Synthesis and Reactivity of Dithienopyrazines**

Heteroacenes developed to widely used building blocks in organic semiconductors for application in organic electronics due to their tunable structures and properties concomitant with inherent stability. Here, we report efficient preparation and investigation of so far unknown heterotriacenes, basic anti- and syn-dithienopyrazine 5 and 6 . The comparison of the two isomers with respect to electronic properties and follow-up reactions gives insights into structure-property and -reactivity relationships. Examples of transition metal-catalyzed C−C cross-coupling reactions of corresponding halogenated derivatives show the practical impact for extended π-conjugated systems applied in optoelectronic devices.     

Thermal Reactions of Guaiazulene and Its 3-Methyl Derivative with Dimethyl Acetylenedicarboxylate

The thermal reaction of 7-isopropyl-1,3,4-trimethylazulene (3-methylguaiazulene; 2 ) with excess dimethyl acetylenedicarboxylate (ADM) in decalin at 200° leads to the formation of the corresponding heptalene- ( 5a/5b and 6a/6b ; cf. Scheme 3) and azulene-1,2-dicarboxylates ( 7 and 8 , respectively). Together with small amounts of a corresponding tetracyclic compound (‘anti’- 13 ) these compounds are obtained via rearrangement (→ 5a/5b and 6a/6b ), retro-Diels-Alder reaction (→ 7 and 8 ), and Diels-Alder reaction with ADM (→ ‘anti’- 13 ) from the two primary tricyclic intermediates ( 14 and 15 ; cf. Scheme 5) which are formed by site-selective addition of ADM to the five-membered ring of 2 . In a competing Diels-Alder reaction, ADM is also added to the seven-membered ring of 2 , leading to the formation of the tricyclic compounds 9 and 10 and of the Diels-Alder adducts ‘anti’- 11 and ‘anti’- 12 , respectively of 9 and of a third tricyclic intermediate 16 which is at 200° in thermal equilibrium with 9 and 10 (cf. Scheme 6). The heptalenedicarboxylates 5a and 5b as well as 6a and 6b are interconverting slowly already at ambient temperature (Scheme 4). The thermal reaction of guaiazulene ( 1 ) with excess ADM in decalin at 190° leads alongside with the known heptalene- ( 3a ) and azulene-1,2-dicarboxylates ( 4 ; cf. Schemes 2 and 7) to the formation of six tetracyclic compounds ‘anti’- 17 to ‘anti’- 21 as well as ‘syn’- 19 and small amounts of a 4:1 mixture of the tricyclic tetracarboxylates 22 and 23 . The structure of the tetracyclic compounds can be traced back by a retro-Diels-Alder reaction to the corresponding structures of tricyclic compounds ( 24--29 ; cf. Scheme 8) which are thermally interconverting by [1,5]-C shifts at 190°. The tricyclic tetracarboxylates 22 and 23 , which are slowly equilibrating already at ambient temperature, are formed by thermal addition of ADM to the seven-membered ring of dimethyl 5-isopropyl-3,8-dimethylazulene-1,2-dicarboxylate ( 7 ; cf. Scheme 10). Azulene 7 which is electronically deactivated by the two MeOCO groups at C(1) and C(2) shows no more thermal reactivity in the presence of ADM at the five-membered ring (cf. Scheme 11). The tricyclic tetracarboxylates 22 and 23 react with excess ADM at 200° in a slow Diels-Alder reaction to form the tetracyclic hexacarboxylates 32 , ‘anti’- 33 , and ‘anti’- 34 (cf. Schemes 10–12 as well as Scheme 13). A structural correlation of the tri- and tetracyclic compounds is only feasible if thermal equilibration via [1,5]-C shifts between all six possible tricyclic tetracarboxylates ( 22, 23 , and 35–38 ; cf. Scheme 13) is assumed. The tetracyclic hexacarboxylates 32 , ‘anti’- 33 , and ‘anti’- 34 seem to arise from the most strained tricyclic intermediates ( 36–38 ) by the Diels-Alder reaction with ADM.     

