Frontier Orbital Considerations

A knowledge of the composition of frontier orbitals gleaned from molecular orbital theory provides helpful insights into the structures and formation of 

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According to these consideration the diamino-substituted phosphenium (an alternative suggestion for its nomenclature is phosphanylium) cation, 5, and the phosphanetriylammonium (iminophosphenium) cation, 6, possess the largest intrinsic (gas phase) stabihties. Since in the X-ray structures the molecules are to a first-order isolated, this theoretical stability scale determined for the gas phase should also mimic the various trends of the stabilities of the cations and their chelation behaviour. The methylenephosphenium, 7, and the PjH cations, 8, suffer from poor stabihties. On the other hand the phosphirenium cation, 11, is considered to be fairly well stabilized. It is due to n-electron delocalisation of the positive charge in the phosphirenium cation. Intermediate cases in stabihty are the PO+ (9) and PS+ cations (10). Of further interest are the frontier orbital considerations, as shown in Fig. 2. 

Chemical Reactivity Aspects and Frontier Orbital Considerations... 

Theoretical calculations have also permitted one to understand the simultaneous increase of reactivity and selectivity in Lewis acid catalyzed Diels-Alder reactions101-130. This has been traditionally interpreted by frontier orbital considerations through the destabilization of the dienophile s LUMO and the increase in the asymmetry of molecular orbital coefficients produced by the catalyst. Birney and Houk101 have correctly reproduced, at the RHF/3-21G level, the lowering of the energy barrier and the increase in the endo selectivity for the reaction between acrolein and butadiene catalyzed by BH3. They have shown that the catalytic effect leads to a more asynchronous mechanism, in which the transition state structure presents a large zwitterionic character. Similar results have been recently obtained, at several ab initio levels, for the reaction between sulfur dioxide and isoprene1. ... 

Padwa et al. (44) studied the diazo-decomposition of 119 and found that the cyclic ylide 120 could be trapped by a variety of heterodipolarophiles such as ethyl cyanoacetate (Mander s reagent) to provide aminal 121 or with benzaldehyde to generate the bicyclic acetal 122. In both cases, only a single isomer was formed, with the regiochemistry easily predicted from frontier orbital considerations. Nair et al. (45) were able to employ the highly functionalized o-quinone 125 for the trapping of carbonyl ylide 124 to provide the highly complex cycloadduct 126 in 76% yield. 

These methods appear rather simple, yet they were the starting point of a long evolution. Gilles Klopman, whose research interests at Case-Western Reserve University later turned to modeling bioactive molecules, was the first to use Sandorfy s methods. Kenichi Fukui made extensive use of them in his well-known work on the structures and reactions of saturated hydrocarbons and their derivatives. Fukui added his frontier orbital considerations. Around 1959 the milieu of developments in quantum chemistry contributed to inspire William N. Lipscomb to conceive the extended Hiickel method, which was subsequently implemented by Lawrence L. Lohr and Roald Hoffmann.83 Soon thereafter, John Pople and his coworkers introduced self-consistent field methods based on the zero-differential overlap approximation.815... 

What I have tried to do in this book and the published papers behind it is to move simultaneously in two directions—to form a link between chemistry and physics by introducing simple band structure perspectives into chemical thinking about surfaces. And I have tried to interpret these delocalized band structures from a very chemical point of view—via frontier orbital considerations based on interaction diagrams. 

This point may appear to be semantics to one interested primarily in the connection between stoichiometry and geometric structure however, for more detailed considerations the model used matters. For example, the two-center-two-electron bond model with a lone pair on P suggests electrophilic attack at a single P atom, e.g., protonation leading to a P-H terminal bond. On the other hand, in the delocalized MO model the degenerate e symmetry HOMOs of P4 are centered on the P-P edges. Frontier-orbital considerations, then, suggest electrophilic attack would... 

