Origin of the synchronicity in bond formation in polar Diels-Alder reactions: an ELF analysis of the reaction between cyclopentadiene and tetracyanoethylene

The origin of the synchronicity in C-C bond formation in polar Diels-Alder (P-DA) reactions involving symmetrically substituted electrophilic ethylenes has been studied by an ELF analysis of the electron reorganization along the P-DA reaction of cyclopentadiene (Cp) with tetracyanoethylene (TCE) at the B3LYP/6-31G* level. The present study makes it possible to establish that the synchronicity in C-C bond formation in P-DA reactions is controlled by the symmetric distribution of the electron-density excess reached in the electrophile through the charge transfer process, which can be anticipated by an analysis of the spin electron-density at the corresponding radical anion. The ELF comparative analysis of bonding along the DA reactions of Cp with ethylene and with TCE asserts that these DA reactions, which have a symmetric electron reorganization, do not have a cyclic electron reorganization as the pericyclic mechanism states. Due to the very limited number of cases of symmetrically substituted ethylenes, we can conclude that the synchronous mechanism is an exception of DA reactions.     

How Ionization Catalyzes Diels-Alder Reactions

The catalytic effect of ionization on the Diels-Alder reaction between 1,3-butadiene and acrylaldehyde has been studied using relativistic density functional theory (DFT). Removal of an electron from the dienophile, acrylaldehyde, significantly accelerates the Diels-Alder reaction and shifts the reaction mechanism from concerted asynchronous for the neutral Diels-Alder reaction to stepwise for the radical-cation Diels-Alder reaction. Our detailed activation strain and Kohn-Sham molecular orbital analyses reveal how ionization of the dienophile enhances the Diels-Alder reactivity via two mechanisms: (i) by amplifying the asymmetry in the dienophile's occupied π-orbitals to such an extent that the reaction goes from concerted asynchronous to stepwise and thus with substantially less steric (Pauli) repulsion per reaction step; (ii) by enhancing the stabilizing orbital interactions that result from the ability of the singly occupied molecular orbital of the radical-cation dienophile to engage in an additional three-electron bonding interaction with the highest occupied molecular orbital of the diene.     

Lewis碱稳定的硼代苯与亲二烯体的Diels-Alder反应的理论研究

采用密度泛函理论方法在B3LYP/6-31G(d)水平上研究了Lewis碱稳定的硼代苯与一些亲二烯体的两种可能的Diels-Alder反应的微观机理和势能剖面, 并研究了反应的溶剂效应和取代基效应. 计算结果表明, 一部分反应以直接的近同步的协同方式进行, 而在另一部分反应中, 两个反应物分子先形成分子间复合物, 然后再经过协同的过渡态生成产物. 与气相中相比, 二氯甲烷溶剂使所研究的大部分反应的活化能垒有所增加. 在乙炔或乙烯分子中分别引入吸电子基团CO2Me或CN能显著降低反应的活化能垒. 形成一个C—B键的杂Diels-Alder反应都比相应的Diels-Alder反应在热力学和动力学上容易进行, 这与实验结果一致.     

A substituent effects study reveals the kinetic pathway for an interfacial reaction

This paper describes the use of a substituent effects study to understand the mechanistic basis for an interfacial Diels-Alder reaction that does not proceed with standard second-order kinetics. Cyclopentadiene (Cp) undergoes a Diels-Alder reaction with a chemisorbed mercaptobenzoquinone to yield an immobilized Diels-Alder adduct. The pseudo-first-order rate constants are not linearly related to the concentration of diene, but they reach a limiting value with increasing concentrations of diene. The results of a substituent effects study support a mechanism wherein the electrochemical oxidation of hydroquinone produces two states of quinone. The first form, Q*, either reacts with Cp or isomerizes to Q, a form that is significantly less reactive with the diene. The interfacial reaction reaches a maximum rate when the concentration of diene is sufficiently high so that Q* undergoes complete Diels-Alder reaction and does not isomerize to Q. This work provides an example of the use of physical organic chemistry to understand an interfacial reaction.     

