Quinuclidine halonium ions

A second set of experiments were performed after the treatment of 10 or 11 with added pyridinium ion whereby the mixed quinuclidine pyridine halonium ion (12) that was created in situ was subsequently treated with a varying [4-penten-l-ol] or [4-hexene-l-ol] in the presence of a varying [pyridine]. Inspection of the process shown in Figure 8 suggests that, once having made 12... 

An elecrophilic Br+ or I+ can be successfully transferred to hydroquinidine (13) and two of its commercially available derivatives (4-chlorobenzoate and 9-phenanthryl ether hydroquinidines) simply by mixing two equivalents of the hydroquinidine with one equivalent of sym(co d ne)2-X+ perchlorate in methylene chloride or acetonitrile. H NMR studies (31) showed that the iodonium ion was associated with the nitrogen at the quinuclidine portion of the hydroquinidine instead of the aromatic nitrogen and also that all of the sym-collidines were removed from the X+ since only free collidine and no collidine-I+ peaks were observed. The (hydroquinidine)2-halonium ion is stable in solution for more than 30 minutes at room temperature these ions (and their parent amines) are more soluble in methylene chloride than in acetonitrile, and having R group other than hydrogen also improves the solubility. 

As with non-aromatic sp or sp nitrogens, there are not many examples (Fig. 6) of N... X. ., N complexes where an sp nitrogen acts as the halogen bond acceptor for the halonium ions. Representative examples are the iodonium and bromonium complexes of quinuclidine (GEXGOF and BUWMOV, respectively) and the iodonium complex of hexamethylenetetramine (HTMA, refcode HMTITI). 

One of the possible ways to stabilize the amine-halonium complexes is to increase the basicity of the amine, bearing in mind that an appropriate one must also not have easily removable P-hydrogens which will lead to oxidation of the amine and formation of an imine. Quinuclidine (pKa of quinuclidinium ion is 11.3 (55)) is 105-106-fold more basic than the pyridines and both the bromonium (10 (36)) and iodonium (11 (57)) BF4 salts have been made and characterized by X-ray crystallography. Interestingly, although the reaction must generally occur as outlined in Figure 7, neither of these ions shows any observable reaction... 

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