Molecular Orbital Correlations of Some Simple Radical Reactions

13 Molecular Orbital Correlations of Some Simple Radical

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Reactions GERALD JAY GLEICHER Department of Chemistry, Orgeon State University, Corvallis, OR 97331

The a p p l i c a t i o n o f relatively simple molecular orbital theory t o problems i n v o l v i n g the formation and r e a c t i v i t y o f organic r a d i c a l s has lagged behind r e l a t e d studies on charged t r i g o n a l species. This i s somewhat s u r p r i s i n g f o r although r a d i c a l forming reactions are often judged by the u n i n i t i a t e d to be much less s e n s i t i v e to s t r u c t u r a l changes, post hoc evidence i n d i c a t e s that a j u d i c i o u s choice o f system can obviate such problems. There a l s o e x i s t s a compensating f a c t o r i n that the uncharged r a d i c a l system may be simpler t o t r e a t t h e o r e t i c a l l y than its charged counterparts due to the absence o f a pronounced electric field (1). A too sanguine overview, however, can be adopted. Through the e a r l y s i x t i e s s u r p r i s i n g l y few i n v e s t i g a t i o n s were planned t o make use o f p o t e n t i a l molecular o r b i t a l c a l c u l a t i o n s . Even more d i s t u r b i n g was the observation that in c e r t a i n studies where such c o r r e l a t i o n s were attempted, o v e r a l l agreement was u n s a t i s f a c t o r y . The best o f the e a r l y experimental work is a s e r i e s o f i n v e s t i g a t i o n s i n v o l v i n g r a d i c a l aromatic s u b s t i t u t i o n by a l k y l r a d i c a l s s t u d i e d by Szwarc and co-workers 02-6). This and still earlier work by Kooyman and Farenhorst with the t r i c h l o r o m e t h y l r a d i c a l (7) have allowed f o r systematic v a r i a t i o n in the steric and e l e c t r o n i c p r o p e r t i e s o f the a t t a c k i n g s p e c i e s . Experiments i n d i c a t e that there i s rather l i t t l e v a r i a t i o n i n r e l a t i v e rate trends as a f u n c t i o n o f a t t a c k i n g r a d i c a l . T h e o r e t i c a l l y , it can also be p o i n t e d out that there i s l i t t l e d i f f e r e n c e among c o r r e l a t i o n s utilizing parameters determined from ground s t a t e c a l c u l a t i o n s and those u t i l i z i n g c a l c u l a t e d energy d i f f e r e n c e s (see below). A second type o f i n v e s t i g a t i o n can be typified by the work of Kooyman in the generation o f arylmethyl and s i m i l a r l y d e l o c a l i z e d r a d i c a l s and the attempted c o r r e l a t i o n with c a l c u l a t e d molecular orbital parameters (8). Studies o f t h i s type represent but p a r t o f the o v e r a l l subject o f arylmethyl r e a c t i v i t y which includes not only r a d i c a l generating hydrogen a b s t r a c t i o n processes, Equation 1, but also the corresponding carbonium i o n and ©0-8412-0421-7/78/47-069-227$05.00/0

Pryor; Organic Free Radicals ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

ORGANIC FREE RADICALS

228 carbanion r e a c t i o n s , Equations 2 and CH

0

CH

3

+

*·

— »

0

3.

2

+

h x

1

© CH Y 2

CH

2

Θ CH

Œ

3

2

1

Kooyman s i n v e s t i g a t i o n u t i l i z e d the r e l a t i v e rates o f hydrogen a b s t r a c t i o n from a s e r i e s o f a r a l k y l compounds (and one o l e f i n ) by the t r i c h l o r o m e t h y l r a d i c a l (8). The experimental r e s u l t s are shown i n Table I. R e l a t i v e rates o f Hydrogen A b s t r a c t i o n from S e l e c t e d Hydrocarbons by the T r i c h l o r o m e t h y l R a d i c a l . Hydrocarbon Toluene Diphenylme th ane 2-0ctene Triphenylmethane 3-Phenylpropene Indene

R e l a t i v e Rate 1.0 7.98 11.67 17.90 28.57 111.9

A l i n e a r c o r r e l a t i o n between logs of the r e l a t i v e rate constants and c a l c u l a t e d HMO p i energy d i f f e r e n c e s between the d e l o c a l i z e d product r a d i c a l (assumed equivalent to the t r a n s i t i o n state) and the i n i t i a l .unsaturated compound was attempted. The r e s u l t s were q u i t e poor. A c o r r e l a t i o n c o e f f i c i e n t of only 0.47 was obtained. (A formalism s t r o n g l y analogous to the Hammett equation w i l l be employed throughout the discuss i o n . The slopes o f the c o r r e l a t i o n s , u n l i k e rho values, have l i t t l e recognized p h y s i c a l meaning and are not even comparable i f energy d i f f e r e n c e s are c a l c u l a t e d by d i f f e r e n t methods. The numerical values o f these slopes w e l l t h e r e f o r e be ignored. However, c o r r e l a t i o n c o e f f i c i e n t s s h a l l be given and considered as a measure o f the r e l i a b i l i t y o f the c a l c u l a t i o n employed. A l l such c o e f f i c i e n t s are obtained from l i n e a r l e a s t squares analyses o f the c o r r e l a t i o n s . ) Although t h i s poor c o r r e l a t i o n may be a t t r i b u t a b l e i n p a r t to u t i l i z a t i o n of the o v e r l y simple HMO approach and to e x p e r i -


13.

