Mechanism of cis-enamide formation from N-(alpha-silyl)allyl amides: synthetic potential of stepwise dyotropic rearrangements

A novel transformation of silyl amides to N-cis-propenyl amides was recently reported, the reaction of which is a formal 10-electron double sigmatropic, or dyotropic, rearrangement. Density functional calculations (B3LYP/6-311++G(3d,3p)//B3LYP/6-31G(d)) have been carried out to investigate the mechanism of this reaction. A two-step process involving sequential 1,4-silyl and 1,4-hydrogen shifts is predicted. The 1,3-dipolar azomethine ylide intermediate profits from charge stabilization by allylic resonance and phenyl conjugation. The consecutive thermal migration of two sigma-bonds (stepwise dyotropic rearrangement) is an example of a host of reactions with synthetic potential.     

Analyse conformationnelle a l'aide de la notation des angles de torsion—III: Controle conformationnel de certaines reactions de type sn2' ou sni' de derives cyclohexeniques faisant intervenir un deplacement allylique de l'olefine

To take place stereoselectively S_N2' reactions of cyclohexene derivatives require an axial leaving group either on a half-chair (suprafacial reaction) or on a 1,3-diplanar form or a boat (antarafacial reaction). These conclusions appear generally relevant to concerted reactions of cyclohexenic derivatives involving an allylic migration of the olefinic bond.     

Mechanism of 1,3-migration in allylperoxyl radicals: computational evidence for the formation of a loosely bound radical-dioxygen complex

The three pathways postulated for 1,3-migration of the peroxyl group in the allylperoxyl radical (1a), a key reaction involved in the spontaneous autoxidation of unsaturated lipids of biological importance, have been investigated by means of quantum mechanical electronic structure calculations. According to the barrier heights calculated from RCCSD(T)/6-311+G(3df,2p) energies with optimized molecular geometries and harmonic vibrational frequencies determined at the UMP2/6-311+G(3df,2p) level, the allylperoxyl rearrangement proceeds by fragmentation of 1a through a transition structure (TS1) with a calculated DeltaH++(298 K) of 21.7 kcal/mol to give an allyl radical-triplet dioxygen loosely bound complex (CX). In a subsequent step, the triplet dioxygen moiety of CX recombines at either end of the allyl radical moiety to convert the complex to the rearranged peroxyl radical (1a') or to revert to the starting peroxyl radical 1a. CX shows an electron charge transfer of 0.026 e in the direction allyl --> O(2). The dominant attractive interactions holding in association the allyl radical-triplet dioxygen pair in CX are due chiefly to dispersion forces. The DeltaH(298 K) for dissociation of CX in its isolated partners, allyl radical and triplet dioxygen, is predicted to be at least 1 kcal/mol. The formation of CX prevents the diffusion of its partners and maintains the stereocontrol along the fragmentation-recombination processes. The concerted 1,3-migration in allylperoxyl radical is predicted to take place through a five-membered ring peroxide transition structure (TS2) showing two long C-O bonds. The DeltaH++(298 K) calculated for this pathway is less favorable than the fragmentation-recombination pathway by 1.9 kcal/mol. The cyclization of 1a to give a dioxolanyl radical intermediate (2a) is found to proceed through a five-membered ring transition structure (TS3) with a calculated DeltaH++(298 K) of 33.9 kcal/mol. Thus, the sequence of ring closure 1a --> 2a and ring opening 2a --> 1a' is unlikely to play any significant role in allylperoxyl rearrangement 1a --> 1a'. In the three pathways investigated, the energy of the transition structure is predicted to be somewhat lower in either heptane or aqueous solution than in the gas phase. Although the energy lowering calculated for TS1 is smaller than the calculated for TS2 and TS3, it is very unlikely that the solvent effects may reverse the predicted preference of the fragmentation-recombination pathway over the concerted and stepwise ring closure-ring opening mechanisms.     

