2730
C O M M U N I C A T I O N S TO THE E D I T O R The Biradical Intermediate i n the Addition of the Ground State Oxygen Atoms, O(3P), to Olefins’ Sir: The main and probably the exclusive initial step in the reaction of the ground state oxygen atoms, O(3P), with a number of simple olefins2isZb,cthe addition of the oxygen atom to the olefinic dauble bond. Various features of these reactions have been investigated in considerable detail2-+and a general reaction mechanism has been formulated.2brctg The addition products formed are epoxides and carbonyl compounds. The molecular rearrangements (migration of an atom or a radical group) in the formation of carbonyl compounds have been explainedzcin a consistent manner by postulating a triplet biradical intermediate as the initial adduct. However, in a recent study of the addition of O(3P) atoms to condensed olefins at cryogenic temperatures (77 to 113°K) Scheer and Klein6 observed the same addition products but were unable to reconcile their distribution with a biradical intermediate and proposed instead the following nonclassical “transition state” structures, formed from cis- and trans-2-butene1 respectively
YH3 cT i
CH3
\
c=-.
i
9
,,’
\
H ____. O’-.---H ’\\%,
CH3
In these structures oxygen atom was assumed to be in the plane of the molecule, “interacting loosely” and “immobilizing” both vinylic H atoms in the cis (A) and only one in the trans structure (B). Nigration of H was thus permitted in the trans structure, to give methyl ethyl ketone, but not in the cis structure, in which only CH3 migrated, to give isobutanal. The ketone was actually a very important product of cis-2-butene and it had therefore to be assumed also that a t temperatures as low as -196” rotation about the (only slightly perturbed) double bond occurred in the cis “transition state” to convert it to the trans “transition state” but was forbidden in the opposite direction. In view of the great importance of the finer details of the mechanism of addition of O(3P) atoms to olefins, it is imperative that the postulates of Scheer and Klein be subjected to close scrutiny. I n the present communication it is shown that (1) they are entirely incompatible with the previous experimental observations and with several new experimental results now The Journal of Physical Chemistry, Vola74, No. 18, 1970
obtained, and (2) they are unnecessary since product distributions at cryogenic temperatures can be explained simply by conformational effects in the “triplet biradical” intermediates. Somewhat similar conformational effects have been observed in some types of Wagner-Meerwein rearrangements.7 The term “triplet biradical”2 was not intended to convey the notion of two entirely independent free radicals. Spin conservation requires that the two are antibonding in the initial adduct but some overall electronic interaction is to be expected and perhaps a partial transfer of negative charge to the oxygen atom with acquisition of some zwitterion character.2e However, such interactions should not affect basically the following arguments, which are mainly based on the effects of nonbonded repulsions. With these qualifications, the term “triplet biradical” may be retained until detailed electronic and structural descriptions become available.8 The biradical may therefore be portrayed as having a free radical associated with the oxygen atom, which is bonded to a tetragonal carbon, while the other planar alkyl free radical is centered on a trigonal carbon. The original double bond is now a single bond and rotation about it is possible but it requires an activation energy. Nonbonded repulsions between bulky alkyl substituents, when present , contribute substantially to the establishment of potential energy minima corresponding to the most stable conformations. The conformers invoked here are restricted rotors and their structure is of course not staticg since the internal motions are not frozen even at 77°K: the molecules do not by any means spend all their time in their most stable conformations. The explanation of the strongly stereoselective addition to trans-2(1) Issued as N.R.C. No. 11441. (2) (a) R. J. Cvetanovic, J. Chem. Phys., 23, 1375 (1955); (b) ibid., 25, 376 (1956); (c) Can. J . Chem., 36, 623 (1958); (d) J. Chem. Phys., 30, 19 (1959); (e) Can. J . Chem., 38, 1678 (1960); (f) J . Chem. Phys., 33, 1063 (1960); (9) Advan. Photochem.,l, 115 (1963). (3) S. Sat0 and R. J. Cvetanovi;, Can. J . Chem., 36, 279 (1958); 36, 970 (1958); 36, 1668 (1958); 37, 953 (1959). (4) J. M. 5 . Jarvie and R. J. Cvetanovi4 Can. J . Chem., 37, 629 (1959). (5) R. J. Cvetanovi; and L. C. Doyle, ibid.,38, 2187 (1960). (6) (a) M. D. Scheer and R. Klein, J . Phys. Chem., 73, 597 (1969); (b) R. Klein and M. D. Scheer, ibid., 73, 1598 (1969). (7) (a) P. I. Pollak and D. Y . Curtin, J . Amer. Chem. Soc., 72, 961 (1950); (b) C. K. Ingold, “Structure and Mechanism in Organic Chemistry,” Cornell University Press, Ithaca, N. Y., 1953, PP 506508; (c) “Steric Effects in Organic Chemistry,” M. S. Newman, Ed., Wiley, New York, N. Y . , 1956, pp 10, 270. (8) U s e of alternative names for this species is of questionable value a t this time. (9) M. D. Scheer and R. Klein, J . Phys. Chem. 74, 2732 (1970).
