J. Phys. Chem. 1996, 100, 7471-7479
7471
Rearrangement and Fragmentation Mechanisms of Vibrationally Activated Enolate Ions in the Gas Phase Kristin A. Sannes and John I. Brauman* Department of Chemistry, Stanford UniVersity, Stanford, California 94305-5080 ReceiVed: September 11, 1995; In Final Form: January 2, 1996X
The unimolecular rearrangement and fragmentation mechanisms of several alkyl enolate ions have been investigated using Fourier transform ion cyclotron resonance mass spectrometry and infrared multiple photon activation techniques. We have observed unusual fragmentations induced by infrared multiple activation of several enolate ions that do not follow previously generalized pathways. Two new mechanisms are proposed to explain the unusual fragmentations. First, we propose the intermediacy of an alkyl radical/ketene radical anion complex that is formed by homolytic cleavage. This complex is expected when the alkyl anion is unbound. Within this complex, the alkyl radical can either abstract a hydrogen from the ketene radical anion to form deprotonated ketene anion and an alkane or transfer a hydrogen to the ketene radical anion to form an aldehyde enolate ion and an alkene. Second, we propose that vibrationally activated enolate ions can undergo a 1,3-methyl rearrangement in addition to a 1,3-hydrogen rearrangement. The proposed mechanisms appear to be general and are able to predict the fragmentations of other enolate ions.
Introduction Deprotonation of unsymmetrical ketones can lead to a mixture of isomeric enolate ions that are not distinguishable by mass but that have different structures. Unimolecular rearrangements and fragmentation are characteristic of an enolate ion’s structure. Thus, a clear understanding of the unimolecular rearrangement and fragmentation pathways can provide powerful structural information as well as provide important insights into the reaction chemistry of enolate ions. With this in mind, we began a study of the unusual unimolecular dissociations of vibrationally activated enolate ions. Several groups have studied the collisional activation of simple alkyl enolate ions.1-9 Some generalized pathways were developed to explain the observed unimolecular fragmentations.1 The pathways for the fragmentations of the enolate ions involve either anion/molecule complexes as low-energy intermediates or intramolecular 1,6-hydrogen rearrangements followed by internal elimination reactions. Bowie and co-workers studied the collisional activation of 3-methyl-2-butanone and 3,3-dimethyl-2-butanone enolate ions.5 In addition to fragmentations that followed the generalized pathways, unusual fragmentations that were not easily explained were observed. Bowie and co-workers proposed the intermediacy of a homoenolate ion to explain the unusual fragmentations. The initially formed primary enolate ion rearranges to the homoenolate ion. From the homoenolate ion, several different anion/molecule complexes are formed that lead to the unusual fragmentation products. In this paper, we propose two new mechanisms that can explain most of the unusual fragmentations observed by Bowie and co-workers. First, we propose the intermediacy of an alkyl radical/ketene radical anion complex that is formed by homolytic cleavage.10 In contrast, alkyl anion/neutral complexes formed by heterolytic cleavages are the proposed intermediates in most unimolecular fragmentations. In the alkyl radical/ketene radical anion complex, a disproportionation reaction can occur in which the alkyl radical can either abstract a hydrogen from the ketene radical anion to form the deprotonated ketene anion and an X
Abstract published in AdVance ACS Abstracts, March 1, 1996.