They react easily with electrophiles and add nucleophiles at C-6. In cycloaddition reactions they may react as 2jt, 4n, or 6 i compounds. According to frontier orbital considerations they readily react with electron-deficient dienophiles (e.g., silenes) in Diels-Alder reactions this is due to the strong interaction between the fulvene HOMO and dienophile LUMO [9]. Although the n and n orbitals of silenes are generally 1-2.5 eV higher in energy than is the case for the alkene congeners [10] a normal [4+2] cycloaddition behaviour for 3 is observed in earlier works [3-5]. 

This frontier orbital consideration fits well with our investigations [11] Reaction of 3 with pentafulvene 8 [12] leads to the [4+2] exo/endo-isomers 9. Compared to reactions of 3 with cyclopentadiene [3] it is surprising that the E/Z [2+2] cycloadducts 10 are formed exo/endo E/Z = 45/25 29/11) additionally. 

In agreement with frontier orbital considerations, crotonic esters add to nitrones to produce p-o o esters regioselectively. This reaction provides the basis for a synthesis of the Senecio alkaloid supinidine (7). Reaction of 1 with methyl 4-hydroxycrotonate yields the isoxazolidinone 5, which is easily converted to the... 

Radical reactions can often be rationalized on the basis of frontier orbital considerations for intermediate radical species, the reactivity and stereochemistry of which can certainly be regulated with Lewis acid additives [21-23]. The first appearance of Lewis acids in radical reactions was in polymerization reactions resulting in alternation of copolymers different from that obtained without Lewis acids [24-26]. This concept, Lewis acid-directed radical reactions, has been applied to reductions and alkylations of organic halides or olefins, and has resulted in highly stereospecific processes. 

In all these reactions, frontier orbital considerations explain the observations. Let us take one of them, the [3,3] sigmatropic rearrangement (151 - 152) known as the Cope rearrangement. As usual we simplify the reaction to the minimum (153— 154), and the problem is to identify the components with... 

This chapter describes the structures of aromatic heterocycles and gives a brief summary of some physical properties. The treatment we use is the valence-bond description, which we believe is appropriate for the understanding of all heterocyclic reactivity, perhaps save some very subtle effects, and is certainly sufficient for a general textbook on the subject. The more fundamental, molecular-orbital description of aromatic systems is less relevant to the day-to-day interpretation of heterocyclic reactivity, though it is necessary in some cases to utilise frontier orbital considerations, however such situations do not fall within the scope of this book. 

Table 11.21).3 3 The trans cycloadduet was the major produet in all eases but the selectivity of the cycloaddition varied with reaction conditions, generally favoring 371. The ratio of dimers was independent of the method of triplet generation, also independent of the concentration of the sensitizer and quencher and was unaffected by tbe presence of excess reactant. Tbe solvent bad an effect, and tbe concentration effect was more pronounced in nonpolar solvents. Tbe regiocbemistry of tbe cycloaddition is controlled by tbe orbital coefficients. Interaction of tbe largest coefficients predicts tbe regiocbemistry. Tbe cbemoselectivity can also be predicted by frontier orbital considerations.

In the next section, the basic features of decomposition schemes for bond energies will be discussed. In particular, we wish to stress the importance of other interactions than the charge-transfer type HOMO-LUMO interactions commonly employed in frontier orbital considerations. Important contributions arise also from polarization of the interacting units, "especially in the highly polarizable metallic substrates, and from repulsive interaction with occupied (sub-)valence shells contributing to the steric repulsion. The latter interaction in fact determines the bond distances as it is responsible for the inner, repulsive, part of the potential energy curves. 

As shown in Fig. 7.14, in the CH Activation by an ES pathway the coordination of the CH bond of the alkane to an electrophilic center followed by loss of a proton can also be described as a nucleophilic attack of sol on the coordinated CH bond, leading to CH cleavage and generation of an intermediate E-CH3 species. Of course, it would be expected that as the CC double bond is considerably more electron rich than an alkane CH bond, more reactive electrophi-lies would be required for similar coordination and cleavage of the CH bond. Given the low energy, [Pg.250]

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