Added Evidence for an Intermediate Eno~aion/Olefin-ion/~eutral Complex in the Loss of a Methyl Group From Ionized 2-Hexanone John C. Traeger

Charles E. Hudson and David J. McAdoot

Chemistry Department, La Trobe University, Bundoora, Victoria 3083, Australia

Marine Biomedical Institute and Department of Human Biological Chemistry and Genetics,University of Texas Medical Branch, 200 University Boulevard, Galveston, Texas 77550, USA

SPONSOR REFEREE: T. H. Morton, University of California, Riverside, California, USA

Bouchoux and coworkers' recently concluded that the loss of methyl groups containing carbon atoms other than C1 from metastable 2-hexanone ions (1) follows a 1,2-shift involving an incipient enol ion. Early

attributed this loss of a methyl group to formation of a cyclic ion, but subsequent studies4 demonstrated the product to be protonated 3-penten-2- one (8). Scheme 1 summarizes the pathway proposed by Bouchoux and coworkers' to explain equal losses of methyl groups containing C4 and C6 from 1. Bouchoux and coworkers' suggested that 3-5 "may be seen as ionlmolecule reactions between the products of the McLafferty rearrangement in a transient species [CH3C(OH)CH2. . . CH,CHCH,]+'." A number of mechanistic proposals involving intermediates such as 4 in Scheme 1 have appeared r e~en t ly . ' ~~ ' -~

Evidence for the presence of intermediates such as 4, other than rationalization of decomposition patterns, is clearly desirable. A variety of observations indicate that complex-mediated reactions are usually confined to a relatively narrow energy range above the threshold energy for complex formation. 10-13 Such behaviour has been proposed as an identifying characteristic of complex-mediated decompositions. 14, l5 Confinement to a narrow energy range around the threshold for the associated simple cleavage reaction is a property of alkane eliminations from ionized ketonesi2 and

ethers,I3 probable complex-mediated reactions. The photoionization ionization efficiency (PI) curves for these alkane eliminations reach plateaus at energies several tenths of an electron volt above their onsets. The assumption that a step-function threshold law applies to photoionization leads to the conclusion that alkane elimination does not occur at ion internal ener- gies higher than the onset of the ~1a teau . l~ Therefore, we measured PI curves to shed light on whether shifts of incipient enol ions relative to their olefinic copro- ducts are complex-mediated processes.

PI curves for methyi losses from ionized pentadeuter- ated 1 and 2 (1,1,1,3,3-d5) are shown in Figs 1 and 2, respectively. The PI curve for loss of CH; from the former ion displays a marked plateau with an onset between 10.2 eV and 10.3 eV. The ionization efficiency curve for the corresponding fragmentation of the second ion decreases in slope but does not become level between 10.2 eV and 10.7 eV. Furthermore, the latter curve starts to rise again at about 10.7 eV, whereas the curve for the loss of CH; from ionized 2-hexanone remains level up to the upper energy limit of the measurement. The difference between the shapes of the PI curves for the losses of CH; suggests that the plateau arises from properties of a reaction step in the isomerization of 1 to 2, rather than a step between 2 and 8. The likely explanation of the difference between

r HO+' 1 HO+ i

9 I 0 2

* C H j - u-u 8 7 6

Scheme 1

' Author to whom correspondence should be addressed.

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P H 0 T 0 I 0 N I z R T I 0 N

E F F I C I E N C

INTERMEDIATE ENOL-ION/OLEFIN- ION/NEUTRAL COMPLEX

. . . . . * * . * .**

.:*. * . .. . . * .

.(I *

....

.. I .

1

Figure 1. Photoionization ionization efficiency curves for the losses of CH3 to form an ion of m/z 90 (circles) and of CD; to form an ion of m/z 87 (triangles) from 2-hexanone-1,1,1,3,3-ds. Note the plateau in the curve for loss of CH3 above 10.2 eV.