The Diels-Alder reaction of ethene and 1,3-butadiene: an extended multireference ab initio investigation.

The concerted and stepwise mechanisms of the Diels-Alder reaction between 1,3-butadiene and ethene have been investigated using highly correlated multireference methods (MRAQCC) and extended basis sets. Full MRAQCC geometry optimizations have been performed in all cases. The best estimate for the energy barrier of the Diels-Alder reaction is 22 kcalmol(-1). Anti- and gauche-out minima for the biradical structures and corresponding fragmentation saddle points have been determined. The biradical anti fragmentation saddle point is located 6.5 kcalmol(-1) above the concerted saddle point. The gauche-in structure does not correspond to a local minimum, but leads on geometry optimization directly to cyclohexene.     

Revealing carbocations in highly asynchronous concerted reactions: The ene-type reaction between dithiocarboxylic acids and alkenes

The ene-type reaction between (dithio)carboxylic acids and alkenes has been studied computationally by DFT and topological (analysis of the electron localization function, ELF) methods. The reaction proceeds under kinetic control and the observed differences in regioselectivity are well-explained by the relative stability of the different transition structures. In agreement with experimental observations, electron-rich alkenes lead to Markownikoff adducts while electron-poor alkenes lead to Michael adducts. In all cases the reaction proceeds through an only transition structure (one kinetic step) although a different synchronicity was observed depending on the alkene electronics. The ELF analysis of the reactions corroborates the existence of a transient carbocation (hidden intermediate) in the reactions with electron-rich alkenes. On the other hand, electron-poor alkenes proceed through a more synchronous concerted mechanism. It can be predicted that with electron-rich alkenes bearing highly donating the transient carbocations might be captured by a nucleophile.     

Synthesis of a new stable, neutral organothulium(II) complex by reduction of a thulium(III) precursor

The new stable, neutral Tm(II) complex (Cp(ttt))2Tm [Cp(ttt) = 1,2,4-tris(tert-butyl)cyclopentadienyl] can be obtained either by direct reaction of NaCp(ttt) with TmI2 or by reduction of (Cp(ttt))2TmI in non-polar solvents; this latter route may prove itself useful for the isolation of other neutral non-classical low-valent organolanthanide species.     

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.     

A reactive bond orbital investigation of the Diels-Alder reaction between 1,3-butadiene and ethylene: Energy decomposition, state correlation diagram, and electron density analyses

The reactive bond orbital (RBO) method (Hirao, Chem Phys Lett 2007, 443, 141) is extended and applied to the Diels-Alder reaction between 1,3-butadiene and ethylene, with the aim of understanding the nature of their interaction. The roles of distortion, electrostatic, exchange, polarization, and charge transfer (CT) interaction energies at the transition state of the reaction are evaluated by means of RBO energy decomposition analysis. The effects of the hypothetical interactions on electron density redistribution are also identified by analysis based on the RBO method. CT is shown to play essential roles in the new bond formation between the reacting molecules and their internal bond order alterations. However, each of the CT interactions from butadiene to ethylene and from ethylene to butadiene does not necessarily contribute to the bond-order alteration process effectively. A state correlation diagram approach based on the RBO method is also proposed, and its usefulness in understanding the origin of the barrier in the Diels-Alder reaction is demonstrated.     

