Protonation of ketene and vinylketene

Journal of Molecular Structure (Theo&em), 183 (1989) 319-330 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

PROTONATION

319

OF KETENE AND VINYLKETENE

Relative stabilities of isomeric cationic products

RfiGIS LEUNG-TOUNG, MICHAEL R. PETERSON, THOMAS T. TIDWELL and IMRE G. CSIZMADIA Department of Chemistry, University of Toronto, Toronto, Ontario M5S IA1 (Canada) (Received 15 April 1988)

ABSTRACT Ab initio (3-21G) calculations have been used to study the structure of protonated ketene as well as unprotonatedandpIotonatedvinylketene (CH,(&-CH(y)CH(j?)=C(a)=O). Single-point 6-31G* determinations of the t&al energies of the optimized 3-21G structures of protonated vinylketene were also carried out. Two stable conformations of the ground state of vinylketene were found, with the s-trans conformations being more stable than the s-ci.s ones by 7.3 kJ mol-’ (6-31G*). &Protonation of vinylketene gave CH&HCH==C=O with an s-trans CCCC torsion angle as the most stable structure, with a preference of 66.1 kJ mol-’ (6-31G*) relative to fi protonation forming CH,-CHCH,E=O

and 159.3 kJ mol-’

(6-31G*) relative to (Yprotonation

forming CH,=CHcHCH=O. For ketene, the /3protonated form is more stable than the 0 and C, protonated isomers by 157.1 and 318.2 kJ mol-‘, respectively (3-21G).

INTRODUCTION

Protonation of ketenes and the structures of the resultant cations have been the subject of intense theoretical study [l-7], as well as experimental investigation in the gas phase [8-121 and in solutions [13-211. These studies are in agreement, that proton transfer to C, of the ketene to give acylium ions 1 as the most stable cation is the favored pathway. It has also been argued that acid-catalyzed addition reactions of ketenes, such as hydrations, occur by a pathway involving initial protonation of the ketene to the acylium ion 1 followed by nucleophilic attack, and proposals for single-step concerted additions [ 19-211 have been criticized [ 1,13-161. R\=c=, R’6 4

L

R\CH;=o R’

1

Recently, we have carried out experimental studies of the acid-catalyzed hydration of the vinylketene 2 in aqueous solution [ 161. Several isomeric cati-

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0 1989 Elsevier Science Publishers B.V.

320

onic interme~a~s may be expected to occur in this addition, and to unders~d the course of this reaction we have carried out ab initio c~c~ations on the protonation of ketene ( 3 ) and vinylketene ( 4 1. c=o

CH2=C=0

2

CH2-CHCH=C=O

3

4

Experimentally, the acid-catalyzed hydration of 2 gives the acids 6 and 6 in a ratio of 6~1,even though 6 is the more stable product [ 161. This result may be interpreted in terms of a kinetic preference for protonation at C6 leading to a more stable carbocationic intermediate 7 in preference to the less stable carbocation 8 which leads to the more stable acid (Scheme 1). It was desirable to test this indirection compu~tionally.

G

c=o

6

8 Sefieme

*-

1.

METHOD

Ab initio L~AO-MO-SC~ cal~~ations were performed using the program MONSTERGAUSS [22] in conjunction with a GOULD 32/9705 mini-computer. The basis sets used were the split-valence 3-21G [ 231 and the split valence with single polarization 6-31G* [ 241. The RHF method [ 25 ] was used for the closed-shell systems. The geometries of the singlet ground state for all

321

the molecules considered were energy optimized at the 3-21G basis level using the optimally conditioned (OC ) method [ 261. The gradient optimizations were terminated when the gradient length was reduced to below 0.0005 mdyne. Single-point 6-31G* energy determinations were performed on the optimized 321G geometries. The orders of the critical points were determined by finite differences of gradients for every molecule computed at the 3-21G basis level. The order checks were performed by evaluating the second derivatives of the Hessian matrix by small changes in the optimized dihedral angles, bond lengths and bond angles that would break any symmetry elements possessed by the molecule. RESULTS AND DISCUSSION

Protonated ketene

The structures and stabilities of protonated ketene isomers are illustrated in Scheme 2 at the 3-21G basis level. Table 1 gives the computed total ( - hartree) and relative (kJ mol-' ) energies for all isomers on the potential energy surface (PES) of protonated ketene. The optimized parameters for all isomers are given in Tables 2 and 3. Results at a higher level of calculation (4-31G, 6-31G*) are also available [2]. Results obtained for the optimized geometric parameters in this work are comparable to those of previous work [ 21, except for the oxiranyl cation ( 1 lb).

llb

ii

g

i

+

‘57.1

nlp+H

Scheme 2: Structures and relative energies (kJ mol-‘)

of protonated ketene isomers (3-21G*).