- P H O T O N E N E R G Y (eV) -

Figure 2. Photoionization ionization efficiency curves for the losses of CH3 to form an ion of m/z 90 (circles) and of CD; to form an ion of m/z 87 (triangles) from 4-methyl-2-pentanone-1,1,1,3,3-ds.

the two curves is that 1 but not 2 must pass through 4 on the way to losing CH;, and that at higher energies, formation and isomerization of 4 cannot compete with the dissociation pathway 3-9 10.

The difference between the energy required to reach the onset of the plateau and the appearance energy (AE) of 8 is about 0.6 eV. We have presented evidence that this difference, the energy range over which reac- tion takes place, is a function of the polarity and polarizability of the neutral species and the size of the ion formed, by studies" of fragmentations such as those illustrated in Scheme 2. The complex 13 is very similar to 4 both in the size of the ion and in the polarity and polarizability of the neutral species. (Both alkyl radi- cals and olefins have very small dipole moments, and

12 13 14

Scheme 2

Table 1. Photoionization appearance energies

CR,(C= O)CDZCH&H,CH, 1-1,1,1,3,3-dS 9.32 CSH4D50 + ( 8-ds) 9.62

C3HDs0+.( 10-dS) 9.98 CD3(C = O)CD2CH(CH& 2-1,1,1,3,3-d, 9.31

CSHdDs0' (8-ds) 9.65

Precursor Ion AE (eV)

CSH7DZO' (11-dz) 9.95

CsH7D20' (1 1-d2) 9.95 C;HDSO+'(lO-dS) 9.86

alkanes, alkenes and alkyl radicals of corresponding sizes have very similar polarizabilities. 12. ") Thus, assuming that the shapes of the PI curves are deter- mined by the strength of the attractive forces in 4 and 13, the energy dependences of the loss of a methyl group from 1 and propene elimination from 12 should be similar. The difference between the onset of the plateau and the AE for propane elimination from ionized 2-methyl-3-pentanone (Scheme 2), 0.7 eV," fulfils this prediction and therefore supports the occur- rence of 1-+3-94-95 prior to the loss of a methyl group.

Reactions of iodneutral complexes take place at energies at which the appropriate covalent bond is broken, with the partners still held in association by electrostatic attractions. In complexes with partners that are comparable in size, polarity and polarizability to 4, reactions are able to take place as much as 0.3- 0.4 eV below the threshold for the associated simple dissociation.", l2 For 4 the dissociation pathway is 3 4 10. AE(C,HD,O")-AE(C,D,H,O+) for ionized 2-hexanone is 0.36eV (Table 1), which is within the anticipated range.

In summary, the PI curve for the loss of a methyl group from 1 demonstrates that the migration of an incipient McLafferty product enol relative to the accompanying olefin takes place over a relatively narrow energy range. This provides additional evidence for pr~posak?~ that this recently recognized type of isomerization occurs through ionheutral complexes.

It is of interest to compare our appearance energies (Table 1) to related ones measured much earlier by photoionization by Murad and Inghram." Our AE for loss of CH; from 1-d5 (9.62 eV) is in reasonable accord with the previous AE for loss of CH; from 1, 9.66 eV. This demonstrates that the previous measurement provided the threshold for formation of 8 rather than 9, in contrast to what has been assumed.' The previous AE for CH3 loss from 2, 9.80eV, is intermediate between our values for loss of CH; and CD; from 2-1,1,1,3,3-d5. This suggests that the earlier value is a composite for the two processes. AE(10) from 1 coin- cides reasonably with the previous value.'* However, AE(10) from 2 is about 20 kJ/mol lower than the value obtained by Murad and Inghram and the result from 1. There is a low energy tail on the AE curve for the reaction 2-+ 10, perhaps due to an impurity; extrapola- tion ignoring the tail gives an AE very close to 10.10eV, in better agreement with results from 1 and the literature.'*

The appearance energies determined here provide heats of formation for 8, 9 and 11. These and related thermochemical data are given in Table 2. Using AE(8)

