Molecular-orbital calculations rearrangements

Tsuda and Oikawa (1989) investigated the photolysis of the 1,2-isomer of 10.89 (1,2-benzoquinone diazide) by means of MINDO/3 molecular orbital calculations with configurational interaction. These authors came to the conclusion that no ketocarbene of the type of 10.90 is formed, but that the rearrangement into the cyclopentadienyl ketene 10.94 is a concerted reaction in which the elimination of nitrogen and the rearrangement take place simultaneously. In the opinion of the present author the theoretical result for 1,2-quinone diazide is not necessarily in contradiction to the experimental investigations of Sander, Yankelevich et al., and Nakamura et al., as the reagents used were not exactly the same. 

Radical ions - charged species with unpaired electrons - are easily generated by a number of methods that are discussed in more detail below. Their properties have been characterized by several spectroscopic techniques, and their structures and spin density contributions have been the subject of molecular orbital calculations at different levels of sophistication. The behaviour of radical ions in rearrangement and isomerization reactions as well as in bond-cleavage reactions has been extensively studied [for recent reviews see Refs. 11-13 and references cited therein]. Useful synthetic applications, such as the radical-cation-catalyzed cycloaddition [14-20] or the anfi-Markovnikov addition of nucleophiles to alkenyl radical cations [21-25], have been well documented. In... 

Addition of DBr to 1,3-cyclohexadiene occurs by a 1,2-anti process and a 1,4-syn process,133 a conclusion also predicted by molecular orbital calculations.134 Stereospecific allylic rearrangement of the 1,2-trans product, however, eventually produces a reaction mixture containing predominantly cis products. 

The characteristic features of hydroboration of alkenes—namely, regioselec-tivity, stereoselectivity, syn addition, and lack of rearrangement—led to the postulation of a concerted [2 + 2] cycloaddition of borane353,354 via four-center transition state 37. Kinetic studies, solvent effects, and molecular-orbital calculations are consistent with this model. As four-center transition states are unfavorable, however, the initial interaction of borane [or mentioned monobridged dimer, Eq. (6.56)] with the alkene probably involves an initial two-electron, three-center interaction355,356(38, 39). 

Several alternative transition structures for the [2.3]-Wittig rearrangements (Z)-(48) —> threo- 49), and ( )-(50) — erythro-(51), have been explored by ab initio molecular orbital calculations at the 6-31G level in an attempt to explain why the observed stereo selection is opposite to that for alkenes which do not bear a 1-carboxylic group.49 It has been concluded that coordination of lithium cation to two oxygen atoms and the C(4) carbon plays a significant role in reactions of (48) and (50), thereby making it easier to break the 0(2)—C(3) bond. 

Ab initio molecular orbital calculations on these systems have been confined to the 1,2,3-triazolo[4,5-d]pyrimidines (7), the so-called 8-azapurines , and references to this subject may be found in the previously mentioned review <86AHC(39)ii7>. In 1989, quantum mechanical perturbation methods have been used to study the activity of 8-azapurine nucleoside antibiotics in transcription processes <89Mi 7i3-oi>. The l,2,3-thiadiazolo[5,4-d]pyrimidine derivative (51), a rearrangement product of 8-aza-6-thioinosine, has been used in a molecular modeling study of the antitumor activity of sugar derivatives of pyrimidopyrimidines <89PNA(86)8242>. 

Silvestre and Hoffmann have considered the conversion of ruthenium acetylide complexes to the corresponding vinylidene species using extended Hiickel molecular orbital calculations (69). Although the rearrangement of free acetylene to its vinylidene isomer is thermodynamically disfavored, their results indicate that the transformation becomes thermodynamically... 

The problem is conventionally sidestepped by assuming that nuclear and electronic motions are decoupled, but despite many efforts this condition has never been shown to yield a rigid molecular shape either. The insurmountable problem is permutational invariance. In molecular-orbital calculations that decouple electronic from nuclear motion the nuclei are identified in order to support the definition of molecular structure, but then permutation of identical nuclei implies rearrangement of bonds and a new set of calculated electronic energies. There is little hope of ever overcoming these problems ... 

The evidence for the mechanisms of the mass-spectrometric and photochemical reactions leading to the eliminations of an olefin from a ketone [equation (120)] have been summarized (Section VIIDl). If it is accepted that the structure of the fragment ion from this process has an enolic structure, it is possible to discuss the mechanism of the reaction theoretically. The reaction appears to consist of two parts, first the transfer of hydrogen and second, the elimination of olefin. There has been considerable conjecture as to whether these parts of the mass-spectrometric McLafferty rearrangement are stepwise or concerted. Prom their self-consistent field calculations, Boer et al. (1968) have concluded the reaction is step-wise. From perturbation and valence-electron molecular orbital calculations, Dougherty (1968b) has concluded the reaction is concerted. The above results depend on the adjustable parameters fed into the equations one set of parameters may eventually prove to be better. 

Csizmadia, I. G., Gunning, H. E., Gosavi, R. K., Strausz, O. P. Mechanism of the Wolff rearrangement. V. Semiempirical molecular orbital calculations on a-diazo ketones, oxirenes, and related reaction intermediates. J. Am. Chem. Soc. 1973, 95, 133-137. 

KEY WORDS PIXE, X-ray satellite. Electronic structure. Molecular orbital calculation, DV-Xo, Resonant orbital rearrangement... 

Hiickel molecular orbital calculations have shown that the 5-position in this ring system has associated with it a greater tt-electron density than is present in the corresponding positions in s-triazolo[4,3-a]pyridine, s-triazolo[4,3-a]pyrimidine, and s-triazolo[4,3-c]pyrimidine. This fact has been used to rationalize the difficulty in rearranging this heterocycle to its s-triazolo[l,5-a] analogue, in contrast to the ease with which the other s-triazolo systems rearrange. 

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