Nucleophilic Reactivity in Gas-Phase Anion-Molecule Reactions

4 Nucleophilic Reactivity in Gas-Phase Anion-Molecule Reactions Richard N . McDonald, A. K. Chowdhury, W. Y. Gung, and K. D . DeWitt

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Department of Chemistry, Kansas State University, Manhattan, KS 66506 Three different topics of nucleophilic anions reacting with neutral molecules are discussed. (1) O2.- is established intrinsically and kinetically as a super nucleophile in SN2 reactions withCH3Xmole cules. (2) The intrinsic reactivity scale of nucleophilic additions to carbonyl centers is developed by using C6H5N.- as the nucleophile and can be extended with (C6H5)2C.-. (3) The phosphoryl anion (CH3O)2PO- is shown to be a poor nucleophile in SN2 reactions with CH3X molecules but reacts rapidly with ICF3 and BrCF3 by initial electron transfer; mainly (CH3O)(X)P2- negative ions result.

TOPICS O F T H E G E N E R A L A R E A O F N U C L E O P H I L I C REACTIONS O F ANIONS with neutral substrates in the gas phase included in this chapter are (1) the nucleophilicity of 0 * in S 2 reactions, (2) development of an intrinsic reactivity scale for nucleophilic reactions with organic carbonyl-containing molecules, and (3) investigations of ( C H 0 ) P O ~ in S 2 reactions with C H X reactants and electron-transfer processes with X C F molecules. 2

N

3

2

N

3

3

Experimental Section Experiments were carried out in a previously described (1, 2) flowing afterglow (FA) apparatus at 298 Κ (see Figure 1). Briefly, the ion of interest is produced continuously in a fast flow of helium buffer gas in the upstream end of the flow tube by electron impact on small concentrations of added reagents via inlets 1-5. The fast flow (v = 80 m/s, P = 0.5 torr) is maintained by a large, fast pumping system. Following thermalization of the ion of interest by collisions with the buffer gas in the next 20-45 cm of the flow tube, neutral reactant molecules are added via the inlet located about halfway down the flow tube, and the ion-molecule reaction occurs in the final 65 cm of the flow tube. The flow is sampled into a differentially pumped compartment (10" torr) containing the quadrupole mass filter and electron multi plier, which continuously monitor the ion composition of the flow. Kinetics of these bimolecular ion-molecule reactions are determined under pseudo-first-order condi tions with the concentration of the added neutral reactant in large excess compared to the ion concentration by methods already given (1). H e

7

0065-2393/87/0215-0051$06.00/0 © 1987 American Chemical Society

Harris and McManus; Nucleophilicity Advances in Chemistry; American Chemical Society: Washington, DC, 1987.


ELECTRON GUN

INLET 2

HELIUM BUFFER GAS —

HELIUM INLET

PRODUCTION INLETS

ΙΟΝ

ION-MOLECULE REACTION REGION

u

REAGENT INLET TO VERIFY PRODUCT ION STRUCTURE

t[

Figure 1. Diagram of the flowing afterglow apparatus.

ION-MOLECULE NEUTRAL INLET

ROOTSMECHANICAL PUMP


6" DIFFUSION PUMP

1Γ

VACUUM FEEDTHROUGHS

500 L/s TURBOMOLECULAR PUMP

4.

Gas-Phase Anion-Molecule Reactions

MCDONALD ETAL.

0'

as a Super S 2

2

53

Nucleophile

N

Ion Generation. The generation of 0 · ~ in the F A involved the se quence of reactions (3, 4) 2

NH

+ e" -> H N " + - H

3

(D

2

H N" + CH CH=CH -> C H " + NH 2

3

2

1 0

3

3

where k = 2.5 X 10 c m molecule and ΔΗ° = - 1 4 ± 5 kcal/mol, and

1

5

(2)

3

1

s , reaction efficiency equals 0.43,

C H " + 0 -> Ο , · " + C H - Downloaded by RUTGERS UNIV on January 11, 2018 | http://pubs.acs.org Publication Date: July 1, 1987 | doi: 10.1021/ba-1987-0215.ch004

