Hyperconjugation systems with

Comparison of all reported cleavages gives the apparent ordering H > Ph (or aryl) > alkyl in terms of tendency for groups to be cleaved from silicon, with the exception that at least one aryl substituent is retained on silicon in the anion VI. It is clear that reduction of aryl silanes leads to significant weakening of Si—R bonds that have appropriate symmetry for hyperconjugative interactions with the reduced n systems, a point that has been cited as evidence for the importance of n — type interactions in unsaturated silanes (86). 

CT-participation and hyperconjugation. The great variation in nuclear motion depends on the systems. With 3-nortricyclyl ions Olah has proved the nonvertical stabilization to be more significant than the vertical hyperconjugation this is deary seen from the NMR spectra... 

All these well-established concepts obviously are useful for the discussion of the planar cumuienes, such as ketenes, diazocompounds, thioketenes, and butatrienes. For nonplanar cumuienes (allenes, ketene imines, carbodiimides, pentatetraenes) the situation is not so simple, as the proper classification of electronic states depends essentially on the substitution patterns of the molecules (lc,24,25,73). The usual concepts based on VB arguments have led toclassiH-cations and analytical descriptions of allenic electronic systems (or orbitals) in terms of two perpendicular, isolated tt systems, especially in terms of two-center ethylenic tt systems (24). Sometimes, hyperconjugation of the eth-ylenic C=C tt system with the CH2 group orbitals of corresponding symmetry has been taken into consideration (24). 

The electron density distributions in monosubstituted allenes are discussed in Ref. 9. In allenes the tt system of the cumulenic moiety comprises the C=C and the CH2 subunits, that is, in VB terms there is a hyperconjugation of the C=C TT system with the CH2 group orbitals of corresponding symmetry. 

Figure 8.14 Cross-conjugated systems connected with a n-system and cross-hyperconjugated systems connected with a pseudo-7t system.

The most stable conformer of the 1-methylallyl cation, with an eclipsed conformation of the methyl group with the planar allyl system, is a consequence of a a-a hyperconjugative interaction of the C-H bond parallel to the allyl system with an allyl C-C [Pg.203]

MM2 was, according the web site of the authors, released as MM2 87). The various MM2 flavors are superseded by MM3, with significant improvements in the functional form [10]. It was also extended to handle amides, polypeptides, and proteins [11]. The last release of this series was MM3(%). Further improvements followed by starting the MM4 series, which focuses on hydrocarbons [12], on the description of hyperconjugative effects on carbon-carbon bond lengths [13], and on conjugated hydrocarbons [14] with special emphasis on vibrational frequencies [15]. For applications of MM2 and MM3 in inorganic systems, readers are referred to the literature [16-19]. 

Neighboring group participation (a term introduced by Winstein) with the vacant p-orbital of a carbenium ion center contributes to its stabilization via delocalization, which can involve atoms with unshared electron pairs (w-donors), 7r-electron systems (direct conjugate or allylic stabilization), bent rr-bonds (as in cyclopropylcarbinyl cations), and C-H and C-C [Pg.150]

Metal Alibis and Alkoxides. Metal alkyls (eg, aluminum boron, sine alkyls) are fairly active catalysts. Hyperconjugation with the electron-deficient metal atom, however, tends to decrease the electron deficiency. The effect is even stronger in alkoxides which are, therefore, fairly weak Lewis acids. The present discussion does not encompass catalyst systems of the Ziegler-Natta type (such as AIR. -H TiCl, although certain similarities with Friedel-Crafts systems are apparent. 

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