S0022-3654(95)02649-9 CCC: $12.00
alkane, or transfer a hydrogen to the ketene radical anion to form an aldehyde enolate ion and an alkene. Second, we propose that vibrationally excited enolate ions can undergo a 1,3-methyl rearrangement before the initially formed enolate ion can fragment. Fragmentation products from enolate ions other than the initially formed enolate ions are then observed. In order to test the proposed mechanisms, we investigated the unimolecular rearrangement and fragmentation of several enolate ions using Fourier transform ion cyclotron resonance mass spectrometry and infrared multiple photon activation techniques. The infrared multiple photon activation of 2-butanone enolate ions and its deuterated analogs (i.e., 2-butanone4,4,4-d3 enolate ions and 2-butanone-1,1,1,3,3-d5 enolate ions) are consistent with the proposed alkyl radical/ketene radical anion complex. The infrared multiple photon activation of 3-methyl-2-butanone and 3-pentanone enolate ions are consistent with the proposed 1,3-methyl rearrangement as well as the alkyl radical/ketene radical anion complex. Experimental Section Chemicals. Nitrogen trifluoride was purchased from Ozark Mahoning and 2-butanone-4,4,4-d3 wa purchased from Cambridge Isotope Laboratories, Inc. 2-Butanone, 3-methyl-2butanone, 3,3-dimethyl-2-butanone, 3-pentanone, 4-methyl-2pentanone, and 4,4-dimethyl-2-pentanone were purchased from Aldrich. All chemicals were used without further purification. All neutrals were degassed on the forelines by several freezepump-thaw cycles before introduction into the high-vacuum chamber. Synthesis of Trimethylsilyl Enol Ether. The primary trimethylsilyl enol ether of 2-butanone-4,4,4-d3 was synthesized by modifying the procedure outlined by House et al.11 Diisopropylamine (14.7 mmol) and n-butyllithium (13.3 mmol) were added to 20 mL of dry distilled THF at 0 °C maintained under nitrogen. This solution was cooled to -78 °C using a dry ice/ acetone bath and 2-butanone-4,4,4-d3 (13.3 mmol) was added slowly. After 5 min, trimethylsilyl chloride (14.7 mmol) was syringed in. The solution was allowed to warm slowly to room temperature. The solution was then washed four times with cold aqueous sodium bicarbonate and once with a cold saturated © 1996 American Chemical Society
7472 J. Phys. Chem., Vol. 100, No. 18, 1996
Sannes and Brauman
sodium chloride solution. The solution was dried and distilled. The primary trimethylsilyl enol ether of 2-butanone-4,4,4-d3 was isolated by preparative gas chromatography (Hewlett-Packard 5790 gas chromatograph equipped with a thermal conductivity detector, 10% SE-30 on Chrom WHP 80/100 mesh column at 90 °C). The structure of the primary trimethylsilyl enol ether of 2-butanone-4,4,4-d3 was confirmed by the observation of the vinyl hydrogens and the methylene hydrogens in the 200 MHz NMR spectrum. The methyl hydrogens are not observed since the hydrogens have been replaced with deuteriums. No peaks corresponding to the secondary isomer were observed in the NMR spectrum. Synthesis of Deuterated Ketones. 2-Butanone-1,1,1,3,3d5 and 3-methyl-2-butanone-1,1,1,3-d4 were synthesized.12 Each ketone was mixed with deuterium oxide in a 1:4 ratio and with sodium deuteroxide (5 mmol). The mixture was stirred for 36 h. The ketone was then fractionally distilled into another sample of basic deuterium oxide. After another 36 h, the mixture was fractionally distilled again. The collected deuterated ketone was not purified further. In the negative mass spectrum of each deuterated ketone, only the M-D peak was observed, indicating that the ketone was g95% pure. The positive ion mass spectrum of the 2-butanone 1,1,1,3,3-d5 showed peaks corresponding to CD3CO+ and CH3CD2CO+. The positive ion mass spectrum of the 3-methyl-2-butanone-1,1,1,3-d4 showed peaks corresponding to CD3CO+ and (CH3)2CDCO+. The positive mass spectra of the deuterated ketones, also, indicates a purity g95%. Ion Generation. The primary ion, F-, was generated from electron impact on nitrogen trifluoride (eq 1). A mixture of isomeric enolate ions was then generated by deprotonation of the appropriate ketone (eq 2). The proton transfer reaction was NF3