3

5

10

2

3

3

-1

(3)

5

-1

where k = 3.0 X 10~ c m m o l e c u l e s , reaction efficiency equals 0.43, and ΔΗ° = —1.4 ± 0.7 kcal/mol (5, 6). Rate constants, reaction efficiencies (defined as k /k where k is the calculated collision limited rate con stant), and reaction exothermicities are given (3-6). Ammonia was added via inlet 1, propene via inlet 2, and dioxygen via inlet 4 in Figure 1. ohsd

co]

col

S 2 Reactions of 0 · ~ with C H X Molecules. Bohme and co-workers (7) established a kinetic nucleophilicity scale for gas-phase anions in S 2 reactions based on their rates of reaction with C H X (X = Br, C l , or F). For this comparison, the reactions of 0 · ~ with C H B r and C H C 1 (Table I) were used because the displacement of F " from C H F by 0 · ~ is strongly endothermic. Both of these reactions occurred at close to the collision limit; thus, 0 · ~ is placed in the category of gas-phase anions of high nucleophilicity. Other members are H " , F ~ , C H 0 ~ , H O " , and H N ~ . However, the considerably lower exothermicities for these two 0 · ~ reactions distinguish it from the other high nucleophilic anions in that all of them have much larger reaction exothermicities with these two C H X molecules. From the Pellerite and Brauman (8) application of Marcus theory to S 2 methyl-transfer reactions, the kinetic barrier is made up of an intrinsic barrier for the reaction that is decreased by the magnitude of the reaction exothermicity. Because the exothermicities of the reactions with 0 · are the lowest (by upward of 20 kcal/mol) compared to the other anions of high kinetic nucleophilicity, the intrinsic barriers for these S 2 reactions with 0 · are the smallest. Thus, 0 · ~ can be called a super S 2 nucleophile. The remaining reactions of 0 · ~ with C H X substrates in Table I occur primarily or exclusively by S 2 displacement. Their rates given as reaction efficiencies vary from reaction occurring on essentially every collision with C F C 0 C H to C H C 0 C H where 2 out of every 1000 collisions, on the average, yield C H C 0 ~ . In accordance with the S 2 mechanism for their reactions, the reaction efficiencies are correlated with the reaction exother micities and by the anionic leaving group ability as modeled by the proton affinity of the departing anion. N

2

3

N

3

2

3

3

3

2

2

3

2

2

3

N

2

N

2

2

N

2

3

N

3

2

3

3

3

2

2

3

N


54

NUCLEOPHILICITY

,_

Table I. Kinetic and Thermochemical Data for Reactions of 0 with CH X Molecules 2

S2

ΔΗ°

Channel

(kcal/mol)

N

Reaction

o,ooo-

+ CF C0 CH -> CF C0 - + CH 0 -+ CH Br - » B r " + CH 0 -" + CHC1 Cl" + CH 0 -+ HC0 CH -* HC0 ~ + CH 0 " Q r + HC==CHC0CH-> HC = CHC0 - + CH 0 -o,- + CH C0 CH -• CH C0 - + CH 0 -3

2

3

2

3

2

2

2

3

3

3

2

3

2

3

2

3

2

2

2

3

3

2

2

2

2

3

3

a

b


c

2

3

3

2

3

3

2

2

ΔΗ°^

(HA) h

(kcal/mol)

0.97 1.00 1.00 0.90

-32.5 -24.9 -17.0 -12.1

0.92 0.76 0.39 0.05

323 324 333 345

1.00 0.79

— - 7.7

0.03 0.002

— 349

c

c

^obsd^coi equals reaction efficiency ( £ „ „ ) . Reference 12; errors are ±2 kcal/mol. is unknown. p (H 2 C 2

A

= CHC0 -)

