Stereochemical Analysis of an Aromatic Triplet Di-π-methane Rearrangement

It is shown that (−)-(S)-N,N-dimethyl-2-(1′-methylallyl)aniline ((−)-(S)- 4 ), on direct irradiation in MeCN at 20°, undergoes in its lowest-lying triplet state an aromatic di-π-methane (ADPM) rearrangement to yield (−)-(1′R,2′R)- and (+)-(1′R,2′S)-N,N-dimethyl-2-(2-methylcyclopropyl)aniline ((−)-trans- and (+)-cis- 7 ) in an initial trans/cis ratio of 4.71 ± 0.14 and in optical yields of 28.8 ± 5.2% and 15 ± 5%, respectively. The ADPM rearrangement of (−)-(S)- 4 to the trans- and cis-configurated products occurs with a preponderance of the path leading to retention of configuration at the pivot atom (C(1′) in the reactant and C(2′) in the products) for (−)-trans- 7 and to inversion of configuration for (+)-cis- 7 , respectively. The results can be rationalized by assuming reaction paths which involve the occurrence of discrete 1,4- and 1,3-diradicals (cf. Schemes 10, 12, and 13). A general analysis of such ADPM rearrangements which allows the classification of these photochemical reactions in terms of borderline cases is presented (Scheme 14). It is found that the optical yields in these ‘step-by-step’ rearrangements are determined by the first step, i.e. by the disrotatory bond formation between C(2) of the aromatic moiety and C(2′) of the allylic side chain leading to the generation of the 1,4-diradicals. Moderation of the optical yields can occur in the ring closure of the 1,3-diradicals to the final products, which may take place with different trans/cis-ratios for the individual 1,3-diradicals. Compounds (−)-trans- 7 as well as (+)-cis- 7 easily undergo the well-known photochemical trans/cis-isomerization. It mainly leads to racemization. However, a small part of the molecules shows trans/cis-isomerization with inversion of configuration at C(1′), which is best explained by a photochemical cleavage of the C(1′)–C(3′) bond.     

Ab initioSTO-3G optimization of planar,perpendicular, and twisted molecular structures of biphenyl

The complete molecular structure of biphenyl, characterized by 12 independent parameters, has been derived by ab initio gradient techniques using a STO -3G basis set for coplanar, perpendicular, and minimum energy conformations with the constraint of planar phenyl ring units and a C₂ symmetry axis along the CC interring bond. The minimum torsional angle obtained was ? = 38.63° with torsional energy barriers of 8.59 and 10.04 kJ/mol for ? = 0° and ? = 90°, respectively.     

Zur Photochemie von 1, 2-Benzisoxazolen in stark saurer Lösung

On the Photochemistry of 1, 2-Benzisoxazoles in Strongly Acidic Solution The 1, 2-benzisoxazoles 1a, 1b and 1d when dissolved in 96% sulfuric acid and irradiated through a quartz filter with a mercury high-presure lamp yield, after work-up, mixtures of 2, 5- and 2, 3-dihydroxy-acylbenzenes ( 2 and 3 , respectively; cf. Schemes 1 and 3 and Table 1). Irradiation of 3, 5-dimethyl-1, 2-benzisoxazole ( 1c ) in 96% sulfuric acid leads to the formation of 2, 3-dihydroxy-5-methyl-acetophenone ( 3c ) in only 6% yield (cf. Table 1). It is assumed that the 1, 2-benzisoxazolium ions react in the excited singlet state by heterolytic cleavage of the N, O-bond to yield the corresponding aryl oxenium ions 7 in the singlet ground state (see Scheme 5). Reaction of 7 with HSO ₄ ? ions, present in 96% sulfuric acid, yields, after hydrolysis, the dihydroxy compounds 2 and 3 . Photolysis of 3-methyl-1, 2-benzisoxazole ( 1b ) in diluted sulfuric acid (0,5 to 9 M ) in methanol or water leads only to the formation of 2-amino-phenol ( 6 ; see Scheme 3), presumable via photo-isomerization of 1b to 2-methylbenzoxazole ( 5b ) which then is hydrolyzed to give 6 .     

Synthesis of Benz[a]azulenes Substituted at the Benzo Ring

It is shown that the thermal electrocyclic ring-closure reaction of 1,2-di[(E)-prop-1-enyl]benzene to yield 2,3-dimethylnaphthalene (cf. Scheme 1) [10] can successfully be applied also to the synthesis of benz[a]azulenes (cf. Schemes 2 and 3). Starting materials are methyl 4,6,8-trimethylazulen-2-yl ketone ( 6 ) and the corresponding 2-carbaldehyde 5 , which, in a Horner-Emmons reaction, are transformed into the (azulen-2-yl)-acrylates (E)- 8 and (E)- 7 , respectively. Vilsmeier formylation of these compounds, followed by the Horner-Emmons reaction leads to the formation of the bisacrylates (E,E)- 11 and (E,E)- 12 , respectively. In an alternative reaction, (E)- 8 , on treatment with dimethyl acetylenedicarboxylate (ADM) in the presence of [RuH₂(PPh₃)₄], can be transformed into the methoxycarbonyl-substituted bisacrylates (E,E)- and (E,Z)- 17 . All three bisacrylates, on heating at 180–190° in p-cymene, undergo cyclization to yield the corresponding dihydrobenz[a]azulenes 13 , 14 , and 18 , respectively, which could easily be dehydrogenated on heating in the presence of Pd/C. The new benz[a]azulenes 15 , 16 , and 19 are fully characterized.     