Mass spectral study of ring E of taraxasterol and compounds with similar ring substitution

The origin of the [M–69]⁺ and [M–111]⁺ signals in the mass spectrum of taraxasterol was studied through the use of C(18), (19), (21), (22) and/or (30) deuteriated derivatives. The generality of these signals for ring systems with an exocyclic methylene group and a methyl moiety on an adjacent carbon was verified with 2-methylmethylenecyclohexane, 1-methyl-2-methylene-trans-decalin, 1,10-dimethyl-2-methylene-trans-decalin and some of their deuteriated derivatives. The most plausible mechanism for the formation of the [M–69]⁺ ion appears to involve cleavage of both bonds allylic to the exocyclic methylene group with a 1,3-hydrogen transfer from the adjacent ring. Genesis of the [M–111]⁺ ion is more complicated but a five-membered allylic ion generated from ring D is proposed.     

Acid-catalyzed conversion of caryolan-1-ol to isoclovene: A computational investigation of the multi-step carbocation rearrangement

Acid-catalyzed conversion of caryolan-1-ol to isoclovene involves a multi-step carbocation rearrangement. Electronic structure calculations show that the pathway proceeds through an initial 3° carbocation, as well as a series of three other 3° carbocations. The key stage in which the ring structure is rearranged occurs not as might initially be imagined, in two separate steps with the intermediacy of a 2° carbocation, but rather in a single, concerted but highly asynchronous dyotropic rearrangement. The transition structure for this dyotropic rearrangement strongly resembles the 2° carbocation that would be involved in a stepwise mechanism. However, the dyotropic rearrangement is stereochemically unusual. While one of the bond migrations is suprafacial, as expected, the other is effectively antarafacial. This unusual stereochemical outcome is enforced by the geometric constraints of the polycyclic structure.     

Reaction mechanism and chemoselectivity of intermolecular cycloaddition reactions between phenyl-substituted cyclopropenone ketal and methyl vinyl ketone

In this paper, the mechanisms of the intermolecular [3+2] and [1+2] cycloaddition reactions of 1,1/1,3-dipolar π-delocalized singlet vinylcarbenes, which is obtained from cyclopropenone, with an electron-deficient C═O or C═C dipolarophile, to generate five-membered ring products are first disclosed by the density functional theory (DFT). Four reaction pathways, including two concerted [3+2] cycloaddition reaction pathways and two stepwise reaction pathways (an initial [1+2] cycloaddition and then a rearrangement from the [1+2] cycloadducts to the final [3+2] cycloadducts), are investigated at the B3LYP/6-31G(d,p) level of theory. The calculated results reveal that, in contrast to the concerted C═O [3+2] cycloaddition reaction pathway, which is 7.1 kcal/mol more energetically preferred compared with its stepwise reaction pathway, the C═C dipolarophile favors undergoing [1+2] cycloaddition rather than concerted [3+2] cycloaddition (difference of 5.3 kcal/mol). The lowest free energy barrier of the C═O concerted [3+2] cycloaddition reaction pathway shows that it predominates all other reaction pathways. This observation is consistent with the finding that the C═O [3 + 2] cycloadduct is the main product under experimental conditions. In addition, natural bond orbital second-order perturbation charge analyses are carried out to explain the preferred chemoselectivity of C═O to the C═C dipolarophile and the origins of cis-stereoselectivity for C═C [1+2] cycloaddition. Solvent effects are further considered at the B3LYP/6-31G(d,p) level in the solvents CH(3)CN, DMF, THF, CH(2)Cl(2), toluene, and benzene using the PCM model. The results indicate that the relative reaction trends and the main products are insensitive to the polarity of the reaction solvent.     