Intrinsic Reactivity Scale for Nucleophilic Addition Reactions at Carbonyl Centers The intrinsic reactivity scale is a topic that led us to begin studies of gasphase reactions in 1978. However, the problem of reversibility of the nu cleophilic addition from the tetrahedral intermediate is even more severe in the gas-phase pressure regimes than it is in solution. Our approach to this problem involved generation of a new class of reactive intermediates called hypovalent anion radicals. Hypovalent anion radicals contain less than the normal number of substituents attached to the central atom found in the corresponding neutral free radical, and these anion radicals have both the electron pair of the anion and the spin unpaired electron of the radical formally located on the central atom. Phenylnitrene anion radical ( C H N ~ ) is a member of this class of nitrogen-centered species and is readily formed from C H N added at inlet 1 (Figure 1) by dissociative electron attachment (equation 4) (9). The idea was to shut down the reverse of the nucleophilic addition to the carbonyl group from the tetrahedral intermediate by allowing the faster follow-up chemical reaction of radical β fragmentation to occur; the acylanilide anions plus the radicals R or R are obtained: e

6

5

6

5

3

x

2

C H N 6

Ο PhN-~ +

Ri-C-Rj—-

5

3

+ e"

C H N-- + N , mlz 91

οΙ PhN—C-R. I R,

6

(4)

5

*-*• PhN=C(0")R + -R, â

(5) I— PhN=C(0")R, + -R


2

4.

55

Gas-Phase Anion—Molecule Reactions

MCDONALD ET AL.

We examined the reactions of C H Ν · " with the series of carbonylcontaining molecules listed in Table II (10). Because H transfer is a compet ing reaction channel in some of these reactions, we factored out of the total rate constants that part due to carbonyl addition and radical fragmentation, and these rate constants were made relative to that for acetone; these fc i values are given in the middle column of Table II. 6

5

+

C=0

re

Table II. Relative Rate Constants and Reaction Exothermicities of Carbonyl Addition-Radical Fragmentation of Carbonyl-Containing Molecules with C H N Downloaded by RUTGERS UNIV on January 11, 2018 | http://pubs.acs.org Publication Date: July 1, 1987 | doi: 10.1021/ba-1987-0215.ch004

6

5

0

ΔΗ (kcal/mol)

c=o k rel 1 4 11 80 83 12 31 67 0.2 0.02 157 108 113 a

Substrate

K

CH3COCH3

CH COC H Cyclobutanone 3

2

5

CH3COCF3

CF COCF 3

3

CH3CHO

C H CHO (CH ) CCHO HC0 CH 2

5

3

3

2

3

CH3CO2CH3

CF C0 CH CH COCOCH CH COC0 CH 3

2

3

3

3

3

a

2

3

-19 -22 b -22 -31 -16 -18 -24 -6 -4 -15 -32 b

These relative rate constants are & (sum of the fractions of those channels yielding addition adducts or acylanilide anions) with each substrate relative to k ° for acetone. Not determined. total

c =

h

Three observations from these data are noteworthy. (1) The range of fc ° is about 8,000, which is nearly the full range of kinetic reactivity available in our F A experiments, from reactions occurring on every collision (k = fc ) to those occurring in one out of 10 collisions. (2) We observe that for "normal" substituents on the carbonyl center, the order of reactivity is aldehydes ( C H C H O ) > ketones [ ( C H ) C = 0 ] > esters ( C h C 0 C H ) . This reactivity difference holds for the intramolecular comparison with methyl pyruvate where the total reaction occurred on every other collision, but 7 times more addition-fragmentation occurred at the keto C O than at the ester C O (10). (3) No correlation was observed between the k values and the overall reaction exothermicity in forming the acylanilide anion and the radical (see equation 5). This latter point is most clearly seen by comparing the slow reaction of C H N ·" with acetone, which is 19 kcal/mol exothermic, with the very fast reaction of C H N * ~ while C F C 0 C H is only 15 kcal/mol exothermic. c=

rel

4

ohsd

col

3

3

2

3

2

c=0

re]

