Cycloaddition reactions bimolecular reaction

A full development of the rate law for the bimolecular reaction of MDI to yield carbodiimide and CO indicates that the reaction should truly be 2nd-order in MDI. This would be observed experimentally under conditions in which MDI is at limiting concentrations. This is not the case for these experimements MDI is present in considerable excess (usually 5.5-6 g of MDI (4.7-5.1 ml) are used in an 8.8 ml vessel). So at least at the early stages of reaction, the carbon dioxide evolution would be expected to display pseudo-zero order kinetics. As the amount of MDI is depleted, then 2nd-order kinetics should be observed. In fact, the asymptotic portion of the 225 C Isotherm can be fitted to a 2nd-order rate law. This kinetic analysis is consistent with a more detailed mechanism for the decomposition, in which 2 molecules of MDI form a cyclic intermediate through a thermally allowed [2+2] cycloaddition, which is formed at steady state concentrations and may then decompose to carbodiimide and carbon dioxide. Isocyanates and other related compounds have been reported to participate in [2 + 2] and [4 + 2] cycloaddition reactions (8.91. 

CgH (n = 6, 7, 8). A novel collision-induced isomerization of CgH7 (10a), which has a sttained allenic bond, to (lOyS) has been reported to occur upon SIFT injection of (10a) at elevated kinetic energies (KE) and collision with helium. In contrast, radical anions (9) and (11) undergo electron detachment upon collisional excitation with helium. Bimolecular reactions of the ions with NO, NO2, SO2, COS, CS2, and O2 have been examined. The remarkable formation of CN on reaction of (11) with NO has been attributed to cycloaddition of NO to the triple bond followed by eliminative rearrangement. 

Compared to micellar bimolecular reactions involving reactive surfactant counterions, considerably less work has been done on micellar bimolecular reactions involving two neutral reactants. We will discuss here micellar effects on cycloaddition reactions though this is by no means the only system for which micellar catalysis has been investigated (see, e.g., Bonollo et al. °). 

There are several reports in the literature dealing with the bimolecular [3 + 2] cycloaddition reactions of alkynyl-substituted diazo compounds. Propargyl diazoacetate 212, when stored for 2 weeks at 0 °C, was transformed into an oligomer to which the constitution 213 was assigned (273) (Scheme 8.50). The alkynyl-diazoketone 214 requires a much higher temperature and is transformed into pyrazole 215, which probably arises from intermolecular cycloaddition, pyrazole tautomerization, and carbenic N/H insertion (274). The inter-intramolecular... 

K. N. Houk, N. G. Rondan, and J. Mareda, Theoretical Studies of Halocarbene Cycloaddition Selectivities. A New Interpretation of Negative Activation Energies and Entropy Control of Selectivities, Tetrahedron 1985, 41, 1555. Calculations on carbene addition reactions led to a general explanation of why it is possible for very exothermic, bimolecular reactions to have negative activation enthalpies. 

Orbital Symmetry Conservation in Bimolecular Cycloadditions. The cycloaddition reactions of carbonyl compounds to form oxetanes with ethylenes, as well as those of enones and their derivatives to form cyclobutanes, are examples of reactions which originate from triplet excited states and lead in the first step to biradical intermediates. Such reactions are of course not concerted, and they show little or no stereo-specificity. 

Although actual diffusion in solids is not significant within the lifetimes of excited molecules, bimolecular reactions can take place when molecules are kept in close contact in a polymer or crystal lattice. In some crystals the molecules are ideally spaced for cycloaddition, as in the example of cinnamic acid (Figure 4.80). The geometrical requirements are quite stringent and the reaction cannot proceed if the interplane separation of the molecules exceeds about 4A. 

Seelig, B., Keiper, S., Stuhlmann, F., Jaschke, A. Enantioselective ribozyme catalysis of a bimolecular cycloaddition reaction. Angew. Chem.. hit. Ed. Engl. 39, 4576 4-579 (2000). 

