3942
Journal of the American Chemical Society
I. Posible hlechanisni for the \Vatu Gas Shift Reaction Catalyzed by Pt[P(i-Pr),] ,
Schenie
1 100:12 /
June 7 , 1978
(19) The decomposition of trafl~-PtH(CO~K)[P(i-Pr)3]~ occurred above 70 OC.
(20) T. Yoshida and S. Otsuka, J. Am. Chem. SOC.,99, 2134-2140 (1977). (21) u(C0) 1840 (w), 1770 (VS)cm-'; 6 1.17 (CH3 dd, JH-P= 13.5, J+H = 7.0 Hz), -2.5 (CH, m). The equivalence of four P(i-Pr)3 units ma be ascribed to the ligand dissociation as observed for Pt3(C0)3(PPh&2Y (22) J. Chatt and P. Chini, J. Chem. SOC.A, 1538-1542 (1970). (23) The water gas shift reaction catalyzed by 4 under the same conditions (in acetone, 100 " C ) shown in the text produced 46 mol of H2 and 40 mol of COz/mol of catalyst (24) Employing Pt[P(kPr)3]3(0.1 mmol) in a mixture of acetone (15 mL)and H20 (1 mL). HCOzH (5 mmol) decomposed at 20 O C with a haif-time of 15 min. (25) u(Pt-H) 2200, u(OC0) 1530, 1310 cm-'; 6 -21.8 (Pt-H, t, Jp-n = 14.8, &-H = 1012 HZ), 1.33 (CH3, q, '+-H '&H = 14.0, &-H = 6.8 HZ), -2.4 (CH. m), 9.5 (OzCH, br). The platinum(l1) hydrido formate complex has a precedent, trans-PtH(OzCH)[P(c-~eHl~)~]~: A. lmmirzi and A. Musco, Inorg. Chim. Acta, 22, L35-L36 (1977).
+
yield) by the reaction of trans-{PtH(CO)[P(i-Pr)3]2)BPhdwith CH3ONa a t room temperature. Facile formation of Pt(R)(C02H)(diphos) fro,m Pt(R)(OH)(diphos) (R = CH3, cC6H9) is also known.I7 The C 0 2 evolution should occur from thermal decompositionIs of the unstable PtH(C02H)L2.I9 A dihydride species tran~-PtH2[P(i-Pr)3]2~~ (4) will then be formed. The reaction of 4 with CO leading to Pt3(C0)3L4 is possible. However, this route in the catalysis is excluded since the isolated Pt3(CO),[P(i-Pr)3]42' was found almost inactive catalytically. Thus, 4 is thought to undergo hydrogen elimination (Scheme I). Involvement of 4 in the catalytic cycle was confirmed by the dihydride-catalyzed water gas shift reaction23from which was isolated 3 as its BPh4 salt. Alternatively, reductive elimination of H C 0 2 H from PtH(CO,H)L2 with concomitant formation of Pt[P(i-Pr)3]2 is possible. A rapid catalytic decomposition of HCO2H into COZ and Hz occurs with 1 a t room t e m p e r a t ~ r e and , ~ ~ the platinum complex was recovered as 4 quantitatively. The decomposition of HC02H probably proceeds through oxidative addition of H C 0 2 H to give trans-PtH(O2CH) [P(i-Pr)3]2, which is followed by @-hydrogen elimination affording 4 and C02. Consistent with this, the decomposition of HC02H was also catalyzed by trans-PtH(O2CH) [P(i-Pr)3]225 prepared separately by COz insertion into the Pt-H bond of 4. The possible pathways are summarized in Scheme I.