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Molecular Orbital Correlations

229

mental problems i n e v a l u a t i o n o f rate constants, there also e x i s t underlying d i f f i c u l t i e s concerning the choice o f systems. B e n z y l i c r a d i c a l s derived from dipheny1methane, triphenylmethane and 3-phenylpropene cannot achieve complete p l a n a r i t y and arguments based on d e l o c a l i z a t i o n i n p l a n a r systems cannot be s a f e l y applied. As s h a l l be seen most subsequent workers i n the f i e l d o f b e n z y l i c r e a c t i v i t y have opted t o study s e r i e s o f compounds o f o v e r a l l greater s i m i l a r i t y than those u t i l i z e d by Kooyman. Benzylic

R e a c t i v i t y and Molecular O r b i t a l C o r r e l a t i o n s .

The formation o f unsubstituted p o l y c y c l i c b e n z y l i c carbonium ions has been s t u d i e d f o r over twenty years (9^ 10) . While much o f the e a r l i e r work was apparently planned with a p p l i c a t i o n o f Hiickel molecular o r b i t a l theory i n mind, the problem continues to a t t r a c t workers and more recent p u b l i c a t i o n s have combined experimentally more p r e c i s e r e s u l t s with advanced t h e o r e t i c a l techniques (11). The corresponding carbanions have also been generated u t i l i z i n g the r e a c t i o n o f p o l y c y c l i c arylmethanes with strong bases (12, 13). I n i t i a l t h e o r e t i c a l c a l c u l a t i o n s were again with HMO theory. I r r e s p e c t i v e o f the complexity o f the molecular o r b i t a l approach employed, i t i s p o s s i b l e t o attempt c o r r e l a t i o n s using some c a l c u l a t e d energy d i f f e r e n c e , e.g. Kooyman s r e s u l t s ( 8 ) , or some parameter derived from the i s o l a t e d aromatic s t a r t i n g material. While the l a t t e r approach has worked w e l l i n c o r r e l a t i n g the r e s u l t s o f e l e c t r o p h i l i e and r a d i c a l aromatic s u b s t i t u t i o n (14) , i t has been less frequently a p p l i e d t o the genera t i o n o f charged b e n z y l i c s p e c i e s . M e c h a n i s t i c a l l y t h i s neglect i s reasonable. The t r a n s i t i o n s t a t e s f o r generation o f the b e n z y l i c ions must show a strong resemblance to the charged intermediate. Any approach which ignores t h i s w i l l probably be i n c o r r e c t . The u t i l i z a t i o n o f energy d i f f e r e n c e s should also be a p p l i c a b l e to r e l a t i v e l y endergonic r a d i c a l forming reactions as w e l l (15). The HMO c o r r e l a t i o n s o f r e l a t i v e rates o f formation o f the p o l y c y c l i c b e n z y l i c ions lead t o some i n t e r e s t i n g conclusions. Compounds o f an α-naphthyl type ( i . e . those having the e x o - c y c l i c methylene u n i t attached to a carbon adjacent to a p o i n t o f annelation) always show a r e a c t i v i t y less than that expected based on compounds o f the β-naphthyl type. A tendency t o t r e a t data i n terms o f dual c o r r e l a t i o n s has been thus developed. The infrequent i n c l u s i o n o f nonalternant ( i . e . non-benzenoid) systems would u s u a l l y generate data which could not be accommodated by e i t h e r the α-naphthyl o r β-naphthyl c o r r e l a t i o n s . These findings f o r the generation o f arylmethyl anions (12, 13) are i l l u s t r a t e d i n Figure 1. A s i n g l e HMO c o r r e l a t i o n i n c l u d i n g data from both α-naphthyl and 3-naphthyl type systems y i e l d s a c o r r e l a t i o n f


ORGANIC F R E E RADICALS

230

log k

ΔΕ

(HMO 3) Journal of the American Chemical Society

Figure 1. calculated

Correlation of the relative rates of formation of arylmethul anions with HMO energy differences: (O), β-naphthyl compounds; O , a-napthyl compounds; (), non-alternant compounds.


13.