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INTERMEDIATE ENOL-IONIOLEFIN -1ONINEUTRAL COMPLEX

Table 2. Relevant thermochemical data Spenes Heat of formation (kJ/mol) CH3(C = 0)CHZCHZCHzCH; - 279.8' CH;(C = O)CHZCH(CH3), - 288.7" CH3CH = CH2 20.0a CH3 143.9b H+ 1530 CH,(C -= 0)CH = CHCH, - 151" CH;(C=O+H)CH=CHCH; (8) 522d +04XH2CHZCHzCH3 (9) 559d +OsCCHZCH(CH;)2 (11) 550d a Ref. 20. bRef. 22. See text. Obtained by the expression22.26 AH,,,(A+) = AEz9, +

AHa9*(AB) - AH,,(B) + AH,,,, where AH,, i s obtained by statistical mechanical calculations and is estimated to be 23 kJ/mol for the present fragmentation reactions.

from 2, places AH&$) at ca 522kJfmol (Table 2), considerably higher than the 490 kJ/mol estimated by Bouchoux and coworkers.' Their gas-phase basicity measurement^'^ used an estimated AH, of - 174 kJ/mol for 3-penten-2-one, whereas we estimate - 151 kJ mol based on AHf(truns-2-butenal) = - 100.6 kJ/mo12" and - 50.2 kJfmol for the Benson group equivalent difference*' between C-(HJ(C0) + CO(Cd)(C) and CO(Cd)(H). We, like Bouchoux and coworkers,1 assumed that CO(Cd)(C) can be approximated by CO(Cb)(C). We could not find the discrepancy between our and their calculations. A small portion of the discrepancy arises because previous workers' used 1528 kJ/mol instead of 1530 kJ/mol for AHdH'). Alternatively, AHf(3-penten-2-one) can be estimated to be -154kJ/mol from AHf(4-methyl-3-penten-2-one(liq)) = - 219.2 kJ/mo1,23 AHvap(4-methyl-3-penten-2-one) = 35.2 kJ/m01,'~ and taking the contribution of the methyl substituent to be 29.3 kJfrn01.'~ Our estimate for AH,(S), using AHf(3-penten-2-one) = - 151 kJ/rnol, is 512 kJ/mol, in reasonable agreement with the value of 522 kJ/mol derived from our AE measurements.

Consistent with the conclusions of Bouchoux and coworkers, the heat of formation of the protonated alcohol 8 is below that of its acyl isomers. This causes 8 rather than 9 and 11 to be the dominant product of decomposition of metastable ions. It also suggests a trend for the protonated alcohols to increase in stability more rapidly with increasing size than their acyl isomers, as the propanoyl cation is more stable than protonated a~rolein.*~ Reflecting this, metastable decompositions to C3H50+ produce the propanoyl ion?8 unless they cannot easily reach a reacting configu- ration to produce that Protonated methyl vinyl ketone and the butanoyl cations seem to be formed with about equal facility:(' reflecting comparable stabi- lities. In the present system, the more stable product is the protonated unsaturated carbonyl, and it is the dominant product of decomposition of metastable ions. This change in the isomeric product formed as its size increases reflects that fact that the many isomerization reactions available to C,H2nO+' isomers28c. d. 303 31 usually enable them to find their lowest energy fragmentation pathways on the time scale for fragmentation of metas- table ions.

Acknowledgements We wish to thank Thomas Morton for helpful suggestions, Debbie Pavlu for preparation of the manuscript and the Australian Research Grants Scheme and the Robert A. Welch Foundation (Grant H-609) for financial support.

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REFERENCES 1. G. Bouchoux, J. Tortajada, J. Dagaut and J. Fillaux, Org. Mass

2. A. N. H. Yeo and D. H. Williams, J . ,4m. Chem. Soc. 91,3582

3. F. W. McLafferty, D. J. McAdoo and J. S. Smith, J . Am. Chem.

4. G. Bouchoux, R. Flammang, Y. Hoppilliard, P. Jaudon, A.

5. D. J . McAdoo and C . E. Hudson, Adv. Mass Spectrom. 11, 823

6. M. Masur, A. Sprafke and H-F. Grutzmacher, Org. Mass

7. C . E. Hudson, T. Lin and D. J. McAdoo, Org. Mass Specfrom.

8. G . Bouchoux, F. Bidault, F. Djazi, B. Nicod and J. Tortajada,

9. D. J. McAdoo, C. E. Hudson, M. Skyiepal, E. Broido and L. L.

Spectrom. 22,451 (1987).

