Orbitals overlap, cyclopropane

In the interaction of the local 2pv orbitals, two more bonding molecular orbitals are formed against one less bonding. In all previous cases the opposite occurred. This is due to the negative overlap between adjacent 2py orbitals—whether, by convention, all positive lobes point in the clockwise direction, or whether all positive lobes point in the anticlockwise direction. The two bonding 2pv combinations in fact fall below the two antibonding (hybrid 2s, 2px) combinations. The former each have two electrons while the latter are empty. The six electrons of the three C—C bonds are nicely accounted for. The method creates simultaneously the acc and or c molecular orbitals of cyclopropane (note that the latter three lie relatively close in energy). 

FIGURE 4.6 Orbital overlap in cyclopropane. The arrows point toward the center of electron density. 

Figure 4.10 (a) Orbital overlap in the carbon-carbon bonds of cyclopropane cannot... 

The C—C bonds of cyclopropane are bent => orbital overlap is less... 

We rationalize the involvement of the less substituted lateral bond of 23 instead of its more highly substituted internal bond as reflecting the need for orbital overlap. The bicyclohexene system has limited mobility, allowing the lateral cyclopropane bond more significant orbital overlap with the alkene p-orbitals (23 ). The partici-... 

Significantly less regioselectivity was observed for benzobicyclo[3.1.0]hexene (47). The isolated products, methanol adducts and NOCAS products of structures 48 and 49, suggest that nucleophilic attack occurs at C5 and C6. These results were explained by the orbital overlap between the benzene ring and the 3 °-3° versus 3 -2 cyclopropane orbitals. ... 

Problem 9.6 Account for the ring strain in cyclopropane in terms of geometry and orbital overlap. ... 

Problem 9.25 Although cyclopropanes are less reactive than alkenes, they undergo similar addition reactions, (fl) Account for this by geometry and orbital overlap, (b) How does HBr addition to 1,1-dimethylcyclopropane resemble Markovnikov addition <... 

Because of the ring strain in cyclopropane (Problem 9.6), there is less orbital overlap (Fig. 9-1) and the sigma electrons are accessible to attack by electrophiles. 

Schier and Schmidbaur93 performed a clever experiment that addressed part of this question does the orientation of the carbanion relative to the phosphorus atom play any role Scheme 2 shows two syntheses of ylides involving cyclopropyl substituents. In the first reaction, since the pKa of cyclopropane is considerably below that of propane, the expected product is the cyclopropylide. However, the isopropylide is the only recovered product. The second reaction also demonstrates the avoidance of the cyclopropylide product. The cyclopropylide possesses a very pyramidal carbanion that is directed away from phosphorus, allowing for minimal orbital overlap. The isopropylide is much less pyramidal and phosphorus can better assist in stabilizing the carbanion. While this stabilization does not require explicit orbital overlap (the electrostatic interaction of the carbanion with the onium is expected to be smaller in the cyclopropylide since it is directed away from P), it does suggest that some orbital interactions are involved. Hence, although the ylene contribution is small, it is unlikely that the ylene contribution is nil. 

In these C-H insertion reactions, the similarity with cyclopropane formation by intramolecular cycloadditions to alkenes is clear, and the mechanisms mirror one another quite closely. As with the cyclopropanation reactions, the path of the reaction differs according to whether the carbene is a singlet or triplet. Singlet carbenes can insert in a concerted manner, with the orbitals overlapping constructively provided the carbene approaches side-on. 

This distortion is obviously caused by conjugation of the cyclopropane ring with the C=C double bond. This leads to a maximum interaction (orbital overlap) [50] in the bisected conformation which is apparent in the crystal structure of 3. 

Cyclopropyl ketones (29) behave as weakly conjugated compounds, with significant orbital overlap between one ty-bond (or sometimes two) of the cyclopropane ring and the jr-orbital of the carbonyl group (p. 242). The three-membered ring is reduced by lithium in liquid ammonia, with rupture of whichever cr-bond most effectively overlaps the carbonyl tt-orbital [280]. The reduction probably involves transfer of an electron to the jS -carbon atom of this cr-bond, so that conjugative overlap can be maintained during a smooth transition to the species (30) which is both a carbanion and a resonance-stabilised enolate radical. A second electron trans-... 

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 effects observed for bicyclo[3.1.0]hex-2-ene, 86, were particularly clear-cut, because the H spectrum is fully resolved [218]. The polarization pattern supported a species with spin-density on C3 and C6, indicating the delocalization of spin and charge into the lateral cyclopropane bond. The bicyclohexene system has limited mobility, enabling more significant orbital overlap of the lateral cyclopropane bond with the alkene p-orbitals (86 +). The participation of the lateral bicyclohexene bond is supported by ab initio calculations, carried to the MP2/6-31G level of theory. The lateral cyclopropane bond is lengthened (C1-C6 = 1.748 A), and carbons C3 and C6 carry prominent spin densities, whereas lower spin densities were found at C2 and Cl [220]. 

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