Protonated Forms of Iminopropadienones, RN:C:C:C:O, and

Protonated Forms of Iminopropadienones, RN:C:C:C:O, and Cyanoketenes: Combined ab initio MO and Mass Spectrometry Studies. Robert Flammang, Yves ...
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4814

J . Phys. Chem. 1994,98, 4814-4820

Protonated Forms of Iminopropadienones, RN=C=C=C=O, Initio MO and Mass Spectrometry Studies

and Cyanoketenes: Combined ab

Robert Flammang' and Yves Van Haverbeke Department of Organic Chemistry, University of Mons- Hainaut, B-7000 Mons, Belgium Ming Wah Wong,' Andreas Riihmann, and Curt Wentrup' Department of Chemistry, The University of Queensland, Brisbane, Queensland 4072, Australia Received: October 29, 1993; In Final Form: February 25, 1994'

The structures and unimolecular fragmentation reactions of protonated iminopropadienones, RN=C=C=C=O (R=H and CH3), were investigated by a combination of mass spectrometry based experiments (collisional activation (CA), neutralization-reionization (NR), and chemical ionization (CI) mass spectrometry and flash vacuum pyrolysis) and high-level a b initio molecular orbital calculations, at the QCISD(T)/6-3 1 1+G(2dYp) level of theory. The C- and N-protonated forms, RN+=C-CH=C=O and R(H)N+=C=C=O , were separately generated from a variety of precursors. The experiments and calculations indicate that the most favorable site of protonation of iminopropadienones is the central carbon atom (C2), and the resulting ions are best regarded as N-alkylated or N-protonated cyanoketenes. The N-protonated iminopropadienone is close in energy to the C-protonated one, while the 0-protonated form is significantly higher in energy. The proton affinities (298 K) of HNCCCO, CH3NCCC0, N C C E C O , and NCC(CH3)CO are predicted to be 861,920, 784, and 798 kJ mol-', respectively.

1. Introduction A new class of organic molecules, the iminopropadienones, R N = C = C = C = O , isoelectronic with carbon suboxide, C3O2, was discovered in our laboratories recently.' As part of our investigation of these exciting compounds, we report here the gas-phase generation of the C- and N-protonated species using a combination of collisional activation (CA) mass spectrometry,2 neutralization-reionization (NR) mass ~pectrometry,~ chemical ionization (CI),4 flash vacuum pyrolysis (FVP),4 and high-level ab initio molecular orbital (MO) calculations.5 In particular, we wished to determine the most favorable site of protonation in iminopropadienones. 2. Theory

Method and Results. Standard ab initio molecular orbital calculationsSwere carried out with the GAUSSIAN 926 system of programs. Geometry optimizations were performed with the standard polarized split-valence 6-3 lG* basis sets at the HartreeFock (HF) and second-order Morller-Plesset perturbation level^.^ Harmonicvibrational wavenumbers and infrared intensities were predicted at these equilibrium geometries using analytical second derivatives. The directly calculated H F wavenumbers and zeropoint vibrational energies (ZPVE) were scaled by 0.8929 and 0.9 135, respectively, to account for the overestimation of vibrational wavenumbers and ZPVEs at this level of theory.' Heat capacity corrections (H298'HO) were calculated at the HF/ 6-31G' level and used to correct proton affinities to 298 K. Spinrestricted calculations were used for closed-shell systems and spin-unrestricted for open-shell systems. Improved relative energies were obtained through quadratic configuration interaction with singles, doubles, and augmented triples (QCISD(T))8 calculations with the larger 6-31 1+G(2d,p)s basis set, based on the MP2/6-31G* optimized geometries. This level of theory is evaluated with the use of the additivity approximation, Abstract published in Aduance ACS Abstracts, April 1, 1994.

0022-36541941209848 14%04.50/0

AE(QCISD(T)/6-31 l+G(Zd,p)) = AE(QCISD(T)/6-31G*) - AE(MP2/6-31G*) AE(MP2/6-31 1+G(2d,p)) (1)

+

Our best relative energies discussed within the text correspond to QCISD/6-31 l+G(Zd,p) values with zero-point contributions. We have determined how relative energies obtained from eq 1 comparewith thedirectlycalculated QCISD(T)/6-3 1 l+G(Zd,p) values for several representative open- and closed-shell systems considered in this paper. The relative energies calculated on the basis of the additivity scheme are fairly close (f5 kJ mol-I) to the directly computed values. The frozen-core approximation was employed for all correlated calculations. The full set of optimized (MP2/6-3 1G*) CjHNO, CjHzNO+, C4H3N0,and C4H4NO+equilibrium structures is shown in Figure 1. Calculated total and relative energies of the C3H*NO+ and C4H4NO+ isomeric ions and related species are given in Tables 1 and 2, respectively. Vibrational wavenumbers and infrared intensities of C3HzNO+ equilibrium structures (3a-c) are presented in Table 3. Equilibrium Structures of Protonated Iminopropadienones. Iminopropadienones,R N = C I = C ~ = C ~ = O R=H , (1) and CH3 (2), have five possible sites of protonation. In a recent theoretical study, we have shown that the central carbon (C2) and the terminal N and 0 atoms of iminopropadienones bear strong negative charges and are the most probable sites of protonation.Ic Thus, we have considered three protonated species of iminopropadienones: C-protonated (a), N-protonated (b), and 0protonated (c). Preliminary calculations at the MP216-3 lG* level have confirmed that protonation at C I and C3 atoms is less favorable, -300 kJ mol-' above the most stable structure. The most stable C3H2NO+ion corresponds to the C-protonated form (3a). 3a is more stable than the N- (3b) and 0-protonated (3c) forms (Figure 1) by 53 and 127 kJ mol-', respectively. Likewise, the C-protonated form (4a) is the lowest-energy structure for protonated N-methyliminopropadienone (4). The N-protonated species (4b) is significantly less stable than the C-protonated form (4a), by 89 kJ mol-1. Note that the isomeric structure HNCC(CHs)CO+ (7) lies close in energy (+17 kJ mol-I) to the 0 1994 American Chemical Society