Quantum mechanical study of the ring-closing reaction of the hexatriene radical cation

The ring-closing reaction of hexatriene radical cation 1(*)(+) to 1,3-cyclohexadiene radical cation 2(*)(+) was studied computationally at the B3LYP/6-31G* and QCISD(T)/6-311G*//QCISD/6-31G* levels of theory. Both, concerted and stepwise mechanisms were initially considered for this reaction. Upon evaluation at the B3LYP level of theory, three of the possible pathways-a concerted C(2)-symmetric via transition structure 3(*)(+) and stepwise C(1)-symmetric pathways involving three-membered ring intermediate 5(*)(+) and four-membered ring intermediate 6(*)(+)-were rejected due to high-energy stationary points along the reaction pathway. The two remaining pathways were found to be of competing energy. The first proceeds through the asymmetric, concerted transition structure 4(*)(+) with an activation barrier E(a) = 16.2 kcal/mol and an overall exothermicity of -23.8 kcal/mol. The second pathway, beginning from the cis,cis,trans rotamer of 1(*)(+), proceeds by a stepwise pathway to the cyclohexadiene product with an overall exothermicity of -18.6 kcal/mol. The activation energy for the rate-determining step in this process, the formation of the intermediate bicyclo[3.1.0]hex-2-ene via transition structure 9(*)(+), was found to be 20.4 kcal/mol. More rigorous calculations of a smaller subsection of the potential energy hypersurface at the QCISD(T)//QCISD level confirmed these findings and emphasized the importance of conformational control of the reactant.     

Evidence for diazenyl allyl diradicals in the thermolysis of 4-alkylidene-1-pyrazolines

Secondary deuterium kinetic isotope effects indicate that the CN bond anti to the Me group on 4-ethylidene-1-pyrazoline breaks preferentially to the syn bond. It is suggested that the product determining step involves an intramolecular radical displacement of nitrogen, as well as an electrocyclic rotation of the allylic methylene groups of the diazenyl allyl diradical, to generate the cyclopropane ring concerted with the loss of nitrogen. The position of deuterium in the methylmethylenecyclopropane can best be rationalized by this latter step.     

Thermal [1,3] sigmatropic rearrangements of bicyclic and tricyclic vinylcyclobutanes: a gray zone between the concerted and stepwise extremes

Four typical thermal [1,3] sigmatropic rearrangements of bicyclic and tricyclic vinylcyclobutanes and one fancied analogous reaction (R2 in Scheme 1) were examined using CASSCF, CASPT2 and CAS+1+2 methods to discern the reaction mechanisms. Computed results indicate that it is difficult to simply designate these reactions as traditional single-step concerted or stepwise mechanisms, but a situation locating between these two extremes seems to be reasonable. The extent the reaction exhibits as a single-step concerted or stepwise path is much dependent on the geometrical constraints of reactant. For example, the system with three-member ring will tend to behave like a single-step concerted process, where only one rotation movement around C–C bond could be found when the bridged C–C is broken. However, the species with four-member ring will be much closer to the stepwise mechanism involving diradical varieties, because there are two different rotation movements exist when the bridged C–C is broken. Our calculation will also rationalize that only suprafacial retention product could be yielded for the thermal [1,3] sigmatropic rearrangement of tricyclic vinylcyclobutane.     

3-Aza-cope rearrangement of quaternary N-allyl enammonium salts. Stereospecific 1,3 allyl migration from nitrogen to carbon on a tricyclic template

