Asymmetric Diels–Alder reaction with >C=P– functionality of the 2-phosphaindolizine-η¹-P-aluminium(O-menthoxy) dichloride complex: experimental and theoretical results

• Rajendra K. Jangid,
• Nidhi Sogani,
• Neelima Gupta,
• Raj K. Bansal,
• Moritz von Hopffgarten and
• Gernot Frenking

Beilstein J. Org. Chem. 2013, 9, 392–400, doi:10.3762/bjoc.9.40

• computational calculations using the hybrid functional of Becke [46] and Lee, Yang and Parr [47]. Geometry optimizations of the reactants, the transition states and the cycloadducts were performed at the B3LYP/6-31+G* level. Stationary points were analysed by frequency calculations at the same level to confirm

Presence or absence of a novel charge-transfer complex in the base-catalyzed hydrolysis of N-ethylbenzamide or ethyl benzoate

• Shinichi Yamabe,
• Wei Guan and
• Shigeyoshi Sakaki

Beilstein J. Org. Chem. 2013, 9, 185–196, doi:10.3762/bjoc.9.22

• minimal model of TS1 was proposed and was found to be insensitive to n. Keywords: basic hydrolyses; DFT calculations; ethyl benzoate; N-ethylbenzamide; reactive intermediates; transition states; Introduction Basic hydrolyses of esters and amides have been extensively studied experimentally [1]. Use of

• adopted method or not in relation to the former part of point 3. Transition states (TSs) were sought first by partial optimizations at bond-interchange regions. Second, by the use of Hessian matrices, TS geometries were optimized. They were characterized by vibrational analyses, which checked whether the

Metal-mediated aminocatalysis provides mild conditions: Enantioselective Michael addition mediated by primary amino catalysts and alkali-metal ions

• Matthias Leven,
• Jörg M. Neudörfl and
• Bernd Goldfuss

Beilstein J. Org. Chem. 2013, 9, 155–165, doi:10.3762/bjoc.9.18

• enantioselectivity, DFT calculations on the enantio-determining transition states were performed by employing the nonempirical TPSS-functional and BP86. Two possible pathways were found computationally (pathway A, B, Scheme 4). The first essential step in the catalytic process is the formation of the imine

• . The addition of 4-hydroxycoumarin to the activated Michael system 12 could now occur along two slightly different mechanistic pathways (A, B, Scheme 4) including the enantio-determining transition states corresponding to TS-13 or TS-14. Both pathways require the formation of a carbon–carbon bond and

• )-enantiomer of 17 should be generated in majority. The experimentally favored enantiomer is indeed (R)-configured with 83% ee (Table 2, entry 7). This value is in good agreement with an energetic differentiation of the enantio-determining transition states of 2.3 kcal/mol although no solvent dependent effects

Polar reactions of acyclic conjugated bisallenes

• Reiner Stamm and
• Henning Hopf

Beilstein J. Org. Chem. 2013, 9, 36–48, doi:10.3762/bjoc.9.5

• involving polar intermediates and/or transition states has been investigated on a broad scale for the first time. The reactions studied include lithiation, reaction of the thus formed organolithium salts with various electrophiles (among others, allyl bromide, DMF and acetone), oxidation to cyclopentenones

The chemistry of bisallenes

• Henning Hopf and
• Georgios Markopoulos

Beilstein J. Org. Chem. 2012, 8, 1936–1998, doi:10.3762/bjoc.8.225

Synthesis and evaluation of new guanidine-thiourea organocatalyst for the nitro-Michael reaction: Theoretical studies on mechanism and enantioselectivity

• Tatyana E. Shubina,
• Matthias Freund,
• Sebastian Schenker,
• Timothy Clark and
• Svetlana B. Tsogoeva

Beilstein J. Org. Chem. 2012, 8, 1485–1498, doi:10.3762/bjoc.8.168

• . Extensive DFT calculations, including solvent effects and dispersion corrections, as well as ab initio calculations provide a plausible description of the reaction mechanism. Keywords: bifunctional organocatalyst; DFT calculations; guanidine-thiourea; Michael addition; organocatalysis; transition states

• and H-transfer. The thermodynamics of both routes are comparable at all levels. Once ternary complex Init10 is formed, there are few C–C bond-forming pathways. Based on the extensive conformational search, we were able to find eleven competing transition states (Figure 3 and Figure 4), six of which