References and Notes "Catalyst Handbook", Spring-Verlag, London, 1970. J. P. Collman. J. Halpern, J. Norton, and J. Roth, "Workshop on FunQmentai Research in Homogeneous Catalysis as Related to U.S. Energy Problems", Stanford University, Stanford, Calif., Dec 4-6, 1974. R. M. Laine, R. G. Rinker, and P. C. Ford, J. Am. Chern. SOC.,99,252-253 (1977). H. Kang, C . H. Mauldin, T. Cole, W . Slegeir, K. Cam. and R. Pettit, J. Am. Chem. Soc., 99, 8323-8325 (1977). C.-H. Cheng. D. E. Hendriksen, and R. Eisenberg, J. Am. Chern. SOC.,99, 2791-2792 (1977). T. G. Appleton, H. C . Clark, and L. E. Menzer, Coord. Chern. Rev., 1 0 , 335-422 (1973). T. Yoshida, T. Okano, and S. Otsuka, J. Chern. SOC.,Dalton Trans., 993-999 (1976). S . Otsuka, T. Yoshida, M. Matsumoto, and K. Nakatsu, J, Am. Chem. SOC., 98, 5850-5858 (1976). BF4- salt: v(Pt-H) 2230 cm-'; 6 -18.9 (R-H, t, Jp+ = 14.2, JR-H= 1005 HZ), 1.20 (CH3, q, 3 + . a : ~f 5 J p - ~= 14.7, JH-H= 7.3 HZ), -2.1 (CH, m), 7.0 -8.3 and 8.70 (pyridine, m). 4Pt-H) 2178, u(C0) 2058 cm-l; d -4.55 (Pt-H. t, JP+ = 11.3, JR+ = 881 Hz), 1.32 (CH3, q, 3 J ~ - 5~ J ~ =- ~15.0, JH-H= 7.5 Hz), -2.7 (CH, m). KdlSs= 1.4 X lo-' M (20 " C ) in THF, which was determined by the electronic spectra with and without added P( i-Pr)3.Two isosbestic points were observd at 363 and 374 nm. M) measured in aqueous pyridine ([H20] The pHappof NaOH (9.8 X = 22.2 M) at 20 O C is 13.5. D. H. Gerlach, A. R. Kane, G. W. Parshall. J. P. Jesson, and E. L. Muetterties, J. Am. Chem. Soc., 93,3543-3544(1971). H. C. Clark and K. R. Dixon, J. Am. Chern. SOC.,91, 596-599 (1969). u(I'-H) 1943, u(C0) 1575 cm-'; 6 -9.16 (Pt-H, t, JP-H = 17.6, JR-H = 609 HZ), 1.23 (CH3, qs 3+-H 5 J p - ~= 14.2, JH-H= 7.1 HZ), -2.3 (CH, m). i4Pt-H) 1985, u(C0) 1617, u(C-0) 1008 cm-'; 6 -8.98 (Pt-H, t, JP-H = 1 7 . 7 , & - ~ = 6 3 0 H Z ) , 1.17(CH3,q,3Jp_H+5Jp_~=14.0, JH-H=~.OHZ), -2.1 (CH, m), 3.67 (CH30, s , JR.H = 4.5 Hz). M. A. Bennett and C. A. Appleton, J. Organornet. Chern., 5 5 , C88-C90 (1973). A . J. Deeming and B. L. Shaw. J. Chem. SOC. A, 443-446 (1969).
