Surfaces, cycloadditions

Because of the similarity between the structures of the Ge(100)-2 x 1 and Si(100)-2x1 surfaces, cycloaddition products like those observed on Si(100)-2 x 1 are also expected to form at the Ge surface. Indeed, studying butadiene, Teplyakov et al. [240] showed that a similar Diels-Alder product formed on the surface of Ge(100)-2 x 1. Studies of alkenes have also revealed the formation of [2 + 2] cycloaddition products on germanium. For example, cyclopentene has been shown to form the [2 + 2] cycloaddition product on both surfaces [224,296,297]. In further studies of several other dienes and alkenes (including ethylene [298-303], acetylene [304-306], and cyclohexadiene [307]), cycloaddition products were found for Ge(100)-2 x 1 similar to those observed for Si(100)-2 x 1. 

Hovis, J. S. and Hamers, R. J. Structure and bonding of ordered organic monolayers of 1,3,5,7-cyclooctatetraene on the Si(001) surface Surface cycloaddition chemistry of an antiaromatic molecule. Journal of Physical Chemistry 102, 687 (1998). 

Barriocanal, J. A. and Doren, D. J. Cycloaddition of carbonyl compounds on Si(100) New mechanisms and approaches to selectivity for surface cycloaddition reactions. Journal of the American Chemical Society 123, 7340-7346 (2001). 

FIGURE 3. (a) Ordinary [2 + 2] cycloaddition reaction, (b) [2 + 2] Surface cycloaddition reaction, (c) Surface electrocyclic reaction... 

Another important issue related to surface cycloaddition reactions is the stereochemistry of the products. Advances in this area may open new methods for developing new chemical sensors that may be used for complex molecular recognition tasks. In one study, Wolkow and coworkers54 showed that a scanning tunneling microscope can be used to determine the absolute chirality of individual molecules of cis- and trans-2-butene adsorbed on... 

Flamers R. J., Coulter S. K., Ellison M. D., Flovis J. S., Padowitz D. F., Schwartz M. P., Greenlief C. M., Russell J. N. Jr Cycloaddition Chemistry of Organic Molecules With Semiconductor Surfaces Acc. Chem. Res. 2000 33 617 624 Keywords carbonyi group, semiconductor materiais, surface reaction, aikenes, aromatic compounds, azo compounds, cycioaikadienes, isothiocyanates, unsaturated compounds... 

Houk K. N., Wilsey S. L., Beno B. R., Kless A., Nendel M., Tian J. Retro-Cycloadditions and Sigmatropic Shifts The C7H8 and C7H10 Potential Energy Surfaces Pure Appl. Chem. 1998 70 1947-1952... 

Keywords Cycloadditions, Chemical orbital theory. Donor-acceptor interaction. Electron delocalization band. Electron transfer band, Erontier orbital. Mechanistic spectrum, NAD(P)H reactions. Orbital amplitude. Orbital interaction. Orbital phase. Pseudoexcitation band. Quasi-intermediate, Reactivity, Selectivity, Singlet oxygen. Surface reactions... 

Diels-Alder reactions are allowed by orbital symmetry in the delocalization band and so expected to occur on the surface. In fact, [4-1-2] cycloaddition reaction occurs on the clean diamond (100)-2 x 1 surface, where the surface dimer acts as a dienophile. The surface product was found to be stable up to approximately 1,000 K [59, 60], 1,3-Butadiene attains high coverage as well as forms a thermally stable adlayer on reconstructed diamond (100)-2 x 1 surface due to its ability to undergo [4h-2] cycloaddition [61],... 

Diels-Alder reactions also take place on the Si(100)-2 x 1 [62] and Ge(100)-2 x 1 [63,64] surface. The experiments by Hammers and his colleagues [65] indicate that the [4-1-2] cycloaddition reactions of 1,3-cyclohexadiene and 1,3-dimethylbutadiene on the Si(OOl) surface compete with the [2-1-2] cycloaddition reactions. 

Cycloaddition reactions with the Si(lOO) surface have been investigated for the purpose of designing microelectronics, nonlinear optical materials, sensors, and biologically active surfaces. The features of the [2+2] cycloadditions characteristic of the reactions in the pseudoexcitation band [133] predicts that [2+2] cycloadditions of electron-donating alkenes with Si(100)-2 x 1 surface could proceed with retention of configurations, in agreement with the observation [134]. Such stereospecific functionalizations of surfaces are of potential use for specific applications. 

The pseudoexcitation is induced by the delocalization from alkenes to the Si(100)-2 X 1 surface [133]. Electron-accepting alkenes undergo different reactions. For acrylonitirile, a [4+2] cycloaddition reaction was found to be kinetically most favorable [135]. 