All of the photochemical cycloaddition reactions of the stilbenes are presumed to occur via excited state ir-ir type complexes (excimers, exciplexes, or excited charge-transfer complexes). Both the ground state and excited state complexes of t-1 are more stable than expected on the basis of redox potentials and singlet energy. Exciplex formation helps overcome the entropic problems associated with a bimolecular cycloaddition process and predetermines the adduct stereochemistry. Formation of an excited state complex is a necessary, but not a sufficient condition for cycloaddition. In fact, increased exciplex stability can result in decreased quantum yields for cycloaddition, due to an increased barrier for covalent bond formation (Fig. 2). The cycloaddition reactions of t-1 proceed with complete retention of stilbene and alkene photochemistry, indicative of either a concerted or short-lived singlet biradical mechanism. The observation of acyclic adduct formation in the reactions of It with nonconjugated dienes supports the biradical mechanism. 

Although it has been established that the HOMO (diazoalkane)-LUMO (alkene) controlled concerted cycloaddition occurs without intervention of any intermediate for the reactions of simple diazoalkanes with alkenes, Huisgen once proposed a mechanistic alternative 4 namely an initial hypothetical nitrene-type 1,1-cycloaddition reaction of phenyldiazomethane to styrene followed by a vinylcyclopropane-cy-clopentene-type 1,3-sigmatropic rearrangement Control experiments, however, excluded this hypothesis for the bimolecular 1,3-dipolar cycloaddition reaction of diazomethane (Scheme 60).204... 

In this section we will summarize bimolecular reactions of silenes with alkenes, alkynes and dienes which might be regarded nominally as cycloaddition reactions. 

While studies of reactions in supercritical fluids abound, only a few researchers have addressed the fundamental molecular effects that the supercritical fluid solvent has on the reactants and products that can enhance or depress reaction rates. A few measurements of reaction rate constants as a function of pressure do exist. For instance, Paulaitis and Alexander (1987) studied the Diels Alder cycloaddition reaction between maleic anhydride and isoprene in SCF CO2. They observed bimolecular rate constants that increased with increasing pressure above the critical point and finally at high pressures approached the rates observed in high pressure liquid solutions. Johnston and Haynes (1987) found the same trends in the... 

Looking more at host-guest type systems, the cucurbiturils are often very efficient catalysts in cases where they simultaneously bind two guests, thus increasing effective concentration and changing a bimolecular reaction into a guas/ -unimolecular one. To take one recent example, cucurbit[6]uril (Section 6.2.4) proves to be a very efficient catalyst for the click 1,3-dipolar cycloaddition reaction of an azide with an alkyne, particularly where the substrates are suitable guests for the cavity, as in Scheme 12.20. Cucurbituril binds very effectively to ammonium ions and addition of a small amount... 

Frontier orbital treatment of (p + q + r) cycloadditions is fairly simple. We first combine p and q into one system s. The reaction between s and r is then a bimolecular reaction. To find the symmetry of the FOs of s, no calculation is required. If three-orbital interactions are neglected, it is clear that ... 

Absolute kinetic data have been reported for four of the characteristic bimolecular reactions of disilenes 1,2-addition of alcohols and phenols (equation 72), [2 + 2]-cycloaddition of ketones (equation 73), [2 +4]-cycloaddition of aliphatic dienes (equation 74) and oxidation with molecular oxygen (equation 75). As with silenes, the addition of alcohols has been studied in greatest detail. 

An exciplex can exert a favorable effect on cycloaddition reactions in two ways. First, the prior association favors the bimolecular process. Second, exciplex formation will bias the system toward bond-formation of the donor-acceptor pair, and away from competing chemical processes, such as dimerization of the donor. 

Thermal reactions and 1,3-dipolar cycloaddition reactions Thermally mesoionic oxatriazoles are relatively stable. Heating of (85) in tolane (97) at 200 °C for 20 d in the presence of LiCl induces C02 fragmentation and formation of cycloadduct (99) in 37% yield. There is no bimolecular reaction as found in the case of the sydnones and isosydnones (equation 56) (68CB536). 

Pyrrolo[3,4-dipolar cycloaddition reactions, lose nitrogen under mild conditions to give aziridines (71), cyclopropanes (73) and phosphiranes (75), respectively. Ar... 

The intramolecular Diels-Alder reaction promises to become widely used in the synthesis of natural products from reports of recent research (142-144). In natural alkaloid synthesis by a Diels-Alder reaction, an enamide was utilized as either a suitable dienophile or diene group. There exist numerous examples of bimolecular and intramolecular Diels-Alder reactions of enamides with various dienophiles as well as bimolecular (4 + 2) cycloadditions of enamines with electron-deficient dienes. On the other hand, there appear to be only few reports of the (4 + 2) cycloaddition reactions of enamides with unactivated dienes. 

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