Bridged onium ions

Figure 6.15. A representation of a bridged onium ion generated by electrophilic addition...

During the time of the Olin reports, the first examples of oligomeric boron-bridged (l-pyrazolyl)borate systems appeared from the laboratory of Trofimenko at DuPont Chemicals 24 He reported the synthesis of poly(l-pyrazolyl)borates (6) (Fig. 5) from the reactions of alkali metal borohydrides with the pyrazole ligand. The (l-pyrazolyl)borate ligand was obtained from two pyrazole units when bridged by a BR2 unit on one side and by a metal or onium ion on the other. Even though reports... 

What is retained nowadays of the initial mechanism (Scheme 1) is the occurrence of a cationic intermediate. But bromine bridging is not general, and its magnitude depends mainly on the double bond substituents (Ruasse, 1990). For example, when these are strongly electron-donating, i.e. able to stabilize a positive charge better than bromine, / -bromocarbocations are the bromination intermediates. The flexibility of transition state and intermediate stabilization puts bromination between hydration via carbocations and sulfenylation via onium ions. 

The position of the equilibrium A depends upon which provides the greater stabilization, hyperconjugation or onium ion bridging. 

The following observations have been made (a) UVA sible (UVA IS) spectroscopy (following e.g., disappearance of the color of bromine [Br2]) suggests that a n-complex is formed (b) in what is apparently the rate-determining step, the electron-rich 7i-cloud attacks the bromine molecule (Br2) forming a bromonium ion (the onium ion) and bromide anion (Br, or its equivalent. The bromine atom in the bromonium ion apparently bonds equivalently to both carbon atoms of what was the symmetrical alkene, forming a bridged ion (c) bromide anion (Br ,or its... 

Notice that there are three curved arrows here. For some reason, students drawing this mechanism commonly forget to draw the third curved arrow (the one that shows the expulsion of Br ). The product of this hrst step is a bridged, positively charged intermediate, called a bromonium ion ( onium because there is a positive charge). In the second step of our mechanism, the bromonium ion gets attacked by Br (formed in the hrst step) ... 

Superelectrophilic onium dications have been the subject of extensive studies and their chemistry is discussed in chapters 4-7. Other multiply charged carbocationic species are shown in Table 2. These include Hogeveen s bridging, nonclassical dication (14)26 the pagodane dication (15)27 Schleyer s l,3-dehydro-5,7-adamantane dication (16)28 the bis(fluroenyl) dication (18)29 dications (17 and 19) 19a trications (20-21)19a,3° and tetracations (22-23).31 Despite the highly electrophilic character of these carbocations, they have been characterized as persistent ions in superacids. 

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