Decomposition of Diazenes and Diazene N-Oxides. Consideration of

8 Decomposition of Diazenes and Diazene N-Oxides. Consideration of Three-Electron Stabilization.

Downloaded by FUDAN UNIV on February 9, 2017 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0069.ch008

F R E D E R I C K D. G R E E N E , JAMES D. BURRINGTON, and A B R A H A M M . K A R K O W S K Y D e p a r t m e n t of C h e m i s t r y , Massachusetts Institute of T e c h n o l o g y , C a m b r i d g e , MA 02139

The carbon-nitrogen bond is a strong l i n k of average bond energy 75 kcal/mole. There are, of course, many examples i n which homolytic cleavage of t h i s bond may be e f f e c t e d under r e l a t i v e l y mild c o n d i t i o n s . Diazenes (azo compounds) comprise a well-documented example (1). Decomposition of diazenes a f f o r d s d i n i t r o g e n and r a d i c a l s . The q u e s t i o n of whether the r a t e Ra-N.

Ra-

N

0

.R,

(eq. 1) (eq. 2)

Ra—N.

determining step i n v o l v e s synchronous two-bond cleavage (eq. 1) or one-bond cleavage (eq. 2) has received a great d e a l of study (1, 2). For a l k y l d i a z e n e s (including a r y l - s u b s t i t u t e d a l k y l ) the evidence s t r o n g l y favors cleavage of both carbon-nitrogen bonds in the rate-determining step. A convincing l i n e of evidence is the dependence of r a t e on s u b s t i t u e n t [e.g. Table 1 (3)]. Table 1 Rate of Decomposition of Diazenes, Ra—N=N—R, r e l k, predicted,200°C Synchron Stepwise r e l k, Ra Rb (eq. 1) (eq. 2) Observed 1-Norbornyl 1-Norbornyl 1 ^ 1-Norbornyl t-Butyl . , 6.6xl0 t-Butyl t-Butyl (6.6x10 ) 2(6.6x10 ) 7 χ 10 9

?

Indeed, these f i n d i n g s might appear to be a logical conse quence of a c o n s i d e r a t i o n of the e n e r g e t i c s of the r e a c t i o n . The enthalpy change, Δ Η ° , f o r eq. 1 is approximately +25 kcal/mole f o r R = a l k y l , e.g. t - b u t y l ( 4 ) ; the enthalpy of a c t i v a t i o n is +42 kcal/mole ( 5 ) . In comparison w i t h the average bond energy of 75 kcal/mole f o r the carbon-nitrogen bond, the considerably lower a c t i v a t i o n energy (+42) can be achieved by some synchronous © 0-8412-0421-7/78/47-069-122$05.00/0

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

8.

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Diazenes and Diazene N-Oxides

123

cleavage of both carbon-nitrogen bonds with the attendant recoup ing of the energy cost by p a r t i a l formation of d i n i t r o g e n . Ratedetermining cleavage of j u s t one of the carbon-nitrogen bonds could be considered only i f the r e s u l t i n g a l k y l r a d i c a l and d i azenyl r a d i c a l (RN ») were s t a b i l i z e d to the extent of 30 k c a l / mole (the average bond energy minus the observed enthalpy of activation). A t e r t i a r y a l k y l r a d i c a l i s s t a b i l i z e d to the extent of only a few kcal/mole; a d i a z e n y l r a d i c a l u s u a l l y has been con s i d e r e d to possess no s t a b i l i z a t i o n ( 6 ) . With h i g h l y unsymmetrical diazenes, and p a r t i c u l a r l y when Ra i s phenyl, decomposition does appear to take p l a c e v i a d i a z e n y l r a d i c a l s (eq. 2 ) . S e v e r a l l i n e s of evidence provide strong sup port f o r t h i s view (_7) » i n c l u d i n g CIDNP observations of ΡηΝ · , and s t u d i e s of the dependence of r a t e on v i s c o s i t y and on pres sure. Consider the case of phenyltriphenylmethyldiazene (eq. 3 ) . The a c t i v a t i o n enthalpy i s +27 kcal/mole (1). There w i l l be some