e–
O F–+
(1)
F – + NF2 O
O
+
–
(2) –
complete in 200-300 ms. All of the proton transfer reactions are exothermic. Reaction of the primary trimethylsilyl enol ether of 2-butanone-4,4,4-d3 with F- generated the primary enolate ion of 2-butanone-4,4,4-d3 (eq 3); no secondary enolate ion of 2-butanone-4,4,4-d3 is formed.13 Any impurity peaks were ejected prior to infrared multiple photon activation. –
F +
OTMS CD3
Results
O –
The 9.6 µm, R(22) branch, corresponding to 1079.85 cm-1 (3.1 kcal/mol per photon), and the 9.6 µm, P(18) branch, corresponding to 1048.66 cm-1 (3.0 kcal/mol per photon), were used. The laser lines were measured by a CO2 laser spectrum analyzer (Optical Engineering Model 16-A). The laser beam spot size is reduced by an iris before entering the analyzer cell through a potassium chloride window. Passing through a potassium chloride window does not appreciably decrease the intensity of the laser beam. The intensity of the laser beam can be attenuated by passing the beam through CaF2 flats of varying thicknesses. Because the laser beam is reflected back through the ion cloud by a molybdenum mirror (i.e., the laser beam passes through the ion cloud twice), the effective fluence is twice the measured fluence. The energy of the laser pulse was measured using a Scientech 365 power and energy meter with a Scientech 380102 volume-absorbing disk calorimeter. The area of the laser beam spot is calculated from the image left by the laser pulse on thermal paper. The fluence (J/cm2) is calculated by dividing the laser pulse energy (in joules) by the area (in cm2) of the laser beam spot and then multiplying by two. The range of measured fluences was ∼5-7 J/cm2. Reproducibility. The results reported here are based on a small number of working days (1-2 days) in which quantitative data with a high degree of reproducibility was obtained. Several different data sets of multiple scans were obtained at each different fluence value. The reproducibilities of these photoproduct yields were within (1-2%. Additional attempts at data acquisition were made for each compound and qualitative agreement (photoproduct yields within 10-15% of reported values) was observed. The low degree of reproducibility was due to instability in the ion signal prior to irradiation and/or misalignment of the laser beam. Because of possible systematic errors and experimental difficulties associated with the interaction of the high-intensity laser pulse with the analyzer cell, an estimate of the absolute accuracy of the photodissociation yield measurements is taken to be within approximately (5%. However, some cancellation of errors should occur for the photoproduct branching ratios. It is important that the basic conclusions of this paper are not strongly dependent on the quantitative results. The basic conclusions are drawn from the absence or presence of a specific photoproduct and the relative abundances of the photoproducts.
CD3 + FTMS
(3)
Instrumentation. All experiments were performed with a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR)14 equipped with impulse excitation and controlled by the IonSpec OMEGA system.15-17 The analyzer cell was modified for photochemical experiments. In order to allow the laser beam to enter the cell, the front excitation plate was replaced with a plate that has a 7/8 in. mesh covered hole. The back excitation plate was replaced with a molybdenum mirror to reflect the beam back out. Typical operating conditions were -1.20 V trapping voltage, 3.0 A filament current, 3 eV electron energy, and 0.8 T magnetic field. Pressures of the neutrals in the analyzer cell were monitored with a nude ion gauge (Granville-Phillips 307). The background pressure was ∼2 × 10-8 Torr. Typical operating pressures were (1.2-2.0) × 10-7 Torr for NF3 and (2-3) × 107 Torr for the ketones and the primary trimethylsilyl enol ether of 2-butanone-4,4,4-d3. The total pressure was (3.2-5) × 10-7 Torr. A tunable pulsed CO2 laser (Lumonics 103-2 CO2 TEA laser) was used for the infrared multiple photon activation experiments.