GLEiCHER


231

c o e f f i c i e n t of only 0.82. I f a dual c o r r e l a t i o n i s employed c o r r e l a t i o n c o e f f i c i e n t s o f 0.97 (α-naphthyl) and 0.99 (3-naphthyl, excluding one p o i n t ) are found. I n c l u s i o n o f the data from non-alternant systems, as p o i n t e d out, decreases the reliability. In order to e x p l a i n t h i s d u a l i t y , recourse was made to s p e c i a l s t e r i c f a c t o r s operative i n the α-naphthyl s e r i e s which could increase the energy d i f f e r e n c e between ground and t r a n s i t i o n s t a t e s . This p e r i e f f e c t i s a s p e c i f i c r e s u l t o f non-bonded i n t e r a c t i o n between the methylene u n i t and the atom o r group on the "other s i d e " o f the p o i n t o f annelation as i s shown f o r the α-naphthylmethyl carbanion i n Figure 2A. A rotation of ninety degrees around the e x o c y c l i c carbon-carbon bond shown i n Figure 2B can remove t h i s unfavorable i n t e r a c t i o n but only with complete loss o f conjugation between the e x o c y c l i c carbon and the remainder of the system. A s e m i - q u a n t i t a t i v e estimate of these i n t e r a c t i o n s may be gleaned i f recourse i s made to models o f the systems and standard values f o r hydrogen-hydrogen and carbonhydrogen r e p u l s i o n terms (16). I n i t i a l l y models were constructed using an average aromatic carbon-carbon bond length o f 1.40 A and a carb on-hydro gen bond length o f 1.085 λ. The p e r t i n e n t i n t e r n u c l e a r distances and r e p u l s i v e i n t e r a c t i o n s g i v i n g r i s e to the p e r i e f f e c t are shown i n Figure 2. I t i s assumed that some p r a c t i c a l compromise i s reached with the r e s u l t i n g lessened d e l o c a l i z a t i o n causing a decreased r e a c t i v i t y . In the above carbanion case, f o r example, a r o t a t i o n of seventeen degrees from p l a n a r i t y i s estimated (13) . I t i s p o s s i b l e , however, that i n t u i t i v e l y a t t r a c t i v e as the p e r i e f f e c t i s , there i s no need to invoke i t f o r simple, u n s u b s t i t u t e d , p o l y c y c l i c arylmethyl intermediates. The complete neglect o f e l e c t r o n r e p u l s i o n which u n d e r l i e s HMO c a l c u l a t i o n s cannot be j u s t i f i e d . While t h i s approach can y i e l d reasonable r e s u l t s f o r a l t e r n a n t hydrocarbons because o f the uniform e l e c t r o n d i s t r i b u t i o n , there i s less reason to b e l i e v e t h a t i t w i l l be s a t i s f a c t o r y f o r odd-alternant ions (17) as, i n f a c t , i s observed above. Dewar and Thompson (18) have a p p l i e d the r e s u l t s o f SCF c a l c u l a t i o n s to the problem of carbanion formation. These workers were able to i n c o r p o r a t e α-naphthyl, 3-naphthyl and non-alternant d e r i v a t i v e s i n t o a s i n g l e r e l a t i o n s h i p with a c o r r e l a t i o n c o e f f i c i e n t o f 0.97. This i s shown i n Figure 3. The SCF approach can a l s o make use o f a bond order-bond length r e l a t i o n ship to determine two center resonance and r e p u l s i o n s i n t e g r a l s (19). I t i s thus p o s s i b l e to a r r i v e at c o n s i s t e n t molecular s t r u c t u r e s as w e l l as energies. Such s t r u c t u r e s f o r α-naphthyl species also tend to p r e d i c t a s l i g h t l y decreased importance f o r the p e r i i n t e r a c t i o n , r e l a t i v e to that obtained from the usual HMO approximation. This i s due to g r e a t e r c a l c u l a t e d i n t e r n u c l e a r separations being obtained f o r those atoms causing the repulsive peri e f f e c t . Although

i t i s c l e a r t h a t some of the discrepancy between



A) Planar, standard bond lengths C---H H---H E-

„

B) As A, CH C---H H---H

2.44 A 1.71 A 0.725 kcal/mole

E

2

r o t a t e d 90°

2.44. A 2.74 A (2)

C--H

0.725 kcal/mole

H--H

0.050 kcal/mole

strain

0.725 kcal/mole

L—π E E

H

Η

4.175 kcal/mole

strain

4.900 kcal/mole

oYq C) Planar, SCF bond lengths C Η B

Η Η

Ρ

H—Η strain

2.62 A 1.97 A 0.261 kcal/mole

C--H

3.520 kcal/mole

E

Figure

C—-H Η — Η

2.49 A 1.76 A 0.529 kcal/mole

C-H

D) As C, 3° in-plane bends

1.484 kcal/mole

E

4.049 kcal/mole

2.

Various

s tH--H rain angle s t r a i n

1.745 kcal/mole 0.544 kcal/mole

total strain

2.289 kcal/mole

geometries of the a-naphthylmethyl system with peri interactions and calculated strains

accompanying


13.

GLEiCHER

233


0.0

0.4

0.8 ΔΔΕ

1.2

(SCF eV) Journal of the American Chemical Society

Figure

3.

