5709
J. Am. Chem. SOC.1993,115, 5709-5715
The Search for the Gas-Phase Negative Ion Pinacol Rearrangement Suresh K. Dua,? Robert B. Whait,? Margaret J. Alexander,+Roger N. Hayes,* Albert T. Lebedev,$ Peter C. H. Eichinger,? and John H. Bowie'*t Contribution from the Departments of Chemistry, The University of Adelaide, Adelaide, South Australia 5001, Australia, and University of Nebraska, Lincoln, Nebraska 68588 Received July 13, 1992. Revised Manuscript Received December 3, I992
Abstract: Deprotonated 1,2-diols and 8-methoxyhydrins often eliminate ROH (R = H, Me) on collisional activation in the gas phase. These losses do not involve a negative ion pinacol rearrangement in acyclic systems in which there are no conformational restraints on the relative positions of the reacting groups. In the case of 8-methoxyhydrins, labeling studies show that the product ion is formed by the losses of a methoxide ion and a proton from adjacent positions. The absence of a deuterium isotope effect for this process precludes the operation of an elimination process: we propose that the loss of methanol proceeds via an epoxide cyclization viz. MezC(OMe)C(O-)(Me)z (MeO-)-
-
-
/Oh
+
-CH3(Me)C--C(Me)Z CHz=C(Me)C(O-)(Me)2 MeOH. In contrast, epoxide cyclization does not occur when the two oxygenated substituents cannot adopt an anti orientation. Thus deprotonated cis-2-methoxycyclohexanolloses methanol via a pinacol rearrangement, while loss of methanol from the trans isomer produces deprotonated cyclohex2-en- 1-01, presumably by an epoxide mechanism.
Introduction We have recently described how the Beckmann rearrangement of oximes, a reaction which occursunder acid-catalyzedconditions in the condensed phase,' may occur in the gas phase for negative ions formed by deprotonation of oximes.2 This has led us to investigate whether there are any other examples of such a scenario, and in this paper, we consider whether there is a gasphase negative ion counterpart to one of the oldest and most famous of all acid-catalyzed rearrangements, the pinacol/ pinacolone rearrangement (eq l).3.4 The condensed-phase reaction involves a simple Whitmore 1,2-methyl shift,s and the driving force for the reaction is the stabilization of the final carbonium ion intermediate by elimination of a proton to form the neutral product. Many "pinacol-type" reactions are known: the migrating group may be alkyl, aryl, or h y d r ~ g e n . ~ Me2C-CMe2
I
t
---H+
OH OH
MezC-CMe2
I
I
-+
Me2C$Me2
I
OH
OH YHz
MedCMeJ
I
OH Mez@:
--m
OH
(Me3CCOMe)HO-
01u
b
-
Experimental Section
- H+
rearrangement to form ion complex b, which if formed, should most certainly decompose to yield deprotonated pinacolone c. Ion c may be readily identified from its characteristic fragmentation behavior.6 Base-catalyzed reactions of the pinacol type are not common in the condensed phase: a major reason is presumably that HO- is a poor leaving group under these conditions. Even so, such reactions have been observed, but they involve the release of ring strain?.* e.g. the reaction between HOand 1,l'-dihydroxybicyclopropane yields 2-hydroxy-2-ethylcyclobutanone.7 A few base-catalyzed pinacol-type reactions have been reported for &chlorohydrins, Le. where the leaving group is Cl-.*.9 The aim of the work described in this paper is to determine the mechanisms by which H20 and MeOH are lost in the gas phase from deprotonated 1,2-diolsand 8-methoxyhydrins, respectively. In particular, do these losses occur by negative ion pinacol rearrangements such as that shown in eq 2?
Collisional activation and chargereversal (positive ion)1° mass spectra
MeCOCMe3
(1)
-
(MS/MS) of deprotonated neutrals, and of some source-formed product anions, were determined with a VG ZAB 2HF instrument. Full experimental details for the operation of this instrument have been reported." Specific details are as follows: the chemical ionization slit was used in the ion source, the ionizing energy was 70 eV, the ion source temperature was 150O C , and theacceleratingvoltage was 7 kV. Samplcs were introduced through the septum inlet (which was maintained at 100 "C) to give a measured source pressure of 5 X lCr7 Torr. Deprotonation
a
(MeaCCOCH2)-
+
H20
(2)
C
(5) Whitmore, F. C. J. Am. Chem. Soc. 1932,54,3274. (6)Alexander, M. J.; Bowie, J. H.; Hayes, R. N. Aust. J . Chem. 1990,43, * . . . * . I
It seems possible that in the gas phase, collisional activation of deprotonated pinacol (a, eq 2) could effect a 1,2-methyl anion The University of Adelaide.
t University of Nebraska. I On leave at the University of Adelaide from Moscow State University.
