1614
Free-Radical Chemistry of Organophosphorus Compounds. V. 1,2 Reactions of Thiyl Radicals with Alkylphosphonites. Effects of Activation Energies and A Values on the cy vs. p Scission Competition Wesley G . Bentrude* and Peter E. Rogers Contributionfrom the Department of Chemistry, University of Utah, Salt Lake City, Utah 841 12. Received June 3, 1975
Abstract: The reactions of RS. with certain diethyl alkylphosphonites, R’P(OEt)2, were studied as a function of temperature. Over a 100 OC range of temperatures the ratio of oxidation product to substitution product, Le., R’P(S)(OEt)2/ RSP(S)(OEt)2, was examined to establish values of E s u b - E,, and A s u b / A o x . On reaction with PhCH2P(OEt)2 E s u b - E,, for a series of three RS- was: i-PrS-, 0.35 kcal/mol; ?-Bus., 0.48 kcal/mol; p-MeC6H&H2S., 1.9 kcal/mol. Accompanying changes in A s u b / A o x were minor. The reaction of i-PrS- with t-BuP(OEt)z had E s u b - E,, of 3.2 kcal/mol. Thus E s u b - E,, responds to structural changes in both RS. and R’P(0Et)Z. Arguments are presented in favor of a mechanism which involves formation of an intermediate phosphoranyl radical, R’(RS)P(OEt)2, which gives product via competitive a or /3 scission processes, the relative rates of which depend on the strengJhs of the C-P and C-S bonds undergoing scission. Excluded are mechanisms in which the initial configuration of R’(RS)P(OEt)2 or its rate of permutational isomerization solely control the a / @scission (substitution/oxidation) ratio.
Reaction of alkoxy and thiyl radicals with trivalent organophosphorus derivatives may give both oxidation and substitution products (eq 1). T h e ratios oxidation/substitution RA.
+ PXYZ
-
a scission /3 scission
/
RA-P’
\
Y
S
1I
+ X. substitution
Z
RSP(OEt& 2 RSP(OEt), (1)
‘Z
A = S,O
in these systems a r e strongly structurally dependent. W e suggested earlier3 that this competition be rationalized, to a first approximation, in terms of the effects of the strengths of the R-A and P-X bonds of the likely phosphoranyl radical intermediate (1)4 on the a/P scission ratio. Since product studies a t a single temperature neglect entropy factors, and thereby may lead to spurious conclusions, we have examined the temperature dependence of the oxidation/substitution ratio in the reaction of a series of thiyl radicals with certain alkylphosphonites, eq 2. T h e results of the R’P(S)(OEt),
RS.+ R P ( 0 E t h
2 ‘b)
that initial RSP(OEt)2 was cleanly converted to the stable, oxygen-insensitive 0.0-diethyl S-alkyl phosphorodithioate (eq 3) and that R. was rapidly trapped as R H . T h e latter prevented any of the known5 potential side reaction 4 or the
RSP(OEt),
+ R. (2)
+ R’.
work reported here strongly support the prior postulates and exclude possible explanations based solely on the configuration of the initially formed phosphoranyl radical or on the rate of its subsequent permutational isomerization.
Results As seen in Table I the reactions of PhCH2P(OEt)* with three thiyl radicals (i-PrS., t-Bus., and p-CH3C6H4CH&) were studied along with those of i-PrS- with two alkylphosphonites, PhCH2P(OEt)2 and t-BuP(OEt)l. In this way both the effects of R - stability with constant R’ and vice versa could be investigated. Product ratios were measured a s a function of temperature over the temperature range 0-100 O C or 20-100 O C . Use of an excess of RSH ensured Journal of the American Chemical Society
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98:7
/
R.
+ RP(OEt),
RP(OEt)?