Correction

of the retotive rates of formation of arylmethyl calculated SCF energy differences

anions


with


234

α-naphthyl and 3-naphthyl type systems h i t h e r t o assigned to p e r i e f f e c t s i s not s t e r i c but rather e l e c t r o n i c i n o r i g i n , such an e f f e c t might s t i l l be important i n arylmethyl r e a c t i v i t y . A q u a n t i t a t i v e e v a l u a t i o n o f the p e r i e f f e c t has been undertaken (20). Based upon bond lengths determined from SCF c a l c u l a t i o n s , i t i s shown i n Figure 2C that the non-bonded i n t e r a c t i o n s i n v o l v i n g the e x o - c y c l i c atoms were smaller than thought. These i n t e r a c t i o n s could be f u r t h e r diminished by s l i g h t (three degree) in-plane deformations without any loss o f d e l o c a l i z a t i o n . This i s shown i n Figure 2D. Out o f plane bending o f hydrogen atoms may f u r t h e r reduce p e r i i n t e r a c t i o n s . I n c l u s i o n of atoms or groups l a r g e r than hydrogen w i t h i n the p o t e n t i a l p e r i i n t e r a c t i o n t i o n s , however, may w e l l n e c e s s i t a t e a r o t a t i o n from c o - p l a n a r i t y o f the e x o - c y c l i c group. Formation of Arylmethyl

Radicals by Hydrogen A b s t r a c t i o n .

unlike the corresponding i o n s , b e n z y l i c r a d i c a l s are true a l t e r n a n t hydrocarbon s p e c i e s . I t was deemed o f i n t e r e s t to see whether rates o f formation o f such species could be r e l a t e d to changes i n d e l o c a l i z a t i o n and a l s o whether a s i m i l a r dichotomy o f r e s u l t s between HMO and SCF c o r r e l a t i o n s would be observed. To examine these questions a s e r i e s of arylmethanes was reacted with bromotrichloromethane at 70° ( 2 1 , 2 2 ) . The reactions were i n i t i a t e d with trace amounts of benzoyl peroxide, Equation 4. ArCH

3

+

BrCCl

3

peroxide

y

ArQ^Br

+

HCCI3

4

Although subsequent research i n other l a b o r a t o r i e s have l e d to claims that bromine atom i s the chain c a r r y i n g species i n hydrogen abstractions u t i l i z i n g bromotrichloromethane (28), our o r i g i n a l p r e s e n t a t i o n of r e s u l t s was based on the rate deter mining step shown i n Equation 5. ArCH

3

+

,

CC1

3

>

ArCH

# 2

+

HCC1

3

5

The r e l a t i v e l y endothermic nature o f t h i s process should allow f o r u t i l i z a t i o n o f of a c a l c u l a t e d energy d i f f e r e n c e as a s t r u c t u r a l parameter with which to c o r r e l a t e r e a c t i o n r a t e s . This p a r t i c u l a r r e a c t i o n i s not free from complications. The t r i c h l o r o m e t h y l r a d i c a l i s known t o attack the rings o f p o l y c y c l i c aromatics (7). In order to c o r r e c t f o r t h i s , i t was assumed that any enhanced r e a c t i v i t y o f the methylarene r e l a t i v e to the parent arene r e f l e c t e d only hydrogen a b s t r a c t i o n s . A competition between these two species can thus be used to deter mine (allowing f o r s t a t i s t i c a l c o r r e c t i o n ) the amounts o f hydrogen a b s t r a c t i o n and r i n g s u b s t i t u t i o n f o r each arylmethane. Such an approach presumes that the methyl s u b s t i t u e n t does not e l e c t r o n i c a l l y i n f l u e n c e attack i n the aromatic u n i t . At the time, however, both experimental (24) and t h e o r e t i c a l (25)


13.

GLEiCHER

235

Molecular Orbital Corrections

r e s u l t s seemed to j u s t i f y t h i s view f o r the t r i c h l o r o m e t h y l radical. Table II presents the o r i g i n a l l y obtained percent o f r e action at the methyl group and c o r r e c t e d r e l a t i v e rates o f hydrogen a b s t r a c t i o n . A l s o shown are the c a l c u l a t e d r e l a t i v e energy d i f f e r e n c e s between the arylmethyl r a d i c a l and the i n i t i a l arene. I t can be seen that a large range of experimental r e a c t i v i t i e s , n e a r l y three powers of ten, i s encountered. Table I I .

R e l a t i v e Rates o f Hydrogen A b s t r a c t i o n from a S e r i e s o f Unsubstituted Arylmethanes by T r i c h l o r o m e t h y l Radical at 70°.