General Observations. Because of the nature of the process, infrared multiple photon activation usually results in reactions through the lowest energy pathway.18 For these enolate ions, several fragmentation pathways and vibrational-induced electron detachment were observed. The decrease in the total ion intensity that occurs in the spectra taken with the laser on is attributed to electron detachment which was always the major channel. Consequently, it was not possible to achieve mass balance between the “laser off” spectra and the “laser on” spectra. For the fragmentation pathways, the neutral products are inferred based on mass balance and thermochemical considerations. Products are shown in Table 1. 2-Butanone Enolate Ions. Infrared multiple photon activation (IRMPA) gave ∼50% electron detachment and ∼1.5% fragmentation. The fragmentation channels are (1) ethane loss to produce deprotonated ketene anion; (2) ethylene loss to produce acetaldehyde enolate ion; (3) methane loss to produce deprotonated methylketene anion; and (4) hydrogen loss to produce 3-buten-2-one enolate ion. 2-Butanone-4,4,4-d3 Enolate Ions. IRMPA gave ∼50% electron detachment and ∼3% fragmentation. The fragmenta-
J. Phys. Chem., Vol. 100, No. 18, 1996 7473
56.6 23.5 24.7
18.7
16.0 8.6
18.7
32.4
6.1
6.2
7.8
42.8
6.8
6.6
59.9
7.5 12.7 19.6 6.0
6.5 5.2
40.7
5.7
13.3
13.8 12.9
8.0
9.7 6.8
4.6
20.3
19.3
2.8
3.2 7.7
0.0
0.9 1.1
2.5
7.9 6.7
10.8
37.9
38.2
55.5
•CH CD 2 3
5.8
6.6
tion channels are (1) ethane-d3 loss to produce deprotonated ketene anion; (2) ethyl-d3 radical loss to produce the radical ketene anion; (3) ethylene-d2 loss to produce acetaldehyde-d1 enolate ion; (4) methane loss to produce deprotonated methylketene-d3 anion; and (5) HD loss to produce 3-buten-2-one4,4-d2 enolate ion. A small peak at the mass corresponding to ethylene-d3 loss was also observed. Primary Enolate Ion of 2-Butanone-4,4,4-d3. IRMPA gave ∼35% electron detachment and ∼2.5% fragmentation. The fragmentation channels are (1) ethane-d3 loss to produce deprotonated ketene anion; (2) ethyl-d3 radical loss to produce the radical ketene anion; (3) ethylene-d2 loss to produce acetaldehyde-d1 enolate ion; (4) methane loss to produce deprotonated methylketene-d3 anion; (5) HD loss to produce 3-buten-2-one-4,4-d2 enolate ion; and (6) HD2 loss to produce an ion with m/z 69. 2-Butanone-1,1,1,3,3-d5 Enolate Ions. IRMPA gave ∼32% electron detachment and ∼0.4% fragmentation. The fragmentation channels are (1) ethane-d3 loss to produce deprotonated ketene-d1 anion; (2) ethylene-d2 loss to produce acetaldehyded2 enolate ion; and (3) HD loss to produce 3-buten-2-one-1,1,3d3 enolate ion. No methane-d4 loss was observed. 3-Methyl-2-butanone Enolate Ions. IRMPA gave ∼44% electron detachment and ∼1.5% fragmentation. The fragmentation channels are (1) propane loss to produce deprotonated ketene anion; (2) propylene loss to produce acetaldehyde enolate ion; (3) methane loss to produce 2-methyl-methyleneketene anion; and (4) ethylene loss to produce propionaldehyde enolate ion. 3-Methyl-2-butanone-1,1,1,3-d4 Enolate Ions. IRMPA gave only electron detachment. No fragmentation products were observed. 3-Pentanone Enolate Ions. IRMPA gave ∼80% electron detachment and ∼1.5% fragmentation. The fragmentation channels are (1) propane loss to produce deprotonated ketene anion; (2) ethylene loss to produce propionaldehyde enolate ion; (3) ethane loss to produce deprotonated methylketene anion; and (4) hydrogen loss to produce 1-penten-3-one enolate ion. 3,3-Dimethyl-2-butanone Enolate Ions. IRMPA gave ∼50% electron detachment and ∼1.5% fragmentation. The fragmentation channels are (1) isobutane loss to produce deprotonated ketene anion; (2) isobutylene loss to produce acetaldehyde enolate ion; (3) propylene loss to produce propionaldehyde enolate ion; and (4) methane loss. 4-Methyl-2-pentanone Enolate Ions. IRMPA gave ∼50% electron detachment and ∼27% fragmentation. The fragmentation channels are (1) isobutane loss to produce deprotonated ketene anion; (2) propylene loss to produce acetone enolate ion; and (3) methane loss to produce deprotonated isopropylketene anion. 4,4-Dimethyl-2-pentanone Enolate Ions. IRMPA gave ∼50% electron detachment and ∼20% fragmentation. The fragmentation channels are (1) neopentane loss to produce deprotonated ketene anion; (2) isobutylene loss to produce acetone enolate ion; and (3) methane loss to produce deprotonated tert-butylketene anion.