新的α-单取代环十二酮肟的合成及晶体结构

单晶X射线衍射分析表明,几个新的α-单取代环十二酮与氨衍生物羟胺发生反应得到三种母体构象分别为[3333]和[2334],而取代基为边外向或角反向的α-单取代环十二酮肟.利用底物的"角位羰基参与反应"原理,"记忆效应"及进攻试剂与底物是否形成氢键解释了这一实验结果.通常取代基体积较大以及α-取代基与羰基形成分子内氢键情况下,试剂从空间障碍小以及远离氢键的一面进攻羰基生成α-角反取代环十二酮肟;当试剂与底物的取代基之间能够形成强的分子间氢键时,生成α-边外取代环十二酮肟;当试剂与底物的取代基之间只能形成弱的分子间氢键以及底物的取代基较小时,试剂从两面进攻羰基同时生成α-角反取代环十二酮肟和α-边外取代环十二酮肟.     

Thermal Reaction of Azulene and Some of Its Symmetrically Substituted Methyl Derivatives with Dimethyl Acetylenedicarboxylate

It is shown that azulene ( 1 ) and dimethyl acetylenedicarboxylate (ADM) in a fourfold molar excess react at 200° in decalin to yield, beside the known heptalene- ( 5 ) and azulene-1,2-dicarboxylates ( 6 ), in an amount of 1.6% tetramethyl (1RS,2RS,5SR,8RS)-tetracyclo[6.2.2.2^2,50^1,5]tetradeca-3,6,9,11,13-pentaene-3,4,9,10-tetracarboxylate(‘anti’-7) as a result of a SHOMO (azulene)/LUMO(ADM)-controlled addition of ADM to the seven-membered ring of 1 followed by a Diels-Alder reaction of the so formed tricyclic intermediate 16 (cf. Scheme 3) with a second molecule of ADM. The structure of ‘anti’-7 was confirmed by an X-ray diffraction analysis. Similarly, the thermal reaction of 5,7-dimehtylazulene ( 3 ) with excess ADM in decalin at 120° led to the formation of ca. 1% of ‘anti’- 12 , the 7,12-dimethyl derivative of‘anti’-7, beside of the corresponding heptalene- 10 and azulene-1,2-dicaboxylated (cf Scheme 2). The introduction of Me groups at C(1)and C(3)of azulene ( 1 ) and its 5,7-dimethyl derivative 3 strongly enhance the thermal formation of the corresponding tetracyclic compound. Thus, 1,3-dimethylazulene ( 2 ) in the presence of a sevenfold molar excess of ADM at 200° yielded 20% of ‘anti’- 9 beside an equal amount of dimethyl 3-mehtylazulene-1,2-dicarboxylate ( 8 ;cf. Scheme 1), and 1,3,5,7-tetramethylazulene ( 4 ) with a fourfold molar excess of ADM AT 200° gave a yield of 37% of‘anti’- 15 beside small amount of the corresponding heptalene- 13 and azulene-1,2-dicarboxylates 14 (cf.Scheme 2).     

α-单取代环十二酮肟及缩氨基硫脲的合成及晶体结构

单晶X射线衍射分析表明, α-单取代环十二酮与氨衍生物羟胺和氨基硫脲发生缩合反应得到两种母体构象均为[3333], 而取代基为边外向或角反向的α-单取代环十二酮肟或缩氨基硫脲. 利用底物的“角位羰基参与反应”原理, “记忆效应”及进攻试剂与底物是否形成氢键解释了这一实验结果. 通常情况下, 试剂从空间障碍小的一面进攻羰基而生成α-角反取代环十二酮肟或缩氨基硫脲. 当试剂与底物的取代基之间能够形成分子间氢键时, 则生成α-边外取代环十二酮肟或缩氨基硫脲.     

Bis[1]benzothieno[1,4]thiazines: Planarity,Enhanced Redox Activity and Luminescence by Thieno-Expansion of Phenothiazine

Twofold Buchwald–Hartwig aminations selectively furnish three regioisomers of bis[1]benzothieno[1,4]thiazines; X-ray structure analyses and DFT calculations were corroborated for correlation of their electronic properties. All regioisomers outscore the parent compound phenothiazine with respect to a low-lying oxidation potential and reversible redox activity. The anti-anti bis[1]benzothieno[3,2-b:2′,3′-e][1,4]thiazines possess the lowest oxidation potentials in this series and displayed pronounced green luminescence in solution (Φ_F≈20 %) and in the solid state. Syn-anti regioisomers were only weakly luminescent in solution, but showed aggregation-induced emission enhancement and solid-state luminescence. Most interestingly, X-ray structure analyses revealed that anti-anti derivatives have an amazingly coplanar structure of the pentacyclic anellated 1,4-thiazine system, emphasizing a structural similarity to heteroacenes. The calculated theoretical nucleus-independent chemical shifts additionally suggested that these 8π-electron core systems can be considered as the first electronically unbiased anellated 1,4-thiazines with antiaromatic character.     