2

2

Ph-N-N—CPh

3

+

Ph—Ν=Ν·

+

-CPh

3

(eq. 3)

d e s t a b i l i z a t i o n of the ground s t a t e ( s t e r i c ) and c o n s i d e r a b l e s t a b i l i z a t i o n i n the t r i p h e n y l m e t h y l r a d i c a l . Are these two sources s u f f i c i e n t to account f o r the observed 50 kcal/mole d i f ference between the average carbon-nitrogen bond energy and the enthalpy of a c t i v a t i o n ? Probably not; i t appears reasonable to a t t r i b u t e part of t h i s lowering to s t a b i l i z a t i o n i n the phenyld i a z e n y l r a d i c a l , Ρ η Ν · . Although there have been numerous attempts to observe t h i s s p e c i e s i n the e s r , they have not been s u c c e s s f u l , a s c r i b a b l e i n p a r t to the high exothermicity of the conversion of a d i a z e n y l r a d i c a l , R N « , to R« and N . However, a d i a z e n y l r a d i c a l possesses an aspect that provides c o n s i d e r a b l e s t a b i l i z a t i o n i n other systems, - an odd e l e c t r o n l o c a t e d on an atom adjacent to an atom c o n t a i n i n g a lone p a i r , ! ί — δ . The s t a b i l i z a t i o n provided by t h i s " t h r e e - e l e c t r o n " bond i s most e a s i l y seen i n r e f e r e n c e to the " f o u r - e l e c t r o n " counterpart, A—B. I n t e r a c t i o n between lone p a i r s r e s u l t s i n formation of new energy l e v e l s , one bonding and one antibonding. In the 2

2

" 4 - e l e c t r o n " case

-H

-H

2

v. « λ

" 3 - e l e c t r o n " case

—1

—ft-

" f o u r - e l e c t r o n " case, the net r e s u l t i s d e s t a b i l i z i n g (the a n t i bonding combination i s somewhat more d e s t a b i l i z i n g than the bond ing combination i s s t a b i l i z i n g ) . In the " 3 - e l e c t r o n " case, the net r e s u l t i s s t a b i l i z i n g . The magnitude of the s t a b i l i z a t i o n obviously depends on the extent of the i n t e r a c t i o n of the odd


124

ORGANIC F R E E RADICALS

e l e c t r o n with the adjacent lone p a i r , i n turn dependent on the p a r t i c u l a r atoms i n v o l v e d , the geometry of the system, and the s t a t e o f h y b r i d i z a t i o n a t each atom. Some examples are shown i n Chart 1. Chart 1


/ -Ô-b: A q u a l i t a t i v e resonance r e p r e s e n t a t i o n o f the i n t e r a c t i o n i s À—Β « - ^ λ — S " ; i . e . one might expect the strength o f the i n t e r a c t i o n to increase with i n c r e a s i n g e l e c t r o n e g a t i v i t y o f Β and i n c r e a s i n g s h a r e a b i l i t y o f the lone p a i r of A. Of those shown i n Chart 1, the most s t a b l e " t h r e e - e l e c t r o n " species are the n i t r o x y l s , R N0 ( 8 ) , i n keeping with the simple g e n e r a l i z a t i o n expressed above. Evidence i s a v a i l a b l e on a l l o f the systems shown i n Chart 1 but w i l l not be reviewed here. S u f f i c e i t t o say that the adjacency of a lone p a i r may be a s t r o n g l y s t a b i l i z i n g f a c t o r i n r a d i c a l s t a b i l i t y . The question o f s t a b i l i t y i s a com plex one. In some instances the species i n question may be sub j e c t t o d i r e c t thermochemical measurement, e.g. some n i t r o x y l s (9). I n some instances, two species i n question may be isomeric, and the question o f r e l a t i v e s t a b i l i t y may be answered unambigu ously, e.g. R N0 more s t a b l e than RNOR. In other cases, apparent s t a b i l i t y may be r e l a t e d t o drawbacks t o d i m e r i z a t i o n . For example, d i m e r i z a t i o n o f n i t r o x y l s would e n t a i l the formation o f the weak oxygen-oxygen bond, and the change from " t h r e e - e l e c t r o n " s t a b i l i z a t i o n i n n i t r o x y l s to e l e c t r o n d e s t a b i l i z a t i o n i n the dimer from the four contiguous atoms each holding a lone p a i r . Other comparisons of s t a b i l i t y i n such cases might come from r e l a t i v e r e a c t i v i t y i n atom t r a n s f e r r e a c t i o n s . "Three-electron" bonding i n v o l v i n g carbon r a d i c a l s deserves f u r t h e r a t t e n t i o n . Much evidence e x i s t s to show that hydrogen a b s t r a c t i o n by a l k o x y l r a d i c a l s or halogen atoms i s f a c i l i t a t e d by the adjacency o f n i t r o g e n or oxygen (10). Much o f t h i s f a c i l i t a t i o n may be due t o i o n i c c o n t r i b u t i o n s t o the t r a n s i t i o n s t a t e o f the a b s t r a c t i o n process. Few experiments bear d i r e c t l y on the matter o f how much ground s t a t e s t a b i l i z a t i o n i s present i n r a d i c a l s such as R KdR (11). Another index of " t h r e e - e l e c t r o n " s t a b i l i z a t i o n i s the r e l a t i v e ease o f homolytic bond cleavages. I n the Stevens rearrangement (12) a carbon n i t r o g e n bond i s broken h o m o l y t i c a l l y at a c t i v a t i o n energies w e l l below the carbon-nitrogen bond strength. The " t h r e e - e l e c t r o n " bond i n the n i t r o g e n - c o n t a i n i n g 2

2

2

2

moiety provides much s t a b i l i z a t i o n thereby lowering

the energy


8.