O
CH3CCH2C(CH3)3
O
CH3CCH2CH(CH3)2
CH3CC(CH3)3
O
O
CH3CH2CCH2CH3
O
CH3CCH(CH3)2
CD3CCD2CH3
O
OTMS
CCH2CD3 CH2
O
CH3CCH2CD3
O
CH3CCH2CH3
Discussion
ketone
neutral product loss
CHDCD2 CH2CD2 CH2CH2 CH4 HD2 HD H2 fluence (J/cm )
2
TABLE 1: Photoproduct Yields from the Infrared Multiple Photon Dissociation of Enolate Ions
CH3CH3
70.8 65.0
CH3CD3
10.7
CH3CHdCH2
62.6
(CH3)2CH2
7.2
(CH3)2CdCH2
65.5
(CH3)3CH
44.1
(CH3)4C
Fragmentation Mechanisms of Enolate Ions
Background. The unimolecular fragmentation reactions of simple alkyl enolate ions have been studied by both collisional activation and infrared multiple photon activation techniques in mass spectrometers.1-9,19-21 The major fragmentation reactions are well understood and explained by generalized pathways.1 These pathways fall into two classes. In the first class, anion/molecule complexes which are low-energy intermediates,
7474 J. Phys. Chem., Vol. 100, No. 18, 1996
Sannes and Brauman
are formed by simple cleavage reactions or internal elimination reactions. Within the anion/molecule complex, the anion can deprotonate the molecule followed by dissociation of the complex. In some cases, the initial anion/molecule complex can dissociate to form the anion and the molecule competitively with proton transfer. For example, simple cleavage in 1,1,1trifluoroacetone enolate ion forms the trifluoromethyl anion/ ketene complex.21 The trifluoromethyl anion deprotonates ketene and the complex dissociates to form trifluoromethane and deprotonated ketene anion (eq 4). In addition, the complex dissociates to form trifluoromethyl anion and ketene (eq 5). In O
O
– CF3CCH2
C
CO– + CF3H
HC
CH2
– CF3
–
CF3 + CH2
C
O
(4) (5)
another example, internal elimination of a hydride ion from 4-heptanone enolate ion forms the hydride ion/2-hepten-4-one complex.7 In the complex, the hydride ion deprotonates 2-hepten-4-one to form hydrogen and 2-hepten-4-one enolate ion (eq 6). O
O –
O
+ H2 (6)
–
H–
In the second class, long-chain alkyl enolate ions undergo an intramolecular 1,6-hydrogen rearrangement followed by an internal elimination reaction. For example, 4-heptanone enolate ion undergoes an intramolecular 1,6-hydrogen rearrangement followed by internal elimination of ethylene to form the primary 2-pentanone enolate ion (eq 7).4,7 The elimination of ethylene O
O – + C2H4
– H
(7)
presumably proceeds through a concerted mechanism. The primary carbanion is not formed as an intermediate. In shortchain alkyl enolate ions, intramolecular 1,3-hydrogen rearrangements can occur followed by formation of an anion/molecule complex. For example, an intramolecular 1,3-hydrogen rearrangement converts the primary 2-butanone enolate ion into the secondary 2-butanone enolate ion.22 A methyl anion/methylketene complex is formed by heterolytic cleavage of the methyl group in the secondary 2-butanone enolate ion. Then, the methyl anion abstracts a proton from methylketene to form methane and deprotonated methylketene anion (eq 8)
O – CH2CCH(CH3)2
O CH3CCHCH3
different anion/molecule complexes that lead to the observed fragmentation products are then formed from the homoenolate ion. Although the proposed mechanism accounts for the observed fragmentation products, there are some problems with it. First, the homoenolate ion is a high-energy intermediate. Protons R to the carbonyl group are acidic; those that are β are surprisingly acidic, but much less so than those that are R.23 The work of Harrison and Ni indicates that the homoenolate ions of 3-methyl2-butanone and 3,3-dimethyl-2-butanone cannot be formed directly by deprotonation of the respective ketones with hydroxide anion.24 Harrison and Ni studied the H/D exchange reactions of deprotonated 3-methyl-2-butanone and 3,3-dimethyl-2-butanone ions with CH3OD and CH3CH2OD. A