Protonated Forms of Iminopropadienones

nw

The Journal of Physical Chemistry, Vol. 98, No. 18, 1994 4815

1.011 N

171.4

1.278 179.6

1.159 1393

1319

H

1331

1.022 1329

1249

-I+

0

144.6

l+

l.mH'%, 1519 133.0 1.485

1548

c l z - qn

1.039

w.7

8 (Cl)

9 (Cl)

Figure 1. Optimized geometries (MP2/6-3 1G*) of CjHNO, C s H z N O + , C d H j N O , and C d H 4 N O + equilibrium and transition structures (bond lengths in angstroms and bond angles in degrees).

most stable C-protonated form 4a. The high stability of the C-protonated form has also been observed for the isoelectronic analogue, carbon suboxide ( C ~ O Z ) In . ~ summary, the central carbon ((22) is the preferred site of protonation for iminopropadienones. The structures of 3a and 7 are best considered as the N-protonated forms of cyanoketene (5) and cyano(methy1)ketene (6), respectively. The C-N bond lengths are significantlyshorter in the protonated forms, by 0.015-0.017 A (Figure 1). The N-protonated forms (3b and 4b) are predicted to have a linear NCCCO skeleton. The C & and (23-0 bond lengths in 3b and 4b are longer than those in the corresponding neutral iminopropadienones (1 and 2, respectively) and vice versa for the N-C, and C2-C3 equilibrium distances. To assist future characterization of the protonated iminopropadienones,the HF/6-31G* IR spectra of the three isomeric structures of C3H2NO+are reported in Table 3. Our predictedvibrationalwavenumbers wereobtained by scaling the directly calculated values by a factor of O.892ge7 We have considered the rearrangement of the N-protonated ion (3b) to the more stable C-protonated form (3a). The fourcenter transition structure for the 1,3-hydrogen shift (8, Figure 1) is calculated to lie 513 kJ mol-' above 3b. Thus, the N-protonated iminopropadienone represents a stable structure on the C J H ~ N O +potential energy surface and should be observable in the gas phase.

Unimolecular Dissociations of CdzNO+ Isomeric Structures. The unimolecular fragmentation reactions of the three stable protonated forms of iminopropadienone, 3a-q have been examined. As can be seen in Table 2, dissociation to CO + HCCNH+ ( m / z 40) represents the lowest-energy fragmentation process of the C-protonated form ( 3 4 . Loss of a hydrogen atom is the next most favorable dissociation reaction. The simple bond cleavage processes leading to NC-CHCO'+ and HCCO+ ( m / z 67 and 41, respectively) are also favorable. Fragmentation of 3a to N H CCHCO+ ( m / z 53), the characteristic dissociation of 3a, is energetically competitive. However, loss of NH+ ( m / z 15) is highly unfavorable (Table 2). Note that there is a stable cyclic n structure HNCCHCO+ (9, Figure 1) which lies 489 kJ mol-1 above 3a. Fragmentation of 9 may lead to C*H+ ( m / z 25) HNCO (endothermic by 468 kJ mol-'). Thus, a peak at m / z 25 may also represent a unique dissociation of 3a. As with 3a, the losses of CO and H', giving HzNCC+ ( m / z 40) and HNC3O'+ ( m / z 67), respectively, are the most favorable fragmentation processes for the N-protonated form (3b) (Table 2). Fragmentation of 3b to NH2 CpO+ ( m / z 52), the characteristic dissociation of 3b, is also favorable. Dissociation to C 2 0 and to H2NCC both result in ions of m / z 28. For the 0-protonated species (3c), loss of a hydrogen atom is the energetically most favorable fragmentation process (Table 2). Fragmentation to HNCC + COH+ ( m / z 29), the charac-

+

+

+

4816 The Journal of Physical Chemistry, Vol. 98, No. 18, 1994

Flammang et al.

