Library

Notes: The structures of the norbornenyl and norbornyl sulfones exo-5, endo-5 and endo-6 have been determined experimentally, by X-ray analysis, and theoretically by ab initio calculations (HF/6-31+G*). X-ray crystal structure analyses of the lithiated allylic norbornenyl and norbornyl sulfones endo-3/ent-endo-3·2diglyme and endo-4/ent-endo-4·2diglyme revealed dimeric O-Li contact ion pairs devoid of C-Li bonds. The anions of endo-3/ent-endo-3·2diglyme and endo-4/ent-endo-4·2diglyme adopt both the endo conformation (C2-S) and are characterized by in the exo direction pyramidalized anionic C atoms. The degree of pyramidalization of the C2 atom of 3 is higher than that of 4. Ab initio optimizations (HF/6-31+G*) of the structures of the anions of methylenenorbornene I and methylenenorbornane II resulted in local minima featuring non-planar C2 atoms which are pyramidalized in the exo direction in both cases, but to different degrees. In both cases cryoscopy of 3 and 4 in THF at -108.5 °C revealed approximately 1:1 mixtures of monomers and dimers. The sulfones exo-5, endo-5, exo-6 and endo-6 as well as the lithiosulfones 3 and 4 were studied by NMR spectroscopy. 1H-NMR (400 MHz), 13C-NMR (100 MHz) and 6Li-NMR (44 MHz) spectroscopy of 3 and 4 at -100 °C in [D]8THF revealed in each case only one set of signals, independent of the configuration of the starting sulfones. This indicates in both cases that attainment of both the monomer-dimer and the endo/exo equilibria of 3 and 4 is fast on the NMR time scale. According to 6Li{1H}- and 1H{1H}-NOE experiments of 3 and 4 the monomeric and dimeric species endo-3 and endo-4, having endo anions, seem to be preferred in THF solution. Ab initio calculations of the anions of 3 and 4 resulted in structures endo-3(-Li+), exo-3(-Li+), endo-4(-Li+) and exo-4(-Li+) (HF/6-31+G*), whose atomic point charges were calculated by the method of Kollman et al. The C2 atoms of endo-3(-Li+) and endo-4(-Li+) are pyramidalized in the exo direction whereas the C2 atoms of exo-3(-Li+) and exo-4(-Li+) are pyramidalized in the endo direction. According to the calculations, the endo anions are more stable than the exo anions. There is good agreement between the optimized structures of the free anions and the experimentally determined structures of the anions of the contact ion pairs in the crystal. Reactions of 3 and 4 with DX, MeI, EtI, nPrI and nHeI occurred at the C2 atom under the selective formation of the corresponding endosulfones endo-8a-e and endo-9a-e, respectively, in all cases. Thus, an earlier report on the selective formation of the exosulfone exo-9b in the reaction of 4 with MeI has to be revised. Product ratios were independent of the configuration of the starting sulfones and varied with the nature of the electrophile. Selectivities were highest in the case of the norbornyl species 4. Reaction of 3 with PhCHO occurred at the α position (C2) to afford the alcohols endo-8f and exo-8f (88:12) as single diastereomers and at higher temperatures at the γ position (C8), whereas reaction of 4 with PhCHO took place at the γ position even at low temperatures. Methylation of endo-5 and exo-5 at -105 °C by both the stepwise method and by the in situ method gave different ratios of exo- and endo-methylation products. The selectivities of reaction of 3 and 4 with electrophiles have been rationalized by the Curtin-Hammett/Winstein-Holness concepts. It is proposed that endo-3 (endo-4) and exo-3 (exo-4) are conformationally labile on the time scale defined by the rate of their reactions with electrophiles, and are attacked by electrophiles with high selectivities from the exo and the endo face, respectively, because of the shielding by the phenyl group and the direction of the pyramidalization of the anionic C atom. Preferential formation of the endosulfones is thus ascribed to exo attack of electrophiles on endo-3 (endo-4) being faster than endo attack on exo-3 (exo-4) because of Houk's staggering effect. Methylation of 3 and 4 by the in situ method showed 3, whose C2 atom is stronger pyramidalized, to be more reactive than 4. The base-catalyzed H/D exchange of sulfones endo-5, exo-5, endo-6 and exo-6 with NaOCD3 in CD3OD proceeded in all cases with a high degree of retention of configuration.