GREENE ET AL.

125

needed to e f f e c t cleavage. Many r e l a t e d rearrangements are a l s o i n t h i s c l a s s (12),, and provide i n f o r m a t i o n on the degree of s t a b i l i z a t i o n i n " t h r e e - e l e c t r o n " bonds. The scope of "threee l e c t r o n " s t a b i l i z a t i o n i s f a r broader than the few cases shown i n Chart 1. Some a d d i t i o n a l important charge types are shown below, and i n c l u d e r a d i c a l anions of o l e f i n s and carbonyl com pounds ( k e t y l s ) , both s u b s t a n t i a l l y more s t a b l e than the c o r r e sponding carbon r a d i c a l or a l k o x y l r a d i c a l .


\·

\.

..

...

\ · ..

C-C C-NC —Ο·* :0—0î / \ / / " '· Mention should be made of " t h r e e - e l e c t r o n " bonding i n v o l v i n g second row elements, e.g. the e f f e c t of adjacent atoms c o n t a i n i n g lone p a i r s on phosphorus or s u l f u r r a d i c a l s ( A — Β i n which Β i s Ρ or S ) , and the e f f e c t of phosphorus or s u l f u r adjacent to a r a d i c a l ( A — Β i n which A i s R Ρ or RS). The great i n c r e a s e i n r a t e of decomposition of symmetrical diazenes c o n t a i n i n g o

3

2

3

2

6

l>


3

2

2

3

-

λ.

—

.

r e l k, 150° Ζ 8 1

Ph CHN(0)=NC00CH Ph CHN=NC00CH Ph CHN(0)=CPha. 2

2

AG

1.0 ("10. ) ~1

3

3

2

, 150°

33.3 (-31.5) 33

Diazene 7, diazene N-oxide 8 and the n i t r o n e J9 a l l decompose at a comparable r a t e . The diazene N-oxide and the n i t r o n e should both be aided by " t h r e e - e l e c t r o n " s t a b i l i z a t i o n i n the d i a z e n o x y l and the iminoxyl m o i e t i e s , r e s p e c t i v e l y (see F i g . 2). The i m p l i c a t i o n (by r e f e r e n c e to the type of diagram shown i n Chart 2 but not repeated f o r the present case) i s that c o n s i d e r a b l e " t h r e e - e l e c t r o n " s t a b i l i z a t i o n a l s o may be present i n the d i a z e n y l r a d i c a l . The comparison of 7 w i t h 8 s u f f e r s from some u n c e r t a i n t y i n the matter of one-bond v s . two-bond cleavage i n the rate-determining step of the model system, benzhydrylphenyldiazene; there may be some weakening of the phenyl-nitrogen bond i n the t r a n s i t i o n s t a t e . The greater r a t e f o r 8 (estimated) than f o r J may be more a consequence of some synchronous character to the decomposition of 8 than a consequence of comparable amounts of " t h r e e - e l e c t r o n " s t a b i l i z a t i o n i n the d i a z e n y l r a d i c a l , R N · and the d i a z e n o x y l r a d i c a l , RN 0-. In e i t h e r case, the c o n c l u s i o n from t h i s example i s that diazenes which decompose by one-bond cleavage i n the r a t e determining step (eq. 6) may decompose at a r a t e comparable to (or perhaps, l e s s than) that of the corresponding N-oxide when the oxygen i s on the n i t r o g e n of the carbon-nitrogen bond undergoing homolysis ( i . e . i n eq. 6, k < k ) . In summary, the type of s t a b i l i z a t i o n on which a t t e n t i o n has 2

2

2


ORGANIC F R E E

132

RADICALS

been focussed here, " t h r e e - e l e c t r o n " bonding, appears to be an important f a c t o r i n r a d i c a l s t a b i l i z a t i o n , a p p l i c a b l e to many s i t u a t i o n s , and h i g h l y worthy of f u r t h e r i n v e s t i g a t i o n . Acknowledgment. T h i s work was supported, i n p a r t , by P u b l i c Health S e r v i c e Research Grant CA-16592 and T r a i n i n g Grant 5T32 CA-09112 from the N a t i o n a l Cancer I n s t i t u t e .


