Fragmentation of the Tropanes including Mass Spectrometry

The first example of fragmentation was described by Willstatter (see Volume I, p. 287), i.e., alkaline cleavage of tropinone methiodide to 

On the other hand, as mentioned briefly in Volume VI, page 150, 3j8-tropanyl chloride (LXIa Y = C1) is converted by potassium cyanide into a pyrrolidine (LXVIII), while the 3a-epimer gives the 3a-cyano-tropane due, presumably, to a transannular participation of the ring 

A systematic study of the fragmentation of tropanes under electron impact was carried out 8a, b). As a control, the compounds were selectively deuterated either in C-6 and C-7 or in C-2 (C-3) and C-4. When the m/e of an individual fragment was shifted by 2, 4 (or 5) units, its original position in either the five- or six-membered ring was indicated. 

Ketones behave differently from alcohols of that series. The molecular ion LXXII, formed by removal of one electron from tropinone (LXXIa), undergoes homolytic cleavage either of bond C-1—C-7 (or of the equivalent C-5—C-6) typical for ammonium salts, or of bond C-1/C-2 (its equivalent being C-4/C-5) as do simple ketones. In the first case, a radical ion LXXIII is formed, to afford the A-methylpyrroleninium 

In the second case, fission C-l/C-2 gives an a-substituted pyrroleninium ion (LXXVIIa) which loses ketene (cleavage 3/4) with formation of the a-methylene-iV-methylpyrroleninium radical ion (m/e 97). This, in turn, eliminates a proton and appears in the spectrum with m/e 96 (LXXVIII). The latter is probably a 2-methylene-iV-methylpyrrolinium ion since deuteration in either position [2D in C-6 and C-7 (LXXIc) or 4D in C-2 and C-4 (LXXIb)] is marked by a shift by 2 mass units. 

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