TABLE 1: Calculated Total Energies' (hartreea) and Zero-Point Vibrational Energies' (ZPVE, kJ mol-1) molecule state symmetry MP2/6-31GS QC1SD(T)/6-31GS MP2/6-31 l+G(2d,p) HNCCHCO+ 13d H2NCCCO+ (3b)' HNCCCOH+ (3c) n HNCCHCO+ (9) T.S.: 3a 3b (8) HNCCCO (1) NCCHCO (5) CH,NCCHCO+ ( 4 4 HNCC(CH,)CO+ (7) CH,(H)NCCCO+ (4b) CHaNCCCOH+ (4~) CHpNCCCO (2) NCC(CH3)CO (6)

-

occco

OCCHCO+ NCCHCO'+ HNCCCO'+ HCCNH' HCCNH+ HNCCC HNCCC'+ CCHCO' CCHCO+ CCCOH' CCCOH+ H2NCC' H2NCC+ H2NC+ H2NC' HNCO HCCO' HCCO+ CCOH' CCOH+ CaO C3@+

c20

c20'+ C2H+ NH2' NH2+ HNC HNC'+ HOC' HOC+

-244.462 49 -244.441 63 -244.409 37

-244.505 10 -244.483 12 -244.450 86

-244.621 52 -244.603 94 -244.574 36

122.4 126.5 122.8

'A 'A 'A' 'A' 'A' 'A' 1A' 'A' 'A 'A'

-244.260 65 -244.224 96 -244.123 32 -244.155 92 -283.645 76 -283.636 55 -283.610 73 -283.593 06 -283.287 44 -283.324 71 -264.006 57 -264.308 38 -243.777 29 -243.774 59 -131.607 10 -131.270 81 -168.982 38 -168.616 56 -189.429 62 -189.053 43 -189.331 77 -188.980 67 -131.566 23 -131.233 51 -93.340 95 -93.625 10 -168.221 06 -151.479 35 -1 5 1.099 96 -151.363 41 -151.001 86 -188.860 03 -188.452 76 -150.794 11 -150.442 73 -75.842 95 -55.690 86 -55.241 40 -93.125 54 -92.689 34 -1 13.455 66 -113.184 16 -113.021 22 -1 12.509 44 -55.058 78 -54.569 66 -75.521 03 -75.065 77 -0.498 23

-244.319 56 -244.277 21 -244.160 06 -244.196 99 -283.704 07 -283.694 44 -283.668 70 -283.649 91 -283.338 62 -283.381 06 -264.035 20 -264.343 11 -243.841 73 -243.830 50 -131.659 44 -131.316 31 -169.024 97 -168.684 92 -189.468 85 -189.103 60 -189.390 66 -189.049 00 -131.611 35 -131.289 33 -93.376 88 -93.658 35 -168.243 70 -151.514 64 -151.136 86 ,-151.407 80 -151.053 15 -188.896 21 -188.513 06 -150.834 39 -150.483 77 -75.887 49 -55.71 1 56 -55.271 47 -93.150 60 -92.725 98 -113.483 12 -113,210 45 -1 13.038 99 -112.538 24 -55.079 33 -54.597 71 -75.537 17 -75.085 61 -0.498 23

-244.418 90 -244.390 77 -244.289 96 -244.3 15 63 -283.839 72 -283.832 85 -283.807 41 -283.793 36 -283.487 85 -283.521 57 -264.184 70 -264.482 82 -243.924 84 -243.926 83 -131.703 75 -131.352 14 -169.091 31 -168.708 01 -189.552 33 -189.165 58 -189.466 54 -189.096 18 -131.665 87 -131.313 51 -93.403 01 -93.703 47 -168.346 97 -151.589 69 -151.197 08 -1 5 1.480 02 -151.100 59 -188.980 39 -188.559 11 -150.893 26 -150.529 47 -75.880 51 -55.748 58 -55.287 52 -93.191 58 -92.743 45 -1 13.548 80 -113.261 87 -113.100 36 -112.580 83 -55.101 24 -54.604 56 -75.594 44 -75.125 23 -0.499 8 1

122.2 106.4 88.0 91.7 204.1 202.2 207.9 204.2 171.6 172.0 62.6 92.1 84.7 83.5 81.2 82.5 69.0 69.2 69.8 71.7 68.4 69.0 91.8 93.1 73.2 75.8 59.8 50.2 51.2 56.6 54.1 44.1 40.7 26.7 22.7 48.3 54.0 51.7 44.8 43.8 39.9 36.6 14.6 14.3 21.1 20.4 23.9 20.1 0.0

'2*+ 'Ai 2A'f 2A" 2Alf 'A' LA' 22+

2A' 3A' 2A' 3A' 2B~ 'AI 'AI 'B2 'A' 2Afl 'A' 2A 'A'

'2+ 22+

'2+ 2n

'2+ 2B~ 'Ai

co

'2+ *2+ 2A' '2+ '2+

CO'+ NH NH'+ OH' OH+ H'

22+ 3 22n 2n 322 s

a

ZPVE

'A' 'AI 'A'

Based on MP2/6-31G*-optimized geometries. HF/6-31G* values.

teristic dissociation of 312, is endothermic by 612 kJ mol-'. Dissociations leading to CCOH+, HNCC+, and HNC*+ ( m / z 41, 39, and 27, respectively) are also favorable. In summary, the three protonated forms of iminopropadienones (3a-c) are calculated to have different fragmentation reactions. Thus, their structures may be readily deduced from the CA mass spectra. Proton Affinities. To provide an estimation of the likely accuracy of our computed proton affinities (PA) of iminopropadienones a t the QCISD(T)/6-31 l+G(2d,p)+ZPVE level, we have determined also the proton affinity of the isoelectronic analogue, carbon suboxide (C3O2).'O The calculated proton affinity PA a t 298 K (778 kJ mol-') is in excellent agreement with theexperimentalvalue (780 kJ mol-')? This lendsconfidence to our predicted proton affinity of iminopropadienones. The calculated proton affinities (298 K) of HNCCCO (1) and CH3NCCCO (2)are 861 and 920 kJ mol-', respectively, while the

predicted PA for the corresponding cyanoketene isomers, NCCHCO (5) and NCC(CH3)CO (8), are 784 and 798 kJ mol-', respectively.

