Transition structure stabilisation

In Sn2 reactions, substituents at the central atom that can stabilise cationic character will also stabilise the SN2 transition state leading to products. As well as lowering the energy, such stabilisation moves SN2 saddle points in the direction of cationic species resulting in incipient cationic character in the transition structure and longer bonds to both the nucleophile and the leaving group.162 165... 

A clear correlation between the stabilisation of the endo-transition structure and the size of substituent at the 2-position of an 1-oxa-1,3-butadiene is again seen in the cycloaddition of the N-acetyl-enaminoketone 8-20 to 8-12. As expected, the reaction of 8-20 a to give 8-21 a and 8-22 a shows only a very small AAV, whereas with growing bulkiness of R as in 8-20 b and 8-20 c an increase of AAV is observed with the formation of the trans-cycloadduct 8-22 as the major product under high pressure. Because of the pressure effect it can clearly be deduced that 8-22 is formed via an endo-Z-anti-transition structure, presumably due to a strong hydrogen bond in the (Z)-diastereomer and a steric discrimination of the ( )-diastereomer of 8-20. However, an exo-E-anti-transition structure would give the same product (Fig. 8-7) [548]. 

However, homoaromatic stabilisation appears to be absent in neutral systems. Homobenzene (cycloheptatriene) 1.23 and trishomobenzene (triquinacene) 1.26, even though transannular overlap looks feasible, show no aromatic properties. In both cases, the conventional structures 1.23 and 1.24, and 1.26 and 1.27 are lower in energy than the homoaromatic structures 1.25 and 1.28, which appear to be close to the transition structures for the interconversion. 

At the same time, rotation about the formally single bond between N-l and C-2 in these compounds is more restricted than the drawing of a single bond implies, just as it was with amides. The two A-methyl groups in both enamines 2.63 and 2.82 have different chemical shifts and coalescence measurements show that the free energy of activation for rotation is 56 kJ mol 1 (13 kcal mol-1) for the former and 69 kJ mol-1 (16.5 kcal mol-1) for the latter. Decreasing the stabilisation of the anionic centre in the transition structure with a less powerful acceptor than a nitro group, as in the ester 2.83 reduces the barrier to rotation about the N—C bond to 58 kJ mol-1 (14 kcal mol-1). 

A novel class of nucleic acid mimics has been described which possess two ethylenediamine moieties for intermolecular metal co-ordination (25). In the presence of Zn + ions and template DNA, the analogues (25) form relatively stable structures, stabilised by the co-ordination of adjacent chelating moieties with zinc ions. It was shown that with an oligothymidine template and the adenine derivative of (25) that a 2 1 complex was formed, which showed a biphasic melting transition. Short RNA duplexes (3-4 bp) are considerably stabilised if both termini of the duplexes are bridged by non-nucleotidic linkers. For example, the pairing of rGAA with rUUC in such a cyclic system exhibits a Tm of 36°C in IM salt solution. 

Poor Hammett a correlations are often obtained for reactions in which a positively charged centre is formed that can resonate with an electron donating substituent, because of the additional resonance transmission for these substituents This phenomenon is illustrated by the solvolysis of substituted 2-phenyl-2-chloropropane (Figure 5). Scheme 6 shows how a transition structure may obtain extra stabilisation by resonance interaction between carbenium ion and 4-methoxy and 4-dimethylamino groups. In these cases there is no effect on the energy of the reactant molecules. 

The displacement reaction at the substituted 1-phenylethyl centre has a substantial p y value of -0.55 which indicates strong interaction between leaving group and nucleophile as in a four-centre transition structure (Scheme 2). The additional methyl group would assist the mechanistic change from in-line to front-side owing to its stabilisation effect on an incipient carbenium ion. 

The Hammond postulate predicts that stabilisation of the reactants or products moves the transition structure away from the corner which is becoming more stable - an effect which is parallel to the reaction coordinate (see Appendix 1, Figure However, stabilisation of the 0,1 or... 

One 11 Bond. It hardly needs saying that a n bond is not usually free to rotate. The n energy 2En that we saw in Fig 1.26 (280 kJ mol ) would be lost at the transition structure for rotation about the C—C bond, which would have the two p orbitals orthogonal. This value is higher than the energy normally available for a chemical reaction. For rotation about a n bond to become easy in the ground state, either the transition structures like diradical 2.101 or the zwitterion 2.102 must be stabilised or the planar structure 2.100 must be destabilised. 

C, and the cation 2.111 into the W-shaped cation 2.110 with the same half-life at 35 °C. These correspond to enthalpies of activation of 74 and 101 kJ mol 1 (18 and 24 kcal mol-1), respectively. This measurement only sets lower limits to the rotation barrier of an allyl cation, because it is not known whether rotation takes place in the cations themselves or in the corresponding allyl chlorides with which they could be in equilibrium.141 The barrier in cations is also much affected by solvation and by the degree of substitution at the termini, since the transition structure for rotation draws on such stabilisation more strongly than the delocalised allyl cation does. 

We can, however, see in the n molecular orbitals that product-like character in the transition structure favours ortho substitution over para for C-, Z- and X-substituted benzenes. When we look at the sum of the energies for the filled molecular orbitals of the intermediates 4.85 and 4.86, we see (Fig. 4.7) that the total n stabilisation of the former (3.50/3), which is linearly conjugated, is greater than the latter (3.45/3), which is cross-conjugated. Similarly with a Z-substituted benzene, the former gives a n stabilisation of 3.05/3 and the latter 2.93/3. The difference is greater in the Z-substituted case, and this is, in fact, the observed trend (Table 4.5) insofar as... 

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