-
(3)
RP(SXOEt),
(4)
readdition of free R.’ to RSP(OEt)2 from occurring. Reactions were run to total consumption of R’P(OEt)2 and gave excellent product balances. (See Experimental Section for details.) Plots of In [(substitution)/(oxidation)] vs. 1 / T gave the values of E s u b - E,, and Asub/A,x listed in Table I from which the kox/ksubratios shown were calculated. Errors (linear regression analysis) a r e a t the 95% confidence level. Cases 1 and 2 compare results obtained by direct measurement of the product phosphorodithioate (case 1) with those obtained by use of the toluene peak as a measure of the amount of substitution which occurs (case 2). Agreement is excellent. Figure 1 is the plot of log k,/k,j vs. 1 / T for case 1 which utilizes 49 experimental points, given as a n example of the quality of the plots obtained.
Discussion It should first be noted that within the series of reactions of PhCH2P(OEt)z with three RS. there is a systematic variation in Esub - E O x ;that is Esub - E,, increases in the order i-PrS- < t - B u s . < p-CH&H4CH2S-. If the plausible assumption is made that Esub is near constant since P h C H y is displaced in each case, then E,, decreases in the order of increasing Re stability, i.e., i-Pr < t-Bu- < pC H ~ C ~ H ~ C Likewise, H ~ S . a comparison of cases 1 and 2 with case 4 in which E,, is likely unchanged shows a n apparent increase in Esub of about 3 kcal/mol when the less stable tert-butyl radical is the product of substitution instead of the benzyl species. Thus, the competition between the enthalpies of oxidation and substitution in these reactions is apparently influenced by the stabilities of the radicals formed in both processes.
March 31, 1976
1675 Table 1. E, and A-Factor Effects on the Competition between Oxidation and Substitution in the Reaction RS.
1 2 3 4 5
i-PrSc i-PrSd t-BuSd p-MeC6H4CH2Sd i-PrSe
0.36 f 0.01f 0.34 f 0.02g 0.48 f 0.06h 1.9 f 0 . 1 ' 3.2 f 0.2j
PhCH2 PhCH2 PhCH2 PhCHz t-Bu
0.8 I 0.79 0.44
1.3 5.4
+ R'P(OEt)2
0.994 0.955 0.976 0.993 0.995
2.12 2.15 4.74 13.2 22.9
95% confidence limit. Correlation coefficient. Based on GLC measurements of i-PrSP(S)(OEt)z and PhCH2P(S)(OEt)z. Based on toluene . points; temperatures 0, 20, 40, 60, 80, 100 OC;digital elecand PhCH2P(S)(OEt)2. e Based on i-PrSP(S)(OEt)z and ~ - B U P ( S ) ( O Ef~4)9~ total tronic integrator. g 54 total points; temperatures 0, 20,40, 60, 80, 100 OC; electronic digital integrator. 46 total points; temperatures 0, 20,40, 60, 80, 100 OC; electronic digital integrator, ' 33 total points; 0, 20, 40, 60, 80, 100 "C; disk integrator. J 36 total points; temperatures 20, 40, 60, 80, 100 OC; electronic integrator.
Several precise mechanistic alternatives may be considered for these reactions. (1) Competing concerted sulfur transfer and R.' displacement without formation of a discrete intermediate like 1 (eq 2). (2) Oxidation via intermediate 2 in competition with a concerted, one-step, substitution process:
Rs\
&OEt),
1"'+ R!
R.
+ R'P(S)(OEt), (5)
RSP(OEt1,
(3) Substitution via intermediate 2 in competition with a concerted, I -step oxidation process: R. R'P(SXOEt),
-2 4
+
2 1
[a)
2
R.'
LO'
+ RSP(OEt),
p scission: cy
- 2
sciwion RSP(OEt), / \
+ R.'
R'P(S)(OEt),
I
1,T. *c-s
Figure 1. Temperature dependence of reaction 2, case 1 of Table I .
(4)Both oxidation and substitution via 2 with the oxidationlsubstitution ratio determined by the relative rates of a and
I 5
I O
(7)
+ R.