3. Mass Spectrometry Protonated Iminopropadienone (3). Two protonated forms of iminopropadienone, HN+=C-CH=C=O (3a) and H2N+= C=C=C=O (3b),should be accessible by dissociative ionization of 4-chlorouracile (10)and 5-amino-4-methoxycarbonylisoxazole (12),resectively.11Jb Indeed, the molecular ions lo'+,formed by electron impact, fragment by losing HN=C=O in a retro DielsAlder reaction and subsequently eliminating the chlorine atom (Scheme 1). The base peak in the CA spectrum of the resulting C-protonated iminopropadienone ions ( 3 4 is due to decarbonylation, giving m/z 40 (C*H2N+) (Figure 2a). This decarbonylation is already observed, but to a lesser extent, without the collision gas. Just as in the case of the oxygen analogue,

Protonated Forms of Iminopropadienones

The Journal of Physical Chemistry, Vol. 98, No. 18, 1994 4817

TABLE 2 Calculated Relative Energies' (kJ mol-') of Protonated Iminopropadienones and Their Decomposition Products species ~

SCHEME 1

relative energy

~~~

HNCCHCO+(3a) H2NCCCO+ (3b) HNCCCOH+(3c)

-

n

HNCCHCO+ ( 9 ) T.S.: 3a 3b (8) HNCCCO (1) H+ NCCHCO (5) + H+ NCCHCO'+ ( m / r 67) + H' HCCNH+ ( m / r 40) + CO HCCNH' + COB+( m / z 28) HCCO+ ( m / z 41) + HNC HCCO' H N W ( m / z 27) HNCO + C2H+ ( m / z 25) CCHCO+ ( m / z 53) + NH CHCCO' NH'+ ( m / s 15)

+

+

+

0.0 52.8 127.1

11

13

488.9 565.9 854.5 779.0 425.1 366.4 758.9 536.4 654.2 957.0 830.4 1125.6

1

5

H2NCCCO+ (3b) HNC3O'+ ( m / r 67) + H' HzNCC+ ( m / z 40) CO HzNCC' + CO'+ ( m / z 28) H2NC+ ( m / z 28) C20 H2NC' + C2O'+ ( m / z 40) C 3 0 * +( m / z 52) + NH2' C3O NH2+ ( m / z 16)

0.0 388.4 397.6 834.2 692.4 862.3 645.0 789.3

HNCCCOHt(3c) HNC3O'+ ( m / z 67) + H' HNCC COH+ ( m / z 29) HNCC'+ ( m / z 39) + COH' HNC C20H+ ( m / z 41) HNC'+ ( m / z 27) C20H' HN + CpOH+ ( m / z 53) HN'+ ( m / z 15) C3OH' HNC3 OH+ ( m / r 17) HNC3'+ ( m / r 51) OH' CHsNCCHCO+ (4a) HNCC(CH3)CO+ (7) CHs(H)NCCCO+ (4b) CH3NCCCOH+ ( 4 ~ ) NCC(CH3)CO (6) + H+ CH3NCCCO (2) + H+

0.0 314.1 612.1 769.7 627.5 797.0 835.3 1170.9 1099.5 2038.3

+

+

+

+

+

+

+

+

+

a Based on calculated total 31 l+G(Zd,p)+ZPVE] from Table 1.

0.0 17.4 89.2 125.6 811.1 9 12.8

energies

3a

40

[QCISD(T)/6-

TABLE 3 Calculated (HF/6-31G*) Vibrational Wavenumben. (cm-1) and Infrared Intensities6 (km mol-') of Protonated IminoDroDadienones ~~

C-protonated form l a N-protonated form l b 0-protonated form I C A'

A"

3520 3028 2306 2195 1335 1087 959 704 64 1 435 148 612 523 416 368

(1076) (107) (90) ( 571j 170) (42) (11) 151) (4) (13) (4) 148) (2) (50) 187)

A1

B1

B2

3325 2287 2161 1583 1391 707 564 539 505 100 3428 1108 618 419 147

(428) (2222) (1590)

A'

A"

3522 3508 2333 2246 1360 1132 688 603 579 455 149 587 528 503

(341) (1454) (1668) -(66oj (26) (386) (80) (173) (27) (23) (14) (58) (141) (39)

137

(21)

aScaled by a factor of 0.8929 (ref 7). *Intensity values are in parentheses.

O==C=CH-C-+-9 structurally significant peaks are also observed at m / z 53 (loss of NH) and m / z 25 (C2H+). Supportive evidence that the ion produced from lo'+possesses the structure 3a was obtained by protonation of cyanoketene (5) in a CI experiment. Cyanoketene (5) was produced by FVP of the 3,s-dimethylpyrazolide of cyanoacetic acid (13) at 610 O C

Figure 2. (a) CA (02) spectrum of H N + e - C H = C = O ions 38 ( m / z 68) produced by dissociative ionization of 4-chlorouracile (10) (EI, 70 eV); an identical spectrum is obtained by protonation of cyanoketene (5) (CI, methanol). The regions m / z 24-29 and m / z 50-53 are amplified by a factor of 10. (b) CA (02) spectrum of C3H2NO+ ions 3b ( m / z 68) produced by dissociative ionization (EI, 70 eV) of 5-amino-4-methoxycarbonylisoxazole (12).

(Scheme 1). The CA spectrum of the resulting ions was identical with that of the m / z 68 ions from lo'+ shown in Figure 2a. Accordingly, the ions 3a are best regarded as N-protonated cyanoketene. They are identical with the expected result of C-protonation of iminopropadienone ( l ) , but they cannot be produced directly that way because neutral 1, on generation by FVP of 11 or 12, tautomerizes to cyanoketene (5) due to wall collisions (Scheme l ) . l a * b A different m / z 68 ion, assigned the N-protonated iminopropadienone structure 3b, was produced by dissociative ionization of the 5-amino-4-methoxycarbonylisoxazole(12). The ion 12'+ sequentially loses CH30 and HN=C=O1* to produce the m / z 68 ion (Scheme 1 and Figure 2b). Here, the intensities of signals a t m / z 53 and 25 in the CA spectrum are significantly weaker