光诱导有机化合物异构化反应的研究——Ⅹ. 吲哚啉螺吡喃瞬态吸收光谱的研究

研究了1-正-辛基-3,3-二甲基-吲哚啉-2,2′-螺-5′,6′-(2-溴-4-硝基-苯并)吡喃, 1-正-辛基-3,3-二甲基-吲哚啉-2,2′-螺-5′,6′-(4-硝基-苯并)吡喃, 1-正-辛基-3,3-二甲基-吲哚啉-2,2′-螺-5′,6′-(2,4-二硝基-苯并)吡喃及1-正-辛基-3,3-二甲基-吲哚啉-2,2′螺-5′6′-(2-氯-4-硝基-苯并)吡喃在环已烷和甲苯溶液中光致开环过程的瞬态吸收光谱。观察到具有较长寿命的中间体及聚集体的存在。初步提出异构化反应过程的机制中既包含有三重态过程, 也有单重态参与。     

Synthesis and Characterization of Rhenium(V) Oxo Complexes with a New Thiol-Amide-Thiourea Ligand System. X-ray Crystal Structure of [1-Phenyl-3-[2-((2-thioacetyl)amino)ethyl]thioureato]oxorhenium(V)

General methods for preparing Re(V)O complexes with a novel series of thiol-amide-thiourea (TATU) ligands, a new class of N(2)S(2) chelates, were developed. The TATU ligands, the first multidentate systems designed with a bidentate thiourea moiety, have been used to prepare the first high-valent transition metal complexes with bidentate thiourea coordination. Direct reaction of N-(2-aminoethyl)-2-((triphenylmethyl)thio)acetamide (1) with phenyl, 4-methoxyphenyl, 4-chlorophenyl, and methyl isothiocyanate afforded ready access to the corresponding S-protected TATU ligands in one step. A two-step preparation of the N,N-dimethylthiourea TATU ligand derived from 1 was also developed. Deprotection of thiols in trifluoroacetic acid with triethylsilane followed by a ligand exchange reaction with Re(V)O precursors yielded neutral ReO(TATU) complexes. The structure of [1-phenyl-3-[2-((2-thioacetyl)amino)ethyl]thioureato]oxorhenium(V) (6a) was determined by X-ray diffraction methods. Crystal data for 6a: C(11)H(12)N(3)O(2)ReS(2), fw 468.6, orthorhombic, Pca2(1); a = 22.605(5) ?, b = 13.029(3) ?, c= 9.698(2) ?; V = 2856.3(11) ?(3); Z = 8. The coordination environment of 6a was pseudo-square-pyramidal with a deprotonated thiol S, deprotonated amide N, deprotonated thiourea N, and thiocarbonyl S coordinated in the basal plane and the oxo ligand in the apical position. The thiourea function forms a four-membered chelate ring in the multidentate TATU ligands. The two N-C and the S-C bond distances within the monodeprotonated thiourea moiety were typical of bonds with multiple-bond character. Solution (1)H NMR data for all five complexes were consistent with the solid-state structure of 6a. A broad singlet attributable to the uncoordinated NH group of thiourea was observed for the monosubstituted thiourea complexes but was not present for the N,N-dimethylthiourea derivative. Instead, two singlets of equal intensity were observed for the two methyl groups, indicating that there is restricted rotation around the C-N(CH(3))(2) bond and an extended pi system in the thiourea moiety. The four-membered ring might cause difficulty because the M-S distance would be relatively long in an undistorted ligand. This may be the reason such chelate ligands have not been previously investigated. However, the N-C-S angle narrows to approximately 105 degrees, permitting a Re-S bond with a typical bond length to be formed. We conclude that such a ring represents a versatile new building block to create multidentate ligands.     

An Efficient Straightforward Synthesis of Benz[a]azulene

Benz[a]azulene ( 1 ) is synthesized in five steps (cf. Scheme 2) starting from commercially available 2-iodobenzyl alcohol ( 4 ) and tropylium tetrafluoroborate in an overall yield of 44%. The key step (cf. also Scheme 1) is the intramolecular Heck reaction of the 8-phenylsulfonyl-substituted heptafulvene 7 , which leads in nearly quantitative yield directly to 10-(phenylsulfonyl)benz[a]azulene ( 8 ). The desulfonylation of 8 can be accomplished by Julia's method with Na₂S₂O₄/NaHCO₃ in DMF/H₂O at 85–90°, thus leading to pure 1 in 78% yield. The phenylation of 8 with PhLi or PhCul at ?78° in THF occurs regioselectively at C(9). Dehydrogenation of the formed dihydroazulenes with o-chloroanil in toluene at room temperature gives 9-phenyl-10-(phenylsulfonyl)-benz[a]azulene ( 9 ) in 70% yield (cf. Scheme 3), which, again, can be desulfonylated with Na₂S₂O₄/NaHCO₃ in DMF/H₂O in good yields. The addition of PhLi to 1 in THF occurs at temperatures ? ?25°. Ionic dehydrogenation (1. Ph₃C⁺BF/MeCN; 2. Et₃N) of the dihydro forms leads to 3 , as the main product, and its positional isomers.     

Regulation of plastic from a circular economy perspective

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