Orbital moiety

Krivdin and co-workers ° applied a double perturbation theory (DPT) at the second order level of approximation formalism to examine the dihedral angle dependence of the Fermi-contact (FC) contribution to nuclear spin-spin coupling constants. The authors have derived an analytical expression relating the FC term of /(H,H) across the aliphatic single carbon-carbon bond to the dihedral angle describing inner rotation around the C-C bond in the ten-electron ten-orbital moiety H-C-C-H. In particular, the authors have shown that extrema of H,H) are observed at q> = nn [n = 0, 1, 2,...), which provides a theore-... 

The regioselectivity benefits from the increased polarisation of the alkene moiety, reflected in the increased difference in the orbital coefficients on carbon 1 and 2. The increase in endo-exo selectivity is a result of an increased secondary orbital interaction that can be attributed to the increased orbital coefficient on the carbonyl carbon ". Also increased dipolar interactions, as a result of an increased polarisation, will contribute. Interestingly, Yamamoto has demonstrated that by usirg a very bulky catalyst the endo-pathway can be blocked and an excess of exo product can be obtained The increased di as tereo facial selectivity has been attributed to a more compact transition state for the catalysed reaction as a result of more efficient primary and secondary orbital interactions as well as conformational changes in the complexed dienophile" . Calculations show that, with the polarisation of the dienophile, the extent of asynchronicity in the activated complex increases . Some authors even report a zwitteriorric character of the activated complex of the Lewis-acid catalysed reaction " . Currently, Lewis-acid catalysis of Diels-Alder reactions is everyday practice in synthetic organic chemistry. 

Concerted cycloadditions are observed with heterocyclics of all ring sizes. The heterocycles can react directly, or via a valence tautomer, and they can utilize all or just a part of unsaturated moieties in their rings. With three-membered rings, ylides are common reactive valence tautomers. Open chain 47T-systems are observed as intermediates with four-membered rings, and bicyclic valence tautomers are commonly reactive species in additions by large rings. Very often these reactive valence tautomers are formed under orbital symmetry control, both by thermal and by photochemical routes. 

Diene moieties, reactive in [2 + 4] additions, can be formed from benzazetines by ring opening to azaxylylenes (Section 5.09.4.2.3). 3,4-Bis(trifluoromethyl)-l,2-dithietene is in equilibrium with hexafluorobutane-2,3-dithione, which adds alkenes to form 2,3-bis-(trifluoromethyl)-l,4-dithiins (Scheme 17 Section 5.15.2.4.6). Systems with more than two conjugated double bonds can react by [6ir + 2ir] processes, which in azepines can compete with the [47t + 27t] reaction (Scheme 18 Section 5.16.3.8.1). Oxepins prefer to react as 47t components, through their oxanorcaradiene isomer, in which the 47r-system is nearly planar (Section 5.17.2.2.5). Thiepins behave similarly (Section 5.17.2.4.4). Nonaromatic heteronins also react in orbital symmetry-controlled [4 + 2] and [8 + 2] cycloadditions (Scheme 19 Section 5.20.3.2.2). 

The bis(diene) (46) adds dienophiles preferentially on the side syn to the oxirane moiety (Scheme 35) (80X149). This may be due to formation of a charge-transfer complex by donation of electron density from oxygen into an antibonding orbital on the dienophile. 

In many of their complexes PF3 and PPI13 (for example) resemble CO (p. 926) and this at one time encouraged the belief that their bonding capabilities were influenced not only by the factors (p. 198) which affect the stability of the a P M interaction which uses the lone-pair of elecU"ons on p and a vacant orbital on M, but also by the possibility of synergic n back-donation from a nonbonding d , pair of electrons on the metal into a vacant 3d , orbital on P. It is, however, not clear to what extent, if any, the a and n bonds reinforce each other, and more recent descriptions are based on an MO approach which uses all (cr and n) orbitals of appropriate symmeU"y on both the phosphine and the metal-containing moiety. To the extent that a and n bonding effects on the stability of metal-phosphorus bonds can be isolated from each otlier and from steric factors (see below) the accepted sequence of effects is as follows ... 

Tile basic chromophors of indigo and of some tetraazafulvalenes are very similar and therefore the frontier orbitals are comparable (90JPC949). PPP calculations on the 1,3,5,7-tetraazafulvalene 90 (R = Me) showed a good agreement with its UV/VIS spectrum. Tlie substitution of both methyl-sulfanyl groups in 90 by diethylamino/piperidino moieties led to a hypso-chromic shift of about 45 nm. 

Accordingly, the CO moiety acquires negative charge. The consequent exigencies of the electroneutrality principle are then met by the CO group donating this charge back to the metal via its now expanded <7-donor orbital ... 

Dibenzobicyclo[2.2.2]octadienones (34) bearing an aromatic substituent were designed to probe the unsynunetrization of the carbonyl it orbital arising from the aromatic % orbitals [103,104], Reduction of the carbonyl moiety of 2- (R H) and 3-substituted (R3 H) dibenzobicyclo[2.2.2]octadienones (34) was studied by using sodium borohydride in methanol at - 3 °C. The 2- (34a) and 3-nitrodibenzo-bicyclo[2.2.2]octadienones (34d)... 

We postulate that the attack on both sides is accelerated by positive SOI (89a), but an unfavorable orbital interaction along the syn attack trajectory (89b) cancels the acceleration at the syn face [151] as the diene approaches the anhydride moiety (preferentially in endo fashion), unfavorable out-of-phase interaction (SOI) of the n lobes at Cj and of the diene with the tt lobes of the aromatic moiety of the dienophile occurs (89). The unexpected anti-selectivity stems from nnfavorable SOI on the syn side. 

In the presence of out-of-phase spiro interaction of the dienes, there was also a large subsidiary in-phase orbital interaction on the naphthalene moiety of 93a (94). The syn addition observed in the case of 93a can be accounted for in terms of the additional intervention of this SOI. 

When the carbonyl groups are present, the transition state for syn attack is sta-bihzed by interactions between the in-phase combination of the NN lone pairs and the antisymmetric n orbital of the CO-X-CO bridge (100). Although the secondary effect (SOI) operates only during syn approach and contributes added stabilization to this transition state, the primary orbital interaction (see 103) between the HOMO of the cyclohexadiene moiety of 100 and the n orbital of the dienophile (NN, Fig. 16) is differentiated with respect to the direction of attack, i.e., syn or anti, of triazolinedione (NN, Fig. 16). 

The substituent effect of the aromatic nitro group can be accounted for in terms of n orbital unsymmetrization. The LUMO of the dibenzobicyclic lactone (106) can be analyzed as an in-phase combination of three vacant ir orbitals, i.e., those of benzene, nitrobenzene and the 2(5//)-furanone moiety. The energetically... 

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