• ), at all levels employed, compared to the corresponding transition states that lead to the R product. The activation barriers vary between +7.3 kcal·mol−1 (+7.5 kcal·mol−1 DFT-PCM, TS8 S) and +14.1 kcal·mol−1 (+14.5 kcal·mol−1 DFT-PCM, TS7 R). DFT-D and MP2/6–311++G(d,p)//6–31G(d) results support the

A quantitative approach to nucleophilic organocatalysis

• Herbert Mayr,
• Sami Lakhdar,
• Biplab Maji and
• Armin R. Ofial

Beilstein J. Org. Chem. 2012, 8, 1458–1478, doi:10.3762/bjoc.8.166

• reactions [4][73][74][75][76][77][78][79][80]. Quantification of the nucleophilic reactivities of the enamines 32a–e in acetonitrile (20 °C) [83]; a definition of the Dunitz pyramidalization Δ is given in [84]. Proposed transition states for the stereogenic step in proline-catalyzed reactions. Kinetic

Toward unidirectional switches: 2-(2-Hydroxyphenyl)pyridine and 2-(2-methoxyphenyl)pyridine derivatives as pH-triggered pivots

• Christina Tepper and
• Gebhard Haberhauer

Beilstein J. Org. Chem. 2012, 8, 977–985, doi:10.3762/bjoc.8.110

• for use as a unidirectional pivot. If the phenolate 1 is protonated (grey arrow in Figure 1a), the most-stable conformation of the phenol 2 can be reached either by a clockwise or a counterclockwise rotation around the C–C single bond (black arrows in Figure 1a). As the transition states for both

• kJ·mol–1. According to the Boltzmann distribution between the two transition states, the rotation is almost completely (>99.9999%) unidirectional at 298 K. It must be emphasized that the rotation caused by the protonation of 5 and 7 is unidirectional only with respect to a single molecule. In equilibrium

Enantioselective supramolecular devices in the gas phase. Resorcin[4]arene as a model system

• Caterina Fraschetti,
• Matthias C. Letzel,
• Antonello Filippi,
• Maurizio Speranza and
• Jochen Mattay

Beilstein J. Org. Chem. 2012, 8, 539–550, doi:10.3762/bjoc.8.62

• and ρ < 1 factors in that with BS. To verify whether the measured enantioselectivity is determined by the relative stability of the starting diastereomeric complexes or by the transition states involved in the reaction path, the collision-induced dissociation spectra of the [V∙H∙tyrOMe]+ and [V∙H∙amph

Thermodynamic and kinetic stabilization of divanadate in the monovanadate/divanadate equilibrium using a Zn-cyclene derivative: Towards a simple ATP synthase model

• Hanno Sell,
• Anika Gehl,
• Frank D. Sönnichsen and
• Rainer Herges

Beilstein J. Org. Chem. 2012, 8, 81–89, doi:10.3762/bjoc.8.8

• presence of metal cations such as Zn2+, Mg2+ and Mn2+ in the active site [9]. It is generally assumed that the coordination of the metal cations to the phosphate oxygen atoms reduces the Coulomb repulsion in the transition states or in the intermediates, thus, lowering the activation barrier [10]. In

Dependency of the regio- and stereoselectivity of intramolecular, ring-closing glycosylations upon the ring size

• Patrick Claude,
• Christian Lehmann and
• Thomas Ziegler

Beilstein J. Org. Chem. 2011, 7, 1609–1619, doi:10.3762/bjoc.7.189

• observed regioselectivity over the course of the reaction. If the rate-determining step of the reaction is endergonic, then differences in productlike transition states will account for the observed selectivity according to Hammond’s postulate [44]. For the reaction path to the most constrained macrocylic

Asymmetric synthesis of quaternary aryl amino acid derivatives via a three-component aryne coupling reaction

• Elizabeth P. Jones,
• Peter Jones,
• Andrew J. P. White and
• Anthony G. M. Barrett

Beilstein J. Org. Chem. 2011, 7, 1570–1576, doi:10.3762/bjoc.7.185

• gave none of the required product. An aldol type reaction with benzaldehyde (Table 1, entry 9) afforded alcohol 6i in good yield, with complete diastereoselectivity at C-6, but no selectivity at the alcohol stereocenter (1:1). Examination of the two possible Zimmerman–Traxler type transition states for

• equatorial position, as there is now a 1,2-gauche interaction with the bulky di-methoxyaryl group adjacent to it. We consider that neither of these transition states is lower in energy, which would be consistent with the lack of diastereoselectivity. Finally, the addition of (R)-propylene oxide as the