N-Allyl enamines can undergo a [3,3] sigmatropic rearrangement known as a 3-aza-Cope (or amino-Claisen) reaction. We explored a 3-aza-Cope reaction involving 1,3 allylic migration from nitrogen to carbon in N-allyl enammonium quaternary salts, exemplified by benzo[a]quinolizine 8 and pyrrolo[2,1-a]isoquinoline 13, with an interest in stereochemistry and mechanism. Salts 8 and 13 were accessed, respectively, through stereospecific allylation of hydroxy amines 4 and 11a/11b to give 7 and 12a/12b, which were dehydrated with trifluoroacetic acid. Allylic migration in these tricyclic tetrahydroisoquinolines occurred with high stereospecificity, with the major products 9 (from 8) and 15a (from 13) apparently deriving from a concerted suprafacial [3,3] rearrangement. The rearrangement of 8 to 9 was facile at 23 degrees C (t(1/2) = ca. 5 h) and was >98% stereospecific, whereas the rearrangement of 13 to 15a/15b required heating between 50 and 100 degrees C, with ca. 90-95% stereospecificity (t(1/2) = ca. 0.3 h at 100 degrees C). A deuterium-labeling experiment with 21 ((2)H-13) confirmed that allylic inversion accompanies the 1,3 migration en route to major isomer 22a ((2)H-15a), supporting the predominance of a concerted [3, 3] sigmatropic mechanism. However, the 5-10% loss of stereospecificity in the rearrangements of the pyrroloisoquinolines 13 and 21, reflected by formation of minor isomers 15b and 22b, respectively, indicates a minor nonconcerted reaction pathway.     

A density functional theory study clarifying the reactions of conjugated ketenes with formaldimine. A plethora of pericyclic and pseudopericyclic pathways

The reactions of vinylketene (1a), imidoylketene (1b), and formylketene (1c) with formaldimine (2) were studied at the B3LYP/6-31G* level. For the cycloadditions of these conjugated ketenes with 2, several possible pathways to both [4 + 2] and [2 + 2] products were examined. The lowest energy [2 + 2] pathways are, in most cases, calculated to be stepwise, forming the products via rate-determining conrotatory electrocyclization of zwitterionic intermediates. However, concerted transition structures analogous to the ketene plus ethene [2 + 2] cycloaddition reaction were also located; the existence of multiple transition states offers a resolution to a long-standing controversy regarding the mechanism of ketene plus imine cycloadditions. Both stepwise and concerted [4 + 2] pathways were calculated for 2b and for 2c; both these pathways are pseudopericyclic. The inherently low barriers associated with pseudopericyclic transition states provide an explanation of the experimental preference for [4 + 2] cycloadditions of alpha-oxoketenes and predict [4 + 2] cycloadditions should also be favored for imidoylketenes. For a vinylketene constrained to a Z-geometry, the concerted [4 + 2] cycloaddition is also predicted to be the lowest energy pathway. An explanation is offered for the unusual thermal equilibration from a six-membered ring (3d) to a four-membered ring (4d) observed by Sato et al. Transition structures for facile pseudopericyclic 1,3- and 1,5-hydrogen shifts in the zwitterions were also calculated.     

Concerted and Stepwise Mechanisms in Metal‐Free and Metal‐Assisted [4+3] Cycloadditions Involving Allyl Cations

The thermal [4+3] cycloaddition reaction between allenes and tethered dienes (1,3‐butadiene and furan) assisted by transition metals (Au^I, Au^III, Pd^II, and Pt^II) was studied computationally within the density functional theory framework and compared to the analogous non‐organometallic process in terms of activation barriers, synchronicity and aromaticity of the corresponding transition states. It was found that the metal‐mediated cycloaddition reaction is concerted and takes place via transition structures that can be even more synchronous and more aromatic than their non‐organometallic analogues. However, the processes exhibit slightly to moderately higher activation barriers than the parent cycloaddition involving the hydroxyallylic cation. The bond polarization induced by the metal moiety is clearly related to the interaction of the transition metal with the allylic π* molecular orbital, which constitutes the LUMO of the initial reactant. Finally, replacement of the 1,3‐butadiene by furan caused the transformation to occur stepwise in both the non‐organometallic and metal‐assisted processes.     

