7717
preceded and followed by the known standards. The same general procedure was used to prepare control reactions containing the expected products. All products were found to be stable at the reaction temperature. Larger scale reactions from which products were isolated were run on an approximately 1-g PAT scale. All products were isolated by glc and were identified by comparing their physical and chemical properties with authentic samples. Pmr spectra of PAT reaction mixtures showed that the products detected by glc analysis were not formed in the chromatograph. Gaseous products were
withdrawn by a gas-tight syringe and analyzed by glc on a Porapak S column. For the thermal decomposition of PAT in trimethyl phosphite under air, the solutions were prepared and sealed at atmospheric pressure without degassing and then were decomposed and analyzed in the usual manner. Acknowledgment. The research was supported by a grant from the National Science Foundation (GP22885).
Free-Radical Chemistry of Organophosphorus Compounds. 11. Reactivity of Phenyl Radical toward Trimethyl Phosphite and the Mechanism of the Corresponding Photo-Arbuzov Reaction with Phenyl Iodide Juian-Juian L. Fu,' Wesley G. Bentrude,* and Claibourne E. Griffin3
Contribution f r o m the Department of Chemistry, University of Utah, Salt Lake City, Utah 84112, and Department of Chemistry, University of Toledo, Toledo, Ohio 43606. Received January 29, 1972 Abstract: In competitive kinetic experiments, phenyl radical from the thermal decomposition of phenylazotriphenylmethane (PAT) at 60" is shown to react with trimethyl phosphite (TMP) ca. 100 times as rapidly as it abstracts chlorine from CC14. Under the same conditions, hydrogen abstraction from cyclohexene and dimethyl or diethyl phosphite (DEP) is found to be 4.1 and 2.9 times, respectively, as fast as reaction of phenyl with CC1,. Photolysis at 60" of phenyl iodide in solution with various TMP and DEP mixtures gives extrapolated ratios of rate constants essentially the same as found from PAT decompositions. It is concluded that Ph . is the phenylating species in these reactions as well. A second-order rate constant for reaction of Ph. with TMP at 60" of 106-107 mol-' 1. sec-' is estimated. It is suggested that the high reactivity of Ph. and the relative unreactivity of CHI. toward TMP are a result of the greater strength of the P-C bond formed in the phosphoranyl radical intermediate RP(OCH:),, where R = Ph., and possible resonance stabilization of this intermediate by the phenyl substituent.
T
he preceding paper emphasized product studies of the decomposition of phenylazotriphenylmethane (PAT) in trimethyl phosphite (TMP) and mixtures of TMP with dimethyl phosphite (DMP) and diethyl phosphite (DEP). It was concluded that the products formed and their response to variations of medium could be consistently interpreted in terms of rate-determining thermolysis of PAT to give Ph., NP, and Ph3C. (reactions 1 and 2) followed by a very rapid A
+PhN,. + .CPh3 PhNz. + .CPha +Ph. + Nz + .CPha PhN=NCPh,
(1)
reaction (3 and 4) of Ph. with T M P to yield dimethyl Ph.
+ (CHaO),P +PhP(OCH3)3
(3)
1
Ph$OCH3l3
---t
PhP(O)(OCH&
+ CH3.
determine the reactivity of radicals toward trivalent phosphorus. This is because Ph. is of intermediate p ~ l a r i t y being ,~ more electrophilic than alkyl radicals, which d o not react with trialkyl phosphites, but less so than the extremely reactive alkoxy and thiyl species. However, the newly formed carbon-phosphorus bond in the potential phosphoranyl radical intermediate in this reaction, 1, is likely to be considerably stronger than that in similar species resulting from alkyl radical attack." Further, the phosphoranyl radical with
(4)
phenylphosphonate. As discussed previously, the reactivity of P h . toward TMP is of special interest so far as increasing our understanding of the factors which (1) Taken in par? from the Ph.D. thesis of J.-J. L. Fu, University of Utah, June 1971 ; University Fellow, 1967-1970. (2) University of Utah; to whom inquiries should be addressed. (3) University of Toledo. (4) J.4. L. Fu and W. G. Bentrude, J . Amer. Chem. Soc., 94, 7710 (1972).
(5) The order of electronegativities, terf-butoxy > phenyl > methyl > cyclohexyl, has been established: H. Pritchard and H. Skinner, Chem. Reu., 55,745 (1955); A. F.Gaines and F. M. Page, Trans. Faraday Soc., 62, 3086 (1966); Krishnaji and S. Chandra, J . Sci. Znd. Res., 27, 135 (1968) [Chem. Abstr., 69, 46136b (1968)l. Hammett p values for abstraction of hydrogen from substituted toluenes, which presumably measure polarity, are for ChC. , 6 - 1.46 at 50"; for f-CIHBO..7 -0.35 at 40"; for CHI.,* -0.1 at 100"; and for Ph.,9,1o 0 to -0.4 at 60". (6) E. S.Huyser, J . Amer. Chem. Soc., 82,394 (1960). (7) C. Walling and J. A. McGuinness, ibid., 91, 2053 (1969); H. Sakurai and A . Hosomi, ibid., 89,458 (1967). (8) W. A. Pryor, U. Tonellato, D. L. Fuller, and S . Jomonville, J. Org. Chem., 34,2018 (1969). (9) R. F. Bridger and G. A. Russell, J . Amer. Chem. Soc., 85, 3745 (1963). (IO) W. A. Pryor, J. T. Echols, Jr., and K. Smith, ibid., 88, 1189 (1966). (11) The average bond energy, 5 for Ph3P is 77 kcal/mol, based on gas-phase heats of formation (AHfO) at 298"K, for Ph3P of 78.5 kcal/ mo1,2' and Ph. of 78.5 kcal/mol [S. W. Benson and E. Whittle, private communication. See R. S . Rodgers, D. M. Golden, and S . W. Benson, J . Amer. Chem. Soc., 89, 4578 (1967), for a slightly older value].