Theoretical study of the Cp2Zr-catalyzed hydrosilylation of ethylene. Reaction mechanism including new sigma-bond activation

The Cp(2)Zr-catalyzed hydrosilylation of ethylene was theoretically investigated with DFT and MP2-MP4(SDQ) methods, to clarify the reaction mechanism and the characteristic features of this reaction. Although ethylene insertion into the Zr-SiH(3) bond of Cp(2)Zr(H)(SiH(3)) needs a very large activation barrier of 41.0 (42.3) kcal/mol, ethylene is easily inserted into the Zr-H bond with a very small activation barrier of 2.1 (2.8) kcal/mol, where the activation barrier and the energy of reaction calculated with the DFT(B3LYP) method are given and in parentheses are those values which have been corrected for the zero-point energy, hereafter. Not only this ethylene insertion reaction but also the coupling reaction between Cp(2)Zr(C(2)H(4)) and SiH(4) easily takes place to afford Cp(2)Zr(H)(CH(2)CH(2)SiH(3)) and Cp(2)Zr(CH(2)CH(3))(SiH(3)) with activation barriers of 0.3 (0.7) and 5.0 (5.4) kcal/mol, respectively. This coupling reaction involves a new type of Si-H sigma-bond activation which is similar to metathesis. The important interaction in the coupling reaction is the bonding overlap between the d(pi)-pi bonding orbital of Cp(2)Zr(C(2)H(4)) and the Si-H sigma orbital. The final step is neither direct C-H nor Si-C reductive elimination, because both reductive eliminations occur with a very large activation barrier and significantly large endothermicity. This is because the d orbital of Cp(2)Zr is at a high energy. On the other hand, ethylene-assisted C-H reductive elimination easily occurs with a small activation barrier, 5.0 (7.5) kcal/mol, and considerably large exothermicity, -10.6 (-7.1) kcal/mol. Also, ethylene-assisted Si-C reductive elimination and metatheses of Cp(2)Zr(H)(CH(2)CH(2)SiH(3)) and Cp(2)Zr(CH(2)CH(3))(SiH(3)) with SiH(4) take place with moderate activation barriers, 26.5 (30.7), 18.4 (20.5), and 28.3 (31.5) kcal/mol, respectively. From these results, it is clearly concluded that the most favorable catalytic cycle of the Cp(2)Zr-catalyzed hydrosilylation of ethylene consists of the coupling reaction of Cp(2)Zr(C(2)H(4)) with SiH(4) followed by the ethylene-assisted C-H reductive elimination.     

A joint study based on the electron localization function and catastrophe theory of the chameleonic and centauric models for the Cope rearrangement of 1,5-hexadiene and its cyano derivatives

A novel interpretation of the chameleonic and centauric models for the Cope rearrangements of 1,5-hexadiene (A) and different cyano derivatives (B: 2,5-dicyano, C: 1,3,4,6-tetracyano, and D: 1,3,5-tricyano) is presented by using the topological analysis of the electron localization function (ELF) and Thom's catastrophe theory (CT) on the reaction paths calculated at the B3LYP/6-31G(d,p) level. The progress of the reaction is monitorized by the changes of the ELF structural stability domains (SSD), each being change controlled by a turning point derived from CT. The reaction mechanism of the parent reaction A is characterized by nine ELF SSDs. All processes occur in the vicinity of the transition structure and corresponding to a concerted formation/breaking of C(1)-C(6) and C(3)-C(4) bonds, respectively, together with an accumulation of charge density onto C(2) and C(5) atoms. Reaction B presents the same number of ELF SSDs as A, but a different order appears; the presence of 2,5-dicyano substituents favors the formation of C(1)-C(6) bonds over the breaking of C(3)-C(4) bond process, changing the reaction mechanism from a concerted towards a stepwise, via a cyclohexane biradical intermediate. On the other side, reaction C presents the same type of turning points but two ELF SSD less than A or B; there is an enhancement of the C(3)-C(4) bond breaking process at an earlier stage of the reaction by delocalizing the electrons from the C(3)-C(4) bond among the cyano groups. In the case of competitive effects of cyano subsituents on each moiety, as it is for reaction D, seven different ELF SSDs have been identified separated by eight turning points (two of them occur simultaneously). Both processes, formation/breaking of C(1)-C(6) and C(3)-C(4) bonds, are slightly favored with respect to the parent reaction (A), and the TS presents mixed electronic features of both B and C. The employed methodology provides theoretical support for the centauric nature (half-allyl, half-radical) for the TS of D.     

