Interaction back-lobe

Interactions of C-Cl dipoles with iT-electrons in highly chlorinated bicy-clo[2.2.1]heptenes such as 226 were invoked to explain the chemical shifts of C(2), C(3), and C(7) (358). Similarly, remarkably large upheld ya-SCS(OR) values in 227 (n = 2 to 4) were attributed to a back-lobe interaction of the C(l)-O bond with the p-orbital of C(3) in the distorted double bond (cf. 228) (359). 

The cis isomer 52 was found to react about 450 times faster than its unsilylated analogue 51 in 97% trifluoroethanol. However, the turns isomer 53 showed essentially no acceleration. The secondary deuterium isotope effect observed with the fi tetradeuterated analogue of the cis isomer 52 confirmed that the molecule reacts via the diequatorial conformation. In this conformation the back lobes of the Si—C bond at the 3-position can interact with the developing p orbital at the 1-position. This through-space interaction is often referred to as homohyperconjugation. 

The nature of the frontier orbitals is also of interest and a Mulliken population analysis of the constituent orbitals establishes that the highest occupied molecular orbital (HOMO) of each phosphine consists primarily of a lone pair sp hybrid on phosphorus. The orbital energy ordering PMe3 < PH3 < PF3 also parallels the percentage phosphorus s-character of the HOMO of PMe3 (11% s and 60% p), PH3 (14% s, 67% p) and PF3 (29% s, 32% p). In each case the back lobe of the sp hybrid interacts with the substituent attached to phosphorus in a (T-bonding fashion. 

Due to the above differences, stereoelectronic effects in O- and S-heterocycles cannot be automatically transferred to the N-heterocycles and vice versa. Differences in hybridization were shown to be important for stereoelectronic hyperconjugative interactions. ° Not only is hybridization of a donor orbital related to its absolute energy, but hybridization determines the direction in which non-bonding orbitals are projected in space. Consequently, directionality of sp" hybrids can change considerably as a function of substitution. Furthermore, hybridization controls the relative size of the two lobes of a lone pair. The front and back lobes... 

As shown in Fig. 1, the big lobes of these hybrids point toward each other. Therefore, if the nucleophile approaches the substrate from the front side, its HOMO overlaps in phase with the big lobe of 0c and out-of-phase with the big lobe of 0x-Numerical calculations show that the unfavourable (nucleophile-leaving group) interaction usually overrides the favourable (nucleophile - reaction center) interaction in this front-side approach, so that back-side attack is finally preferred, leading to inversion of configuration. 

This pathway is favorable because the lowest empty molecular orbital of the cy. lic methyl ethylene phosphate ester consists principally of lobes on the back sides of the P-0 ring bonds which can readily interact with the approaching electron pair from water. 

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