maximum of three and two deuteriums were incorporated into the deprotonated 3-methyl-2-butanone and 3,3-dimethyl-2-butanone ions, respectively. Only R-hydrogens were exchanged; no β-hydrogens were exchanged. Deprotonation of 3-methyl-2butanone and 3,3-dimethyl-2-butanone with hydroxide anion formed the respective enolate ions of the ketones and not the respective homoenolate ions. Thus, the enolate ions of 3-methyl-2-butanone and 3,3-dimethyl-2-butanone are significantly more stable than the homoenolate ions of 3-methyl-2-butanone and 3,3-dimethyl-2-butanone. It should be noted that under certain experimental conditions, the formation of homoenolate ions has been observed in the gas phase. Squires23a and Kass23b have synthesized homoenolate ions directly. This work shows that while homoenolate ions can be isolated and allowed to undergo reactions, homoenolate ions are significantly less stable than normal enolate ions. In addition, Nibbering and co-workers found that deprotonated tert-butanal can exchange up to 8 hydrogen atoms for deuterium atoms indicating the existence of a homoenolate ion.25,26 Second, the homoenolate ion will probably rearrange to the more stable enolate ion before fragmenting.27 Bowie and coworkers tried to generate the homoenolate ion of 2-butanone using three different methods: (1) collision-induced loss of formaldehyde from the appropriate alkoxide ion (eq 10); (2) collision-induced decarboxylation of the appropriate carboxylate anion (eq 11); and (3) the SN2 displacement of a trimethylsilyl group with methoxide ion (eq 12). The collisional activation O
O
CH3CCH2CH2CH2O– CID O
–
O
O –
C
CHCH3
CID
CH3CCH2CH2CO2–
(10)
CH3CCH2CH2– + CO2
(11)
O
CH3CCH2CH2Si(CH3)3 + CH3O–
–
CH3CCH2CH2– + CH2O O
O O
(9)
– CH2
CH3CCH2CH2– + (CH3)3SiOCH3 (12)
CF3
CH3C
CO– + CH4
(8)
Bowie and co-workers observed unusual fragmentations in the collisional activation of 3-methyl-2-butanone and 3,3dimethyl-2-butanone enolate ions that do not follow the above generalized pathways.5 In order to explain the unusual fragmentations, Bowie and co-workers proposed the homoenolate ion as a key intermediate. A partial equilibrium between the primary enolate ion and the homoenolate ion is established by an intramolecular 1,4-hydrogen rearrangement (eq 9). Several
of the ions formed by the three methods are very similar to the collisional activation of 2-butanone enolate ions. Therefore, under these conditions, the homoenolate ion must rearrange to the primary 2-butanone enolate ion before dissociating (eq 13). O
O –
CH3CCH2CH2
–CH
2CCH2CH3
(13)
Since the three methods form the homoenolate of 2-butanone with excess internal energy, it is possible the homoenolate ion
Fragmentation Mechanisms of Enolate Ions
J. Phys. Chem., Vol. 100, No. 18, 1996 7475
can rearrange to the enolate ion immediately. It is also possible that the homoenolate ion rearranges during collisional activation. Third, the intermediacy of the homoenolate ion does not completely explain the fragmentations of the deuterated analogs of 3-methyl-2-butanone and 3,3-dimethyl-2-butanone enolate ions. The homoenolate ion mechanism predicts products that are not observed. In addition, this mechanism predicts unusual and inconsistent isotope effects (see Appendix). We propose two new mechanisms to explain some of the unusual fragmentations that are observed. First, we propose an alkyl radical/ketene radical anion complex as an intermediate. Second, we propose that a 1,3-methyl rearrangement can occur before the ion fragments. Some of the other fragmentations are explained by the previously generalized pathways. There are still some fragmentations, however, that are not easily explained. The “mechanism” proposed here is a plausible postulate consistent with observations and other mechanistic