322 TABLE 1 Protonated ketene

9 10

lla llb

Total energy ( - hartree)

Relative energy (kJ mol-‘)

3-21G”

4-3lG”

3-21G”

4-31Gb

151.20130 151.14146 151.08010 151.08035

151.81794 151.79523 151.71161 151.70559

0.0 157.1 318.2 317.6

0.0 151.0 279.0 295.0

a This work. b Ref. 2.

Although the energies differ significantly at the various levels considered, one important feature of the calculations is that the p protonated isomer (9) is more stable than the 0 (lo),or the C, (11) protonated isomers (Table 1). Both structures 1 la and 1 lb of the (Yprotonated isomer 11 were calculated to be energetically unfavorable. The bridged conformer ( 1 lb) is more stable than the open planar cation lla by only 0.6 kJ mol-’ (3-21G), but at the 431G basis level [ 2,7b] 1 la is more stable than 1 lb by 15.9 kJ mol-‘, whereas at the 6-31G** level llb is calculated to be more stable than lla by 42.7 kJ mole1 [ 7b]. 0

I +C >

,/“’

%H

cIT II

I

‘5 H

H/L* 9

Vinylketene

H$+H8

\

s H8

lla

10

(CH,=CHCH=C=O,

o/ I + \c ii b

4)

The optimized parameters of the vinylketene conformers are given in Tables 2 and 3. The computed total energies (-hartree) at the 3-21G and 6-31G* single-point energy determinations are tabulated in Table 4. Semi-empirical (CNDO/B and MNDO) studies on vinyl ketene have been reported previously wherein parameters from standard tables were used [ 271. The most stable conformer (4a) is planar, with an s-truns (CCCC) torsional angle. It is more stable than the planar (4b) conformer with an s-cis (CCCC) torsional angle by 5.5 (3-21G) and 7.3 (6-31G*) kJ mol-‘.

0(4)-H(3) 0(4)-w C(2)-H(3) C(l)-H(7) C(l)-H(8) C(l)-H(9) C(l)-C(2) C(l)-C(5) CGW(6) C(5)-H(8) C(5)-H@) C(6)-H@) C(6)-H(10) C(6)-H(U)

(A,

Bonddistance

4b

1.3028 1.4707 1.3208 1.0733

1.0718 1.0718 1.0736 1.0745

1.3022 1.4659 1.3200 1.0749

1.0725 1.0718

1.1621 1.1611

4a

1.0879 1.0879 1.4450

1.1095

9

1.2660

1.0787 1.0787

1.2269

0.9889

10

1.4762

1.0776

1.2164 1.0719

lla 12a

1.4493

1.0794 1.0875 1.0875

1.3808 1.3498 1.4879 1.0768

1.2382 1.1205 1.0691 1.0736 1.0711

llb 0.9873

13

1.0900 1.0900

1.0714 1.0753

1.0717

1.2758 1.4550 1.4759 1.5369 1.3207 1.3122

1.0820

1.0703 1.0786 1.0865 1.0715 1.0865 1.0731

1.3794 1.3512 1.4956 1.0762

1.0741

1.1109

14a

1.0718 1.0739

1.0715

1.4531 1.5480 1.3122

1.0896 1.0896

1.1112

14b

1.0741 1.0750

1.5064 1.3675 1.3718 1.0714

1.2053 1.0744 1.0753

16a

16a

1.0740 1.0751

1.4941 1.3685 1.3713 1.0722

1.0770 1.0770

1.3087 1.4948 1.4309 1.1084 1.1084

1.2058 1.1492 1.0769 1.0762 1.0719

15b

[ numberingscheme0(4)-C(2)-C(l)-C(5)-C(6)]

1.1207 1.2350

12b

Calculated bond distances (A) for ketenes and protonated ketenes (3-21G//3-21G)

TABLE 2

1.0780 1.0773

1.3042 1.4945 1.4315 1.1086 1.1086

1.0715

1.1531

16b

7 (2)

7 (1)

119.36 121.19 120.95 122.13 122.73 120.00

90.00

10

179.80 180.05 180.00 180.00 123.12 123.36 124.46 124.01 127.23 109.01 116.86 116.32 109.02 120.34 109.02 120.34 116.34