Alkene selenenylation: A comprehensive analysis of relative reactivities, stereochemistry and asymmetric induction, and their comparisons with sulfenylation

• Vadim A. Soloshonok and
• Donna J. Nelson

Beilstein J. Org. Chem. 2011, 7, 744–758, doi:10.3762/bjoc.7.85

• intermediates usually of rate-determining transition states, exhibit characteristic patterns in their plots of log krel versus ionization energy (IE) or highest occupied molecular orbital (HOMO) energy [72][73][74][75][76]. These patterns were shown to be particularly useful to reveal the steric forces within

• , and electronic characteristics of, the rate determining transition states. Thus, plotting log krel values against IEs or HOMO energies of alkenes allows a comparison of the significance of steric and electronic effects in the rate-determining step of the reaction. In particular, additions to alkenes

• , that proceed via 3-membered intermediates (or transition states), display plots with one trend line if the reaction rate is predominantly dependent upon electronic effects [72][73][74][75][76], and multiple trend lines if both steric and electronic effects are important [72][73][77][78]. In this work

Construction of cyclic enones via gold-catalyzed oxygen transfer reactions

• Leping Liu,
• Bo Xu and
• Gerald B. Hammond

Beilstein J. Org. Chem. 2011, 7, 606–614, doi:10.3762/bjoc.7.71

• + 2] cyclization lying only 4.4 kcal/mol above the starting complex. Further calculations on the barrier for transition states were also consistent with the rapid conversion that was observed in the experiments. Overall, the large energetic preference of the intermediates and transition states for the

The intriguing modeling of cis–trans selectivity in ruthenium-catalyzed olefin metathesis

• Naeimeh Bahri-Laleh,
• Raffaele Credendino and
• Luigi Cavallo

Beilstein J. Org. Chem. 2011, 7, 40–45, doi:10.3762/bjoc.7.7

• propene molecule is roughly perpendicular to the Ru–propylidene bond. From intermediate Co1, two transition states corresponding to productive metathesis, TS1-cis and TS1-trans, can be reached via rather low energy barriers (3.1 and 2.5 kcal·mol−1 for TS1-cis and TS1-trans, respectively). The two

• transition states differ in the relative orientation of the Me groups of the propene group and of the propylidene moiety, syn in TS1-cis, and anti in TS1-trans, with TS1-trans favored by 0.6 kcal/mol The experimentally proved free and fast rotation around the Ru–alkylidene bond allows both TS1-cis and TS1

• -trans to be reached from Co1 [19][20]. The two transition states collapse into the corresponding MCy-cis and MCy-trans metallacycles, at 5.6 and 6.3 kcal·mol−1 below the starting 14e1 intermediate, respectively. The two metallacycles are the most stable structures along the reaction pathway. Release of

Kinetics and mechanism of vanadium catalysed asymmetric cyanohydrin synthesis in propylene carbonate

• Michael North and
• Marta Omedes-Pujol

Beilstein J. Org. Chem. 2010, 6, 1043–1055, doi:10.3762/bjoc.6.119

• complex 2 concentration of 24 mM and for spectra b and d, the concentration of benzaldehyde was 4.8 M. Structures consistent with the 51V NMR spectra. Bimetallic aluminium(salen) complex for asymmetric cyanohydrin synthesis. Rate determining transition states for asymmetric cyanohydrin synthesis: a) when

Catalysis: transition-state molecular recognition?

• Ian H. Williams

Beilstein J. Org. Chem. 2010, 6, 1026–1034, doi:10.3762/bjoc.6.117

• ) boat conformation and in which a single-atom mutation affects the free-energy of activation dramatically. Keywords: catalysis; computational simulation; enzymes; molecular recognition; transition state; Introduction “Molecular recognition of transition states” was the title of a paper presented by

• Kirby [1] at a discussion held in April 1993 on the chemistry of biological molecular recognition; he addressed the fundamental question of how enzymes lower the free energies of the transition states for the reactions they catalyse, with reference to his own elegant experimental studies on catalysis

• . In March 1991, at a workshop held under the auspices of the Science and Engineering Research Council’s Molecular Recognition Initiative, I presented a paper on theoretical modelling of transition states for biochemical processes, which included a computational model for carbonyl reduction catalysed

Synthesis of oxa-bridged derivatives from Diels–Alder bis-adducts of butadiene and 1,2,3,4-tetrahalo-5,5-dimethoxycyclopentadiene