Remote Tricarbonyl(diene)iron Substituent Effect on Ester Heterolysis. The Solvolyses of 5,6,7,8-Tetramethylidenebicyclo[2.2.2]oct-2-yl Methanesulfonate and of its Tricarbonyliron Mono- and Dinuclear Complexes

Buffered acetolyses and hydrolyses of 5,6,7,8-tetramethylidenbicyclo[2.2.2]oct-2-yl methanesulfonate ( 17 ), of its ‘syn-endo’ ( 18 ), ‘syn-exo’ ( 19 ), ‘anti-endo’ ( 20 ), ‘anti-exo’ ( 21 ) tricarbonyliron complexes and of its ‘anti-exo,syn-endo’ ( 22 ) and ‘anti-endo,syn-exo’ ( 23 ) bis(tricarbonyliron) dinuclear complexes have been investigated (product analysis and kinetics). In contract with the solvolyses of the uncomplexed mesylate 17 , the solvolyses of the complexed esters can be highly chemo- and stereoselective. The nature of the products (non-rearranged bicyclo[2.2.2]oct-2yl vs. rearranged bicyclo[3.2.1]oct-2-yl derivatives) depends on the relative configuration of the tricarbonyl(diene)iron moieties and on the medium. The rates of solvolyses of 17 are only slightly affected by complexation of one or both s-cis-butadiene units with Fe(CO)₃ groups, except in the cases where the diene moiety ‘anti’ with respect to the mesylate is complexed onto its ‘endo’ face ( 20,23 ). In these cases, significant rate-retardation effects are observed, consistent with the inductive effect of the Fe(CO)₃ substituent. Such retardation effects are overwhelmed by competing accelerating homoallylic participation by uncoordinated ‘anti’ -diene moieties ( 18,19 ) or, as in the case of the ‘anti-exo’-Fe(CO)₃ complexes 21 and 22 , by possible direct metal participation to the ionization process.     

Beckmann Fragmentation and Rearrangement. Part VII. Fragmentation and cyclization of α-methylthio-ketoximes. Fragmentation reactions no. 27

α-Methylthio-propiophenone anti-oxime p-toluenesulfonate (tosylate) ( 12b ) fragments quantitatively in 80% ethanol yielding benzonitrile and a methylidenesulfonium ion 15 . The syn-isomer, however, undergoes a Beckmann rearrangement. The fragmentation of α-methylthio-isobutyropher one anti-oxime tosylate ( 13b ) is accompanied by cyclization to the 1, 2-thiazetin-1-ium ion 27 , which is hydrolyzed via the sulfimine 29 to the keto sulfide 20 and the keto sulfoxide 30 . A comparison of the rates of the α-alkylthio anti-ketoxime tosylates 12b and 13b and of the homomorphous oxime tosylates 16b and 17b shows that fragmentation and cyclization are strongly assisted by the sulfur atom. Whereas both the anti- and syn-isomers of α-amino ketoxime derivatives fragment quantitatively, only the anti-isomers of α-alkylthio ketoxime derivatives undergo facile fragmentation.     

Bishomochinon

Two isomeric 2,4,6,8-tetrabromo-cyclooctane-1,5-diones ( 8a and 8b ) are formed in the tetrabromination of cyclooctane-1,5-dione ( 7 ). Treatment of a mixture of 8a and 8b with triethylamine gives rise to anti- 1,3-dibromo-bishomoquinone ( 9 ), which is reduced with zinc to anti-bishomoquinone ( 4 ) in a 65% overall yield. Either 8a or 8b , when heated with copper powder in a high vacuum, affords 1-bromo( 11 ) and 1,3-dibromo-anti-bishomoquinone ( 9 ), anti-bishomoquinone ( 4 ) itself as well as its sys-isomer ( 5 ). The anti-configuration was assigned to 4 on the basis of its reduction to two diols, one of which showed NMR. coupling of its two isochronic carbinol protons with one cis-vicinal proton and one trans-vicinal proton. Spectral data of the compounds are discussed. Of particular interest is the inversion of the chemical shifts of exo- and endo-methylene protons when comparing the NMR.-spectra of anti- and syn-bishomoquinone.     