+
+
0002-7863/78/ 1500-3942$01.OO/O
Toshikatsu Yoshida, Yasuhiko Ueda, Sei Otsuka* Department of Chemistry Faculty of Engineering of Science Osaka Unicersity, Toyonaka, Osaka, Japan 560 Receiued December 19, 1977
Complete Kinetic Analysis of Thermal Stereomutations of (+)-( 1 S,ZS,3Sj-r-l-Cyano-
t-2-phenyl-l,c-3-dideuteriocyclopropane Sir: Both one-center ( k i ) and two-center (kij) thermal epimerizations of cyclopropanes are known in special c a ~ e s , l -but ~ there is no reliable method for anticipating the relative importance of these reaction modes in other cyclopropyl systems. Only experiments capable of discriminating among all onecenter and all two-center epimerization possibilities can provide the factual grounds for developing theory appropriate to this task. To synthesize the substrate selected for our kinetic studies, (+)-( 1S,2S,3S)-r- 1-cyano-t-2-phenyl- 1 ,~-3-dideuteriocyclopropane (( +)-1-c), trans-P-de~teriostyrene,~ and ethyl diazoacetate-dl were reacted in the presence of CuSO4; the resultant mixture of esters was epimerized with potassium tert-butoxide in deuterated tert-butyl alcohol6 to afford r - l ethoxycarbonyl-t-2-phenyl-1,~-3-dideuteriocyclopropane. Hydrolysis with dilute acid and resolution through the quinine salt7 gave (+)-( lS,2S,3S)-2-phenyl-I ,3-dideuteriocyclopropanecarboxylic acid. Conversion to the corresponding nitrile, (+)-1-c, was accomplished by way of the acid chloride and the amide.8 Epimerization of (+)-1-c with potassium tert-butoxide in deuterated tert-butyl alcohol gave a 70:30 mixture of (+)-1-c and (-)-2-f, without racemization of ( + ) - l . The rotations in CHC13 of optically pure nitriles were [ m ]+369O ~ for (+)-1 and [ a ]-22.8' ~ for (-)-2, Thermal equilibration9 a t 242.1 "C of 1 and its cis isomer 2, starting with either isomer, was followed by VPC; the concentration vs. time data provided values for the rate constant (kl k2 kl3 k23) = 1.09 X 5-l and theequilibrium constant K = 0.40.
+--FD
+ +
+
Ph
Ph
D
k i ' kpS k i s + k p a
CN
D
i
2
When the thermal isomerizations of 1-c and of 2-t were examined by VPC analysis, followed by preparative VPC separation of cis and trans isomers and N M R analysis to distinguish 1-c from 1-2, and 2-c from 2-2, the concentration vs. time data-44 experimental points-were fit to theoretical curves based on exact solutions to the kinetic expressions ap-
0 1978 American Chemical Society
3943
Communications to the Editor
Table 1. Calculateda and Observed Mole Percent Concentrations of the Cyclopropane Isomers of Scheme 111
Scheme I
(+)-I-c
Time, min
x
I
2 -4
__*
t
d
CN
Scheme I1 Q
( t )- I
-
D
kltk13
N M R Data 98.7 1.3 79.1 8.8 78.5 8.7 95.5 4.5 69.2 14.3 68.4 14.3
0 240 (calcd) 240 (exptl) 0 375 (calcd) 375 (exptl)
k Z +k13
(-1- 2
0 240 240 0 375 375 a
(-)-I4
Rotational 96.7 78.8 78.8 IO0 71.9 70.1
(calcd) (exptl) (calcd) (exptl)
Data 3.3 9.1 8.4 0 11.6 12.5
(-)-24
(+)-2-c
0 8.1 8.6 0 10.6 10.9
0 4.0 4.1 0 5.9 6.4
0 8.1 8.7 0 10.8 11.7
0 4.0 4.0 0 5.7 5.7
According toscheme 111 and the parameters K = 0.40, kl = 0.76 k z = 0.33 X IO-' s-l, k l z = 0.60 X IO-' s-I, and k l $ =
X IO-'s-l,
0.43
x
10-5 s - ~ ,
Scheme 111 p,h
+
+
+
+
9 h CN
+ +
propriate to Scheme I and the parameters K , ( k I k23) = 0.76 X lo-' S - I , ( k 2 k13) = 0.33 X S - I , (k3 kl2) = 0.60 X s-I, and (k3' k , ~ ' =) 0.43 X s-]. The eight rate constants not given explicitly in Scheme I are derivable from these five parameters and the principle of microscopic reversibility. Calculated concentrations matched observed concentrations of 1-c, l - t , 2 - t ,and 2-c with an average deviation of 0.8 mol %. If the thermal isomerization of (+)-1 (or ( - ) - 2 ) were analyzed by VPC, followed by VPC separation of cis and trans isomers and polarimetric analysis of each combination of enantiomers, application of the same exact equations