Fu, Bentrude, Griffin Free-Radical Chemistry of Organophosphorus Compounds
7718 Table I. Decomposition of Phenylazotriphenylmethane (PAT) in Dimethyl Phosphite-Carbon Tetrachloride Solutions at 60"
8.99 7.35 7.59 8.20 7.19 0.568 1.19 2.82 5.33 5.99 8.27 15.0
0.207 0.559 1.01 2.80 6.30 1.01 1.01 1.01 1.01 1.01 1.01 1.01
86.3 79.4 72.3 52.0 34.5 74.9 74.5 73.9 71.3 63.7 59.0 51.3
5.42 14.2 23.8 43.2 56.8 25.0 24.7 24.1 23.8 21.6 20.0 19.6
3.09 2.88 2.81 2.97 3.16 2.87 2.90 3.06 2.80 2.72 2.67 2.23 Av 2.85
0.248
a Mol l.-I X lo2. Mole ratio. Based on the initial amount of PAT added. kH/kci = [Ccla]([C&] - [C6Hs]o)/[(CH3o)zP(o)H]. [CsHjC1]; [CsHs]is the amount of benzene produced in decomposition of PAT in CCla-(CHaO)zP(O)H while [C&]O is the amount of benzene produced in decomposition of PAT in CCl, with similar concentration and can be obtained from Figure 1. e Standard deviation, one u limit.
phenyl on phosphorus may benefit from resonance stabilization. In this paper, we present competitive kinetic data consistent with the presence of free Ph. as the active phenylating species in the reaction with T M P and from which an estimate of the rate constant for phenylation at 60" can be made. It is shown as well that the formation of dimethyl phenylphosphonate on photolysis of PhI-TMP solutions probably also involves free Ph Results Competitive Reactions. Phenyl from PAT with TMP, CCI,, and Hydrogen Sources. In order to obtain an estimate of the reactivity of phenyl radicals with TMP, we have determined the relative rates of reaction of this species toward T M P and toward CC1,. This is a particularly useful measurement, since the comparative reactivities of CC14and a great number of hydrocarbons and other hydrogen donors (kCl/kH)have been previously determinedag Ideally, direct competitive studies would be desirable. However, T M P and other trivalent phosphorus compounds undergo spontaneous ionic and radical reactions with CC1,. I 2 We therefore measured the rate constant for the reaction of phenyl with T M P ( k p ) cs. that of its reaction with several hydrogen donors (klI) and then combined this with kCl/kHvalues previously measured by others or determined as part of this research. Agreement between kp/kH values using the three hydrogen donors, dimethyl phosphite (DMP), diethyl phosphite (DEP), and cyclohexene, was quite good. The first competition studied was that between D M P and CCII. The ratio kH/kCl was determined from the amounts of benzene and chlorobenzene formed at the completion of the reaction and was calculated using expression 5 . Of course, this approach is justified kH/kcI
=
[CCL][C FH6]/[CsH ,Cl][( CHsO),P(O)H]
(5)
only if: (a) reactions 6 and 7 are of the same kinetic order in C6H;. and first order in D M P and CC14; (b) the ratio [DMP]/[CCl.,] remains constant during the (12) See R . E. Atkinson, J. I. G. Cadogan, and J. T. Sharp, J . Chem. SOC.B , 138 (1969), and references cited therein.
Journal of the American Chemical Society 1 94:22
reaction; and (c) reactions 6 and 7 are the only sources of benzene and chlorobenzene. Effects of changing
+ HP(O)(OCHdz +CsH6 + .P(O)(OCH& LH
CsH5.
CgHj.
+ CCI,
c1
----f
C6HbCI
+ clpc.
(6) (7)
the [CC14]/[DMP] ratio on the kH/kC1value are seen in data compiled in Table I. The constancy of kH/kC* and the lack of effect of changes in PAT concentration indicate that assumption a is valid. The large excesses of D M P and CC1, should assure the constancy of the concentration of these reactants (assumption b). A possible complication, however, is reaction between D M P and CC14. Diphenylphosphine oxide has been shown13 to react with CC14 at room temperature, and (RO),P(O)Cl has been isolated as a product in the reaction of dialkyl phosphites with CCll in the presence of a tertiary amine.14 Chloroform is also formed in the reaction which may proceed according to
+ cc13 + (RO)zP(O)H +(RO)zP(O). + HCCI,
(RO)zP(O).
+ CCh
--f
(RO)tP(O)CI
" X I 3