precedents. It is the simplest rationalization of the experimental facts that we have been able to construct. Alkyl Radical/Ketene Radical Anion Complex. The alkyl radical/ketene radical anion complex is a low-energy intermediate. Formed in the cleavage step, it can be considered as an alkyl radical/ketene complex plus an additional electron. Obviously, the wave function that describes this system must incorporate elements of electron occupation on both sites. To first order, however, the “location” of the additional electron depends on the relative electron affinities of the alkyl radical and the neutral ketene. If the electron affinity of the alkyl radical is less than the electron affinity of the ketene, then the intermediate will look like an alkyl radical/ketene radical anion complex. In the ketene radical anion, it is possible that the electron is dipole bound.28-33 This complex is plausible if the alkyl radical has a negative electron affinity; i.e., the corresponding alkyl anion is unbound.34 If the electron affinity of the alkyl radical is greater than the electron affinity of the ketene, then the intermediate will be an alkyl anion/ketene complex. This complex is expected when the alkyl radical has a positive electron affinity, i.e., when the corresponding alkyl anion is bound. Two products can be formed from the alkyl radical/ ketene radical anion complex, while only one product is formed from the alkyl anion/ketene complex. Within the alkyl radical/ ketene radical anion complex, the alkyl radical can undergo a disproportionation reaction to form deprotonated ketene anion and an alkane or to form an aldehyde enolate ion and an alkene. Disproportionation in radical pairs is a commonly observed process. The reaction involves β-hydrogen atom transfer from one radical to the other; transfers in both directions can be observed, eq 14.
R• + R′• f R-H, R + H, R′-H, R′ + H
(14)
These reactions are known to be very fast, and the rates depend, in part, on the number of β-hydrogens.35 2-Butanone. To test the proposal of the alkyl radical/ketene radical anion complex, the infrared multiple photon activation of the 2-butanone enolate ions and its deuterated analogs was studied. Since the ethyl anion is unbound, an ethyl radical/ ketene radical anion complex is the expected intermediate in the fragmentation of the primary 2-butanone enolate ion (eq 15). Ethane loss to form deprotonated ketene anion and ethylene O –CH
2CCH2CH3
nhν
CH2
•
C
O–
HC
CO– + CH3CH3 O–
•CH CH 2 3 CH2
CH + CH2
(15) CH2
loss to form acetaldehyde enolate ion are observed in the infrared
multiple photon activation of a mixture of 2-butanone enolate ions. The observed products are consistent with the ethyl radical/ketene radical anion complex. In order to determine from which carbon the hydrogen is transferred, the infrared multiple photon activation of a mixture of 2-butanone-4,4,4-d3 enolate ions was studied. Consistent with our mechanism, loss of CH3CD3 to form deprotonated ketene anion and loss of CH2CD2 to form acetaldehyde-1-d1 enolate ion are observed (eq 16). This indicates that the transferred HC O –CH CCH CD 2 2 3
nhν
CH2
•
C
CO– + CH3CD3 O–
O–
•CH CD 2 3
CH2
C• + •CH2CD3 (16) O–
CH2
CD + CH2
CD2
hydrogen comes from the C4 carbon. In addition, loss of •CH CD is observed which is consistent with our mechanism. 2 3 Loss of •CH2CD3 is observed in the dissociation of a mixture of 2-butanone-4,4,4-d3 enolate ions while loss of •CH2CH3 is not observed in the dissociation of a mixture of 2-butanone enolate ions because it is more difficult to transfer a deuterium than a hydrogen. Thus, the deuterated ethyl radical/ketene radical anion complex may dissociate before a deuterium can be transferred. A very small peak at the mass corresponding to CHDCD2 loss (
