was no longer an allene (6 a) but the zwitterion 6 b. In the case of 8, the allene structure 8 a is more stable than the zwitterionic form 8 b by only approximately 1 kcal mol(-1). These results suggest a high reactivity of 6 and 8 towards nucleophiles and explains the behaviour of 7. In addition to the ground states, the low-lying excited states of 6 and 8 were considered, which are represented by the diradicals 6 c and 8 c and, as singlets, lie above 6 b and 8 a by 19.1-24.8 and 14.4-17.7 kcal mol(-1), respectively.

Computational assessment of the electronic structures of cyclohexa-1,2,4-triene, 1-oxacyclohexa-2,3,5-triene (3delta(2)-pyran), their benzo derivatives, and cyclohexa-1,2-diene. An experimental approach to 3delta(2)-pyran.

The six-membered cyclic allenes given in the title have been studied theoretically by means of an MR-CI approach. For all compounds, the allene structures were found to be the ground states in the gas phase. In the cases of cyclohexa-1,2-diene (1), the isobenzene 2, and the isonaphthalene 7, the most stable structures having a planar allene moiety are the diradicals 1b, 2b, and 7b, representing the transition states for the racemization of 1a, 2a, and 7a and being less stable than the latter by 14.1, 8.9, and 11.2 kcal/mol, respectively. At variance with this order, the 3delta(2)-pyran 4 and the chromene 5 have the zwitterions 4c and 5c as the most stable planar structures, which lie only 1.0 and 5.4 kcal/mol above 4a and 5a, respectively. According to the simulation of the solvent effect, 4c even becomes the ground state of 4 in THF solution. The frontier orbitals of the respective states of 2 and 4 suggest different rates and sites for the reaction with nucleophiles. For the first time, the pyran 4 has been generated and trapped. As a precursor for 4, 3-bromo-4H-pyran (9) was chosen, the synthesis of which was achieved on two routes from 4H-pyran. The treatment of 9 with potassium tert-butoxide (KOt-Bu)/18-crown-6 gave 4-tert-butoxy-4H-pyran as the only discernible product, whether styrene or furan was present, indicating the interception of 4 by KOt-Bu. Finally, the disagreement between the experiment and the theory concerning the heat of formation and the electronic nature of the isobenzene 2 is resolved by demonstrating that the experimental data can provide only an upper limit of the DeltaH(f) degrees value.

Addition of Bromine to Ketenes and Bisketenes: Electrophilic Attack at Carbonyl Carbon and Neighboring Group Participation.

Addition of bromine to bisketene (Me(3)SiC=C=O)(2) (1) gave the fumaryl dibromide E-7, whose stereochemistry was proven by X-ray structure determination. Upon warming, E-7 rearranged to the furanone 8, and this process was faster in the more polar CD(3)CN compared to CDCl(3), consistent with an ionization pathway for the rearrangement. The bromination of 1 in CH(2)ClCH(2)Cl followed second-order kinetics with a rate constant (2.1 +/- 0.1) x 10(4) M(-)(1) s(-)(1) at 25 degrees C. The first-order dependence of bromine addition to 1 on [Br(2)] is attributed to intramolecular nucleophilic assistance by the second ketenyl moiety in an initial complex of 1 and Br(2) to give E-7. A transition structure for this process has been calculated by ab initio methods. By contrast PhMe(2)SiCH=C=O 11 and gamma-oxoketene 16 underwent bromination by third-order kinetics, second order in [Br(2)], indicating the absence of neighboring group participation in the rate-limiting step. The bisketene Me(2)Si(CH=C=O)(2) (13) underwent bromination by mixed kinetics with both first- and second-order terms in [Br(2)].