Allylic positions, calculated charges

The 2-chloroallyl cation generated by matrix isolation has been studied by FT-IR spectroscopy and calculations [MP2(fc)/6-311G(d,p) level].235 Structure 78 of Cs symmetry with bridging chlorine, proposed earlier,162 was found to be less stable by 7.5 kcal mol 1 than cation 79 of C2v symmetry and could not be found by spectroscopy. Furthermore, in contrast to 1- or 3-chloroallyl cations, the centrally positioned chlorine does not contribute to charge delocalization through back-donation as a consequence of the Ji-orbital noninteraction between the n electrons of Cl and the LUMO of the allyl cation.236... 

The 5n2 reactions of y-substituted allyl chlorides where the nucleophile is a free anion and an ion pair were studied theoretically at the HF/6-31+G level of theory.10 The calculations showed that the free ion, S N2 reaction proceeds through a transition state (7) with significant positive charge on Ca and little conjugation with the n-system. Placing substituents on Cy of the free ion reactions gave computed Hammett p values of -3.3 and -4.6 for the trans- and ds-conformation reactions, respectively. The transition states (8) for the ion pair reactions, on the other hand, have considerable positive charge on both Ca and Cy and the computed Hammett p value is +18.8. 

A 1,3-dipole as shown in Schemes 6-5 and 6-6 corresponds to a system with three parallel atomic p-orbitals, i.e., to an allyl anion, but without net charge. It is, therefore, called an allyl-type 1,3-dipole. The system may contain, however, an additional 7i-bond in the plane perpendicular to the allyl anion type molecular oribtal, and then belongs to the propargyl - allenyl type. Normally, 1,3-dipoles of this type are linear, whereas those of the allyl type are bent. The term 1,3 relates to the reactivity in these positions, not to formal charges. A series of theoretical studies (e. g., by Hiberty and Leforestier, 1978 Yamaguchi et al., 1980 see review of Houk and Yamaguchi, 1984) clearly show, however, that some of these 1,3-dipoles have considerable biradical character (e.g., O3 53% and CH2N2 28% in ab initio calculations at the 4-3IG level). We will return to biradicals in the mechanistic discussion of Sect. 6.3. 

Recent ab initio calculations by Schleyer and coworkers indicate that 4 is strongly stabilized by interaction of the empty 3p Si orbital with the Jt-orbital of one allylic double bond [5]. This interaction leads to a pyramidalized silicon center and relatively short Si-C(sp ) distances and stabilizes the cation 4 by 15.4 kcal moT compared to trimethylsilylium (at MP2/6-31G //MP2/ 6-3IG ). The bonding situation in 4 is best described in terms of three dimensional aromaticity with the 3p(Si) and the C=C double bond obeying the 4n+2 interstitial electron" rule. Due to the reduced positive charge at silicon, cations such as 4 should be also less reactive towards nucleophiles. A facile synthetic approach to this novel typ of silyl cation is outlined in Scheme 1. 

Equation 4.52 can be used to calculate the charge density for each position of the allyl cation, radical, and anion, and the results are shown in Table 4.1. These values are comforting, because they agree with our chemical experience with allylic systems. The resonance description of the allyl cation and radical (Figure 4.13) suggests that exactly half of the charge or unpaired electron density is associated with each of the terminal carbon atoms, and the HMO result is the same. 

---