6 scission Since E S R evidence is available6 for the presence of a phosphoranyl radical similar to 2 when CH& is generated in the presence of (EtO)3P or n-Bu3P, we tend to discount alternative 1, although the E S R work does not prove the intermediacy of species like 2 in such reactions. Furthermore, t h e substitution of PhCH2- and t-Bu. by Me2N- in cyclic phosphonites occurs with inversion of configuration about phosphorus.' For this to occur in a concerted fashion would require a n in-line arrangement of R' and R S , e.g., a s in 3. This could arise only by attack of RS. on
3
pyramidal R'P(OEt)* opposite R'. T h e orientational requirement of 3 could confer upon the substitution process a reduced A value compared to that for oxidation which requires only that RS. attack toward the lone pair of pyramidal (Et0)2PCH2Ph in order to give the known8 retentive stereochemistry. Table I shows, however, the similarity of A,, and A s u b in cases 1-4, a finding inconsistent with alternative 1. Alternatives 2 and 3 a r e unlikely ones in that both the
oxidation and substitution activation energies in Table I respond t o structural variation. In reaction 5 the rate of 2 formation should have little dependence on the nature of RS., and the oxidation activation energy could be influenced by R. stability only if step (a) were rapidly reversible and followed by step (b) in a rate-determining role. Similarly, step ( a ) of reaction 6 would have to be reversible to account for the response of this scheme to changes in R.'. N o experimental evidence is available concerning the reversibility of RS. additions to trivalent phosphorus. However, it seems unlikely that t h e two competing processes would occur one through a n intermediate and one concertedly. In addition, it is known9 that the second-order rate constant of RS-reaction with n-Bu3P, ( E t 0 ) 3 P , and Ph3P is 108-109 a t 60 "C. In these studies only oxidation was found. If reaction 6 obtains in the competitive oxidation/substitution reactions, then for case 5 the rate constant for step 6a would be a t least 5 X lo6. Assuming a n A value of lo9 for such a second-order process (likely a maximum in view of the above arguments about the stereospecificity of substitution) leads to a n E , of 3.5 kcal/mol. This requires that E , for step 6a when R - is PhCHz be nearly zero and thus a near diffusioncontrolled substitution process. Further the A s u ~ / A oratio x in comparing cases 2 and 5 responds just oppositely to what one expects for the replacement of a benzyl by a bulky tertbutyl. For the above reasons we judge that alternative 4 most reasonably and economically depicts the mechanism of reaction of thiyl radicals with RP(OEt)2. It can be estimated3p4 from thermodynamic d a t a that AHr0 (298 OC, g ) for transfer of sulfur from t - B u s - to X3P is favorable energetically by 28-29 kcal/mol. T h e energetics of this oxidation a r e depicted in Figure 2. It is less clear what the energetics of the substitution process should be. Most certainly the
Bentrude, Rogers
/
Reactions of Thiyl Radicals with Alkylphosphonites
1676 that the RS. enters t h e apical position of t h e intermediate phosphoranyl radical, and the electropositive odd electron (or pair) fifth ligand is equatorial a s indicated by ESR,” then both 4 and 5 could be formed initially. Presumably 4
R’
Et0
Eta\P--. I I’ :
\I
R
SR
. . ..
-_L
. R ’ + K’P(S)(@Ef)2
=r*ct,oi
(,,rii.-.?fc
Figure 2. Energy diagram for reaction of RS. w i t h K’P(OEt)*.
