Hyperconjugative stabilization

Methylindole has a p/sTa of -4.6 and it is therefore a weaker base than indole itself this unusual effect has been ascribed in part to the decreased hyperconjugative stabilization of the conjugate acid (38) by the one hydrogen at position 3 compared with the two hydrogens at position 3 in the 3//-indolium ion (39). 

By now the reader will already have been alerted to an important geometrical consequence of the hyperconjugative stabilization of B. Bond 12 should shorten, due to a decreased electron density in the (12 antibonding) Walsh orbital. 

Cieplak stated that the lone pair back-donation of C5 substituent also stabilizes the transition state during the syn approach since it improves hyperconjugation of the C5-H bond (extended hyperconjugation). He pointed out that for the cyclopen-tadienes having C5 substituents such as hydroxy, methoxy, amino, and chloro moiety, the hyperconjugative stabilization and the back-donation work in the same direction (Scheme 27). 

Some ylides 92 among them C-silylated ones have been synthesized in order to compare the stabilization influence on the carbanionic center of various C-substituents (I, SiMej, Ph3P ) [ 113]. It appears from this study that the stabilization due to the electron-withdrawing C-substituents (R or R ) is not so negligible by comparison with the hyperconjugative stabilization between the ylidic carbon and the phosphonium group [114]. This is particularly true for the iodine substituent. 

The anomeric configuration is set in the reductive lithiation step, which proceeds via a radical intermediate. Hyperconjugative stabilization favors axial disposition of the intermediate radical, which after another single electron reduction leads to a configurationally stable a-alkoxylithium intermediate. Protonation thus provides the j9-anomer. The authors were unable to determine the stereoselectivity of the alkylation step, due to difficulty with isolation. However, deuterium labeling studies pointed to the intervention of an equatorially disposed a-alkoxylithium 7 (thermodynamically favored due to the reverse anomeric effect) which undergoes alkylation with retention of configuration (Eq. 2). 

On the contrary, rearrangements of cyclobutylhalocarbenes to the corresponding halocyclopentenes could be anticipated to have lower energy barriers, because of greater relief of ring strain and less hyperconjugative stabilization in the carbenes... 

The relative strengths of hyperconjugative stabilizations could be rationalized with contour plots similar to Fig. 3.49. The most important features of such plots could be predicted from the transferable forms of the NBOs (cf. Figs. 3.16 and 3.25) and expected variations with electronegativity (Sections 3.2.5 and 3.2.8). As shown in Fig. 3.49, the hyperconjugating ax and ax NBOs are typically canted away from the pi NBOs, weakening their interactions compared with ordinary (7r-7r ) con-jugative stabilizations in Table 3.19. However, the dependences on bond/antibond... 

Table 3.22. Some C=C bond orders, 7tcc polarization, ncc and 7tcc NLMOs, and hyperconjugative stabilizations between... 

Figure 3.52 Leading hyperconjugative stabilizations in CFH2CH = CH2, showing the torsional dependence of n-o (solid lines) and a-n interactions (dotted lines) for the C—F (crosses) and two C—FI bonds (triangles, circles) of the—CFF12 group. The sum of all six interactions is shown as the heavy solid line (squares), which may be compared with the total barrier potential in Fig. 3.51.

The principle that maximum hyperconjugative stabilization results from orienting the best vicinal donors (3.161a) anti to the best vicinal acceptors (3.161b) has an immediate corollary. 

Table 3.25 shows how the main NBO descriptors vary in the perp —anti transition of NH2CH2F. As expected, this torsional change leads to strong increases in overlap (from iS ,0 = 0.01 to, Vno = 0.29) and hyperconjugative stabilization (from 0 to 21.7 kcal mol-1), as well as significant population shifts ( 0.07e) fromnN to ocxf. 

In the triplet excited state, an electron is formally removed from the oxygen n0 orbital (the in-plane pv-type nonbonding orbital) of the (3 manifold and added to the 7tco antibond of the at manifold (formally breaking half the pi bond). From the viewpoint of the amine nN donor, this excitation makes available a low-lying half-filled n0 acceptor NBO suitable for strong hyperconjugative stabilization. The nN—nQ interaction... 

The localized product-NBO view also makes clear the importance of the non-planarity of the TS in achieving favorable hyperconjugative stabilizations, because interactions of 7t-planar geometry. Indeed, one can see that favorable vicinal n-a or a-7r overlap should primarily involve one end (hybrid) of each NBO, oriented, if possible, in the anti conformation for maximum stabilization. This viewpoint allows one to recognize the importance of angular and orientational factors that would not be evident in a purely topological framework. 

The relatively long C(l)-C(6) (2.247 A) and short C(l)-C(2) (1.427 A) bond calculated for 3 were in agreement with the hyperconjugative stabilizing interaction n(CO) oC(l),C(2) <- pC(6) which can be represented by the limiting structures 3 3 . These features were not present in the case of 5-cyano- and 6-cyanobicyclo[2.2.1]hept-2-yl cations, the former ion being calculated to be more... 

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. 

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