基于Diels-Alder点击反应构建壳聚糖-O-聚乙二醇接枝共聚物

通过Diels-Alder(D-A)反应,合成了具有规整化学结构的接枝共聚物,壳聚糖-O-聚乙二醇(CS-O-PEG).D-A反应所需双烯体(呋喃环)通过糠基硫醇与端甲基丙烯酸酯聚乙二醇之间的巯基-丙烯酸酯(thio-acrylate)反应合成得到;马来酰亚胺基丙酸通过活泼酯法偶联到十二烷基硫酸钠-壳聚糖复合物(SCC)羟基上,从而获得亲双烯体.采用红外光谱(FTIR)和核磁共振(1H-NMR)表征了中间产物与最终产物的结构,并用原位核磁监测D-A反应及其逆反应过程.结果表明,聚乙二醇双烯体可在水介质中温和条件下定量接枝到壳聚糖羟基上,反应具有点击特征;同时,聚乙二醇与壳聚糖之间的连接键在高温下(90℃)可通过D-A逆反应而发生断裂.     

Enantioselective organocatalytic Diels-Alder reactions: a density functional theory and kinetic isotope effects study

The mechanism and enantioselectivity of the organocatalytic Diels-Alder reaction were computationally investigated by density functional theory at the B3LYP/6-31G(d) level of theory. The uncatalyzed Diels-Alder reaction was also studied to explore the effect of the organocatalyst on this reaction in terms of energetics, selectivity, and mechanism. The catalyzed reaction showed improved endo/exo selectivity, and the free energy of activation was significantly lowered in the presence of the catalyst. Both uncatalyzed and catalyzed reactions exhibited concerted asynchronous reaction mechanism with the degree of asynchronicity being more evident in the presence of the catalyst. The Corey's experimentally derived predictive selection rules for the outcome of the organocatalytic Diels-Alder reaction were also theoretically analyzed, and an excellent agreement was found between experiment and theory.     

Conformational selectivity in the diels-alder cycloaddition: predictions for reactions of s-trans-1,3-butadiene

Diels-Alder cycloaddition of s-trans-1,3-butadiene (1) should yield trans-cyclohexene (7), just as reaction of the s-cis conformer gives cis-cyclohexene (9). Investigation of this long-overlooked process with Hartree-Fock, Moller-Plesset, CASSCF, and DFT methods yielded in every case a C(2)-symmetric concerted transition state. At the B3LYP/6-31G (+ZPVE) level, this structure is predicted to be 42.6 kcal/mol above reactants, while the overall reaction is endothermic by 16.7 kcal/mol. A stepwise diradical process has been studied by UBLYP/6-31G theory and found to have barriers of 35.5 and 17.7 kcal/mol for the two steps. Spin correction lowers these values to 30.1 and 13.0 kcal/mol. The barrier to pi-bond rotation in cis-cyclohexene (9) is predicted (B3LYP theory) to be 62.4 kcal/mol, with trans-cyclohexene (7) lying 53.3 kcal/mol above cis isomer 9. Results suggest that pi-bond isomerization and concerted reaction may provide competitive routes for Diels-Alder cycloreversion. It is concluded that full understanding of the Diels-Alder reaction requires consideration of both conformers of 1,3-butadiene.     

4 + 2] Cycloadditions of rigid s-cis dienes to C(60). A synchronous Diels-Alder reaction

[reaction: see text] The Diels-Alder reaction of rigid s-cis dienes with C(60) occurs by a concerted mechanism, via a symmetrical transition state.     

