Methylene hyperconjugation effects

The UV photoelectron spectra of several chromans have been compared with other cyclic and acyclic ethers. The chroman spectra are clearly distinguishable from those of the other ethers and the differences have been interpreted in terms of the decreased conjugative effects of oxygen and increased hyperconjugative effects of the 4-methylene group (76T167). 

The importance of hyperconjugative effects on A sx was rejected on the basis of various GVB approaches and Mulliken population analyses (see Electronic Wavefunctions Analysis) for methylcarbene. It was concluded that it is not the singlet state stabilization by the methyl group but rather the triplet state destabilization which results in a smaller singlet-triplet separation with respect to methylene. The GVB Mulliken population analyses show that the hybridizations of the a orbitals are almost identical in the singlet states of methylene and methylcarbene. For the triplet states, however, Mulliken population analyses indicate a decrease in p character when... 

Concerning steric factors, 43 is attacked in the most hindered position ( inverse effect of substitution ) likewise, 39 is attacked at the most hindered carbon. Obviously, the transition states for the formation of 44 or 50 show limited sensitivity to the degree of substitution, and the relief of ring strain is a more significant factor than the steric hindrance in the transition state. On the other hand, steric factors are important in systems such as P-phellandrene radical cation 40 which is attacked at the xo-methylene carbon (most easily accessible), or the tricyclane radical cation 56 which is attacked at the less hindered 3° carbon further removed from the dimethyl-substituted bridge (approach a). Both reactions also benefit Irom the formation of the most highly substituted, hyperconjugatively stabilized free radicals. 

A. Methyl and methylene groups alpha to carbonyl systems nndergo weak hyperconjugation, which weakens the C H bond and drops the frequencies. The effect is best observed on the symmetric bending modes. 

There is a prevalent misconception that hyperconjugation with the CH bonds of a methylene group can only be effective when one is above and the other below the nodal plane. This is of course true of the antisymmetric combination of orbitals (< >a — 0b), but not of the symmetric combination (0a -I- 0b), which is not excluded from entering into hyperconjugation with a ir-electron system unless it is either in the nodal plane or perpendicular to it. 

The striking feature of Table XIII is the dramatic decrease in the effect of methylene deuteration in the most sterically hindered substrate, which is reasonably ascribed by Shiner (158) to a steric prevention of a certain important configuration in the transition state. Shiner (60) believes the latter to resemble (a) of Figure 12, which is the least hindered possible conformation. It might be noted that, for reasons similar to those given in small print in Sec. VA, 1, this transition state is capable of some hyperconjugative stabilization, perhaps to about the same extent as the more hindered alternative staggered conformation (b). 

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