Notes: No abstract

Notes: A formal asymmetric synthesis of pentalenolactone E (1b) and pentalenolactone F (1a) has been accomplished. Ozonolysis of the diphenyl-substituted triquinane 3 and Kauffmann methylenation of ketone 5 with WOCl3-2 MeLi yielded the unsubstituted triquinane 9. The crucial rearrangement of the linear triquinanoid lactone 11 to the angular triquinanoid lactone 14a was accomplished using orthoformate and acid in methanol. Subjecting triquinanes 14a/b to the selenoxide method gave triquinene 15. Homologation of γ-lactone 15 to the angular diquinanoid δ-lactone 2 via a Horner-Wadsworth-Emmons or Peterson reaction of hemiacetals 16a/b was, however, not successful. Chemoselective reduction of 14a afforded hemiacetals 21a/b, reaction of which with the phosphonate salt 17a ultimately led to the ketene dithioacetal 22. The angular intermediates 25a/b were obtained from 22 by reduction to give the linear hemiacetals 24a/b, which rearranged to the dithio ortholactones 25a/b in the presence of acid. Introduction of the double bond and deprotection were accomplished via selenation of 25a/b with N,N-diethylbenzeneselenylamide and treatment of selenides 30a/b with silver nitrate. The unsaturated aldehydes 28 and 29 thus obtained were converted to 2 and 31, respectively, by oxidation with manganese dioxide in the presence of sodium cyanide, methanol and acetic acid. Alkene 2 was isolated by crystallization.

Notes: Tetraarylsuccinonitriles 2 were synthesized via oxidative dimerization of diarylacetonitriles 1 in basic media. The thermal decomposition of 2 results in two identical diarylcyanomethyl semi-radicals 3. The dissociation energy (Ea) measured using the thiophenol radical scavenging technique was found to amount to 94,3 ± 9,2 kJ/mol in the case of tetraphenylsuccinonitrile 2a. Tetraarylsuccinonitriles 2 show an atypical “stepwise” initiation mechanism in free-radical polymerization, particularly in the case of methyl methacrylate (MMA) as monomer. In the initial phase of the polymerization reaction a very high concentration of diarylcyanomethyl primary radicals 3 leads to the formation of short-chain telechelics with both end groups originating from the initiator. In the further course of the polymerization these MMA telechelics are able to re-form radicals by the scission of thermolabile carbon-carbon bonds and by the release of initiator end groups, and so the MMA telechelics ‘re-initiate’ the free-radical polymerization (“resuscitable free-radical polymerization”). In styrene polymerization, tetraarylsuccinonitriles 2 indeed cause a pronounced primary radical termination effect, but the styrene telechelics formed in contrast to MMA telechelics are thermostable, and therefore no “re-initiation” effect occurs. The X-ray structure determination of the styrene telechelic 2,2,3,5,5-pentaphenyladiponitrile 4a exhibits a bond length of the thermolabile carbon-carbon bond of 158,7 pm as compared to 162,8 pm of the corresponding MMA telechelic 2,2,5,5-tetraphenyl-3-methoxycarbonyl-3-methyladiponitrile and emphasizes the higher thermal stability of 4.

Notes: The tricarbonylchromium complexes chlorobenzene 1 reacts with terminal alkynes 2 through a palladium-copper-catalyzed coupling to give a variety of Cr(CO)3-complexed phenylethynyl arenes, heteroarenes and ferrocene 3 in good to excellent yield. Due to the electron-withdrawing nature of the Cr(CO)3 group these novel complexes can be regarded as organometallic push-pull chromophores. Analogously, the corresponding free ligands 4 (phenylethynyl arenes, heteroarenes and ferrocene) were synthesized by coupling iodobenzene and 2. The crystal structure analysis of the singly Cr(CO)3-complexed tolane 3e reveals a strong deviation from coplanarity of both phenyl rings by an angle of 50.9(2)° presumably due to crystal packing. Correlations are established between selected substituent parameters (σP, σI, σR, σP+ and Δπ) and the carbonyl carbon resonances in the 13C-NMR spectra for the complexes 3. The overall electronic substituent effect is transmitted to the carbonyl groups by both mesomeric and inductive mechanisms. The push-pull complexes 3 display relatively small negative solvochromicities of longest wavelength absorption band (MLCT band).

Notes: The palladium-copper-catalyzed coupling of the tricarbonyl-chromium-complexed chlorobenzene 1 with (trimethylsilyl)-acetylene gives tricarbonyl{η6-[(trimethylsilyl)ethynyl]-benzene}chromium(0) (2) in high yield. After desilylation of 2 tricarbonyl[η6-(ethynylbenzene)]chromium(0) (3) is obtained quantitatively. Using the palladium-copper-catalyzed methodology, we can readily introduce Cr(CO)3-complexed phenylethynyl units by a multifold coupling of 3 with iodobenzene, 1,2-, 1,3-, 1,4-di- and 1,3,5-tiiodobenzene (4a-e) or 1 to give polynuclear Cr(CO)3-complexed (phenylethynyl)benzenes 5a-e and doubly Cr(CO)3-complexed tolane 6 in good to moderate yield. The crystal structure analyses of μ3-{η6:η6:η6[1,3,5-benzenetriyltris(2,1-ethynediyl)]tris(benzene)}tris[tricarbonylchromium(0)] (5e) and μ-{η6:η6-[1,2-ethynediylbis(benzene)]}bis[tricarbonylchromium(0)] (6) reveal that the tricarbonylchromium tripods in the same molecule are arranged in antiparallel syn-eclipsed conformations. The Eglington coupling of 3 affords a doubly Cr(CO)3-complexed diphenylbutadiyne 7 in excellent yield.