(1) Bcclrmann, E. Chem. Eer. 1887,20, 1507. (2)Adams, G. W.;Bowie, J. H.; Hayea, R. N. J. Chem. Soc., Perkin Trans. 2 1989,2159; 1990, 1279. (3) Fittig, R. Justus Liebigs Ann. Chem. 1859,110, 17; 1860,114,54. (4)For reviews see: Collins, C. J. Q. Rcv., Chem. Soc. 1960, 14, 357. Pocker, Y,In Molecular Rearrangements; de Mayo, P., Ed.; Interscience: New York, 1963;Vol. 1, pp 15-25.
LUL I .
(7) Denis, J. M.; Conia, J. M . Tetrahedron Lett. 1972,4593. (8) Barlett, P. D.; Rosenwald, R. H. J . Am. Chem. Soc. 1934,56, 1990. Geissman,T. A.;Akawie, R.I. J. Am. Chem.Soc.1951,73,1993.Mousseron, M.; Jaquier, R. J.; Mousseron-Canet, M.; Zagdoun, R. Bull. Soc. Chim. Fr. 1952,1042. Miller, R. D.;Dole., D. Tetrahedron Lett. 1973,1151. (9)DeKimpe, N.; Verhe, R.; DeBuycke, L.; Schamp, N. Bull. Soc. Chim. Eelg. 1979,88,719.Poslinska, H.; Palanowski, R. Przem. Chem. 1985,64, 478;Chem. Abstr. 105,42424h. (10)Bowie, J. H.; Blumenthal, T. J. Am. Chem. Soc. 1975,97, 2959. Szulejko, J. E.;Bowie, J. H.; Howe, I.; Beynon, J. H. Int. J. Mass Spectrom. Ion Phys. 1980,13,76. (1 1) Stringer, M. B.; Bowie, J. H.; Holmes, J. L. J. Am. Chem. Soc. 1986, 108,3888.
OOO2-7863/93/1515-5709$04.00/00 1993 American Chemical Society
Dua et al.
5710 J. Am. Chem. SOC.,Vol. 115, No. 13, 1993 Table I. Collisional Activation Mass Spectral Data for 1,2-Diols m / z (loss or formation) abundance
parent ( m / z )
116 (H') 100,101 (CHd) 70,57 (MezCHOH) 4 117 (H') 100,101 (CHID) 72,57 (MeZCHOD) 4 167 (Hz) 10,151 (HzO) 19,141 (CzH4) 42,85 (CsHsO) 12,83 (C5HloO) 100
MezC(OH)C(O-)(Mez) (1 17) Me2C(OD)C(O-)Me)2 (1 18)
168/167" (Hz, HD) 15,151 (HOD) 25,142 (CzH4) 38,86 (CsHaO) 18,83 (C5H9DO) 100
a
Unresolved.
Table II. Mass Spectral Data for Selected Product Ions from Deprotonated 1,2-Diols parent ion (m/z)
product ion (m/z)
MezC(OH)C(O-)Mez (117)
-CHI (101)
(169)
CA MS/MS/MSa
CA MS/MS/MS
57 (MeZCHOH) 100,43 (C4HloO) 42 59 (55), 43 (loo), 42 (79), 39 (32), 31 (20), 29 (16), 27 (12), 15 (15) 100 (H') 3,86 (Me') 2,85 (CHI) 3,83 (C& Hz) 1, 57 (Me2CHOH) 100,43 (C4HloO) 36,41 (HCzO-) 1,15 (Me-) 0.1 59 (54), 57 (8), 55 (4), 53 (5), 51 (2), 43 (loo), 42 (78), 39 (34), 31 (21), 29 (15), 27 (lo), 15 (12) 149 (Hz)9,83 (CsHs) 100,67 (C5H7-) 7
CA MS/MS
149 (Hz)100,147 (2H2) 4,123 (CzHi) 8,109 (42) 6,107 (44) 2
CA MS/MS
149 (Hz) 11,83 (C5H8) 100,67 (C5H7-) 9
CR MS/MS/MSa
[MeCOC(OH)Mez - HI(101)
CA MS/MS CR MS/MS -HzO (151)
C A m / z (loss or formation) abundance C R m / z (abundance)
spectrum type
+
OH 0-
-@ a Spectra
(151)
are weak-peaks