would lead to experimental values for ( k l k13), ( k 2 + k23), kl2, and klz' (Scheme 11). Although six experimental values would then be available for isomerizations from a single starting material such as (+)-1-e, and there are six rate constants to be deduced, the kinetic problem would not be solved: values of k 1 2 ,k3, (kl k23), (k2 k13), ( k l k13), and ( k z kz3) would be available, and additional experimental data would be required to overcome the linear dependency of the four sums of two rate constants . Analysis of samples derived from kinetic runs starting from (+)-1-c through N M R spectroscopy with the aid of the optically active shift reagent tris[3-(heptafluoropropylhydroxymethylene)-d-camphorato]europium( III)lo provided the new data required. In its presence, the enantiotopic protons C(3) H in 1-t and 2-t can be distinguished: the more downfield of the two C(3) H doublets is due to the (+) isomer of 1-t and the (-) antipode of 2-r. The cis products from (+)-1-c were found to contain (-)-2-t but not ( + ) - 2 - t ,and therefore k23 = 0. This, together with the observed rotation of 2 and the NMR-determined ratio of 2-t to 2-c, implied that the concentration of (-)-2-c in the product mixture was zero, within likely experimental error; hence k 13 = 0. When the trans isomer recovered from a kinetic run was examined with the aid of the optically active shift reagent, the C(3) H doublet from (+)-1-t was so small it could be ascribed to the stereochemical impurity of the starting material, which suggested that k3 = 0. Finally, the trans material recovered from kinetic runs and then epimerized a t C( 1) with KOBut/DOBu-t afforded (-)-2-t but no ( + ) - 2 4 ;thus k123, the triple
+
D
+
+
epimerization process in the thermal rearrangement, is, as expected,3c not competitive, and indirect routes from (+)-I-c to ( - ) - l - c are not effective. If k3, k13, k23,and kl23 are indeed 0, then the stereomutations of the 1-cyano-2-phenyl- 1,3-dideuteriocyclopropanes involve just four isomers, not eight, and kinetic Scheme I l l applies. To test the adequacy of Scheme I11 quantitatively, two samples of (+)-1-c were isomerized and the experimentally observed concentrations of the four isomers as derived by N M R data, and as derived from optical rotation data, were compared with calculated concentrations (Table I). The agreement between experimental and calculated concentrations supports the formulation of Scheme 111. These results support the oft employed assumption that the stereomutations of a cyclopropane such as 1 would proceed through k l , k2, and k l l processes alone." Uncertainties in interpretation of data for trans-dialkenylcyclopropanes12and 1-cyano-2-isopropenylcyclopropane~ Ig caused by reports of the dominance of ki, processes in 1,2-dide~teriocyclopropane~~ and 1 -phenyl-2-de~teriocyclopropane3~~c are thereby diminished.
Acknowledgement.This work was supported by the National Science Foundation. References and Notes (1)(a) 8. S.Rabinovitch, E. W. Schlag, and K. B. Wiberg, J. Chem. Phys., 28, 504 (1958); (b) E. W. Schlag and B. S . Rabinovitch, J. Am. Chem. Soc.,82, 5996 (1960). (2)(a) J. A. Berson, Annu. Rev. Phys. Chem., 28, 1 1 1-132 (1977):(b) R. A. Bergman in "Free Radicals", Vol. I, J. Kochi, Ed., Wiley, New York, N.Y.,
1973,Chapter 5. (3)(a) J. A . Berson and L. D.Pedersen, J. Am. Chem. Soc., 97,238 (1975); (b) J. A. Berson, L. D. Pedersen, and B. K. Carpenter, b i d . , 97,240 (1975); (c)J. A. Berson, L. D. Pedersen, and B. K. Carpenter, ibid., 98,122 (1976); (d) J. P. Chesick, ;bid., 84,3250(1962); (e) J. E. Baldwin and K. E. Gilbert,
3944
Journal of the American Chemical Society
/
100:12
/
June 7, 1978
ibid., 98, 8283 (1976);(f) W. C. Dauben and M. S. Kellogg, ibid., 94, 8951
(1972). D. Seyferth, L. C. Waughan, and R. Suzuki, J. Organomet. Chem., 1, 437 ( 1964).