( 1969).

Soc. 91, 5400 (1969).

Maquestiau and P. Meyrant, Spectros. Int. J . 3, 1 (1984).

( 1986).

Spectrom. 22,307 (1987).

22, 311 (1987).

Org. Mass Spectrom. 22, 748 (1987).

Griffin, J . Am. Chem. SOC. 109, 7648 (1987). 10. P. Longevialle, J . Chem. Soc. Chem. Commun. 823 (1980). 11. C. E. Hudson and D. J. McAdoo, Int. J . Mass. Spectrom. Ion

Proc. 59, 325 (1984). 12. J. C. Traeger, C . E. Hudson and D. J. McAdoo, J . Phys. Chem.

92,1519 (1988) 13. D. J. McAdoo, J. C. Traeger, C. E. Hudson and L. L. Griffin,

J. Phys. Chem. 92, 1524 (1988). D. J. McAdoo and C . E. Hudson, Ore. Mass Suectrom. 22.615 " (1987). D. J. McAdoo, Mass Spectrom. Rev. (in press). W. A. Chupka, J . Chem. Phys. 30, 191 (1959). K. J . Miller and J . A. Savchik, .I. Am. Chem. Soc. 101, 7206

E. Murad and M. G. Inghram, J . Chem. Phys. 40, 3263 (1964). G. Bouchoux, Y. Hoppilliard, P. Jaudon and R. Houriet, Org. Mass Spectrom. 19. 394 (1984). J. B. Pedley, R. D. Naylor and S. P. L. Kirby, Thermochemical Data of Organic Compounds, Chapman and Hall, 2nd Edn, London (1986). S . W. Benson, F. R. Cruickshank, D. M. Golden, G. R. Haugen, H. E. O'Neal, A. S. Rodgers, R. Shaw and R. Walsh, Chem. Rev. 69, 279 (1969). J. C. Traeger and R. G. McLoughIin, 1. Am. Chem. Soc. 103, 3647 (1981). N. D. Lebcdeva, N. M. Gutner and N. N. Kiseleva, J. Org. Chem. USSR 12, 1594 (1976). International Critical Tables, McGraw-Hill, New York, V, 132 (1929). AH~tram-2-butene)-AHf(2-methyl-2-butene) = 30.5 kJImol. J. L. Holmes, M. Fingas and F. P. Losing, Can. J . Chem. 59, 80

J. C . Traeger, R. G. McLoughlin and A. J. C . Nicholson, J . Am. Chem. Soc. 104, 5318 (1982). (a) J. H. Vajda and A. G. Harrison, Int. J . Mass. Spectrom. Ion Phys. 30,293 (1979); (b) J C. Traeger, Org. Mass Spectrom. 20, 223 (1985). (a) D. J. McAdoo, F. W. McLafferty and T. E. Parks, J . Am. Chem. Soc. 94, 1601 (1972); (b) C. E. Hudson and D. J. McAdoo, Org. Mass Specfrom. 17,366 (1982); (c) D. J . McAdoo and C. E. Hudson, Org. Mass Spectrom. 18,466 (1983); (d) C. E. Hudson and D. J . McAdoo, Org. Mass Spectrom. 19, 1 (1984). C. Bouchoux, Y. Hoppilltard. R. Flammang, A. Maquestiau and P. Meyrant, Org. Mass Spectrom. 18,340 (1983). D. J. McAdoo, C. E. Hudson, F. W. McLafferty and T. E. Parks, Org. Mas.\ Spectrom. 19. 353 (1984). L. 1,. Griffin, K. Holden, C . E. Hudson and D. J . McAdoo, Org. Mass Spectrom. 21, 175 (1986).

( 1979).

(1981).

Received 11 May 1988; accepted 12 May 1988.

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