Flammang et al.

4818 The Journal of Physical Chemistry, Vol. 98, No. 18, 1994

SCHEME 2

4b

a

54

”%

b

than in the spectra of the isomeric ion 3a discussed above. Thus, in contrast to the C3H20+case? it appears that post-collisional isomerization of ions 3b into 3a is not an important process as evidenced by the low intensity of the m/z 53 peak in the spectrum of 3b (compare Figure 2a and 2b) and the high calculated barrier for interconversion of the ions (section 2). Finally, a third C3HNO+ isomer presenting the NCCCO connectivity, NC-CH2-CO+, has already been described in the literature.” However, this ion features a clearly distinct CA spectrum dominated by an intense loss of CN’ (giving m/z 42), which is not observed for 3a or 3b.I4 Protonated Methyliminopropadienone (4). The problem of tautomerization of neutral iminopropadienone (1 5) can be overcome by studying a homologous molecule, the methyliminopropadienone (2) which can be cleanly prepared by FVP of the Meldrum’s acid derivatives 14 and 15.1a.b At 600 OC, 14 loses Me2NH, acetone, and C02 (Scheme 2). Online chemical ionization of 2 with isobutane affords [CH3NCCCO + H]+ ions (mlz 82) whose CA spectrum is shown in Figure 3a. Intense signals a t m/z 54 (loss of CO) and 41 [CH=C=O+ together with charge-stripped ions CH3NCHCC02+]are indicative of a C-protonated structure 4a. This was confirmed by the preparation of the same ion and hence a very similar spectrum by dissociative ionization of 4-chloro- 1,3-dimethyluracile (16) (Scheme 2 and Figure 3b). Furthermore, this spectrum was also produced by N-methylation of cyanoketene (5) using chemical ionization with chloromethane following FVP of 13 (Scheme 2) (an additional small peak at m/z 42 was also observed in this spectrum). However, a different spectrum is obtained on dissociative ionization of 14 or 15 (Figure 3c). 15’+eliminates acetone, C02, and CHjS‘ to produce an m/z 82 ion assigned the structure 4b (Scheme 2). The major difference between the isomeric ions 4a and 4b is that the latter shows a much stronger signal at m/z 67 (HN=C=C=C=O*+, l*+)due to loss of a CH3’ radical. Conclusive corroboration of the assignment of structures to 4a and 4b is given by the following MS/MS/MS experiments. The N-protonated ions, m/z 82 (4b),obtained by dissociative ionization of the Meldrum’sacidderivative 19+,weresubjected tocollisional activation with He. The resulting m / z 67 ions formed by loss of CH3’wereidentical with themolecular ionof theunsubstituted iminopropadienone HN=C=C=C=O (lo+),reported earlier (Figure 4a).Ib Loss of N H (to give m/z 52) and a very strong signal due to the doubly charged ion (mlz 33.5) are characteristic of lo+.Thus, the precursor ions likely had the structure 4b. FVPof 15 at 480 OC followed by E1 produced another structure of m/z 82 ions which on CA (He) eliminated CH3’. The m/z 67 ions so produced now possessed a new structure (Figure 4b), identical with that of cyanoketene (9+) (Figure 4c). Note in particularanewpeakatmlz 53 (lossofN), theincreasedintensity

b.

I

C

67

1

-

Figure 3. CA ( 0 2 ) spectra of C,H4NO+ ions ( m / z 82) produced by (a) FVP (480 “C)of 14 followed by CI (isobutane), giving 4a; (b) dissociative ionization of uracile 16 (giving 4a); (c) dissociative ionizationof IS (giving 4b);and (d) FVPof 17(61O0C),givingcyano(methyI)ketene(6),followed by CI to 7.

ofmlz41 (CH=C==O+),and thereducedintensityofthechargestripping signal a t m/z 33.5. Thus, in this case, the precursor m/z 82 ions are concluded to have the structure 4a. Moreover, we confirmed the sequence 15 2 4a 5’+ (Scheme 2) by subjecting the pyrolyzate from 15 at 700OC to chemical ionization with methanol, followed by an MS/MS/MS experiment (m/z 82- m/z 67). Thecyanoketene molecule ion (So+)was identical with that described in Figure 4b. Finally, a third isomer of 4a and 4b, the N-protonated cyano(methy1)ketene (7) was produced by chemical ionization of the ketene 6,itself formed by FVP of the pyrazolide 17 (Scheme 3). This spectrum (Figure 3d) is clearly different from those of 4a and 4b. The major differences are the absence of a peak at m/z