Concerted vs stepwise mechanisms in dehydro-Diels-Alder reactions

The Diels-Alder reaction is not limited to 1,3-dienes. Many cycloadditions of enynes and a smaller number of examples with 1,3-diynes have been reported. These "dehydro"-Diels-Alder cycloadditions are one class of dehydropericyclic reactions which have long been used to generate strained cyclic allenes and other novel structures. CCSD(T)//M05-2X computational results are reported for the cycloadditions of vinylacetylene and butadiyne with ethylene and acetylene. Both concerted and stepwise diradical routes have been explored for each reaction, with location of relevant stationary points. Relative to 1,3-dienes, replacement of one double bond by a triple bond adds 6-6.5 kcal/mol to the activation barrier; a second triple bond adds 4.3-4.5 kcal/mol to the barrier. Product strain decreases the predicted exothermicity. In every case, a concerted reaction is favored energetically. The difference between concerted and stepwise reactions is 5.2-6.6 kcal/mol for enynes but diminishes to 0.5-2 kcal/mol for diynes. Experimental studies on intramolecular diyne + ene cycloadditions show two distinct reaction pathways, providing evidence for competing concerted and stepwise mechanisms. Diyne + yne cycloadditions connect with arynes and ethynyl-1,3-cyclobutadiene. This potential energy surface appears to be flat, with only a minute advantage for a concerted process; many diyne cycloadditions or aryne cycloreversions will proceed by a stepwise mechanism.     

The mechanism of photochemical 1,3-silyl migration of allylsilane

The photochemical reaction mechanisms of model compounds for 4-tert-butyl-1-(4-phenylphenyl)-1-(1,1-dimethylallyl)silacyclohexane are investigated using a complete active space comprised of six electrons in six orbitals with the standard 6-31G(d) basis set. It is concluded that the stereochemistry in the photochemical 1,3-silyl migrations of allylsilanes has a retention preference, in accord with the Woodward-Hoffmann rules. The calculated conical intersection (CI) structure suggests a dissociation path to radicals in addition to a 1,3-shift path. The bulkiness and rigidness of a silacyclohexane moiety does not affect the stereochemistry, but a slightly elongated Si-C bond length in the CI structure would promote the dissociation path.     

Ground state sigmatropic and electrocyclic rearrangements in some monoterpenes: The pyrolysis of α-pinene

Alloocimene 7, obtained via ocimene 6 by heating α-pinene 2, was further pyrolysed to a complex mixture of products derived primarily from antarafacial 1,7- and suprafacial 1,5-hydrogen migrations, and 6-eleetron disrotatory electrocyclizations.     

Computational studies on the cyclization of polycyclic aromatic hydrocarbons in the synthesis of curved aromatic derivatives.

Computational studies on the cyclization reactions of some polycyclic aromatic hydrocarbons (PAHs) were performed at the DFT level. Compounds C26H14 and C24H14, which show the connectivity of C60 fullerene fragments, were chosen as suitable models to study the formation of curved derivatives by six- or five-membered ring formation, upon oxidation to their radical cations. Four possible pathways for the cyclization process were considered: a) initial C-C bond formation to afford a curved derivative, followed by dehydrogenation; b) homolytic C-H cleavage prior to cyclization; c) initial concerted H2 elimination and subsequent cyclization; and d) deprotonation of the radical cations prior to cyclization. Computed reaction and activation energies for these reactions show that direct cyclization from radical cations (pathway a) is the lowest-energy mechanism. The formation of five-membered rings is somewhat more favourable than benzannulation. After new cycle formation, homolytic C-H dissociation to afford the corresponding cations is the most favourable process. These cations react with H* without barrier to give H2* Intermediate deprotonations are strongly disfavoured. The relatively low activation energies compared with carbon cage rearrangements suggest that ionization of PAHs can be used for the tailored preparation of nonplanar derivatives from suitable precursors.     

