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J . Org. Chem. 1998, 63, 408-410
Cycloa lk en e Bu d d in g: A Un iqu e
Rea r r a n gem en t Obser ved in th e Ma ss
Sp ectr a of N,N-Dim eth ylh yd r a zon es of
Un sa tu r a ted Ald eh yd es
Athula B. Attygalle,* Kithsiri B. Herath, and
J errold Meinwald
Baker Laboratory, Department of Chemistry,
Cornell University, Ithaca, New York 14853
Received October 24, 1997
In the course of studying the electron-ionization mass
spectra of unsaturated aldehyde derivatives, we encoun-
tered a new mass spectral rearrangement and fragmen-
tation that is both diagnostically useful and fundamen-
tally interesting from a mechanistic viewpoint. When a
double bond is present at position 5 or 6 in an unsatur-
ated straight-chain aldehyde, under electron-ionization
conditions its N,N-dimethylhydrazone undergoes an un-
precedented combination of bond-making and bond-
breaking reactions, for which we suggest the term
“cycloalkene budding”. The spectra obtained are analyti-
cally useful, since the position of the double bond can be
deduced by the appearance of characteristic peaks at m/z
(M - 68)+ or (M - 82)+, representing losses of C5H8 or
C6H10 for 5- and 6-alkenal derivatives, respectively.
Although fragmentations of gas-phase ions resulting in
hydrocarbon elimination, not initiated by the charged
site, have been well established,1-3 these examples have
usually involved a 1,4-elimination process which is not
analogous to the charge-mediated process which we now
report.
The electron-ionization (EI) mass spectrum of the (E)-
5-tetradecenal derivative depicted in Figure 1a provided
us with the first example of this novel fragmentation. It
shows several peaks that are generally expected in EI
spectra of N,N-dimethylhydrazones.4,5 An intense mo-
lecular ion and peaks representing losses of 15 and 44
mass units from the molecular ion are characteristic of
these derivatives.5 A peak at m/z 86 which represents a
γ-hydrogen transfer by a McLafferty rearrangement
(Scheme 1) is particularly prominent.
F igu r e 1. Electron-ionization mass spectra (70 eV) of N,N-
dimethylhydrazones of (E)-5-tetradecenal (a), and (E)-6-tet-
radecenal (b).
of 82 mass units is observed in the spectra of 6-alkenal
derivatives. For example, the spectrum of (E)-6-tetrade-
cenal dimethylhydrazones, illustrated in Figure 1B,
shows an (M - 82)+ signal at m/z 170.
The elemental composition of the m/z 184 ion observed
in the spectrum of (E)-5-tetradecenal dimethylhydrazone
was obtained by high-resolution mass spectrometry (R
) 10000). The measured mass of 184.1931 (calculated
for C11H24N2 184.1940) recorded for this ion established
that the 68 Da loss represents a removal of C5H8 from
the molecular ion (252.2565 observed, 252.2566 calcu-
lated). To rationalize the loss of C5H8 from the molecular
ion, we propose an intramolecular cycloaddition followed
by ring cleavage of the bicyclic intermediate, which
results in the elimination of cyclopentene (Scheme 2).
Similarly, the accurate mass of the m/z 170 ion
observed in the spectrum of 6-tetradecenal dimethylhy-
drazone is 170.1778 (R ) 10000; calculated for C10H22N2,
170.1783). This observation established that the 82 Da
loss represents a loss of C6H10 from the molecular ion,
which can be rationalized analogously as a cyclohexene
extrusion (Scheme 3).
To test this hypothesis, several deuterium labeled 5-
and 6-alkenals capable of providing diagnostically useful
results were synthesized. The mass spectra of the
dimethylhydrazones of (E)-5-[1-2H1]tetradecenal and (E)-
6-[1-2H1]tetradecenal showed the expected rearrange-
ment peaks still at m/z 184 and 170, respectively (Figure
2, parts a and c), confirming that the C-1 deuterium atom
is lost as anticipated. Further support for the proposed
fragmentation was provided by the mass spectrum of
derivative of (Z)-5-[5,6-2H2]tetradecenal (Figure 2b). In
this spectrum, a peak was observed at m/z 185, corre-
sponding to a 69-Da loss from the molecular ion. Ac-
cording to the proposed mechanism, the C-5 deuterium
In addition to these anticipated ions, however, there
is a prominant peak at m/z 184 (M - 68)+ in the mass
spectrum of (E)-5-tetradecenal dimethylhydrazone which
cannot be explained by known fragmentation mecha-
nisms. Indeed, we found this loss of 68 mass units from
the molecular ion to be characteristic of dimethylhydra-
zones bearing a double bond at the 5 position. Mass
spectral data presented in Table 1 shows that this unique
loss appears in the mass spectra of derivatives of both
the cis and trans isomers of 5-decenal, 5-dodecenal,
5-tetradecenal, and 5-hexadecenal. A homologous loss
* Author for correspondence. Phone 607 2554525, fax 607 2553407,
e-mail aba1@cornell.edu
(1) Adams, J . Mass Spectrom. Rev. 1990, 9, 141.
(2) J ensen, N. J .; Tomer, K. B.; Gross, M. L. J . Am. Chem. Soc. 1985,
107, 1863.
(3) Tomer, K. B.; Gross, M. L. In Ion formation from Organic Solids
(IFOS III); Benninghoven, A., Ed.; Springer-Verlag: Berlin, 1986; pp
134-139.
(4) McDaniel C. A.; Howard, R. W. J . Chem. Ecol. 1985, 11, 303.
(5) Goldsmith, D.; Djerassi. C. J . Org. Chem. 1966, 31, 3661.
S0022-3263(97)01960-9 CCC: $15.00 © 1998 American Chemical Society
Published on Web 01/23/1998
Attygalle, Athula B.