A r y l me thane Toluene 1-Methyltriphenylene 2- Me thy11 riphenylene 3-Methylphenanthrene 1-Methylphenanthrene 2-Me thyIn aphthalene 9-Me thylphen an th rene 1-Me thylnaphth alene 6-Methy1chrysene 2 - Me thy 1 an th r acene 1 - Me thy 1 an th r acene 1-Me thylpy rene 9 - Me thy 1 anth racene

% methyl Hydrogen abstraction 100 84.6 69.1 86.9 69.5 68.4 91.0 94.3 77.0 74.5 54.3 76.5 86.4

0.,172 0.,362 0.,395 0.,547 0,,569 0,,682 0.,845 ( i . ,00) 2, ,19 5,,89 13,,1 18,.7 112, .0

H abstr + + + ± ± ± ±

0.008 0.027 0.014 0.027 0.035 0.057 0.056

± ± ± ± ±

0.04 0.47 1.4 1.0 5.0

ΔΔΕ (SCF) (0.000) 0.076 0.068 0.060 0.139 0.076 0.178 0.175 0.214 0.215 0.340 0.352 0.519

Reprinted with permission o f the J o u r n a l of Organic Chemistry, 2008, (1971). Copyright by the American Chemical S o c i e t y .

93»,

Although the arylmethyl r a d i c a l s are a l t e r n a n t hydrocarbons, the Hiickel method again proved inadequate i n c o r r e l a t i n g the data. The compounds f e l l i n t o the usual two s e t s . While the separate c o r r e l a t i o n c o e f f i c i e n t s o f 0.948 f o r the α-naphthyl p o i n t s and 0.904 f o r the 3-naphthyl p o i n t s were good and f a i r , the o v e r a l l c o r r e l a t i o n c o e f f i c i e n t of 0.86 was unacceptable. The s i n g l e SCF c o r r e l a t i o n i s shown i n Figure 4. The o v e r a l l c o r r e l a t i o n c o e f f i c i e n t i s now 0.977. Some s l i g h t improvement can s t i l l be made i f a dual c o r r e l a t i o n i s u t i l i z e d with the two new c o r r e l a t i o n c o e f f i c i e n t s now being greater than 0.99. Perhaps t h i s does r e f l e c t a small p e r i e f f e c t . Why does the Hiickel method s t i l l f a i l i n t r e a t i n g arylmethyl r e a c t i v i t y even i n n e u t r a l systems? This must be a t t r i b u t a b l e to the neglect of e l e c t r o n i c i n t e r a c t i o n terms i n general and s p e c i f i c a l l y neglect of s p i n p o l a r i z a t i o n . The l a t t e r i s p a r t i c u l a r l y important i n odd e l e c t r o n systems and i s due to d i f f e r e n t i a l i n t e r a c t i o n s between e l e c t r o n p a i r s o f the same and opposite s p i n . Huckel c a l c u l a t i o n s cannot account f o r f i n e



236

9-Anthracy 2.0

ΓΠΐ-Pyrenyl 1.0 l 0

S

k

J — ' 1-Anthracyl

2-Anthracyl

ο

rel 3-Phenanthryl

6-Chrysenyl

1-Naphthyl 0.0 k/2-Naphthyl

Phenanthryl

_ Phenanthryl 11-Triphenylenyl 2-Triphenylenyl Phenyl 0.0

0.1

±

0.2

0.3

0.4

0.5

ΔΔΕ (SCF eV) Journal of the American Chemical Society Figure 4. Correlation of the relative rates of hydrogenen abstraction from arylmethanes by the trichloromethyl radical with calculated SCF energy


polycyclic differences

13.

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Molecular

Orbital

Correlations

237

s t r u c t u r e i n the e s r s p e c t r a o f a l l y l and benzyl r a d i c a l s caused by s p i n p o l a r i z a t i o n while SCF c a l c u l a t i o n s can (26). Although i t was f e l t that a c o r r e l a t i o n with a c a l c u l a t e d energy d i f f e r e n c e was i n keeping with the view that a large degree o f carb on-hydrogen bond breaking occurs i n hydrogen a b s t r a c t i o n by t r i c h l o r o m e t h y l r a d i c a l , c o r r e l a t i o n with s e v e r a l ground s t a t e parameters (14) was a l s o undertaken. In a l l cases the r e s u l t i n g c o r r e l a t i o n s were much poorer than that discussed above. The use o f negative evidence i s suspect o f course. However, i t i s f e l t that these r e s u l t s give some support to the view that a l a t e t r a n s i t i o n s t a t e i s encountered i n t h i s process. (22) I t was at t h i s p o i n t that concern over one of the i n i t i a l assumptions developed. The problem that a methylarene might undergo r i n g s u b s t i t u t i o n at a s i g n i f i c a n t l y d i f f e r e n t r a t e than the parent system, could not be ignored. This question seemed to be p a r t i c u l a r l y important f o r 9-methylanthracene. This compound should not only be most s u s c e p t i b l e to r i n g s u b s t i t u t i o n , but, because of i t s high r e a c t i v i t y i n the hydrogen a b s t r a c t i o n r e a c t i o n , can d i s p r o p o r t i o n a t e l y i n f l u e n c e that- o v e r a l l corre lation. In order to evaluate the importance of t h i s p o s s i b l e e f f e c t the t r i c h l o r o m e t h y l a t i o n of a s e r i e s of 9 - s u b s t i t u t e d anthracenes was s t u d i e d i n accord with Equation 6.

