substituted / Cyanohydrins, 0-(trimethylsily1)-/ Hydrogen fluoridephosphorus oxychloridepyridine, elimination of trimethylsilanol by / Phenyl cuprate reagent, conjugate addition of / Phase-transfer catalysis I Chlorination by hexachloroethanesodium hydroxide I Cyclization by dehydrochlorination I Endoperoxides by triplet oxygen I 2H-Pyran, derivative of I Thermochromism Synthesis of Dicyanodiphenylsemibullvalene 4The synthesis of 4 parallels the straightforward route which recently has led from the unsaturated diketone 5 to the dicyanodiphenylbarbaralane 6L7].
The very useful singlet-oxygen ene reaction of reluctant alkenes, e.g. l , 5 , 7, 8 , and, in particular, the mono(ally1 hydroperoxides) 10 and 11, is handicapped by the instability of the sensitiser TPP. A comparative study of TPP, TPFPP, and TDCPP in the singlet-oxygen ene reaction of these alkenes shows that 1) the persistence increases in the order TPP < TPFPP < TDCPP, rendenng TDCPP the sensitiser of choice for the generation of singlet-oxygen in non-polar solvents, 2) the persistence of the tetraarylporphyrins decreases in the presence of an alkene, 3 ) this decrease strongly depends on ~ ~ the nature of the alkene, being most pronounced in the case of cyclohexene. These results are interpreted in terms of unknown substrate-denved species which induce oxidative destruction of the tetraarylporphynns Alternatively, abstraction of allylic hydrogen atoms from the alkenes by the excited sensitisers may give rise to the observed substrate-dependent photobleaching.-Because the singlet-oxygen ene reaction is the key step of a 1,2-carbonyl transposition with concomitant dehydrogenation, the scope and usefulness of this sequence are distinctly improvedRecently, we have devised a novel method for the 1,2-carbonyl transposition with concomitant dehydrogenationL21 of symmetrical ketonesL31 involving their conversion into alkenes which react with singlet-oxygen to afford ally1 hydroper~xides[~I. These were dehydrated in the next step to afford unsaturated carbonyl compounds [5]. When this sequence was applied to 1,5-dimethylbicyclo[3.3.0]octane-3,7-dione, problems were encountered in the twofold photooxygenation of the dienes 7a f Sa, because the mono(ally1 hydroperoxides) 10a and l l a react only very slowly with singlet-oxygen. The reluctance of 10a and l l a , apparently caused by electronic and steric effects, necessitated a high excess of singlet-oxygen and prolonged reaction times. Therefore, the only moderate persistence of the sensitiser 5,10,15,20-tetraphenylporphyrin (TPP) under the vigorous reaction conditions became the critical factor, even when small amounts of pyridine were added as stabiliser and the immersion well was cooled with an aqueous solution of potassium dichromate as light filterL31. Thus, for complete conversion of the readily formed hydroperoxides 10a and l l a , often repeated renewal of TPP was necessary, whose accumulating decomposition products were detrimental to the progress of the reaction, however. Furthermore, the unknown decomposition products appeared to enhance the rate of photobleaching of TPP [4c,6]. Results obtained with Rose Bengal bis(triethy1ammonium) in dichloromethane as solvent were even worse[*]. Clearly, the search for i I more stable singlet-oxygen sensitiser, soluble in aprotic solvents, was advisable. To this end, we tested 5,10,15,20-tetra(pentafluoropheny1)porphyrin (TPFPP) and 5,10, 1 5,20-tetra(2,6-dichlorophenyl)porphyrin (TDCPP). We now report that TDCPP is the sensitiser of choice for the generation of singlet-oxygen in aprotic organic solvents.
