Maximum orbital overlap

Radicals with adjacent Jt-bonds [e.g. allyl radicals (7), cyclohexadienyl radicals (8), acyl radicals (9) and cyanoalkyl radicals (10)] have a delocalized structure. They may be depicted as a hybrid of several resonance forms. In a chemical reaction they may, in principle, react through any of the sites on which the spin can be located. The preferred site of reaction is dictated by spin density, steric, polar and perhaps other factors. Maximum orbital overlap requires that the atoms contained in the delocalized system are coplanar. 

Carbanions which have substituents capable of conjugative delocalisation of the electron pair will perforce be planar (sp2), in order to allow the maximum orbital overlap of the p orbital with those of the substituent, e.g. (4) and (10) ... 

This system covers concerted reactions of the n electron systems on two reactants to form new a bonds yielding carbocyclic rings with a single unsaturation. If the reaction follows the rule of maximum orbital overlap, then it is a suprafacial, suprafacial process and is termed a [,r4 + r t] reaction. By the Woodward-Hoffmann rules this is a symmetry-allowed thermal reaction [13]. 

There are four possible transition states in the reduction of 27 wherein maximum orbital overlap can be maintained with respect to the attacking hydride reagent and the developing electron pair on nitrogen. Two of these (cf. dotted arrow in 29 and 30) require boat-like transition states in order to satisfy the stereoelectronic requirements and are unfavorable kinetic-ally. Of the two possible chair-like transition states (cf solid arrow in 29 and 30) the latter suffers from a strong steric interaction between the nucleophile and the C-8 pseudo-axial hydrogen. The process 30 + 32 is thus disfavored by comparison with the process 29 31. 

The 1,6-addition to a,s, y, a-dienones is also subject to stereoelectronic effects. Addition on the bottom face of dienone 137 leads to a chair-like intermediate while that on the top face leads to a boat-like intermediate 140 in order to maintain maximum orbital overlap. Also, in 140 the R group encounters an eclipsed 1,2-R/H interaction and more importantly, a 1.4-CH3/R steric interaction which resembles the bowsprit flagpole arrangement of a twist-boat form of cyclohexane. This analysis of Marshall and Roebke (48) predicts that the trans product 139 should prevail over the cis product 141. 

Fig. 14. The electronic transfer is strongly adiabatic if the orbital interactions are strong. This is the case when the separation distance is small or the orientation of the two reactants results in maximum orbital overlap. In contrast, if the separation distance is large, the electron transfer is nonadiabatic...

In molecular orbital terms, the unshared /r-electrons at the carbanion site are donated into the a orbital of the adjacent C-F bond when the orbitals are in an anfi-periplanar configuration which ensures maximum orbital overlap [19, 20]. 

The bonding in vinylcyclopropane (3) is such that an s-trans-gauche conformational equilibrium exists to allow for maximum orbital overlap of the asymmetric component of cyclopropane orbitals with the IT- or 7T -orbitals of the ethylene unit, as shown in (3a). From thermochemical stuches it appears that conjugation of an alkene with cyclopropane stabilizes the system by 1.2 kcal mol". The conformational equilibrium for vinylcyclopropane was shown to consist of an s-trans minimum (3b) and two gauche conformers that are equ in energy and destabilized by 1.43 kcal mol with respect to the s-trans conformation. The barrier to interconversion has been determined to be 3.92 kcal mol . ... 

The relative rates of concerted ring opening for substituted cyclopropyl tosylates can be explained on the basis of the selection rules and the principle of maximum orbital overlap. The relative rates of solvolysis for two different dimethylcyclopropyl tosylates are shown in the accompanying diagram ... 

In a pericyclic reaction, the pathway predicted by the selection rules is the one that allows maximum orbital overlap along the reaction pathway, including the transition state. Maximum orbital overlap corresponds to the path of minimum energy and is achieved if the orbitals involved are similar in energy and if the symmetry of the orbitals is maintained throughout the reaction path. 

In Eq. (45) unimolecular reaction rates are introduced. They arise from the assumption of similar structures of the precursor complex for all three reaction channels, which seems to be allowed in the view of the minimum energy and maximum orbital overlap rules. The reaction rate, 3, corresponding to population of the excited triplet, is multiplied by the statistical factor relating to spin multiplicity. 

Existing evidence indicates that C-alkylations of metal enolates with common electrophiles proceeds by an SN2-type mechanism that is, the highest occupied molecular orbital (HOMO) of the enolate attacks the lowest unoccupied molecular orbital (LUMO) of the alkylating agent. Scheme 16 illustrates the principle of stereoelectronic control, which states that the electrophile should approach in a plane perpendicular to the enolate to allow maintenance of maximum orbital overlap in the transition state (24) between the developing C—C bond and the ir-orbital of the carbonyl group. 

Planar cis and trans isomers (Figure 2.1(a)) are the most stable configurations, because the planar structure involves maximum orbital overlap. For the majority of peptides built up from a-amino acids, the amide bond adopts the trans geometry, a-Imino acids (notably proline but also /V-methylamino acids), as well as a-methyl-a-... 

Brinker et al. have studied the insertion of dihalocarbenes into the C-H bonds of bicyclo[3.1.0]hexane and bicyclo[4.1.0]heptane derivatives, both experimentally and theoretically. The reactivity of a specific C-H bond was shown to depend on the ability of the effective delocalization by the alkyl moiety of the partial positive charge that builds up during the insertion step (141 and 142). As a consequence, all factors stabilizing the corresponding carbocation would stabilize the transition state. For example, the insertion of dichlorocarbene into cyclopropane derivatives occurs regio- and stereospecifi-cally [Eqs. (6.129) and (6.130)]. The predominant formation of products 145 and 146, that is, the insertion of CbC into the exo and endo a-C-H bonds, respectively, results from the maximum orbital overlap of the Walsh orbital of the cyclopropane ring and the cx-C-H bond of suitable steric arrangement. 

A single C-C o-bond can undergo free rotation at room temperature, but a Jt-bond prevents free rotation around a C=C bond. For the maximum orbital overlap in a Jt-bond, the two p-orbitals need to be parallel to one another. Any rotation around the C=C bond will break the Jt-bond. 

For maximum orbital overlap attack at 90° is required. However, attack at -107° is observed because of the greater electron density on the carbonyl oxygen atom, which repels the lone pair on H2O... 

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