The meso p o s i t i o n ( s ) o f anthracenes i s known t o be most prone to r e a c t i o n and has been shown to account f o r about ninety percent o f r a d i c a l s u b s t i t u t i o n (27). The r e s u l t s are given i n Table I I I . As can be seen there i s a s u b s t i t u e n t e f f e c t with e l e c t r o n donating groups f a v o r i n g , as might be expected, the t r i c h l o r o m e t h y l a t i o n of the r i n g (28) . This data may be t r e a t e d w i t h i n a standard Hammett type c o r r e l a t i o n to produce a rho value o f -0.83 ( c o r r e l a t i o n c o e f f i c i e n t of 0.970) when p l o t t e d against σ*. The data f o r 9-methylanthra cene, however, shows a s i g n i f i c a n t upwards d e v i a t i o n from the c o r r e l a t i o n . This i s i n d i c a t i v e o f an a d d i t i o n a l mode o f disappearance f o r t h i s compound which must be hydrogen a b s t r a c t i o n from the methyl group. The t o t a l r e a c t i v i t y of 9-methylanthracene i s due mostly (65%) to t h i s l a t t e r process. The o r i g i n a l amount of r e a c t i o n at t h i s s i t e was b e l i e v e d to be 86% o f the t o t a l r e a c t i v i t y o f t h i s compound. Most s e r e n d i p i t o u s l y , however, the o r i g i n a l c o r r e l a t i o n f o r hydrogen a b s t r a c t i o n from arylmethanes i s only s l i g h t l y changed. There i s only a two percent decrease i n the slope o f the c o r r e l a t i o n and a change i n the c o r r e l a t i o n c o e f f i c i e n t from 0.977 t o 0.973.



238 Table I I I .

R e l a t i v e R e a c t i v i t i e s o f 9-Substituted Anthracenes toward T r i c h l o r o m e t h y l Radical A d d i t i o n at 70.0°. Substituent N0 CN

2

C 0

2 Br CI H C

C H

3

H

6 5 i-C Hy 3

C

H

2 5 CHOCH3

k /k. Y

0.71 0.34 0.74 1.00 1.02 1.00 2.28 2.38 3.39 6.85 5.52

+ + ± + +

0.10 0.04 0.07 0.04 0.04

± + ± + +

0.43 0.30 0.44 0.89 0.60

Reprinted with permission of the Journal of the American Chemical S o c i e t y , 9(5, 787, (1974). Copyright by the American Chemical Society. I t i s o f i n t e r e s t that 9-ethyl and 9-isopropylanthracene do not show the same large d e v i a t i o n from the c o r r e l a t i o n f o r r i n g s u b s t i t u t i o n . The simplest conclusion i s that e x o c y c l i c hydrogen a b s t r a c t i o n i s l e s s l i k e l y here because of p e r i e f f e c t s caused by r e p l a c i n g one or both o f the methylene hydrogens by methyl groups. Two other studies i n v o l v i n g hydrogen a b s t r a c t i o n from p o l y c y c l i c arylmethanes have also been reported i n the l i t e r a t u r e . G i l l i o m and coworkers have s t u d i e d many o f the same compounds discussed above i n r e a c t i o n with bromine atom (29). I t was observed that methylanthracenes and methylpyrenes underwent e x c l u s i v e r i n g r e a c t i o n s . The other compounds employed showed r e a c t i v i t i e s i n t h i s r e a c t i o n which p a r a l l e l e d those observed i n the t r i c h l o r o m e t h y l r a d i c a l r e a c t i o n . One must again conclude that appreciable r a d i c a l character i s again developed i n the t r a s i t i o n s t a t e based upon the good c o r r e l a t i o n with SCF calcul a t e d energy d i f f e r e n c e s . The r e a c t i o n of the arylmethanes with t - b u t y l h y p o c h l o r i t e was a l s o undertaken (22) . I t was hoped that the less s e l e c t i v e t-butoxy r a d i c a l might cause a r e a c t i v i t y trend which could be c o r r e l a t e d with ground s t a t e p r o p e r t i e s . Although c h l o r i n e atom traps were employed, e x c l u s i v e r i n g c h l o r i n a t i o n was observed f o r s e v e r a l of these molecules. The remaining systems showed marginal s e l e c t i v i t y . A l l attempt at c o r r e l a t i o n with e i t h e r c a l c u l a t e d energy d i f f e r e n c e s or ground s t a t e parameters were unsuccessful. Formation of Arylmethyl

Radicals by

Addition.

In an attempt to determine whether ground s t a t e molecular o r b i t a l parameters could c o r r e l a t e a r e l a t i v e l y exothermic


13.

