Transition structure alkene

Nitrones are a rather polarized 1,3-dipoles so that the transition structure of their cydoaddition reactions to alkenes activated by an electron-withdrawing substituent would involve some asynchronous nature with respect to the newly forming bonds, especially so in the Lewis acid-catalyzed reactions. Therefore, the transition structures for the catalyzed nitrone cydoaddition reactions were estimated on the basis of ab-initio calculations using the 3-21G basis set. A model reaction indudes the interaction between CH2=NH(0) and acrolein in the presence or absence of BH3 as an acid catalyst (Scheme 7.30). Both the catalyzed and uncatalyzed reactions have only one transition state in each case, indicating that the reactions are both concerted. However, the synchronous nature between the newly forming 01-C5 and C3-C4 bonds in the transition structure TS-J of the catalyzed reaction is rather different from that in the uncatalyzed reaction TS-K. For example, the bond lengths and bond orders in the uncatalyzed reaction are 1.93 A and 0.37 for the 01-C5 bond and 2.47 A and 0.19 for the C3-C4 bond, while those in... 

The theoretical investigations of Lewis acid-catalyzed 1,3-dipolar cycloaddition reactions are also very limited and only papers dealing with cycloaddition reactions of nitrones with alkenes have been investigated. The Influence of the Lewis acid catalyst on these reactions are very similar to what has been calculated for the carbo- and hetero-Diels-Alder reactions. The FMOs are perturbed by the coordination of the substrate to the Lewis acid giving a more favorable reaction with a lower transition-state energy. Furthermore, a more asynchronous transition-structure for the cycloaddition step, compared to the uncatalyzed reaction, has also been found for this class of reactions. 

Figure 7.29. (Top) Molecular representations based on X-ray structural data of the diazo compound 88N2 and the alkene product 89Z (the migrating hydrogen is shown in black in both reactant and product). (Bottom) Schematic reaction path showing the minimal structural changes in the transition from the diazo compound to the product, via the probable transition structure 88TS.

Kinetics and isotope effects are consistent with this mechanism.92 The reagent is electrophilic in character and reaction is facilitated by ERG substituents in the alkene. A B3LYP/6-31G computation found the transition structures and Ea values shown in... 

Scheme 6.40. Transition structures proposed for the polymerization and hydrogenation of alkenes by (ebthi)ZrX2/MAO.

The presence of this bulky group leads to a higher diastereoselectivity than in the unsubstituted case because interactions of the alkene with the titanocene group lead to the exclusive formation of one diastereoisomer, presumably through the most favored transition structure shown in Scheme 12.19, in which steric interactions should be minimized. 

DFT calculations have been used to study the mechanistic pathway of the intramolecular Diels-Alder cycloaddition involved in the biosynthesis of natural products paraherquamide A and VM55599. The cycloaddition involves a dihydropyrolo[l,2- ]pyrazine as the azadiene and a standard alkene as the dienophile (Scheme 1). Analysis of the results reveals that these cycloadditions take place through concerted transition structures associated with [4+2]... 

The triplet instability of both RB3LYP and RHF solutions is responsible for the difference in the symmetry of the ethylene-peroxynitrous acid transition structure. In general, an unsymmetrical transition structure with alkene epoxidation when an alternative symmetrical structure is possible, appears to be associated with an unstable wave function. 

A typical closed-shell transition structure for DMDO epoxidation is exemplified by the epoxidation of E- and Z-2-butene. Baumstark and Vasquez have reported experimental studies that demonstrate the greater reactivity of Z-alkenes in the DMDO epoxidation of E/Z-pairs of alkenes . As anticipated, approach of the dioxirane ring to the Z-double bond in the less hindered manner, away from the methyl groups of DMDO,... 

At that period of time, this rather primitive transition structure for the peracid epoxidation of ethylene was sufficiently novel to warrant publication as a communication Today, calculating transition structures for epoxidation of a variety of complex alkenes with the actual peracid used experimentally, such as meta-chloroperoxybenzoic acid m-CPBA), is commonplace . ... 

TABLE 8. Differences (A) in the O—Cl and O—C2 calculated distances of transition structures for the epoxidations of alkenes with peroxyformic acid using different methods... 

A systematic examination of the transition structures for the peroxyformic acid epoxidation at various levels of theory confirms the above suggestion that the MP2 method does provide adequate geometries for a conjugated carbon-carbon double bond in 1,3-butadiene (Figure 23). However, the inverse KIEs measured by HanzUk and Shearer " for 4-vinylbiphenyl are specific to conjugated alkenes and should not be used as evidence of an asynchronous epoxidation TS for simple nonconjugated aUcenes. ... 

Alkenes strained by twist or r-bond torsion, such as E-cyclooctene, exhibit much lower barriers due to relief of strain in the TS for the oxygen transfer step. While the epoxidation of symmetrically substituted alkenes normally involve a symmetrical approach to the TT-bond, the TSs for epoxidation of E-cyclooctene and E-l-methylcyclooctene exhibit highly asymmetric transition structures. The AAE = 3.3 kcalmol" for E- versus Z-cyclooctene is clearly a reflection of the relative SE of these two medium ring alkenes (16.4 vs 4.2 kcalmol ) ". The classical activation barrier (AE ) for the highly strained bicyclo[3.3.1]non-l-ene is also quite low (Table 10, Figure 26). In these twist-strain alkenes, the approach of the peracid deviates markedly from the idealized spiro approach suggesting fliat this part of the potential energy surface is quite soft. 

FIGURE 26. Transition structures for the epoxidation of selected cyclic alkenes with peroxyformic acid (PEA), optimized at the B3LYP/6-31+G(d,p) level of theory. The classical activation barriers are given at B3LYP/6-311+G(3df,2p)//B3LYP/6-31+G(d,p)... 

FIGURE 3. Preferred transition structures for the oxygen transfer in the reaction of alkenes, sulfides and alkanes with dioxiranes... 

The results of the dioxirane epoxidation of some 3-alkyl-substituted cyclohexenes and of 2-menthene indicate that the diastereoselectivity control is subject to the steric interactions of the dioxirane with the substituents of the substrate, while the size of the dioxirane substituents has only a minimal effect . In the favored transition structure, the alkyl groups of the dioxirane cannot interact effectively with the substituents at the stereogenic center of the chiral alkene . ... 

The transition state of singlet carbene cycloaddition to alkenes involves an electrophilic approach of the vacant p orbital to the n bond of alkenes. By contrast, the first step of the triplet addition process may involve the in-plane a orbital of the carbene. As in the case of C—H insertion (see Section 5.1), the difference in the transition structure between the singlet and triplet cycloaddition becomes important in the intramolecular process, especially when approach to a double bond is restricted by ring strain. Direct photolysis of ( )-2-(2-butenyl)phenyldiazomethane (99) in the presence of methanol gives l-ethenyl-l,la,6,6fl-tetrahydrocycloprop [fljindene [100, 29%, (E/Z)= 10 1] and l-(2-butenyl)-2-(methoxymethyl)benzene (101, 67%). Triplet-sensitized photolysis results in a marked increase in the indene (52%, EjZ) = 1.3.T) at the expense of the ether formation (4%) (Scheme 9.30). On the other hand, direct photolysis of phenyldiazomethane in an equimolar mixture of... 

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