-- -

The Journal of Physical Chemistry, Vol. 98, No. 18. 1994 4819

Protonated Forms of Iminopropadienones

cycloaddition reaction^.^^ However, to the best of our knowledge, no spectroscopic observation of 6 has been published previously.

4. Conclusions The C-protonated iminopropadienone 3a is obtained by dissociation of the uracile cation lo’+ and by N-protonation of cyanoketene (5).The isomeric N-protonated iminopropadienone (3b)is obtained by dissociative ionization of the isoxazole 12. Neutral N-methyliminopropadienone (2)is obtainable by FVP of Meldrum’s acid derivatives 14 and 15. Chemical ionization of 2 as well as dissociative ionization of uracile 16 produce N-methylated cyanoketene 4a (identical with C-protonated methyliminopropadienone). Demethylation of this ion gives the cyanoketene molecular ion (5’+),and methylation of cyanoketene (5)with chloromethane again produces 4a. The N-protonated methyliminopropadienone (4b)is obtained by dissociative ionization of the Meldrum’s acid derivatives 14 and 15.16 Demethylation of 4b in an MS/MS/MS experiment (from 15) gives rise to the molecular ion of iminopropadienone

--30039

(lo+). Thus, experiments demonstrate that protonation at C2 is preferred in (methy1)iminopropadienone;however, both C- and N-protonated forms are stable on the microscecond time scale. Theory agrees that the C-protonated ions are of the lowest energy; the N-protonated forms are accessible and stable; and the 0-protonated forms are substantially higher in energy and have not, so far, been observed experimentally.

-

Fipre4. (a) MS/MS/MS of 15’+: m / z 82 (EI) m / z 67 [CA (He)]. The CA ( 0 2 ) spectrum of m / z 67 (l*+) is shown. (b) FVP (480 OC)/EI of 15 produces m / z 82. C A (He) of m / z 82 produces m / z 67 (5’+),the C A (02)spectrum of which is shown. (c) C A ( 0 2 ) of cyanoketene (5’+) produced from 13 by FVP/EI [*=m/z 33.5 (charge-stripping peaks)].

SCHEME 3

17

6

7

40, the enhanced intensity of the peak a t m / z 28 corresponding probably to H N C H (loss of propadienone), and the cleanly resolved peak for [M-HI2+ ions. Evidence for the structurecyano(methy1)ketene (6) was also adduced by Ar matrix isolation of the pyrolyzate formed from 17 a t 810 OC. The FT-IR spectrum of 6 (Ar, 12 K) featured a strong band at 2145 cm-1 (UC-C-O) together with a peak at 2231 cm-1 (Y”). On isolation without Ar, followed by warmup, the ketene was stable until ca. 120 K. Isolation of the pyrolyzate at room temperature resulted in recombination of 6 and dimethylpyrazole, regenerating 17. The ketene 6 was also trapped with methanol to give methyl 2-cyanopropanoate. Cyano(methy1)ketene (6)has been generated before, from different precursors, and its existence has been inferred from trapping with nucleophiles as well as [2,+2,]

+

5. Experimental Section A six-sector VG Analytical AutoSpec 6 F spectrometer of E1B1E2E3B2E4geometry (E electric sector, and B magnetic sector)17 was used for obtaining the collisional activation (CA) spectra and the neutralization-reionization (NRMS) spectra. For all experiments, a beam of fast ions (8 keV) was mass-selected by a combination of the first three sectors (EIB1E2) and the spectra were recorded by scanning E3 and collecting the ions in the fifth field-free region. CA spectra were obtained using oxygen as the collision gas (80% transmission). For the NRMS, neutralization of the ions with NH3 (30% T) precedes reionization with 0 2 (80% T), unreacted ions being eliminated by floating a t 9 kV the intermediate calibration ion source inserted between the twocells. CI experiments used methanol or isobutane (giving same result) for protonation, and chloromethane for methylation. The pyrolysis device consisting of a quartz tube (3-mm i.d., 50-mm length) installed in the source housing of the spectrometer has been described el~ewhere.