9

O(4)-C(2)-C(1) C(2)-0(4)-H(3) C(2)-C(l)-C(5) C(l)-C(5)-C(6) C(l)-C(2)-H(3) C(l)-C(2)-H(7) C(l)-C(2)-H(8) C(l)-C(5)-H(8) C(l)-C(5)-H(9) C(5)-C(6)-H(8) C(5)-C(6)-H(9) C(5)-C(6)-H(lO) C(5)-C(6)-H(l1)

4b

4a

Bondangle (“)

llb

12a

12b

13

14a

14b

116.34

112.73 109.12 109.12 90.71 121.94

16b

16a

16b

122.35 122.54 125.16 119.69 117.24 121.69 117.15 115.74 116.85 117.44

120.92 121.24 111.37 109.85 120.38 119.96 120.09 122.08 121.77 122.84 123.69 119.68 119.34 109.85 123.01 124.65 123.37 120.87 177.09 180.00 180.00 122.85 124.81 89.87 121.82 121.75

118.94 120.29 114.03 111.57 114.03 111.57

119.60 120.02 116.91 118.50

118.87 118.65 179.67 181.04

15a

[ numberingscheme0(4)-C(2)-C(l)-C(5)-C(6)]

76.30 179.33179.36179.33180.64180.22 121.47 120.59120.53126.30111.88110.94 123.22122.39120.54126.51120.38 118.87 150.08 106.97106.66 116.34 119.07115.51115.55 120.05 119.07 114.77106.97106.66 119.97119.66 117.40111.20117.41

lla

Calculated bond angles for ketenes and protonated ketenes (3-21G//3-21G)

TABLE 3

E

TABLE 4

Vinylketene (CH,-CHCHmC-0

) conformations

Total energy ( - hartree)

4a 4b

3-21G//3-21G

6-31G*//3-21G

227.33003 227.32793

228.60620 228.60341

4a

Dipole moment

Critical point order

1.44 1.76

0 0

4b

There is a slight preference for the s-truns configuration 4a over the s-cis (4b), and 4b has a wider C1C5Csangle by 3.2’. Protonated isomers of vinylketene Scheme 3 illustrates the structures considered for the isomers and conformers of protonated vinyl ketene. The total energies and relative stabilities at the 3-21G basis level and 6-31G* single-point energy determinations are tabulated in Table 5. The optimized parameters for all the structures considered at the 3-21G basis level can be found in Tables 2 and 3. There are no previously published theoretical or experimental results for the protonated isomers of vinylketene. 0-Protonated vinylketene The C,-C2, Cz-0, and 04-H3 3-21G-optimized bond lengths of 0 protonated vinylketene (13) are comparable to those found for the parent protonated ketene 10 (Table 5). The parameters of the vinyl substituent have not sensibly changed compared to unprotonated 4, as expected for CH,=CH-CH= &OH with the positive charge not delocalized in the dienyl system. ,Ho+

13

326 0

Y 6 II

if

H

16a

F

P “A

+

16b

“/C+\C/C\H

“4;r NC\

H/ ‘1”

H ”

0’”

5.3

13

e II

r “\C/C\” 23.0

II liAC--H

15a

H

O\/

I I

H

4

0

\CI

65.6

“y+”

NC&-,

‘5b

II

II l/---H

H~Chl

1.5 I

t

0 93.2

14a

[+

14b

I H

o+ H/C*c/CqhH I H

H-hi 5.9

12b

Scheme 3: Structures and relative energies (kJ mol-‘) 31G*//3-21G).

of protonated vinylketene isomers (6-

cyProtonated vinylketene Two local minima have been located on the PES of the a! protonated vinyl ketene. Conformer 15a possessing anti (CCCC) and syn (CCCO) dihedral angles is more stable than conformer 15b which has anti (CCCC) and anti (OCCC) dihedral angles, by 7.4 kJ mol-’ at the 3-21G basis level. This may be rationalized in terms of O-H8 intramolecular hydrogen bonding in 15a,and an unfavorable H,-H, interaction in 15b. However, at the 6-31G” basis set level 15a is only 1.5 kJ mol-’ more stable than 15b,a negligible difference. It

327 TABLE 5 Energies and relative stabilities of protxmated vinylketenes Total energy ( - hartree )

Critical point order

3-21G//3-21G

6-3lG*//3-21G

227.67898 227.67689

0 1 0

0

Relative stability (kJ mol-‘) 3-21G//3-21G

6-31G*//3-21G 0.0 6.7 226.6 66.1 72.0 159.3

160.8 249.6 254.9

13

227.60598

228.96411 228.96155 228.87782

14a 14b 1Sa 1Sb 16a 16b

227.65409 227.65239

228.93895 228.93670

227.61523

228.90343

1 0

0.0 5.5 191.7 65.3 69.8 167.4

227.61241 227.58521 227.58329

228.90287 228.86903 228.86703

0 2 2

174.8 246.2 251.2

12a 12b

has been suggested [ 291 that ring forming and hydrogen bonding are favored by sp-type basis sets.