• Faiz Ahmed Khan and
• Karuppasamy Parasuraman

Beilstein J. Org. Chem. 2010, 6, No. 64, doi:10.3762/bjoc.6.64

• endo-endo addition. The proposed transition states for the formation of bis-adducts are shown in Figure 3. The initial endo-mono adduct (4 or 11) gives rise to two possible endo-transition states leading to 5, 6 or 12, 13. The corresponding exo-transition states suffer from severe steric congestion due

Radical cascades using enantioenriched 7-azabenzonorbornenes and their applications in synthesis

• David M. Hodgson and
• Leonard H. Winning

Beilstein J. Org. Chem. 2008, 4, No. 38, doi:10.3762/bjoc.4.38

• through early transition states and, as such, steric effects are not usually significant [7]. Therefore, the lack of reactivity with 3-methyl-2-butenal could be due to the positive inductive effect of the terminal methyl groups rendering the β-position of the aldehyde less electrophilic. In an attempt to

Diels- Alder reactions using 4,7-dioxygenated indanones as dienophiles for regioselective construction of oxygenated 2,3-dihydrobenz[f]indenone skeleton

• Natsuno Etomi,
• Takuya Kumamoto,
• Waka Nakanishi and
• Tsutomu Ishikawa

Beilstein J. Org. Chem. 2008, 4, No. 15, doi:10.3762/bjoc.4.15

• -opened product 27. Transition states supposed for the regioselective DAR via quinone route. Representative LUMO coeffients of quinones 8 and 12 (a) and their reaction courses with diene 7 (b). Retrosynthesis of kinamycins. Synthesis of quinones 8 and 12 and the acetals 13 and 14. Reagents and conditions

The enantiospecific synthesis of (+)-monomorine I using a 5-endo- trig cyclisation strategy

• Malcolm B. Berry,
• Donald Craig,
• Philip S. Jones and
• Gareth J. Rowlands

Beilstein J. Org. Chem. 2007, 3, No. 39, doi:10.1186/1860-5397-3-39

• epimer, with the methyl group residing in the axial position. Naturally, we had assumed that the diastereoisomer in which all the substituents adopted a pseudo-equatorial orientation would have been formed preferentially. Yet inspection of the possible transition states for the cyclisation 25 vs. 27

Electronic differentiation competes with transition state sensitivity in palladium- catalyzed allylic substitutions

• Dominik A. Lange and
• Bernd Goldfuss

Beilstein J. Org. Chem. 2007, 3, No. 36, doi:10.1186/1860-5397-3-36

• distances (Å) of transition states and Pd-ene product complexes (pb = phosphabenzene; py = pyridine)[a] Acknowledgements We are grateful to the Fonds der Chemischen Industrie for financial support as well as for a Dozenten-Stipendium to B.G. We especially thank the Deutsche Forschungsgemeinschaft (DFG) for

Conformational rigidity of silicon- stereogenic silanes in asymmetric catalysis: A comparative study

• Sebastian Rendler and
• Martin Oestreich

Beilstein J. Org. Chem. 2007, 3, No. 9, doi:10.1186/1860-5397-3-9

• enantiomer of 4 is preferentially funnelled out via diastereomeric transition states (11 → 12). There is one major difference between the diastereoselectivity-determining steps in these catalytic cycles: (ii) in Scheme 3 and (iv) in Scheme 4. In the migratory insertion (ii, 7 → 8), carbon-silicon bond

Asymmetric aza-Diels- Alder reaction of Danishefsky's diene with imines in a chiral reaction medium

• Bruce Pégot,
• Olivier Nguyen Van Buu,
• Didier Gori and
• Giang Vo-Thanh

Beilstein J. Org. Chem. 2006, 2, No. 18, doi:10.1186/1860-5397-2-18

• asymmetric induction is strong intermolecular interactions like electrostatic attraction and hydrogen bonding between ionic solvents and intermediates or transition states of the diastereoselective reaction step (Figure 1). This observation was made by our group[25] and further confirmed by Leitner and co

High stereoselectivity on low temperature Diels- Alder reactions

• Luiz Carlos da Silva Filho,
• Valdemar Lacerda Júnior,
• Mauricio Gomes Constantino,
• Gil Valdo José da Silva and
• Paulo Roberto Invernize

Beilstein J. Org. Chem. 2005, 1, No. 14, doi:10.1186/1860-5397-1-14

• product. This would increase the usual difference between the two transition states, normally due mainly to π orbital overlaps. As already mentioned, we are performing HOMO-LUMO and transition states calculations to verify some structural factors influences on the course of these reactions. Conclusion