~ 4-Chlorouracile (10)wascommercially available (Sigma). The other samples were prepared according to the literature: 5- [amino(methylthio)methylene]-2,2-dmethyl1,3-dioxane4,6dione (1 1),18 5-amino-4-methoxycarbonylisoxazole(l2),I9 2,2-dimethyl-5[methylamino(dimethylamino)methylene]-1,3-dioxane-4,6,dione (14),18and 2,2-dimethyl-5-[methylamino(methylthio)methylenel- 1,3-dioxane-4,6-dione (15),18 1,3-dimethyl-4-chlorouracile (16),20 and 1-(2-cyanoacetyl)-3,5-dimethylpyrazole(13), and cyanoketene (5).21 1-(2-Cyanopropanoyl)-3,5-dimethylpyrazole(17). To a solution of 1.94 g (20.1 mmol) of 3,5-dimethylpyrazole and 4.17 g (20.2 mmol) of dicyclohexylcarbodiimide in 18 mL of dry methylene chloride, a-cyanopropionic acid22 (2.0 g, 20.2 mmol) in 2 mL of dry methylene chloride was added dropwise a t 0 O C . The reaction mixture was stirred overnight a t room temperature and then filtered off. The filtrate was evaporated in vacuo and the crude product was distilled in high vacuum. 17 (2.32 g, 65% yield) was isolated as a pale yellow liquid (bn.m8 85 “C): FTI R (neat) 2930 (w), 2250 (vw), 1735 (vs), 1700 (w), 1650 (w), 1590 (m), 1560 (w), 1550 (w), 1490 (w), 1460 (m), 1440 (m), 1410 (m), 1390 (s), 1355 (vs), 1300 (m), 1240 (w), 1170 (w), 1140 (w), 1110 (w), 1030 (w), 990 (w), 960 (m) cm-1; IH NMR

4820 The Journal of Physical Chemistry, Vol. 98, No. 18, 1994

Flammang et al.

(5) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio (CDCl3) 6 1.66 (d, 3 H, J = 7.3 Hz, H-3'),2.21 (s, 3 H, 3-CH3), Molecular Orbital Theory; Wiley: New York, 1986. 2.53(d,3H,J=l.OH~,5-CH3),4.88(1,1H,J=7.3H~,H-2'), (6) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; 6.01 (d, 1 H, J = 1.0 Hz, H-4); '3C NMR (CDCl3) 6 13.79 (q, Wong, M. W.; Foresman, J. 8.; Johnson, B. G.; Schlegel, H. B.; Robb, M.

3-CH3), 14.12 (5-CH3), 15.56 (q, C-3'), 31.92 (d, C-2'), 112.47 (d, C-4), 117.73 (s, - C z N ) , 144.70 (s, C-3), 153.68 (s, C-5), 166.12 (s, C-1'); UV (EtOH) Amax(€) 261 nm (2584); MS m / r (re1 intensity) 177 (25, M+), 123 (13), 122 (22), 96 (loo), 95 (90), 81 (22), 61 (83), 56 (24), 54 (34), 52 (19), 44 (53), 43 (64); HRMS calcd for C9HllN30177.0902, found 177.0906; R ~ 0 . 5 3 (n-hexaneldiethy1 ether, 7:3). FVPof 17. At room temperature, 10-15 mg (0.06-0.08 mmol) of 17 was gently evaporated into the pyrolysis apparatus23(10cm quartz tube; i.d. 8 mm; 810 "C). The product was deposited on a BaF2 disk at 17 K, and FT-IR spectra were recorded periodically to monitor the formation of the matrix. After 20 min, a strong and a weak absorption at 2142 cm-1 (YC=+O) and at 2228 cm-1 ( v - N ) , respectively, were observed, due to the formation of ketene 6. In an argon matrix (4 X 10-5 mbar, 100 mbar of Ar in 20 min), these bands were slightly shifted to higher wavenumbers (2145 and 2231 cm-1). These two bands disappeared when the solid sample was warmed to 120 K. When a preparative pyrolysis23 was performed under the same conditions, only the starting material 17 was isolated, thus suggesting recombination of the intermediate ketene 6 with the 3,5dimethylpyrazole. The formation of 6 was confirmed in a trapping experiment with methanol which was continuously deposited onto the cold finger (-196 "C) together with thepyrolysisproducts. Subsequent lH NMR analysis of the cold red solution showed the presence of methyl 2-cyanopropanoate and 17 in a 4:l ratio. A control experiment at room temperature proved that the alcoholysis of 17 in excess methanol proceeded at a much slower rate (tip 80 min).

-

Acknowledgment. The Mons laboratory thanks the Fonds National de la Recherche Scientifique for its contribution in the acquisition of the new tandem mass spectrometer VG AutoSpec 6F. The Brisbane laboratory thanks the Australian Research Council for financial support and for a Research Fellowship for M.W.W. References and Notes (1) (a) Mosandl, T.;Kappe, C. 0.;Flammang, R.; Wentrup, C. J . Chem. Sac. Chem. Commun. 1992,1571. (b) Flammang,R.;Laurent,S.; FlammangBarbieux, M.; Wentrup, C. Rapid. Commun. Mass Specfrom. 1992,6,667. (c) Mosandl, T.; Stadtmiiller, S.; Wong, M. W.; Wentrup, C. J . Phys. Chem. 1994, 98, 1080. (2) See, for example: (a) McLafferty, F. W. Phil. Trans.R . Sot. London A 1979, 293.93. (b) Levsen, K.; Schwarz, H. Mass Specfrom. Rev. 1933, 2, 77. (c) Holmes, J. L. Org. Mass Spectrom. 1985, 20, 769. (3) For reviews see: (a) Wesdemiotis, C.; McLafferty, F. W. Chem. Rev. 1987,87,485. (b) Terlouw, J. K.; Schwarz, H. Angew. Chem., Int. Ed. Engl. 1987, 26, 805. (c) Holmes, J. L. Mass Spectrom. Rev. 1989, 8, 513. (d) Plisnier, M.; Flammang, R. Chim.Nouu. 1990,8, 893. (e) McLafferty, F. W. Science 1990, 247, 925.