Influence of extreme thermodynamic conditions and pyrite surfaces on peptide synthesis in aqueous media

Free energy landscapes and reaction mechanisms underlying the synthesis of diglycine in water were studied computationally. It was found that amino acid activation by carbonyl sulfide, leading to the formation of a cyclic alpha-amino acid N-carboxyanhydride (NCA, or Leuchs anhydride), preferentially follows an indirect pathway that involves an isocyanate intermediate. Extreme temperature and pressure conditions accelerate peptidization greatly compared to the ambient bulk water environment and are shown to favor, in general, concerted versus stepwise mechanisms. Finally, a pyrite surface, FeS2 (001), is found to lower reaction barriers further by decreasing fluctuations and by assisting the preformation of the cyclic five-membered NCA ring due to scaffolding.     

Acyl Migration versus Epoxidation in Gold Catalysis: Facile,Switchable, and Atom-Economic Synthesis of Acylindoles and Quinoline Derivatives

We report a switchable synthesis of acylindoles and quinoline derivatives via gold-catalyzed annulations of anthranils and ynamides. α-Imino gold carbenes, generated in situ from anthranils and an N,O-coordinated gold(III) catalyst, undergo electrophilic attack to the aryl π-bond, followed by unexpected and highly selective 1,4- or 1,3-acyl migrations to form 6-acylindoles or 5-acylindoles. With the (2-biphenyl)di-tert-butylphosphine (JohnPhos) ligand, gold(I) carbenes experienced carbene/carbonyl additions to deliver quinoline oxides. Some of these epoxides are valuable substrates for the preparation of 3-hydroxylquinolines, quinolin-3(4H)-ones, and polycyclic compounds via facile in situ rearrangements. The reaction can be efficiently conducted on a gram scale and the obtained products are valuable substrates for preparing other potentially useful compounds. A computational study explained the unexpected selectivities and the dependency of the reaction pathway on the oxidation state and ligands of gold. With gold(III) the barrier for the formation of the strained oxirane ring is too high; whereas with gold(I) this transition state becomes accessible. Furthermore, energetic barriers to migration of the substituents on the intermediate sigma-complexes support the observed substitution pattern in the final product.     

Site of nucleophilic attack and ring opening of five-membered heterocyclic 2,3-diones: a density functional theory study

The site of nucleophilic addition to five-membered heterocyclic 2,3-diones (4-iminomethylfuran-2,3-dione A1 and 4-formyl-pyrrole-2,3-dione B1) is studied by density functional theory calculations (B3LYP/6-31G) with water as the nucleophile. Both uncatalyzed and water-assisted 1,2-addition to the lactone (lactam) and the keto carbonyl group, and conjugate addition to C5 of the heterocycle and the heteroatom of the 4-iminomethyl (formyl) moiety are considered. In addition, concerted and stepwise ring fission of the lactone (lactam) ring is also treated. The effect of solvation (aqueous solution) is taken into account by the polarizable continuum model (PCM) and the Poisson-Boltzmann SCRF method (PB-SCRF), as well as explicit water molecules. Only this latter approach yields meaningful activation free energies. Barriers for addition of H2O increase in the order 1,4-addition to C5 < addition to the lactone (lactam) carbonyl < hydration of the 3-keto group. No reaction path for concerted water-assisted ring opening could be found.     

Computational study of the aminolysis of 2-benzoxazolinone

Three possible mechanisms (zwitterionic, neutral stepwise, and neutral concerted) of the ring-opening reaction of 2-benzoxazolinone (BO) upon aminolysis with methylamine were studied at the B3LYP/6-31G* level. In the gas phase, the neutral concerted mechanism is shown to be most favorable, which proceeds via a rate-determining barrier of 28-29 kcal/mol. The transition state, CTS, associated with this barrier is a four-centered one, where 1,2-addition of the N[bond]H of methylamine to the C[bond]O of BO ring occurs. The rate-determining barrier of the neutral stepwise pathway is found to be ca. 42 kcal/mol. The inclusion of solvent effects by a polarizable continuum model (PCM) does not change the conclusions based on the gas-phase study; the barrier at CTS is reduced to 20, 20, and 22 kcal/mol in water, ethanol, and acetonitrile, respectively.