T. Saegusa, Y . Ito, T. Shimizu, and S. Kobayashi, Bull. Chem. SOC. Jpn., 42, 3535 (1969). A. T. Young and R. D. Guthrie, J. Org. Chem., 35, 853 (1970). J. E. Baldwin, J. Loliger, W. Rastetter, N. Neuss, L. L. Huckstep, and N. De La Higuera, J. Am. Chem. SOC., 95, 3796 (1973). A. Burger and W. L. Yost, J. Am. Chem. SOC.,70,2198 (1948). The pyrolyses were carried out by submerging sealed, degassed ampules, containing solutions of 1 or 2 in 1-methylnaphthalene, in a molten salt bath controlled in temperature to fO.l OC. R. R. Fraser, M. A . Petit, and J. K. Saunders, Chem. Commun., 1450
3
OH
4
5
6
strate, the relative yields were found to be 77, 18, and 5%, respectively. Simple free-radical autoxidation of 3 by triplet molecular oxygen catalyzed by di-tert- butyl peroxide produced a ratio for 4, 5, and 6 of 16, 7, and 77%, respectively. The predominance of 6 in radical autoxidation but its production in low yield in the 0 2 - s and peroxide experiments is indicative of the intermediacy of singlet oxygen in the latter.5 Control experiments indicated that 1 and 3 were unreactive toward KO*, benzoyl peroxide, or lauroyl peroxide separately. LikeJohn E. Baldwin,* Charles G. Carter wise, controls with KOH, crown ether, and molecular oxygen Department of Chemistry, Unicersity of Oregon demonstrated no reaction with 1 or 3; KOH, crown ether, and Eugene, Oregon 97403 dibenzoyl peroxide underwent only slow reactions with these Receiced February 6 , 1978 singlet oxygen traps which could not effectively compete with the rapid reaction with KO*. B. Oxygen Evolution. When reactions of 0 2 - a were carried out in the presence of 1, the amount of oxygen liberated was found to be dependent upon the amount of 1 used; oxygen evGeneration of Singlet Oxygen in the Reaction of olution was suppressed completely with excess 1. Although Superoxide Anion Radical with Diacyl Peroxides' oxygen evolution could never be completely inhibited by less Sir. efficient singlet oxygen traps* such as tetramethylethylene or 3, a decrease in the amount of oxygen evolved was observed. There has been considerable debate recently regarding the C. Quenching of Singlet Oxygen by Superoxide. The rate formation of singlet molecular oxygen from superoxide anion constant for quenching of singlet oxygen by 0 2 - . has been radical, 0 2 - . 2 We wish to report the apparently efficient determined2bto be 1.6 X IO9 M-I s-l. This suggests that 0 2 - production of singlet oxygen in the reaction of 0 2 - a with the should compete with even efficient singlet oxygen traps such oxidizing agents benzoyl peroxide and lauroyl peroxide. These as 1 while slower reacting olefinic and other traps should be experiments involved the addition of a solution of the diacyl less effective in our system compared to other methods of peroxide in benzene to a slurry consisting of KO2 and 18generating singlet oxygen. We found this to be the case with crown-6 in benzene. The corresponding carboxylic acids3 were 1,3, and tetramethylethylene; the latter two scavengers proisolated in quantitative yield in accordance with the stoichiduced the singlet oxygen product in comparitively low yields. ometry shown in eq I . Additional substantiation that 0 2 - . competes with 1 for singlet 0 0 oxygen was obtained when experimental conditions were varied II II in the reaction of benzoyl peroxide with 0 2 - - in the presence 2O--. RCOOCR --t 2 0 L + 2RC0,(1) of 1. It was found that the yield of 2 depended logically upon The following observations suggest that a significant fraction