Literature Cited. (1) Koenig, T. in "Free R a d i c a l s " , V o l . 1, Ch. 3, J. K. Kochi, Ed., W i l e y - I n t e r s c i e n c e , New York, N.Y., (1973). See a l s o "The Chemistry of the Hydrazo, Azo, and Azoxy Groups", V o l s . 1 and 2, S. P a t a i , Ed., W i l e y - I n t e r s c i e n c e , New York, N.Y., (1975). (2) Engel, P.S. and Bishop, D.J., J. Am. Chem. Soc., (1975), 97, 6754. (3) Hinz, J . , O b e r l i n n e r , Α., and Ruchardt, C., Tetrahedron L e t t . , (1973), 1975. (4) Engel, P.S., Wood, J.L., Sweet, J.Α., Margrave, J.L., J. Am. Chem. Soc., (1974), 96, 2381. (5) M a r t i n , J.C. and Timberlake, J.W., J. Am. Chem. Soc., (1970), 92, 978. (6) The opposite assumption has a l s o been made i.e. the assumption that the gas phase decomposition of azo compounds proceeds by initial one-bond cleavage (eq.2), [e.g. see Benson, S.W. and O'Neal, H.E., Nat. Stand. Ref. Data Ser., Nat. Bur. Stand. (1970), No. 21]; t h i s assumption (not e s t a b l i s h e d ) would require substantial stabilization i n diazenyl radicals. (7) P o r t e r , N.A., Green, J.G., and Dubay, G.R., Tetrahedron L e t t . , (1975), 3363. Pryor, W.A. and Smith, K., J. Am. Chem. Soc., (1970), 92, 5403. Neuman, R.C., Lockyer, G.D., and Amrick, M.J., Tetrahedron L e t t . , (1972), 1221. (8) F o r r e s t e r , A.R., Hay, J.M., and Thomson, R.H., "Organic Chemistry of S t a b l e Free R a d i c a l s " , Academic P r e s s , New York, N.Y., (1968), p. 180-246. (9) Mahoney, L.R., Mendenhall, G.D., and Ingold, K.U., J. Am. Chem. Soc., (1973), 95, 8610. (10) R u s s e l l , G.A. in "Free R a d i c a l s " , V o l . 1, Ch. 7, J. K. Kochi, Ed., W i l e y - I n t e r s c i e n c e , New York, N.Y., (1973). (11) Nelson, S.F. in "Free R a d i c a l s " , V o l . 2, Ch. 21, J. K. Kochi, Ed., W i l e y - I n t e r s c i e n c e , New York, N.Y., (1973). (12) Lepley, A.R., in "Mechanisms o f Molecular M i g r a t i o n s " , V o l . 3 , p. 297, B.S. Thyagarajan, Ed., W i l e y - I n t e r s c i e n c e , New York, N.Y., (1971). (13) Timberlake, J.W. and Hodges, M.L., Tetrahedron L e t t . , (1970), 4147. (14) G i l b e r t , B.C. and Norman, R.O.C., J. Chem. Soc. (London) B, (1966), 86. A l s o , see ref. 9 (above) and r e f e r e n c e s c i t e d therein.



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(15) W i l t , J.W. i n "Free R a d i c a l s " , V o l . 1, Ch. 8, J. K. Kochi, Ed., W i l e y - I n t e r s c i e n c e , New York, N.Y., (1973). (16) Aston, J.G. and Parker, G.T., J. Am. Chem. Soc., (1934), 56, 1387. (17) Cope, A.C. and Trumbull, E.R., "Organic Reactions", V o l . 11, p. 317, Wiley, New York, N.Y., (1960). (18) Nelson, S.F. and B a r t l e t t , P.D., J. Am. Chem. Soc., (1966), 88, 137. (19) Swietoslawski, W. and Popow, M., J. Chim. Phys., (1925), 22, 397. (20) Olsen, H. and Snyder, J.P., J. Am. Chem. Soc., (1977), 99, 1524. (21) Zabel, D.E. and Trahanovsky, W.S., J . Org. Chem., (1972), 37, 2413. (22) N o r r i s , J.F. and Banta, C., J. Am. Chem. Soc., (1928), 50, 1804. (23) Shelton, J.R. and L i a n g , C.K., J. Org. Chem., (1973), 38, 2301. (24) Cohen, S.G. and Wang, C.H., J. Am. Chem. Soc., (1955), 77, 3628. (25) Grubbs, E . J . , Villarreal, J.Α., McCullough, J.D., Jr., and Vincent, J.S., J. Am. Chem. Soc., (1967), 89, 2234. RECEIVED December 23, 1977.


Decomposition of Diazenes and Diazene N-Oxides. Consideration of

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