6

5

6

5

3

2

3


3

56

NUCLEOPHILICITY

Point 3 and certain other results suggest that the potential energy versus reaction coordinate diagram for these carbonyl addition-radical fragmenta tion reactions with C H N ·" occur by the triple minimum surface shown in Figure 2. The interaction of C H N * ~ with a dipolar carbonyl-containing substrate molecule is attractive even at rather long distances and leads to formation of loose collision complexes given as a in Figure 2. Such complexes are held together by ion-dipole and ion-induced dipole forces with well depths of 10-20 kcal/mol. Closer approach of the complex components is repulsive until net bonding takes over with formation of the tetrahedral intermediate (b). Radical β-fragmentation is considered to have a lower 6

5


6

5

Figure 2. Proposed triple-minimum potential energy vs. reaction coordinate diagram for the reactions of PhN*~ with carbonyl-containing molecules. Reproduced from reference 10. Copyright 1983 American Chemical Society.

barrier than that of nucleophilic addition and leads to the loose complex (c), which separates, primarily due to entropie forces, to yield the observed reaction products. This model can be viewed simply in terms of the conver sion of reaction a —• reaction b being rate-limiting. Because this barrier and the rate of nucleophilic addition have little to do with the overall reaction exothermicity, no correlation between & i ° values and - Δ Η ° is observed (or expected) (10). One problem with the reactions of C H N with organic carbonyl containing molecules was that the carbonyl group reactivity scale could not be extended past the simple esters H C 0 C H and C H C 0 C H because their rates were already at the lower limit of determination in the F A . To get around this limitation, we examined other hypovalent species such as c =

r e

e

6

2

5

3

3

2

3


4.

57


MCDONALD E T AL.

( C H ) C - ~ , which is cleanly generated from ( C H ) C = N added at inlet 1 (Figure 1) by dissociative electron attachment: 6

5

2

6

(C H ) C=N 6

5

2

+ e" -

2

5

2

2

(C H ) 0mlz 166 6

5

+ N

2

(6)

2

Our first efforts were directed toward determination of the proton affinity (A ) of ( C H ) 0 ~ (11). This determination is accomplished by using the bracketing method. Potential proton donors of known gas-phase acidity (Table III) are added to the flow containing ( C H ) 0 " until H transfer is no longer observed with the weaker acids; H transfer is judged to occur by attenuation of the ( C H ) 0 ~ signal and observation of the signal for the conjugate base of the acid. This transition occurs between C H C = C H and p-xylene; assignment of A [(C H ) C-~] = 382 ± 2 kcal/mol results (for A values of other organic anions, see reference 12). Because ( C H ) C H * is the product of protonation and A H / [ ( C H ) C H - ] = 69 ± 2 kcal/mol (13), A H / [ ( C H ) C - ~ ] can be calculated to be 358 ± 2 kcal/mol. The A of ( C H ) C - ~ is similar to that of C H 0 " (A = 379 ± 2 kcal/mol) (12), and ( C H ) 0 ~ is a stronger base than the corresponding carbanion ( C H ) C H ~ (A = 364.5 ± 2 kcal/mol) (12) by 18 kcal/mol.' p

6

5

2

+

6

5

2

+


6

5

2

3

p

6

5

2

p

6

6

6

6

5

6

5

2

2

2

p

2

5

5

5

3

p

2

6

5

2

p

Table III. Data for Bracketing A J ( C H ) C ·-] in H+Transfer Reactions with H A Molecules 6

5

2

0

Δ Η ^ (HA)" H+ Transfer (kcal/mol)

HA C H CH CH OH CH O^CH p-CH C H CH CH CH—CH H 0 6

5

yes yes yes no no no

3

3

3

3

6

4

3

3

2

2

"Reference 12; errors are ±2

381.2 381.4 381.8 382.7 390.0 390.8

kcal/mol.