Thermal Reaction of Highly Alkylated Azulenes with Dimethyl Acetylenedicarboxylate: HOMO(Azulene) vs. SHOMO(Azulene) Control in the Primary Thermal Addition Step

The reaction of highly alkylated azulenes with dimethyl acetylenedicarboxylate (ADM) in decalin or tetralin at 180–200° yields, beside the expected heptalene- and azulene-1,2-dicarboxylates, tetracyclic compounds of type ‘anti’- V and tricyclic compounds of type E (cf. Schemes 2–4 and 8–11). The compounds of type ‘anti’- V represent Diels-Alder adducts of the primary tricyclic intermediates A with ADM. In some cases, the tricyclic compounds of type E also underwent a consecutive Diels-Alder reaction with ADM to yield the tetracyclic compounds of type ‘anti’- or ‘syn’- VI (cf. Schemes 2 and 8–11). The tricyclic compounds of type E , namely 4 and 8 , reversibly rearrange via [1,5]-C shifts to isomeric tricyclic structures (cf. 18 and 19 , respectively, in Scheme 6) already at temperatures > 50°. Photochemically 4 rearranges to a corresponding tetracyclic compound 20 via a di-π-methane reaction. The observed heptalene- and azulene-1,2-dicarboxylates as well as the tetracyclic compounds of type ‘anti’'- V are formed from the primary tricyclic intermediates A via rearrangement (→heptalenedicarboxylates), retro-Diels-Alder reaction (→ azulenedicarboxylates), and Diels-Alder reaction with ADM. The different reaction channels of A are dependent on the substituents. However, the main reaction channel of A is its retro-Diels-Alder reaction to the starting materials (azulene and ADM). The highly reversible Diels-Alder reaction of ADM to the five-membered ring of the azulenes is HOMO(azulene)/LUMO(ADM)-controlled, in contrast to the at 200° irreversible ADM addition to the seven-membered ring of the azulenes to yield the Diels-Alder products of type E . This competing reaction must occur on grounds of orbital-symmetry conservation under SHOMO(azulene)/LUMO(ADM) control (cf. Schemes 20–22). Several X-ray diffraction analyses of the products were performed (cf. Chapt. 4.1).     

Reactive orbital energy theory serving a theoretical foundation for the electronic theory of organic chemistry

It is established that the reactive orbital energy theory (ROET) theoretically reproduces the rule-based electronic theory diagrams of organic chemistry by a comparative study on the charge transfer natures of typical organic carbon–carbon and carbon–heteroatom bond formation reactions: aldol, Mannich, α-aminooxylation, and isogyric reactions. The ROET, which is an expansion of the reaction electronic theories (e.g., the frontier orbital theory) in terms of orbital energies, elucidates the reactive orbitals driving reactions and the charge transferability indices of the reactions. Performing the ROET analyses of these reactions shows that the charge transfer directions given in the rule-based diagrams of the electronic theory are reproduced even for the functional groups of charge transfer destinations in all but only two processes for 38 reaction processes. The ROET analyses also make clear the detailed orbital-based pictures of these bond formation reactions: that is, the use of the out-of-plane antibonding π orbitals in acidic conditions (enol-mode) and in-plane antibonding π orbitals in basic conditions (enolate-mode), which explain the experimentally assumed mechanisms such as the π-bond formations in acidic conditions and σ-bond formations at α-carbons in basic conditions. Furthermore, the ROET analyses explicate that the methyl group initially accepts electrons and then donates them to the bond formations in the target reactions. It is, consequently, suggested that the ROET serves a theoretical foundation for the electronic theory of organic chemistry.     

Synthesis of β‐Hydroxy α‐Sulfanyl Esters by Using Nanocrystalline Magnesium Oxide

Aldol‐type reaction between electron deficient aldehydes and sulfonium salts to afford the corresponding β‐hydroxy α‐sulfanyl esters in moderate‐to‐good yields by using nanocrystalline MgO is described. The sulfanyl group is a useful group for further transformations in organic synthesis. Low R_f‐value isomer is anti‐configured as revealed by X‐ray diffraction study and consistent with the assignment of ¹H‐NMR spectrum.     

A theoretical study of carbazole dimers: Does carbazole form an excimer that undermines the performance of organic light emitting diodes?

Carbazole (Cz) dimers in various cofacial conformations, including staggered (Stg), anti, and syn, were explored by means of ab initio calculations at scaled opposite-spin (SOS)-MP2, SOS-CIS(D₀), and additional coupled cluster calculation levels. Similar to other π-conjugated molecules, strong Cz excimers form in the syn conformation in both S₁ and T₁ states, leading to significantly reduced optical excitation energies. Upon excitation, the dimers in the Stg and anti-conformations remain simple excited dimers, exhibiting similar optical energy gaps to those of the monomer. Being far more stable in the ground state, however, the Stg dimer is nearly isoenergetic to the syn dimer in the S₁ state and even more stable in the T₁ state. Given that the intermolecular interactions in the ground state are expected to govern the dimer conformations of Cz-based materials in the solid-state films of organic electronics, our results strongly demonstrate that the electronic excitation of Cz dimers do not necessarily lead to the strong excimer formation unless Cz molecules are forced to be arranged in the syn conformation.     