GLEiCHER

Molecular Orbital Corrections

239

r a d i c a l r e a c t i o n , a t t e n t i o n was d i r e c t e d away from atom abstrac t i o n and toward an a d d i t i o n process. The a d d i t i o n o f a t h i y l r a d i c a l to v i n y l a r e n e s i s shown i n Equation 7 (30) . No measurable attack by the t h i y l r a d i c a l i n the p o l y c y c l i c p o r t i o n

ArCH=CH + 2

»S~^~~^

>

ArCHQ^S-^

^

7

of the o l e f i n takes p l a c e ; thus making the r e a c t i o n i n theory a simple one to study. The r e s u l t s are presented i n Table IV. The o v e r a l l range o f r e a c t i v i t i e s i s much smaller than that Table IV.

R e l a t i v e R e a c t i v i t i e s o f Vinylarenes toward Thiophenol at 70°. Compound Styrene 2-Vinylnaphthalene 9-Vinylphenanthrene 1-VinyIn aphth alene 6-Vinylchrysene 2-Vinylanthracene 1-Vinylanthracene 1-Vinylpyrene 9 - V i ny 1 an th r acene

k^K^ 1 2.8 2.0 4.0 3.8 5.9 6.5 11.5 1.8

± ± ± ± ± ± ± ±

0 .3 0 .2 0 .5 0 .2 1 .2 1 .5 3.0 1 .2

0 0,,061 0.,125 0.,140 0,,142 0.,167 0,.209 0,.234 0,,424

Reprinted with permission o f the J o u r n a l o f Organic Chemistry, 41, 2327 (1976). Copyright by the American Chemical S o c i e t y . found i n the corresponding hydrogen a b s t r a c t i o n study and i s i n d i c a t i v e o f the expected more exothermic process. F a i l u r e was again encountered, however, i n the attempted u t i l i z a t i o n o f ground s t a t e p r o p e r t i e s t o c o r r e l a t e the experimental data. The two most l i k e l y o f such parameters, free valence on the terminal carbon and bond order o f the e x o c y c l i c double bond, both y i e l d e d c o r r e l a t i o n c o e f f i c i e n t s o f less than 0.5. A much b e t t e r c o r r e l a t i o n could again be obtained with a c a l c u l a t e d SCF energy d i f f e r e n c e . This i s shown i n Figure 5. The c o r r e l a t i o n c o e f f i c i e n t , while only 0.932,is s t i l l f a i r . Improvement i s noted i f the data are separated i n t o the t r a d i t i o n a l two s e t s . C o r r e l a t i o n c o e f f i c i e n t s o f 0.95 (α-naphthyl) and 0.97 (3-naphthyl) are now found. P e r i i n t e r a c t i o n s must be very r e a l i n these systems as the e x o c y c l i c p o r t i o n o f the supposed t r a n s i t i o n s t a t e i s no longer a simple methylene group. Indeed, as can be seen, i t i s impossible to i n c o r p o r a t e the r e s u l t s f o r 9-vinylanthracene w i t h i n any o f the above c o r r e l a t i o n s as no p l a n a r conformation i s p o s s i b l e f o r the intermediate r a d i c a l without a p a r t i c u l a r l y severe p e r i i n t e r a c t i o n . The remaining


240


log k

0.0

0.1

0.2 ΔΔΕ

0.3

0.4

(SCF eV) Journal of Organic Chemistry

Figure

5.

Correction of the relative rates of thiyl radical addition vinylarenes with calculated SCF energy differences

to


polycyclic

13.

GLEiCHER

Molecular

Orbital

Correlations

CH SC H 2

6

241

5

α-naphthyl compounds may adopt a conformation with the bulky e x o c y c l i c group d i r e c t e d away from the p e r i hydrogen. Support f o r t h i s view i s obtained from a c o n s i d e r a t i o n of s i m i l a r t h i y l r a d i c a l a d d i t i o n to a homologous s e r i e s of isopropenylarenes as shown i n Equation 8. (30) . The data i n Table V show a l l

Table V.

R e l a t i v e R e a c t i v i t i e s o f Isopropenyl Arenes toward Thiophenol at 70.0°. Compound a-Methylstyrene 2-Isopropenylnaphthalene 1 -1 s op rop eny In aph th a l ene 9-1s op ropenylphen an th rene 9-1s op ropeny1anthracene