The reactions of ( C H ) C - " with C H B r and C H C 1 were studied to determine the kinetic nucleophilicity of ( C H ) C - ~ in S 2 displacements: 6

5

2

3

3

6

(C H ) C-- + CH Br 6

5

2

3

2

N

Br" + (C H ) CCH

3

10

5

6

-1

5

2

3

1

(7)

where k = 3.9 Χ 1 0 " c m m o l e c u l e s" , reaction efficiency equals 0.35, and ΔΗ° = —65.5 kcal/mol, and

1

_

For a similar relationship in the A

p

values of c - C H ' ~ and c - C H , see reference 1. 5

4

5

5


58

NUCLEOPHILICITY

(C H ) C 6

5

Cl" + (C H ) CCH

CH3CI

2

- 1 1

6

3

-1

5

2

(8)

3

1

where k = 3.2 Χ 1 0 c m m o l e c u l e s" , reaction efficiency equals 0.03, and Δ / Γ = - 6 0 . 0 kcal/mol. These results show that ( C H ) C - " is of medium kinetic S 2 nucleophilicity (7). The reaction of ( C H ) 0 with C F C 0 C H was examined to deter mine the kinetic nucleophilicity of P H 0 toward carbonyl addition in competition with the highly exothermic S 2 methyl transfer: 6

5

2

N

-

6

5

2

3

2

3

-

2

N

O" Ph C-C-OCH Downloaded by RUTGERS UNIV on January 11, 2018 | http://pubs.acs.org Publication Date: July 1, 1987 | doi: 10.1021/ba-1987-0215.ch004

2

0 81 Γ ~ " Ph C=C(CT)CF + O C H 2

3

3

(9)

3

Ph C~+

L ^ P h C = C ( C T ) O C H + -CF 0.14

2

2

CF,

3

3

(10)

CF C0 CH H 3

2

3

1— C F C ( Y + P h C C H 0.05 3

9

2

3

-1

(Π)

3

-1

where k = 1.0 x 10 c m m o l e c u l e s , reaction efficiency equals 0.71, ΔΗ°(9) = - 2 5 kcal/mol, ΔΗ°(10) = - 2 3 kcal/mol, and ΔΗ°(11) = - 7 2 . 5 kcal/mol. Carbonyl addition-radical fragmentation wins out over S 2 dis placement by a factor of 19 to 1 in this very fast reaction. The favored radical fragmentation pathway from the tetrahedral intermediate formed by nu cleophilic addition to the carbonyl center is that of loss of the more weakly bound C H 0 compared to the F C*. The reaction of (C H ) C*~ with H C 0 C H occurred exclusively by the addition-fragmentation pathway: N

3

3

6

5

2

2

(C H ) ( 6

5

IIC0 CH

2

2

- 1 0

3

3

(C H ) C=C(0-)H 6

3

-1

5

(12)

2

1

where k = 1.6 Χ 1 0 c m m o l e c u l e s" , reaction efficiency equals 0.11, and ΔΗ°(12) = - 12.0 kcal/mol. The intriguing result of this reaction is not only the fact of the exclusivity of the addition-fragmentation channel but also that the rate constant is >10 faster than that of the reaction of C H N * ~ with H C 0 C H (10). Thus, the kinetic reactivity scale of carbonyl centers can be readily extended beyond that of the simple esters by using ( C H ) C * and other hypovalent anion radicals presently under investigation. 2

6

2

5

3

6

Proton Affinity, AH °, f

and Reactions

5

2

of(CH 0) PO~ 3

2

Substitution reactions by anions at carbon are also known to occur by initial electron transfer. The mechanism of such transformations was first charac terized by Russell and Danen (14) and Kornblum et al. (15), and Bunnett (16) significantly developed its applications and named it the S 1 reaction; an RN


4.