Trading N and O. Part 2: Exploiting aziridinium intermediates for the synthesis of β-hydroxy-α-amino acids

The β-hydroxy-α-amino acids (S,S)-allo-threonine, (S,S)-β-hydroxyleucine and a range of aryl substituted (S,S)-β-hydroxyphenylalanines were prepared from the corresponding enantiopure anti-α-hydroxy-β-amino esters via a rearrangement protocol, which proceeds via the intermediacy of the corresponding aziridinium ions. The starting anti-α-hydroxy-β-amino esters were prepared in >99:1 dr using our diastereoselective aminohydroxylation procedure, whereby conjugate addition of lithium (R)-N-benzyl-N-(α-methylbenzyl)amide to an α,β-unsaturated ester is followed by oxidation of the resultant enolate with (−)-camphorsulfonyloxaziridine. Subsequent activation of the hydroxyl group within the anti-α-hydroxy-β-amino esters promoted aziridinium ion formation [which proceeds with inversion of configuration at C(2)], and regioselective ring-opening of the intermediate aziridinium ions with H₂O [which proceeds with inversion of configuration at C(3)] gave the corresponding anti-β-hydroxy-α-amino esters as single diastereoisomers (>99:1 dr). Deprotection of these substrates via sequential hydrogenolysis and ester hydrolysis gave the corresponding β-hydroxy-α-amino acids in good yield and high diastereoisomeric and enantiomeric purity.     

Cyclophanes Containing Bowl‐Shaped Aromatic Chromophores: Three Isomers of anti‐[2.2](1,4)Subphthalocyaninophane

The connection of bowl‐shaped aromatic boron subphthalocyanines with anti‐[2.2]paracyclophane resulted in the first observation of electronic communication between convex and concave surfaces. Three isomers of anti‐[2.2](1,4)subphthalocyaninophane, described as concave–concave ( CC ), convex–concave ( CV ), and convex–convex ( VV ) according to the orientation of the subphthalocyanine units, were synthesized and characterized by various spectroscopic techniques, including ¹H NMR, electronic absorption, fluorescence, and magnetic circular dichroism spectroscopy and X‐ray crystallography, together with molecular‐orbital calculations. On going from the CC system to CV and further to VV , the Q band broadened and finally split as a result of through‐space expansion of the conjugated systems, which were also reproduced theoretically.     

Direct and Asymmetric Aldol Reactions of N-Azidoacetyl-1,3-thiazolidine-2-thione Catalyzed by Chiral Nickel(II) Complexes. A New Approach to the Synthesis of β-Hydroxy-α-Amino Acids

A direct and asymmetric triisopropylsilyltrifluoromethanesulfonate (TIPSOTf) mediated aldol reaction of N-azidoacetyl-1,3-thiazolidine-2-thione with aromatic aldehydes catalyzed by a chiral nickel(II)-Tol-BINAP complex has been developed (BINAP=2,2’-bis(diphenylphosphino)-1,1’-binaphthyl). The catalytic protocol gives the corresponding anti α-azido-β-silyloxy adducts with outstanding stereocontrol and in high yields. Theoretical calculations account for the stereochemical outcome of the reaction and lay the foundations for a mechanistic model. In turn, the easy removal of the thiazolidinethione yields a wide array of enantiomerically pure derivatives in a straightforward and efficient manner. Such a noteworthy character of the heterocyclic scaffold together with the appropriate manipulation of the azido group open a new route to the synthesis of di- and tripeptide blocks containing a β-aryl-β-hydroxy-α-amino acid.     