(4) Cf.: Brown, J.; Flammang, R.; Govaert, Y.; Plisnier, M.; Wentrup, C.; Van Haverbeke, Y. Rapid. Commun. Mass Specfrom. 1992, 6, 249.

A.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.;Gonzalez, C.; Martin, R. L.; Fox, D. J.; DeFrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. GAUSSIAN92;Gaussian Inc.: Pittsburgh, PA, 1992. (7) Pople, J. A.; Scott, A. P.; Wong, M. W.; Radom, L. Isr. J . Chem. 1993, 33, 345. (8) Pople, J. A.; Head-Gordon, M.; Raghavachari, K. J . Chem. Phys. 1987.87, 5968. (9) Morizur, J. P.; Provot, G.; Tortajada, J.; Gal, J.-F.; Maria, P.-C.; Flammang, R.; Van Haverbeke, Y. In!. J . Mass Spectrom. Ion Process,

submitted for publication. (10) The calculated H298 - HO values, based on scaled HF/6-31G* wavenumbers, for 1-3a, 4a, 5-7, C302, and OCCHCO+ are 16.6,21.2, 15.8, 20.6, 14.7, 18.9, 20.5, 14.7, and 14.5 kJ mol-', respectively. (1 1) Porter, G. N. Mass Specfromefryof Heterocyclic Compounds; John Wiley and Sons: New York, 1985; p 731. (1 2) 5-Aminoisoxazole-4-carboxylicacid esters of type 12 eliminate CHlO followed by HNCO regardless of the nature of a substituent at position 3. This indicates that the exocyclic nitrogen atom (position 5) is lost. Accordingly, it is isocyanic (HNCO) rather than fulminic (HCNO) acid that is lost. This was also demonstrated by deuterium labeling in the case of 5-amino4-cyanoisoxazoles: Flammang, R.; Laurent, S.; Flammang-Barbieux, M.; Wentrup, C. Org. Mass Spectrom. 1993, 28, 1161. ( 1 3) Flammang, R.; Plisnier, M.; Bouchoux,G.;Hoppilliard, Y.; Humbert, S . ; Wentrup, C. Org. Mass Specfrom. 1992, 27, 317. (14) Recovery signals corresponding to survivor ions were observed ( m / z 68), albeit with reduced intensity, in the NR mass spectra of both ions 3a and 3b. These spectra are in fact quite similar. Intense signals at m / z 67 probably arise from the reionization of neutral iminopropadienone form4 by H' loss in the neutralization step. The recovery signal at m / z 67 is indeed the base peak in the NRMS of iminopropadienone ions.'b By analogy with the CaH02+ case? it is possible that the survivor ions can be ascribed in both cases to the most stable isomer, 3a, following post-collisional isomerization in the case of 3b. Unfortunately, the reduced intensity of the recovery signals precluded the recording of a subsequent collisional activation spectrum of the survivor ions. (15) (a) Weyler, W., Jr.; Duncan, W. G.; Moore, H. W. J . Am. Chem. Sot. 197S97.6187. (b) Kunert,D. M.;Chambers,R.;Mercer,F.;Hernandez, L., Jr.; Moore, H. W. Tetrahedron Left. 1978, 19, 929. (c) Neidlein, R.; Cepera, K. F. Chem. Ber. 1978, 1 1 1 , 1824. (d) Moore, H. W.; Wilbur, D. S. J . Org. Chem. 1980, 45, 4483. Moore, H. W.; Gheroghiu, M. D. Chem. Sot. Rev. 1981, 10, 289. (e) Moore, H. W.; Hughes, G.; Srinivasachar, K.; Fernandez, M.; Nguyen, N. V.; Schoon, D.; Tranne, A. J . Org. Chem. 1985, 50, 4231. (16) In the case of 14, dissociative ionization also gives rise to a m / z 97 peak, which generates m / z 82 by loss of CH3'. Thus, there are two possible sources of 4b ( m / z 82) here: direct fragmentation as for I5 (Scheme 2), and loss of CH3' from m / z 97. (17) Bateman, R. H.; Brown, J.; Lefevere, M.; Flammang, R.; Van Haverbeke, Y. Int. J. Mass Spectrom. Ion Processes 1992, 115, 205. (18) Cf.: (a) Ben Cheikh, A.; Chuche, J.; Manisse, M.; Pommelet, J. C.; Netsch, K. P.; Lorencak, P.; Wentrup, C. J. Org. Chem. 1991, 56, 970. (b) Kappe, C. 0.; Kollenz, G.; Leung-Toung, R.; Wentrup, C. J . Chem. SOC., Chem. Commun. 1992, 487. (19) Kano, H.; Makisumi, Y.; Ogata, K. Chem. Pharm. Bull. 1958, 6, 105. (20) Pfleiderer, W.; Schundehutte, K. H. Liebigs Ann. Chem. 1958,612, 158. (21) Plisnier, M. Ph.D. Thesis, University of Mons-Hainaut, Belgium, 1992. (22) de Hoffmann, C.; Barbier, E. Bull. Sot. Chim. Eelg. 1936,45, 563. (23) Wentrup, C.; Blanch, R.; Briehl, H.; Gross, G. J. Am. Chem. Soc. 1988, 110, 1874.