strength of the P-S single bond formed is less than t h a t for a P - 0 bond, but so far as we are aware no value is known. T h e energetics of P=O formation a r e favored over substitution in reactions of RO. with PX3,3,4and we presume this to be true with RS.-PX3 reactions. This also is shown in Figure 2. Only with reasonably stable R.’, t-Bu, and PhCH2- does substitution by R S - compete with oxidation, and with less stable R.’, substitution may not be even thermodynamically a favorable process overall. The observations of Table I then a r e readily explained within the context of Figure 2, and in Table I one may equate E s u b with E , and E,, with E B . For the series of RS-, increasing R. stability is accompanied by a reduced E , for p scission (Eo), the E , for a scission ( E , ) being presumed to remain essentially constant. W e a r e unwilling t o attach any particular importance to the small changes in A , / A p (Asub/ A O x )observed in cases 1-4. Both scission processes have similar probabilities. T h e decrease in k , seen in case 5 compared to cases 1 and 2 results from a n increase in E , (Ep for 1 and 4 presumed constant) which is somewhat offset by the increase in A , / A p T h e cause of the latter effect is not apparent. Another factor which could affect the ease of either a or p scission is the configurational requirement for such a process. ESR studies of nonphenyl-substituted phosphoranyl radicalsI0 have shown large (600 -1200 G ) isotropic phosphorus hyperfine splittings which have usually been interpreted in terms of a trigonal bipyramidal structure with odd electron equatorial. A very recent report” has pointed out that a trigonal bipyramidal geometry with electron pair equatorial andothe odd electron distributed over t h e axial ligands may more accurately describe such species. T h e geometric considerations which follow apply to either electronic description. T h e usual assumption in pentacovalent phosphorus chemistry is that groups enter and leave the trigonal bipyramid only in the apical position.12 In other workI3 we have obtained evidence that the alkoxy radical enters the (R0)4P. intermediate in a t least a stereoqelective manner and ESR results which lead to a similar conclusion for H. have appeared.I4 If RS. attacks R’P(OEt)2 in such a way
Journul of rhe American Chemical Society
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98:7
SR 5
4
.....
OEt
favors a scission, and the ratio 415 therefore would influence the kol/kSratio. It is seen from Table I that a mechanism in which 4 gives only a scission and 5 only p scission cannot apply (unless t h e ratio 4/5 is influenced by RS and R’ in a way that coincidentally parallels bond strengths). If 4 or 5 gives both a and /3 scission, then relative bond strengths would still contribute to a / & and the role of initial 4/5 ratio would be hard to decipher. Clearly, the ratio a/@is not solely controlled by the configurations of the initially formed intermediates and the statistics of’their formation. A further consideration in these systems is the possibility that a scission is exclusively apical but that a permutation process15 interconverts isomers in such a manner t h a t R’ can attain the apical position prior to a scission. (In other studies in RO. systems we have presented evidence that a pair-wise permutation (MusherI6 mode M I ) prior to scission is not r e q ~ i r e d . ) ~ , ~ ’ S uac possibility h is shown in eq 8.
6
-
Again the data of Table I d o not rule out such a process 6 does occur, then it having some role. However, if 5 must constitute a rapid equilibration step preceding a slower a scission18 unless the effects of changing R’ on k5-6 just happen to parallel R.’ stability. The ratio (Y/P does not depend on the rate of’5 6 alone.
-
Experimental Section Materials. The preparations of all reactant and product phosphorus compounds were described earlier.3 Where necessary pure products for GLC sensitivity calibrations against internal standards were isolated by preparative GLC methods. Kinetic Measurements. Pyrex reaction tubes were serum capped and flushed with nitrogen. Diethyl benzylphosphonite (-0.9 mmol) was transferred to the reaction tubes by means of a syringe and approximately 1.9 ml of nitrogen-flushed o-dichlorobenzene was added (molar concentration of phosphonite, -0.3). The above solutions were equilibrated in a constant temperature bath for 30 min before 0.5 ml of mercaptan was added. After another 15 min, 0.005 mmol of azobisisobutyronitrile (AIBN) in 0.1 ml of o-dichlorobenzene was added, and the reaction tubes were heated for at least 10 decomposition half-lives of AIBN. Control reactions showed that the phosphonites were stable under these reaction conditions although the formation of some of the corresponding phosphonates (