2731
COMMUNICATIONS TO THE EDITOR butene at 77°K (predominant formation of trans-@butene oxide and methyl ethyl ketones) is simple. In the trans conformer of the intermediate, as shown by the Newman projection formula, the nonbonded interactions between CH3 and H are relatively weak (Le., the potential energy minimum in the restricted 0
0
CIS-2-BUTENE ADDUCT
T R A N S - 2 - B U T E N E ADDUCT
0
0
a and@ c 2 M H 3
H
0’
CH3
a! and
2-METHYL-2-BUTENE
H&
3
CH3
C2H5
ADDUCTS
0
c K 5 c H 3 & : 3
CH3 0’
H
2 - M E T H Y L - 2 - P E N T E N E ADDUCTS
rotor is very broad) and the necessary geometrical alignments for H and CH3 migration and for the ring closure can all be attained relatively easily. However, the inherently faster H migration and ring closure occur preferentially (to give the ketone and the trans epoxide) and are also much faster at this low temperature than rotation into the less stable cis conformer. As the temperature is raised to 300”K, on the other hand, the slower processes (CH3migration and rotation into the cis conformer) are accelerated relatively more than the faster processes and the reaction therefore becomes much less selective.2c (The “pressure independent fragmentation’’2 also becomes more important.) In the case of cis-2-butene1 the Newman projection formula shows that, because of repulsion between the bulky CHI groups, the orientation of the p orbital of the free alkyl radical favors CH3 migration. As a result, this inherently slower process and the equally slow rotation to the trans conformer are able to compete with the inherently much faster H migration and ring closure. All the four addition products (cis and trans epoxide, methyl ethyl ketone, and isobutanal) are therefore important even at 77”K.O At 300°K all the processes are accelerated and product distribution is affected only littlea2 With 2-methyl-2-butene two adducts are possible, a and 0, as indicated, and CH3 migration is sterically favored in both. Predominant addition a t the less substituted a positionzcexplains predominant formation of pivalaldehyde a t 90°K.sa At 298”K, on the other
hand, the yield of methyl isopropyl ketone (27%) actually exceeds that of pivalaldehyde (17%).1° The products from 2-methyl-2-pentene at 9O0KBbcan be explained in similar manner. In the a adduct CzHs migration is sterically favored but it is inherently slow and therefore the less favorable H migration occurs also to some extent. In the @ adduct CH3 migration is sterically favored and, although slow, can compete with the ring closure. Conformational effects evidently provide simple and logical explanation of product distribution both a t room temperature and at cryogenic temperatures. It is therefore not necessary to invoke the “transition states” postulated by Scheer and Klein. Their postulate has also other serious shortcomings. It implies a concerted addition which however is inconsistent with the very fast reaction rates in a spin forbidden process (requiring in some cases only a few collisions). Moreover, the physical nature of the “loose interactions” of oxygen atoms with H atoms and with the double bond, completely immobilizing the H atoms in some cases and only partially in others and at the same time permitting rotation around a CC double bond at cryogenic temperatures, is difficult to understand. However, the most serious difficulty arises with the tetrasubstituted ethylenes, for example tetramethylethylene. I n this molecule the four bulky methyl groups preclude a “transition state” of the type postulated by Scheer and Klein, yet its reaction with O(3P) atoms at 25”, 100 times faster than that of ethylene, is in fact the fastest reaction of O(3P) atoms studied so far, occurring at approximately each collision. Furthermore, the rates of oxygen atom addition to olefinszd?gshow a continuous increase with the number of alkyl substituents on the double bond with little dependence on what the substituents are. All these results are entirely incompatible with Scheer and Klein’s “transition states” with oxygen atoms in the plane of the olefin molecule, which, moreover, for uncertain reasons has to be exclusively on the side where vinylic H atoms are (e.g. in cis-2-butene). Scheer and Klein assume that no H migration can occur via their ‘?ransition” state A for cis-2-butene because the two vinylic H atoms are “immobilized” and that methyl ethyl ketone is formed only when A isomerizes into B by rotation about the CC double bond. Such an “isomerization” is evidently impossible in cycloolefins and the mechanism of Scheer and Klein dictates therefore that, for example, cyclopentanone should not be a product of addition of O(3P) atoms to cyclopentene. This is in complete contradiction with our experimental results” (Table I). Formation of cyclopentanone as an important product shows that Scheer and Klein’s mechanism is untenable both at room temperature and at 77°K. It (10)
R.J. Cvetanovib and L. C. Doyle, unpublished results. The Journal of Physical Chemistry, Vol. 74, No. IS, 1970
COMMUNICATIONS TO THE EDITOR
2732
for trans- and cis-Zbutene are also applicable to other internal straight-chain olefins.
Table I ----Addition
Temp, OK
298
77
Cyclopentene oxide
products, %--CyclobutaneCyclopencarboxtanone aldehyde
24 32
54 62
DIVISIONOF CHEMISTRY NATIOXAL RESEARCH COUNCIL O F OTTAWA, CANADA
PIF/totel
RECEIVED SEPTEMBER 3, 1969
reaction
22
0.26
6