Theoretical studies on the ene reaction mechanisms of propene and cyclopropene with ethylene and cyclopropene: concerted or stepwise

The potential energy surfaces of the ene reactions of propene and cyclopropene with ethylene and cyclopropene were studied by ab initio molecular orbital (MO) methods. The reaction mechanisms were analyzed by CiLC method on the basis of CASSCF MOs. The concerted and stepwise reaction pathways of the ene reaction of propene with ethylene as the parent reaction were located. The energy barrier of the stepwise process is about 4 kcal/mol lower than that of the concerted one. The other reactions can be found only the stepwise mechanism. Although the endo-type reaction of propene with cyclopropene, where cyclopropene is the enophile, probably occurs through a one-step process, the mechanism is divided into the CC bond formations and the hydrogen migration as a stepwise reaction. The CiLC-IRC analysis of the concerted process of propene with ethylene shows the different patterns of the electronic state variation for the CC bond formation/breaking and the hydrogen migration.     

Quantum mechanical inspection of the Diels-Alder approach to biaryls mechanism

The Diels-Alder reaction of substituted cyclohexadienes with substituted phenylacetylenes offers an attractive alternative for the synthesis of biaryl compounds via a two-step cycloaddition/cycloelimination pathway. Quantum mechanical calculations using B3LYP and M06-2X density functional methods for the reaction of 2-chloro-6-nitrophenylacetylene with 1-carbomethoxy-cyclohexadiene show the reaction proceeds by a stepwise diradical [4+2] cycloaddition followed by concerted [2+4] cycloelimination of ethylene. [2+2] cycloadducts are also the result of stepwise addition. [2+2] cycloadducts isomerize to [4+2] cycloadducts via diradical pathways, which involve the same diradical intermediate in cycloaddition. There is also a competitive conrotatory ring opening followed by trans-cis double bond isomerization pathway of the [4.2.0] bicycle (the [2+2] cycloadduct) to give the cis,cis,cis-1,3,5-cyclooctatriene.     

Quantum chemical study on the thermal Diels-Alder reaction of cyclohexadiene and methyl crotonate

AM1 molecular orbital method using the unrestricted Hartree-Fock(UHF) calculations has been applied to investigate the thermal reaction of cyclohexadiene and methyl crotonate. The calculated results indicate that the thermal Diels-Alder reaction proceeds to product through the concerted path and two radical pathways.     

Cp*Rh complexes with pyridyloxazolines: synthesis, fluxionality and applications as asymmetric catalysts for Diels-Alder reactions

Half-sandwich complexes [RhCl(pymox)Cp*][SbF(6)](1-7)(pymox = pyridyloxazoline) have been synthesised as single diastereomers. Treatment of these with AgSbF(6) generates dications [Rh(OH(2))(pymox)Cp*](2+) which are fluxional at room temperature and which are enantioselective catalysts for the Diels-Alder reaction of methacrolein and cyclopentadiene. Treatment of the dication [Rh(OH(2))((i)Pr-pymox)Cp*](2+) with [X](-) gives [RhX((i)Pr-pymox)Cp*][SbF(6)](X = Br, I) as single diastereomers whilst reaction with 4-Mepy (4-methylpyridine) gives [Rh(4-Mepy)((i)Pr-pymox)Cp*][SbF(6)] as a mixture of diastereomers. Two complexes, [RhCl((i)Pr-pymox)Cp*][SbF(6)](3) and [RhCl(Bz-pymox)Cp*][SbF(6)](6) have been characterised by X-ray crystallography.     

Une etude perturbationnelle du role des catalyseurs dans les reactions de Diels-Alder

Catalyzed Diels-Alder reactions are examined by a simple perturbational treatment. Two hypotheses are made in the calculations: (1) the mechanisms of catalyzed and non-catalyzed Diels-Alder reactions are essentially the same, i.e., both are concerted, but not necessarily synchronous, frontier-controlled reactions; (2) the principal function of a Lewis acid in the reaction mixture is the complexation of a carbonyl or a nitrile groups by the catalyst. The agreement between theoretical predictions and experimental results is excellent.