Cycloadditions with the Si(lOO) surface were theoretically [133] concluded to be reactions in the pseudoexcitation band. The conclusion is applicable to thermal [2+2] cycloaddition reactions of unsaturated bonds between heavy atoms. In fact, Sekiguchi, Nagase et al. confirmed that a Si triple bond underwent the stereospecific reactions with alkenes [137] along the path typical of [2+2] cycloaddition in the pseudoexcitation band. The stereospecific [2+2] cycloadditions of were designed by Inagaki et al. (Scheme 28) [138]. 

The overall pathway for the conversion of the unsaturated azido ether 281 to 2,5-dihydrooxazoles 282 involves first formation of the dipolar cycloaddition product 287, which thermolyzes to oxazoline 282 or is converted by silica gel to oxazolinoaziridine 288. While thermolysis or acid-catalyzed decomposition of triazolines to a mixture of imine and aziridine is well-documented [71,73], this chemoselective decomposition, depending on whether thermolysis or exposure to silica gel is used, is unprecedented. It is postulated that acidic surface sites on silica catalyze the triazoline decomposition via an intermediate resembling 289, which prefers to close to an aziridine 288. On the other hand, thermolysis of 287 may proceed via 290 (or the corresponding diradical) in which hydrogen migration is favored over ring closure. 

Bernardi F, Olivucci M, Robb MA (1990) Predicting forbidden and allowed cycloaddition reactions potential surface topology and its rationalization. Acc Chem Res 23 405... 

A theoretical study based on PM3 frontier molecular orbital (FMO) and potential energy surface (PES) analysis at the restricted Hartree-Fock (RHF)/6-31+G level was performed to examine the reaction of l-amino-2-ethoxycarbonyl-pyridinium mesitylenesulfonate and acrylonitrile in the presence of Hilnig s base leading to the formation of l,2-dihydropyrido[l,2-A]pyridazinium inner salt 17 <1999JOC9001>. The calculations indicated that both the [3+2] cycloaddition reaction and the ring expansion occurred in a concerted manner rather than through a stepwise mechanism via a zwitterionic intermediate 16 (Scheme 1). 

The covalent methods previously discussed for fullerene modification using cycloaddition reactions also can be applied to carbon nanotubes. This strategy results in chemically linking molecules to the graphene rings on the outer surface of the cylinder, resulting in stable... 

Figure 15.16 Some modification methods that are useful for fullerenes also can be used with carbon nanotubes. The reaction of an N-glycine compound with an aldehyde derivative can result in cycloaddition products, which create pyrrolidine modifications on the nanotube surface.

Sun, X.-L., Stabler, C.L., Cazalis, C.S., and Chaikof, E.L. (2006) Carbohydrate and protein immobilization onto solid surfaces by sequential diels-alder and azide-alkyne cycloadditions. Bioconjugate Chem. 17, 52-57. 

Highly stereoselective intramolecular cycloadditions of unsaturated N-substituted azomethine ylides have been conducted under microwave irradiation. Oritani reported that a mixture of the aldehyde 137 and N-methyl- or N-benzylglycine ethyl ester (138) on the surface of silica gel, irradiated under microwaves for 15 min, generated azomethine ylides 139 that subsequently underwent in situ intramolecular cycloadditions to afford the corresponding tricyclic compounds 140 in 79 and 81% yield, respectively (Scheme 9.42) [93],... 

Earlier, [3+ 2]-cycloaddition reactions of nitronates have been described in terms of the FMO theory. For example, French researchers studied reactions of olefins containing EWG groups with nitronates by the FMO—INDO method (248, 338b, 419). Recently, more modem methods have been used for calculations of FMO and the potential energy surfaces for several analogous reactions (87, 399,... 

Another attempt to perform a general mechanistic consideration of [3 + 21-cycloaddition reactions of nitronates with olefins has been made relatively recently by Prof. Denmark and coworkers (162) using modern quantum-chemical methods, which allow one to correctly calculate the potential energy surfaces for model substrates. Since these data have been summarized in the recent review (335), it is not necessary to consider them as comprehensively as the study in (337). 

But if the approach of the molecular planes is perpendicular, i.e., when one surface of the n orbital of one molecule interacts with both surfaces of the n orbital of the other molecule, then cycloaddition will be allowed here but the geometry of the approach will be the deciding factor and geometrically it is inaccessible. 

Similar oxidative cyclization reactions involving the direct oxidation of acyclic 1,3-dicarbonyl compounds have not been reported. However, the generation of radical intermediates by the direct oxidation of cyclic 1,3-dicarbonyl compounds at an anode surface has been reported. Yoshida and coworkers have shown that the anodic oxidation of cyclic 1,3-dicarbonyl compounds in the presence of olefin trapping groups gives rise to a net cycloaddition reaction (Scheme 10) [23]. These cycloaddition reactions proceeded by initial oxidation of the 1,3-dicarbonyl compound at the anode followed by a radical addition to the second olefin. Following a second oxidation reaction, the material then... 

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