k

k

x/ std

1.55 0.14 0.05 0.02

1.0 ± ± ± ±

0.12 0.01 0.002 0.005

Reprinted with permission o f the J o u r n a l o f Organic Chemistry, 41, 2327 (1976). Copyright by the American Chemical S o c i e t y . members o f the α-naphthyl s e r i e s to now possess g r e a t l y reduced r e a c t i v i t y a t t r i b u t a b l e to s t e r i c e f f e c t s as i t i s now impossible f o r a l l large groups on the e x o c y c l i c carbon to " p o i n t away from" the p e r i hydrogen. Conclusions. C e r t a i n d i s t i n c t conclusions may be drawn from these s t u d i e s . F i r s t l y , as might have been expected, i s the obvious f a c t that d e l o c a l i z a t i o n can have a tremendous e f f e c t on the ease o f b e n z y l i c r a d i c a l formation. The p e r i e f f e c t s , invoked by e a r l i e r mechanicians, seem to be less important than claimed f o r n o n - s u b s t i t u t e d systems, but, may exert a large e f f e c t i f s u b s t i t u e n t s are attached to e i t h e r the e x o c y c l i c atom or the p e r i s i t e . (Although only rate r e t a r d i n g p e r i e f f e c t s have been t r e a t e d above, a c c e l e r a t i v e counterparts can be e a s i l y imagined.) F i n a l l y , from the t h e o r e t i c a l p o i n t o f view, a l l o f the work on formation o f b e n z y l i c l i k e r a d i c a l s c i t e d enforce the view t h a t c a l c u l a t i o n s which do not i n c l u d e i n t e r e l e c t r o n terms are inade quate and that ground s t a t e parameters cannot s u c c e s s f u l l y


242


c o r r e l a t e the data.

Abstract The correlation of the rates of formation of arylmethyl free-radicals by molecular orbital theory w i l l be discussed. Different levels of sophistication among pi-electron methods lead to conflicting conclusions concerning the degree of possible electron localization and the importance of non-bonded interactions with the principal radical s i t e . The arylmethyl radical systems have been generated both by hydrogen abstraction (1) and addition (2) reactions.

The sensitivity toward steric factors varies and is much more pronounced in the latter process.

Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14)

Williams, G. Η., "Homolytic Aromatic Substitution", p. 7, Pergamon Press, New York, 1960. Levy, M. and Szwarc, Μ., J. Chem. Phys., (1954), 22, 1621. Smid, M. and Szwarc, M., J. Am. Chem. Soc., (1956), 78, 3322. Smid, M. and Szwarc, M., J. Am. Chem. Soc., (1957), 79, 1534. Smid, M. and Szwarc, M., J. Chem. Phys., (1962), 29, 432. Stefani, A. P. and Szwarc, M., J. Am. Chem. Soc., (1962), 84, 3661. Kooyman, E. C. and Farenhorst, E., Trans. Faraday Soc., (1953), 49, 58. Kooyman, Ε. C . , Disc. Faraday Soc., (1951), 10, 163. Dewar, M. J. S. and Sampson, R. J., J. Chem. Soc., (1956), 2789. Dewar, M. J. S. and Sampson, R. J., J. Chem. Soc., (1957), 2946, 2952. Streitwieser, Α., Jr., Hammond, Η. Α., Jagow, R. H . , Williams, R. M., Jesuitis, R. G., Chang, C. J., and Wolf, R., J . Am. Chem. Soc., (1970), 92, 5141. Streitwieser, Α., Jr., and Langworthy, W. C., J . Am. Chem. Soc., (1963), 85, 1757. Streitwieser, Α., Jr., Langworthy, W. C. and Brauman, J.I., J . Am. Chem. Soc., (1963), 85, 1761. Greenwood, Η. H. and McWeeny, R., Adv. Phys. Org. Chem., (1966), 4, 73.


13.

GLEiCHER

(15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30)

Molecular

Orbital

Correlations

243

Hammond, G. S., J . Amer. Chem. Soc., (1955), 77, 334. Bartell, L. S., J . Chem. Phys., (1960), 32, 827. Dewar, M. J. S., Rev. Mod. Phys., (1963), 35, 586. Dewar, M. J. S., and Thompson, C. C . , Jr., J. Am. Chem. Soc., (1965), 87, 4414. Dewar, M. J. S. and Gleicher, G. J., J. Am. Chem. Soc., (1965), 87, 685. Gleicher, G. J., J. Am. Chem. Soc., (1968), 90, 3397. Unruh, J. D. and Gleicher, G. J., J. Am. Chem. Soc., (1969) 91, 6211. Unruh, J. D. and Gleicher, G. J., J. Am. Chem. Soc., (1971) 93, 2008. Tanner, D. D., Arhart, R. J., Blackburn, Ε. V . , Das, N.C., and Wada, Ν . , J. Am. Chem. Soc., (1974), 96, 829. Farenhorst, E. and Kooyman, E. C., Recl. Trav. Chim. Pays -Bas, (1962), 81, 816. Greenwood, H. H . , Nature (London), (1955), 176, 1024. Fessenden, R. W., and Schuler, R. H . , J. Chem. Phys., (1963), 39, 2147. Iwamura, H . , Iwamura, Μ., Sato, S. and Kushida, K., Bull. Chem. Soc. Jap., (1971), 44, 876. Arnold, J . C., Gleicher, G. J., and Unruh, J . D., J. Am. Chem. Soc., (1974) 96, 787. Roark, R. B., Roberts, J . M., Croom, D. W. and Gilliom, R. D., J. Org. Chem. (1972), 37, 2042. Church, D. F . , and Gleicher, G. J., J. Org. Chem., (1976), 41, 2327.

RECEIVED December 23, 1977.


Molecular Orbital Correlations of Some Simple Radical Reactions

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