15a

15b

The allylic carbocation resonance in 15 is highly stabilizing, as protonation at C!, of 4 is only 93.2 kJ mol-’ less favorable than protonation at C, whereas for ketene itself Pprotonation is favored by 318.2 kJ mol-‘. This suggests that, in suitably substituted vinylketenes, protonation at C, may be observable experimentally. p Protonated vinylketene Cation 14a with a syn (CCCC ) dihedral angle is calculated to be local minimum that is more stable than conformer 14b with an anti (CCCC ) torsional angle of first-order critical point by 4.5 (3-21G) and 5.1 (6-31G*) kJ mol-l, respectively. Conformer 14a is geometrically strained with C1C5C6and C,C,H,, angles of 126.5” and 124.7”, respectively. The stability of the syn relative to the s-trans conformation can be explained by homoallylic participation of the K electrons of the y,6 double bond with the empty p orbital at C, as shown in 14~. The C,-CG distance in 14a is less than in unprotonated 4b (Table 2). This kind of homoallylic interaction is well known in carbocations [ 281.

328

14a

14b

14c

The possibility of a local minimum for a skewed asymmetrical conformer on the energy hypersurface has not been investigated. For the conformers of protonated 1,3-butadienol (CH,=CHCH$HOH), it was found that the skewed conformer was more stable than the planar structures possessing anti (CCCC ), or syn (CCCC ) torsional angles by around 2% 33 kJ mol-’ at the 3-21G and 6-31G* basis levels [291. y Protonated vinylketene

The open-chain conformers 16a and 16b are both computed to be secondorder critical points. This is not really surprising because these primary cations can form three-membered ring conformers by the formation of a C, -Cg bond, and 16b can also collapse to a five-membered ring by C6 -0 bond formation.

Collapse to the three- and five-membered ring conformers has been computed for y protonated 1,3-butadienol (&H$H&H=CHOH) [ 291. The cyclic structures were found to be more stable than the linear chain isomers by around 175-200 kJ mol-’ both at the 3-21G and 6-31G* basis levels. The lower stability of 16b with a syn (CCCC) torsional angle, over 16a which has a tram (CCCC) torsional angle, is due mainly to strain. There is no participation of the a electrons of the cx,j3double bond to stabilize the positive charge, contrary to the case of 14a.The C2C1C5angle is widened to 125.2” and the C,C,C, angle is 121.7”, a deviation of nearly 12” from a perfect sp3 hybridization. S-protonated vinyl ketene

The secondary carbocation ( 12a),with a s-truns (CCCC ) torsional angle and with the CH3 hydrogens staggered with respect to the y-hydrogen, is com-

329

puted to be the most stable conformer. Allylic delocalization in the carbocation increases the C,- C, bond length to 1.38 A, while C,- C, shortens to 1.35 A.

7 + -

H,

,F*=’

H 12a

Conformer 12a is more stable than its eclipsed conformer (12b)with a methyl rotation barrier of 5.5 (3-21G) and 6.7 (6-31G*) kJ mol-‘. 0

0

H

%’

12a

/C

‘H,

12b

CONCLUSION

This study emphasizes structures possessing truns (CCCC ) torsional angles because ketene 2 has a fixed truns vinylic system. The energy differences at the 3-21G and 6-31G* basis levels are very similar except for 13 and 15. There is a stability increase of around 35 kJ mol-’ for 13, and - 8 and - 14 kJ mol-’ for 15a and 15b,respectively, on going from the 3-21G to the 6-31G* basis level. In summary, a double bond at the CW,~ position to the positive charge disperses the charge in a resonance fashion to stabilize the isomer, as in the case of 12 and 15. When a methylene group separates the positive charge and the double bond (C=C), the preferred conformation between the syn and anti (CCCC) conformers will depend on two factors: (a) long-range x-electron interaction, as in 14; and (b) steric effects, as in 16. In conclusion, the calculations predict that protonation of vinylketene will preferentially occur at C6 giving a conjugated carbocation intermediate, in agreement with experiment. Protonation at C, leading to a non-conjugated product, and C,, leading to an aldehyde, are successively less favored. The possibility of whether suitable substitution of vinylketene might enhance these latter two paths is a worthy topic for further study. ACKNOWLEDGMENT

Financial support by the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged.

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8 9 10

11 12 13 14 15 16 17 18 19 20 21 22

23 24 25 26 27 28 29

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