Bridged cations

A mercurinium ion has both similarities and differences as compared with the intermediates that have been described for other electrophilic additions. The proton that initiates acid-catalyzed addition processes is a hard acid and has no imshared electrons. It can form either a carbocation or a hydrogen-bridged cation. Either species is electron-deficient and highly reactive. 

Rauscher 42 calculated that the protonation of a double bond (Eq. (2)) leads to nonclassical, that means H-bridged, cations if the double bond is substituted symmetrically (Rj = R and Rj = R ). 

C8a (A = 0.077°) and an elongation of the 4a-8a bond (1.41 A), indicating a weakened sp bonding interaction between C4a and C8a in the aromatic system. It is clear that the structural features of 5, and other bridging cations give convincing support to Winstein s 3-center, 2-electron non-classical structures. ... 

Compounds having divalent sulfur and selenium atoms bound to more electronegative elements react with alkenes to give addition products. The mechanism is similar to that in halogenation and involves of bridged cationic intermediates. 

Scheme 1 Analogy between the observable dehydrocyclization of 1 under low temperature superacid conditions, and the possible cyclization of a 2-octyl cation in catalytic dehydrocyclizations at 400-500 C. In the p-H-bridged cations the positive charge is delocalized, so the position of the (+) simply indicates the total charge in these species.

In contrast, secondary p-H-bridged cations are much more fragile, and only the cyclooctyl 8, cyclononyl 9, cyclodecyl 10 and cycloundecyl 11 cations have been characterized. 

Although accurate kinetics have not been obtained experimentally, the AG transition-state energy for the above dehydrogenation step has been estimated as 10 1 kcal/mol (15). Since this measurement starts at the energy level of the 1,5-bridged cation, the actual barrier for the second step should be at least 1 kcal/mol lower, e.g. 9 1 kcal/mol. 

For tiie more stable of the two isomers of each type (1,5 and 1,6-p-H-bridges), cations 16 and 18, one can locate a transition-state for dehydrocyclization, leading to product ions 20 and 21 respectively, plus molecular hydrogen. 

The formation of a 1,6-p-H-bridged cation in this case requires the existence of primary-secondary termini, as sketched in the structural drawings (26,27) below ... 

Finally, in contrast to the reactions reported above, 49 reacts131 with dimethyl(methyl-thio)sulfonium fluoroborate (DMTSF) and triethylamine tris(hydrofluoride) in dichlorome-thane to give only 5% of the 1,2-addition product 122. The main products, present in 73 27 ratio, are the exo-exo and endo-exo adducts 123 and 124, formed by exclusive exo attack of the electrophile on the double bond, followed by transannular -participation in the intermediate bridged cation and final addition of fluoride to the nortricyclic cation from both the exo or endo side (equation 120). 

Adsorption of nonionic compounds on subsurface solid phases is subject to a series of mechanisms such as protonation, water bridging, cation bridging, ligand exchange, hydrogen bonding, and van der Waals interactions. Hasset and Banwart (1989) consider that the sorption of nonpolar organics by soils is due to enthalpy-related and entropy-related adsorption forces. 

McMurry has reported an unusually stable j,-hydrido-bridged cation, the [4.4.4]tetradecyl cation (135). Most of the other such carbocations are highly unstable and could be characterized at very low temperatures, typically around... 

The nature of the intermediate cationic species, whether it is a classical methyl cation or a nonclassical bridged cation, has also been determined. The deamination of optically active e c/o-2-aminobicyclo[2.2.1]hcplane-exo-2-carbonitrile gave 2-hydroxybicyclo[3.1. l]-heptane-1-carbonitrile (19) in 9% yield with 92% retention of the optical activity. Similarly, deamination of e.vo-2-trifluoromethylbicyclo[2.2.1]heptyl-e Jo-2-amine gave 1-trifluoromethyl-bicyclo[3.1.1]heptan-2-ol (20) in 11 % yield with 88% retention of the optical activity. These results are interpreted in terms of intermediate bridged cations.87... 

Partial racemization, however, occurred in the deamination of optically active 2-aminobicy-clo[2.2.1]hept-5-ene-2-carbonitrile yielding a 1 2 mixture of the ring-contraction products 2-hy-droxybicyclo[3.1.1]hept-3-ene-l-carbonitrile (21, 40% retention of optical activity) and 4-hy-droxybicyclo[3.1.1]hept-2-ene-l-carbonitrile (22, 5% retention of optical activity). The chiral bridged cation gave only 21, while from the achiral allyl cation both 21 and 22 were formed.87... 

A systematic study of the alkylation of benzene with optically active 1,2-, 1,3-, 1,4-, and 1,5-dihaloalkanes showed that the stereochemical outcome depends on the halogens and the chain length.142 Retention of configuration in alkylation with (S)-1,2-dichloropropane was explained by invoking double inversion and the involvement of a bridged cation such as 27. (S)-l-Bromo-2-chloropropane, in contrast,... 

Doubly bridged cationic diplatinum(II) complexes [Pt(o-CH2C6H4CN-fip )(2=P os)](BF4)2 have been synthesized by halide replacement. The cyano group remains bonded to platinum both in the solid state and in solution.492... 

---