Reversible Photochemical and Thermal Isomerization of Azaboratabisnorcaradiene to Azaborabenzotropilidene

Two new tricyclic 1,2‐azaboratabisnorcaradiene molecules ( 1 b and 2 b ) generated through the photoisomerization of N‐methyl‐2‐phenylimidazolyl‐chelated dimesitylboranes ( 1 a and 2 a ) have been found to undergo unusual photoisomerization, producing the first examples of 1,2‐azaborabenzotropilidenes ( 1 c and 2 c ), accompanied by a distinct color change, upon irradiation at 350 nm. Compounds 1 c and 2 c contain a conjugated alkylideneborane unit and can be fully reverted back to 1 b and 2 b , and subsequently to 1 a and 2 a upon heating. The mechanistic pathway of the new isomerism has been established to involve “walk” rearrangements by DFT computational studies.     

Unusual Conformational Isomerization of 1,3-Oxathianes

Russian Journal of Organic Chemistry -     

Mechanism of the Thermal Z⇌E Isomerization of a Stable Silene; Experiment and Theory

The E and Z geometric isomers of a stable silene (tBu₂MeSi)(tBuMe₂Si)Si=CH(1‐Ad) ( 1 ) were synthesized and characterized spectroscopically. The thermal Z to E isomerization of 1 was studied both experimentally and computationally using DFT methods. The measured activation parameters for the 1Z ? 1E isomerization are: E_a=24.4 kcal mol^?1, ΔH^≠=23.7 kcal mol^?1, ΔS^≠=?13.2 e.u. Based on comparison of the experimental and DFT calculated (at BP86‐D3BJ/def2‐TZVP(‐f)//BP86‐D3BJ/def2‐TZVP(‐f)) activation parameters, the Z?E isomerization of 1 proceeds through an unusual (unprecedented for alkenes) migration–rotation–migration mechanism (via a silylene intermediate), rather than through the classic rotation mechanism common for alkenes.     

Valence Isomerization of a 1,3-Diphosphacyclobutane-2,4-diyl: Photochemical Ring Closure to 2,4-diphosphabicyclo[1.1.0]butane and Its Thermal Ring Opening to gauche-1,4-Diphosphabutadiene

The bond stretch isomer 1,3-diphosphacyclobutane-2,4-diyl 1 was transformed photochemically to give the previously unknown 2,4-diphosphabicyclo[1.1.0]butane 2 , which itself can be converted thermally into gauche-1,4-diphosphabutadiene 3 . The crystal structures of these three energy-rich valence isomers of 1,2-diphosphete have been determined. R=SiMe₃; Mes*=2,4,6-tBu₃C₆H₂.     

Thermal and Photochemical Isomerization of Tetraaryl Tetrakis(trifluoromethyl)

The isomerization of tetraaryl tetrakis(trifluoromethyl)[4]radialenes was studied. When type II (all-Z) isomers of 5,6,7,8-tetraaryl-5,6,7,8-tetrakis(trifluoromethyl)[4]radialenes were heated in tetralin at 170-200 degrees C, isomerization occurred to give mixtures of four [4]radialenes in a ratio of ca. I:II:III:IV = 1:10:5:1. However, when the isomeric mixtures were heated in the solid state at the same temperature, selective isomerization took place to give type II isomers in good selectivity (>91%). Upon irradiation with light, the type II isomers first isomerized to mixtures of the four [4]radialene isomers (I:II:III:IV = 2:2:48:48) and then rearranged to cyclobuta[b]naphthalenes via a 6pi-electrocyclic reaction followed by 1,3-hydrogen migration.     

Thermal Isomerization of Isoborneols and Dehydroisoborneols to New Chiral Building Blocks in Terpenoid Synthesis

The substituted isoborneols 1a – 1g and 5,6‐dehydroisoborneols 6a – 6c , readily prepared in excellent yields from (+)‐camphor and (+)‐5,6‐dehydrocamphor ( 2 ) by aryl, vinyl, or alkyl Grignard addition in the presence of stoichiometric amounts of CeCl₃, were thermally isomerized in a flow reactor system under DGPTI (dynamic gas‐phase thermo‐isomerization) conditions at temperatures between 480 and 630° to give the enantiomerically pure monocyclic carbonyl compounds 7a – 7d, 19a , b, 23 , and 24 . In all cases, product formation proceeded highly regio‐ as well as stereoselectively. The absolute configurations of the new stereogenic centers were determined by ¹H‐NOE measurements. DGPTI of the aryl substrates 1a – 1d is proposed to effect initial cleavage of the weakest single bond in the molecule under formation of a diradical intermediate state followed by intramolecular H‐abstraction to afford the acetophenone derivatives 7a – 7d . This reaction path was further supported by a ²H‐labeling study showing the OH group to be the exclusive H‐source. In contrast, DGPTI of the vinyl substrates 1e and 6b allowed concerted retro‐ene and oxy‐Cope rearrangements. In the case of 5,6‐dehydro‐2‐phenylisoborneol ( 6a ), concomitant diradical and retro‐Diels–Alder reaction pathways could be observed. In addition, a new route to (+)‐trans‐α‐campholanic acid ( 9 ) and (+)‐trans‐α‐dihydrocampholytic acid ( 14 ) is presented by regioselective Baeyer–Villiger oxidation and subsequent hydrolysis of 7c and 7d , respectively.     

1,2-C4H6→2-C4H6异构化反应机理的理论研究

采用自洽场分子轨道UHF/6-31G**从头算法,研究了1,2-C4H6→2-C4H6异构化反应机理,优化了基态势能面上反应物、过渡态、中间体和产物的几何构型,并对各驻点能量进行了零点能校准.结果表明该反应经历一个1-甲基环丙烯生成产物比经两步氢迁移反应历程更易发生.     

Cleavage of 1,3-oxathiol-2-one rings withn-butyllithium a new route to alkyl thiolates

1,3-Oxathiol-2-ones are converted byn-butyllithium into alkynyl thiolates.

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Spaltung von 1,3-Oxathiazol-2-on-Ringen mitn-Butyllithium. Ein neuer Weg zu Alkylthiolaten (Kurze Mitteilung)

Zusammenfassung 1,3-Oxathiol-2-one werden mitn-Butyllithium zu Alkinylthiolaten umgesetzt.