The enolized p-0x0 ester 9 is reduced by NaB(CN)H, in weakly acidic methanol solution to afford an almost quantitative yield of a mixture of diastereomeric P-hydroxy esters 10. On treatment with phosphorus oxychloride in pyridine, 10 is converted into a mixture of the P-chloro esters exo-11, endo-11, exo-12, endo-12, and 13 which is separated in part by chromatography. Sodium methoxide in methanol eliminates hydrogen chloride from the crude mixture of P-chloro esters producing the a$-unsaturated ester ?a in 84% yield based on 9. -Acid-catalysed equilibration of the known vinyl sulphides C2-and Cs-15 is carried out on large scale furnishing a 7 3~2 7 ratio of C2-and Cs-15. This mixture is oxidized to the sulphones C2-and Cs-16 which are allowed to react with sodium cyanide supported on aluminium oxide affording the u,punsaturated dinitrile 17 which is isolated by chromatography. Thus, 17 is now available on a 30-g scale. Methanolysis of l?by a prolonged treatment with hydrogen chloride in methanol followed by hydrolysis of the Pinner salt leads to ?b. -Both a$-unsaturated esters 7 a and b are readily brominated at the allylic positions by N-bromosuccinimide to yield the y,y'-dibromo esters 19a and b. Zinc-copper couple in tetrahydrofuran converts 19a and b into the semibullvalenedicarboxylates 2 a and b. While 2 a is persistent only in dilute solutions, thus frustrating all attempts at its isolation, 2b exhibits considerable stability in solution and forms lemon-coloured crystals.The semibullvalenedicarboxylates 2 a and b undergo a very rapid degenerate Cope rearrangement in solution. In addition, 2 b exhibits reversible thermochromism in both solution and solid state. The structures of the new compounds are based on spectroscopic evidence, including mass, IR, and NMR spectra. The configurations of exo-11, exo-l2,19a and bare established by X-ray diffraction analyses.The predicted accelerating effect of n-acceptor substituents on the degenerate Cope rearrangement of bridged homotropilidenes, e.g. bullvalenes, barbaralanes, and semibullvalenes[21, was demonstrated for the first time in the barbaralane series, with 2,6-dicyanobarbaralane as the andi id ate^^,^]. Shortly afterwards, the 2,6-dicyanosemibullvalene 1 b has been which not only corroborated this resultr6] but also stimulated new interest by exhibiting exceptional properties, such as being yellow in the absence of a chromophor, showing reversible thermochromism ['], and, in the solid state, a rapid non-degenerate Cope rearrangement between valence tautomers non-equivalent only by virtue of their environment in the crystal lattice"'. The interesting colour and thermochromism of 1 b are shared by the 1,5-bridged semibullvalene-2,6-dicarbonitrile 1 c[lo,lll but not by the 3,7-dicyanosemibullvalene 4a['2-141 nor the corresponding dimethyl 3,7-dicarboxylate 4 b"2,151. Grohmann's series of tetramethyl semibullvalenetetracarboxylates 3 seems to exhibit properties similar to those of the dinitriles 1. Unfortunately, an investigation of the thermochromism of 3 is s...
A series of six 1,5-(ethylmethyl)semibullvalenes (1a <==> 1a', 2 <==> 2', 3 <==>3') and two 4(2)-substituted semibullvalenes (4 <==> 4'), each undergoing Cope equilibria between nondegenerate valence tautomers, was investigated by carbon-13 NMR spectroscopy at a range of temperatures in several different solvents. Gompper's treatment of substituent perturbation was extended, specifically accounting for the effects of the substituents on chemical shifts, to allow the determination of the thermodynamic parameters for these skewed equilibria. These new treatments were used to determine the population difference (f - f ') between the valence tautomers and the perturbation thermodynamic quantities DeltaH(P), DeltaS(P), and DeltaG(P). The slow-exchange limit was reached for the parent 1,5-(ethylmethyl)semibullvalenes 3a<==> 3a' from which it was established that the preferred valence tautomer is 3a with the ethyl group on the cyclopropane ring. Despite considerable effort, the slow-exchange limit could not be reached in any of our other remotely substituted semibullvalenes. Provided that the ethyl group always prefers the cyclopropyl position as in 3a, the 1-ethyl-5-methylsemibullvalenes 1a, 2, and 3 are more stable by DeltaH(P) = 0.7-1.7 kJ mol(-1) than their valence tautomers 1a', 2', and 3'. In the directly substituted semibullvalenes (4 left harpoon ovet right harpoon 4'), the preferred valence tautomers 4a and 4b have the bromine atom or the nitrile group on the vinyl position (C(4)) rather than on the cyclopropane ring (C(2)) and are more stable than 4a' and 4b' by DeltaH(P) = 4.8 and 7.0 kJ mol(-1), respectively.
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