J . Am. Chem. SOC.1985, 107, 5198-5203
5198
to ethane rather than methane.
cleavage, a carbene with enough internal energy to undergo this cleavage will be generated. Since it has been observed that an increase in pressure of added hydrocarbon reactant does not lower C2Hzyields: we must assume that cleavage of vibrationally excited 16 and 18 occurs rapidly before collisional deactivation can occur. A major drawback to experimental studies involving nucleogenic 'IC atoms is the fact that only products bearing the "C label are detected. Thus, while labeled acetylene is observed, the remaining fragments from the cleavage are undetected.' However when C atoms are generated by the chemical decomposition of 5-diazotetra~ole,~'yields are high enough to permit detection of all products. Experiments of this type support the contention that cleavage of the bond p to the carbene carbon occurs in a stepwise manner. Thus reaction of carbon with propane gives mainly products which may be rationalized by assuming initial insertion into the 2O C-H bond to generate isopropyl carbene, which may either cleave or rearrange (eq 9). The major products from the cleavage reaction are CzH2 and CH4. The CHI results from hydrogen abstraction by methyl radicals. A concerted cleavage of the two C-C bonds p to the carbene carbon (eq 10) would lead
Acknowledgment. We thank the Auburn University Computation Center for a generous allotment of computer time. M.L.M. thanks Auburn University for the award of a Grant-in-Aid (82-54). P.B.S. gratefully acknowledges support of this research by the National Science Foundation under Grant CHE-8401198. An insightful comment by a referee regarding the possibility of reaction on the 3A' potential energy surface is acknowledged.
(37) Shevlin, P. B.; Kammula, S. J . Am. Chem. SOC.1977, 99, 2627.
Registry No. C, 7440-44-0; CHI, 74-82-8; CH=CH, 74-86-2; ethylidene, 421 8-50-2.
-
t . C H 3 t H-C=C-CH3
*
C H,,LC3 C/H /
CH3
..
concertit
I
C2H2 t CH3
H
t CH3-CH3
H-CEC-H
(10)
Oxyphosphorane Hydrolysis. Reversible Ring Opening of Spirophosphoranes Containing Six-Atom Rings Glenn H. McGall and Robert A. McClelland* Contribution from the Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 1A I . Received December 17, 1984 Abstract: A kinetic and mechanistic study is reported for the hydrolysis of two spirooxyphosphoranes containing six-atom rings (6-phenyl-1,5,7,1l-tetraoxa-6-phosphospiro[5.5]undecane (2a) and the 3,3,9,9-tetramethyl-substituted compound (2b)). An intermediate is observed at pH 0.01 molar) the phosphonate ester products, once formed, undergo further hydrolysis due to the high hydroxide ion concentration. This interferes with the kinetic study of the phosphorane hydrolysis since it is seen in the UV spectra as a decrease in absorbance, as was confirmed by isolating the product mixture from dilute acids where no hydrolysis occurs, and adding this to the same N a O H solutions. A pH-jump technique (ref 5 and Experimental Section) has been developed to measure rate constnats for the phosphorane when this further hydrolysis interferes. As can be seen in Figure 1, the rate of phosphorane hydrolysis is independent of pH above pH 12. The constant kbo in Table I is the first-order rate constant in this region. This rate constant refers to the noncatalyzed conversion of the phosphorane to the mixture of phosphonate products. pH 9-11. In this region the same absorbance increase is observed as that seen at higher pH. Now, however, a first-order dependency on Hf concentration is observed. The second-order is listed in Table I as kSH.General acid rate constant (k,,/[H']) catalysis by the buffer is also observed. A crude Brernsted plot using NH4+, Et,NHt, and H 3 0 +has a slope of 0.65. The kinetics in this region refer to an acid-catalyzed conversion of the phosphorane to the phosphonate mixture.
8
6
7
5
4
3 ppm
Figure 2. 'H N M R spectra for the phosphorane 2b. The top spectrum has been recorded in CDCI3, and the doublet at 6 3.6 represents the four equivalent C H I groups. The remaining spectra have been recorded in 0.005 M N a O D / D 2 0 (HOD resonance at 6 4) at approximately 30 s, 2 min, 5 min, and 15 min after phosphorane addition. The bottom spectrum represents the diester 4b and is characterized by a CH, doublet at 6 3.7 (POCHI) and a CH, singlet a t 6 3.2 (CH,OH).
pH 9. The rate constant for the decrease is dependent on the base component of the buffer. At zero buffer concentration the rate constant (k," of Table I) is independent of pH. We suspected that, under these acidic conditions, the phosphorane was being rapidly converted to an intermediate phosphonium ion 3 and that what was being observed in the kinetics was the hydrolysis of this cation to the phosphonate products. (See Figure 1 of the accompanying paper for an example of the spectral change which occurs in the hydrolysis of an alkoxyphosphonium ion.) This was verified by OCH,CR,CH,OH Ph-P-
I.
OCH,
I.
Ph-P-0
0
'R Ja:R= H J b ; R : CH,
6a;R:H 6 b ; R : CH,
preparing the model cations 6, by methylation of the cyclic phosphonate esters. In aqueous sulfuric acid solutions'* these undergo hydrolysis with a UV spectral change very similar to that exhibited by the phosphorane. Moreover the rate constant for the model ion is virtually identical with that of the corresponding phosphorane. It also proved possible to form the cations 3 in the absence of nucleophile^,'^ by addition of 1 equiv of trifluoro- ~ - (12) 20% H2S04-50%H2S0,. It was necessary to use these more concentrated acids to retard the rate of the cation hydrolysis in order to perform
kinetic experiments with the model cations. (1 3) For other examples where phosphonium ions I t ve been prepared by ring opening of phosphoranes see: ( a ) van Aken, D. Paulissen, L. M. C.; Buck, H. M . J . Org. Chem. 1981, 46, 3189-3193. (b, Granoth, I.; Martin, J. C . J . A m . Chem. SOC.1981, 103, 2711-2715. (c) Hellwinkel, D.; Krapp, W. Chem. Ber. 1978, 1 1 1 , 13-41.