the following three factors: (i) the amount of KO2 used, (ii) of the oxygen generated in these systems is in the excited singlet the concentration of 18-crown-6, and (iii) and concentration state, presumably 'As. of l.9Increasing the amounts of either KO2 or crown ether resulted in lower yields of 2 while increasing the concentration A. Trapping Experiments. Reaction of 0 2 - a with benzoyl peroxide or lauroyl peroxide in the presence of a slight excess of 1 produced higher yields of the singlet oxygen product. of the common singlet oxygen trap 1,3-diphenylisobenzofuran, D. Quenching of Singlet Oxygen by @-Carotene. When Ihe reaction of benzoyl peroxide with 0 2 - - was carried out in the 1, was found to produce 2 equiv of o-dibenzoylbenzene, 2, per equivalent of peroxide. The yields of 2 determined by ultravipresence of both 1 and 0-carotene, the yield of 2 was found to olet spectroscopy were found to be >97% in both systems; depend upon the amount of ,&carotene, a known quencher of isolated yields of 2 from 0 2 - - benzoyl peroxide averaged 86% singlet oxygen.I0 Moreover, the amount of 2 produced was for three trials. Trapping experiments were also conducted with qualitatively in accord with the respective rate constants for tetramethyleth~lene~ to produce 3-hydroxy-2,3-dimethyl-l- trapping by 1 and quenching by @-carotene. butene in low yield. Our results indicate that singlet oxygen or some species mimicking singlet oxygen2d,l can be efficiently produced by Since there is evidenceh.' that these commonly utilized scavengers may not be unambiguous as detectors of singlet the reaction of 0 2 - . with diacyl peroxides, although the oxygen, we investigated the reaction of 0 2 - - with benzoyl mechanism of production is only speculative a t present. It is peroxide or lauroyl peroxide in the presence of 1,2-dimethylpossible that many of the deleterious effects on metabolic cyclohexene, 3. This compound has been shownSto distinguish systems which have been attributed to 02--or H O - may acbetween oxidation by singlet oxygen and a simple free-radical tually be due to singlet oxygen produced by oxidation of 0 2 - m autoxidation reaction on the basis of a distinctly different with biological oxidizing agents. This hypothesis is particularly distribution of products for the two processes. Under our attractive since there is now a consensus that the Haber-Weiss conditions in the reaction of 0 2 - - and benzoyl peroxide, 4,5, reaction between 0 2 - . and H202 to generate HO. is too slow and 6 (eq 2) were formed in relative yields of 92, 5, and 3%, to be of significance i n biological systems.' respectively, demonstrating the generation of singlet oxygen Additional work on the reaction of 0 2 - . with other oxidizing in this system. When lauroyl peroxide was used as the subagents is in progress. (1971).
(a) R. J. Crawford and T. R. Linch, Can. J. Chem., 46, 1457 (1968); (b) J. A. Berson and J. M. Balquist, J. Am. Chem. Soc., 90, 7343 (1968); (c)R. G.Bergman and W. Carter, ibid., 90, 7344 (1968);(d) R. G. Bergman and W. Carter, ibid., 91, 741 1 (1969);(e) M. R. Wilcott, Ill, and V. H. Cargle. ibid., 91, 4310 (1969); (1) A. Churmy and D. Cram, ibid., 95, 4237 (1973); (g) W. von E. Doering and K. Sachdev, [bid., 96, 1168 (1974);(h) W. von E. Doering, G. Horowitz, and K . Sachdev, Tetrahedron, 33, 273 (1977). (a) M. Arai and R . J. Crawford, Can. J. Chem., 50, 2158 (1972);(b) J. E. Baldwin and C. Ullenius, J. Am. Chem. SOC.,96, 1542 (1974).
+
0002-7863/78/1500-3944$01 .OO/O
0 1978 American Chemical Society