59


MCDONALD ET AL.

example of aromatic substitution is given in equation 13 (17). Following a discussion with Jim Swartz of Grinnell College concerning these aromatic substitution reactions involving the phosphoryl anion (e.g., equation 13), we determined the thermochemical properties and gas-phase reactions of the phosphoryl anion. +

Arl + (EtO) PO- K

ArP(=0)(OEt) + KI

2

(13)

2

G e n e r a t i o n a n d T h e r m o c h e m i c a l P r o p e r t i e s of ( C H 0 ) P O ~ (18a). For our studies, dimethyl phosphonate [ ( C H 0 ) P ( = 0 ) H ] was se lected as our reagent because it is more volatile than the diethyl ester. The phosphonate structure was shown to be 6.5 kcal/mol more stable than that of its trivalent tautomer, ( C H 0 ) P ( O H ) (18b). A variety of gas-phase anionic bases can be used to remove a proton from the phosphonate ester forming the phosphoryl anion (mlz 109), but the use of C H N * ~ as the base produces no other primary product negative ions: 3


3

3

2

2

2

6

5

(CH 0) P(=0)H + C H N-" — (CH 0) PO- + C H NHmlz 109 3

2

6

5

3

2

6

(14)

5

Our initial studies were directed toward determination of the A of mlz 109. The bracketing method previously described was employed for determi nation of A [ ( C H ) C * " ] (Table IV). The transition from yes to no for ob served proton transfer to mlz 109 occurred with C H S H and C H N 0 . Although proton transfer did not occur from C H N 0 , slow formation of a cluster ion ( C H 0 ) P O " - C H N 0 was observed, which is probably the hydrogen-bonded complex negative ion; similar cluster ions are formed when C H S H and C F C H O H are added to the flow containing mlz 109. Thus, A [ ( C H 0 ) P O ~ ] = 358 ± 2 kcal/mol; if proton transfer is assumed to occur at phosphorus, A H ° [(CH 0) P(=0)H] = A [ ( C H 0 ) P O ] . Using Benson's tables (19), we calculated Δ Η / [ ( C H 0 ) P ( = 0 ) H ] = —198.1 kcal/mol. From the equation for ionization of ( C H 0 ) P O ~ in equap

p

6

5

2

2

3

3

3

2

3

3

p

3

5

3

2

2

2

2

acid

3

2

p

3

2

3

3

2

2

Table IV. Data for Bracketing A [ ( C H 0 ) P O ] in H+-Transfer Reactions with HA Molecules p

HA

3

2

+

c-C H C HgSH CH N0 CH SH CF CH OH 5

6

2

2

2

3

3

2

R e f e r e n c e 12; errors are ±2

H Transfer

(kcallmol)

yes yes no no no

356.1 357.4 358.7 359.0 364.4

kcal/mol.


2

60

NUCLEOPHILICITY

tion 15, we calculated Δ Η / [ ( C H 0 ) P O - ] = - 2 0 7 . 3 ± 2 kcal/mol. This value can be used to calculate ΔΗ° for various reactions of the phosphoryl anion. 3

2

Δ//° (CH 0) P(=0)H 3

(CH 0) PO- + H

2

3

+

(15)

2

S 2 Nucleophilicity of ( C H 0 ) P O ~ . To determine the intrinsic S 2 kinetic nucleophilicity of ( C H 0 ) P O ~ , we investigated the reactions of the anion with the series of C H X molecules listed in Table V. The rates of these reactions vary from modest with the most reactive C H I molecules to slow with C H B r to no observed reaction with C H C 1 . From the results, we conclude that the phosphoryl anion is kinetically a poor nucleophile in S 2 displacement reactions. N

3

3

2

N

2

3

3


3

3

N

Table V. Kinetic and Thermochemical Data for S 2 Displacement Reactions of (CH 0) PO- with CH X Molecules N

3

2

3

ΔΗ° Reaction (CH 0) PO-

+ C H I - • I-

+ (CH 0) P(

(CH 0) PO-

+

+

3

2

3

3

2

CH I 3

I-

( C H 0 ) P O - + C H B r — Br3

2

3

3

=0)CH

2

3

3

2

3

+ ( C H 0 ) P ( == 0 ) C H 3

3

3

(CH 0) PO- + CH C1 3

2

3

(CH 0) PO3

û

+ CH C1

2

3

Cl-

2

-45.6

3

Cl"+

== 0 ) C H

2

10" a 10a

11

12

^obsd^col a 0.06 a 0.002 a

13

5.2

3

X

l

Nucleophilic Reactivity in Gas-Phase Anion-Molecule Reactions

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