J . Am. Chem. SOC.,Vol. 107, No. 18, 1985
Oxyphosphorane Hydrolysis
5201
h
ii
2
1 Y
2 0
3 f.8
b*CH: CH;
4
-L
I
3
.1
4
0
.2
I 8
J 7
6
5
4
3
2
1 ppm
-3
Figure 3. ' H NMR spectrum (200 MHz) of the cation 3b in CD2C1,. Assignment of the ring protons has been made on the basis14 of P-H coupling constants (.IHb+ = 5.6 Hz, .IH,+ = 16.2 Hz). 6
I
I
I
I
I
7
0
9
10
11
PH
Figure 5. Absorbance change occurring during the slow kinetic phase. For conditions see caption to Figure 4.
with constants kfo and kfHlisted in Table I. The second phase has rate constants which connect the H+-dependent region at pH 10 to the pH-independent region at pH 7 , so that one continuous curve is obtained from high pH and low pH. As seen in Figure 4, the absorbance level attained after completion of the fast phase varies with pH such that the subsequent slow phase undergoes a changeover from a negative absorbance change at pH 7 to a positive change at pH 10. Figure 5 shows a plot vs. pH of the total absorbance change which occurs in the slow phase. The result is a spectroscopic titration curve, and an acidity constant Ka(spect) (Table I) can be calculated according to eq 3, where AA, AAacidrand hAbase are the absorbance changes at intermediate pH and in acid and in base, respectively. AA - AAacjd (3) Ka(spect) = W+I ubase
~
1
2
3
4
TIME (sac)
Figure 4. Absorbance vs. time curves for the hydrolysis of the phosphorane Za. Experiments were conducted on the stopped-flow spectrophotometer at a wavelength of 265 nm and with an initial phosphorane concentration of 5 X molar. The final pH (from top to bottom) is 5.0, 7.28, 7.7, 8.2, 8.7, and 9.2.
methanesulfonic acid to the phosphoranes 2 in anhydrous CD2C12. The cation 3b displays the expected low-field 31PN M R resonance at +30.5 ppm and a 'H N M R spectrum (Figure 3) consistent with the proposed structure. Addition of solutions of this cation to sulfuric acid results in an absorbance decrease, with rate constants identical with those obtained from the phosphorane 2b. Addition of the cation to sodium hydroxide results in an absorbance increase, again with a rate constant identical with that for the phosphorane. In other words, in base solution the cation undergoes ring closure, regenerating its phosphorane precursor. pH 7-10. More complex kinetic behavior is exhibited in this region. The absorbance-time curves reveal two processes (Figure 4), the first characterized by a rapid rise in absorbance and the second characterized by a slower change giving the absorbance of the ester products. As seen in the upper curve of Figure 1 the first change follows the rate law kOhd(fast)= kfo + kfH[H']
(2)
(14) For P-H coupling constants in 1,3,2-dioxophosphorinanes see: Maryanoff, B. E.; Hutchins, R. 0.; Maryanoff, C. A. Top. Stereochem. 1979, 5, 187-326.
- AA
It can be noted that the behaviors at higher pH and lower pH represent simplified versions of this more complex behavior. At pH >10 the initial rapid phase has a negligible amplitude, so that only the relatively slow absorbance increase is observed. At pH < 7 , the initial rise is present but it has become sufficiently fast that it cannot be observed even by stopped-flow spectroscopy. In consequence only the slower absorbance decrease is observed.
Discussion The mechanism which we propose for the hydrolysis of the phosphoranes 2 is shown in eq 4. In this mechanism the phos-
2
3
phonium ion 3 is an intermediate at all pH, being formed reversibly from the phosphorane. The rate-limiting step at all pH is the cation hydrolysis, which occurs with water addition and, in base, hydroxide addition. The intermediacy of the phosphonium ion is obviously well established in neutral and acid solutions where it can in fact be observed as an intermediate. In basic solutions it is not observed but its presence can be inferred from the kinetic analysis. The most direct evidence for the reversibility in base of the first step is the observation that the phosphonium ion ring closes to the neutral phosphorane in these solutions. It is also possible at intermediate pH to observe the phosph0rane:phosphonium ion
5202 J . A m . Chem. SOC.,Vol. 107, No. 18, 1985
McGall and McClelland
equilibration as a rapid process preceding the overall hydrolysis and on the basis of the spectroscopic changes calculate the equilibrium constant (eq 5 ) . It can be noted that if ring opening [phosphorane] [H'] K,(spect) = K, = [phosphonium ion]
(5)
were not truly reversible throughout the pH range of the measurement of this value, a titration curve would not be produced. According to this model, the lower kobsd-pHprofile in Figure 1 refers to the formation of the phosphonate ester productsI5 from an equilibrating mixture of phosphorane 2 and phosphonium ion 3. The assumptions that this equilibration is rapid and that the phosphonium ion hydrolyzes by addition of water, the k2" process, and hydroxide ion, the kZoHprocess, produce the following kinetic k,'[H+] kobsd
=
+ k20HK,
[H+I +Kl
(6)
expression. This equation has the correct form to account for the rate-pH profile. In acid solutions where [H+] >> K,, the phosphonium ion predominates in the initial equilibrium and the observed kinetics refer to the addition of water to this cation. Thus kobsdis independent of pH and k," = k2". At higher pH where [H']