Face Selectivity in the Diels-Alder Additions and Chelotropic Addition of Sulfur Dioxide to 2-(D)Methylidene-3-methylidenebicyclo[2.2.1]heptane

The stereoselectivity of the cycloadditions of 2-(D)methylidene-3-methylidenebicyclo[2.2.1]heptane ( 4 ) to various dienophiles has been determined. The exo- vs. endo-face selectivity depends on the type of dienophiles, and for olefinic ones, on the mode of attack (Alder- vs. anti-Alder endo rule). It is > 9:1 with N-phenyltriazolinedione (NPTAD) and ethylenetetracarbonitrile (TCNE), < 1:9 with dimethyl acetylenedicarboxylate (DMAD), 30 ± 5:70 ± 5 with DMAD in the presence of AlCl₃, 15 ± 5:85 ± 5 with dehydrobenzene and 40 ± 5:60 ± 5 with ¹O₂ generated photochemically (Table 1). With para-benzoquinone and maleic anhydride, the exo- vs. endo- face selectivity is < 1:9 and 20 ± 5:80 ± 5, respectively, for their anti-Alder mode of attack; it is 50 ± 5:50 ± 5 and 55 ± 5:45 ± 5, respectively, for their Alder mode of reaction. Under conditions of kinetic control, the chelotropic addition of SO₂ to 4 is endo-face selective.     

Donor/Acceptor Dihydroindeno[1,2‐a]fluorene and Dihydroindeno[2,1‐b]fluorene: Towards New Families of Organic Semiconductors

New families of donor/acceptor semiconductors based on dihydroindeno[1,2‐a]fluorene and dihydroindeno[2,1‐b]fluorene are reported. Due to the spiro bridges, this new generation of dihydroindenofluorenes allows a spatial separation of HOMO and LUMO, which retains the high E_T value of the dihydroindenofluorene backbone and excellent physical properties. This control of the electronic and physical properties has allowed a second generation of dihydroindeno[1,2‐a]fluorene to be obtained with strongly enhanced performance in green and sky‐blue phosphorescent organic light‐emitting diodes (PhOLEDs) relative to the first generation of materials. To date, this is the highest performance ever reported for a blue PhOLED by using a dihydroindenofluorene derivative. Through this structure–property relationship study, a remarkable difference of performance between syn and anti isomers has also been highlighted. This surprising behaviour has been attributed to the different symmetry of the two molecules, and highlights the importance of the geometry profiles in the design of host materials for PhOLEDs.     

Single‐Point Remote Control of Flapping Motion in Clothespin‐Shaped Bimetallic Palladium Complexes Based on the N(sp2)–N(sp3) Interconversion in Amide Functionalities

The synthesis, structure, and flapping motion of clothespin‐shaped binuclear trans‐bis(salicylaldiminato)palladium(II) complexes (anti‐ 1 ) with 4‐azaheptamethylene linkers bearing amide ( a – g ), urethane ( h ), or urea ( i ) functionalities are described in this report. Various 2D ¹H NMR experiments and XRD analyses indicate that the amide‐ and urethane‐linked anti‐ 1 a , b , d – h complexes exist as equilibrated mixtures of major and minor conformers I and II in CDCl₃, whereas the complexes anti‐ 1 c and i were observed as a single species. The mapping of NOESY cross‐peaks between conformers I and II revealed that the equilibration of the major and minor conformers of anti‐ 1 a , b , d – h proceeds by two pathways, namely a nonrotatory flapping motion of the coordinated blades and a nonflapping rotation of C?N bonds, whereas the equilibration of anti‐ 1 c proceeds by simultaneous flapping and rotation motions. Kinetic studies carried out by means of ¹H–¹H EXSY experiments revealed that 1) the ΔG^≠_298K values for the flapping motion are controlled remotely by the steric and electronic effects of the RCON functionalities and 2) the activation parameters for the nonrotatory flapping process are identical to those for the nonflapping peptide rotation in the complexes anti‐ 1 a,b,d – h , which indicates that the present multistep conformational transformation induced by the flapping motion is controlled by the rate‐determining pyramidalization/depyramidalization (i.e., sp²/sp³ interconversion) of the nitrogen atoms of the functionalities. The static and controllable molecular mobility of anti‐ 1 bearing peptide linkers has been discussed by comparison with the dynamic behavior of its analogues anti‐ 2 – 4 with flexible polymethylene linkers.     

Methyl substituent effects in the 13C spectra of cyclohexanone O-methyl oximes and N,N-dimethylhydrazones

The ¹³C NMR spectra of a series of 4-t-butylcyclohexane-O-methyl oximes and N,N-dimethylhydrazones containing α-methyl substituents have been measured. The effects of α-methyl substitution (syn-axial, anti-axial and anti-equatorial) on the carbons α, β and γ to the substitution site are similar to those previously found in cyclohexanones. Use has been made of these substituent parameters to determine the conformational equilibria of the mobile 2-methylcyclohexanone derivatives.