Out-of-phase interactions

The carbonyl n orbital is also assumed to be unsymmetrized arising from the out-of-phase interaction of the orbital attached to the more electron-donating aryl group (9 and 10). These unsymmetrizations of the carbonyl k orbital correspond well to syn addition (9) and anti addition (10), respectively. Thus, the electron-donation of the p-a orbitals controls the facial selectivities. The cyclopentane system was more sensitive to stereoelectronic effects, showing larger induced biases, than the adamantanone system [63]. 

Thus, the predictions seem to be in conflict with the observed syn biases. However, along the trajectory of attack of the nucleophile to the carbonyl group of the bicyclo[2.2.2]octane structures (indicated in 23 and 24), out-of-phase interactions between the reagent and the substrate are involved, and this is different from the situation in the bicyclo[2.2.1]heptane structures (15a) [83-87]. Thus, attack on the side opposite to the unsaturated moiety will be favored. This is a kind of SOI (Fig. 3a) which unsymmetrizes the n face. 

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. 

The unsymmetrization of the nitrogen non-bonding orbital (n ) was due to out-of-phase interaction of the electron-rich a orbital at the (3 positions (115), which leads to syn addition. Furthermore, in-phase interaction of the nitrogen non-bonding orbital (n ) with the low-lying vacant a orbitals (116, due to the electron-withdrawing CF3 groups) can contribute to the syn preference. 

Ohwada extends his theory, unsymmetrization of n orbitals, to Orbital Phase Environment including the secondary orbital interaction (Chapter Orbital Phase Environments and Stereoselectivities by Ohwada in this volume). The reactions between the cyclopentadienes bearing spiro conjugation with benzofluorene systems with maleic anhydride exemplified the importance of the phase environment. The reactions proceed avoiding the out-of-phase interaction between dienophile LUMO and the HOMO at the aromatic rings. The diene 34 with benzo[b]fluorene favored syn addition with respect to the naphtalene ring, whereas the diene 35 with benzo[c]fluorene showed the reverse anti preference (Scheme 22) [28]. 

The reactions of 5-aryl-5-phenylcyclopentadiene 23-25 occur on the anti side of the more electron rich aromatic system [19], The selectivity is also consistent with the orbital phase environments. The dienophiles avoid stronger out-of-phase interaction with the aromatic ring with higher HOMO. Halterman and coworkers ascribed the selectivity to the Cieplak effect (Scheme 23). 

The propertiesof DABCO (l,4-diazabicyclo[2.2.2]octane) are of considerable interest. The in- and out-of-phase interaction of the two occupied sp3 hybridized lone pairs on the two nitrogen atoms may be described by an interaction diagram such as shown in Figure 3.5b. 

As with the chloropalladation reaction (vide supra)m the rearrangement of >/2-methyl-enecyclopropane to >/4-TMM was shown experimentally to proceed stereoselectively by disrotatory ring cleavage of the distal frontier molecular orbital considerations, which predict that the out-of-phase interaction between the metal orbital in the distal ring-opening of -methylenecyclopropane complexes can be minimized by bending the bond up away from the metal (equation 348)410 ... 

Substitution of the hydrogens of the C=CH2 group with methyl groups increases the frequency, due in part to out of phase interaction with the attached C—C bonds. 

All the above cyclobutene double bonds are equally strained, but the added out-of-phase interaction with the noncyclic C—C bonds increases the frequency. 

Cyclopropenones have a band at 1865-1840 cm due mainly to the carbonyl stretch, but with some out-of-phase interaction with the cyclopropene ring C=C bond. A second band at 1660-1600 cm is due mainly to the ring C=C stretch but with some in-phase interaction with the C=0. ... 

The C=0 stretch vibration involves some out-of-phase interaction with the C—N stretch. An electron withdrawing group on the nitrogen will reduce the contribution form 2, and a higher frequency carbonyl band will result. In unsubstituted amides, C—CO—NH2, a vibration involving the C—N stretch absorbs somewhere near 1400 cm . A weaker band somewhere near 1150 cm" can sometimes be seen which involves the NH2 rock (in plane). A broad band at 750-600 cm" is due to NH2 wag (out of plane). 

Figure 11-4 Compare this picture of the formation of the tt bond in ethene with Figure 11 -3. in-phase interaction between two paraiiei p orbitais (containing one eiectron each shown in biue) resuits in positive overiap and a fiiied bonding tt orbital. The representation of this orbital indicates the probability of finding the electrons between the carbons above and below the molecular plane. Because rr overlap is less effective than o-, the stabilization energy, is smaller than tsE. The rr bond is therefore weaker than the o- bond. The out-of-phase interaction results in the antibonding molecular orbital tt. ...

---