C. E. CASTROAND R. D. STEPHENS
4358
Vol. 86 TABLEV
80
VARIATIOX OF THE SECOSD-ORDER RATECONSTANT FOR THE OXIDATION OF ~ M E T H O X Y P H E N WITH O L p H AT 25.0' 70
PH
0.105 0.78 1.00 2.04 2.89 2.97 4 05
60
50
w -z
Obsd. k2,'
Cf -1 set.-'
Calcd. k ? , b I f - ' see.-'
27.9 22.9 21.9 12 9 7.83 7.65 6.76
25.0 23.2 22.0 12.0 7.67 7.48 6.72
A
Except for the p H 1 data, all observed kz values are the result of 3 runs a t each p H . kl is calculated from eq. 9 with k, = 25.5 sec.-l and k b = 6.65 2 V - I set.-' for p-methoxyphenol.
'U v1
-
40
2 N
30
20
IO
0
3
2
1
4
5
The kinetic results obtained for the oxidation of hydroquinone and p-methoxyphenol contrast markedly with those previously reported for aliphatic glycols. We have found no evidence for the formation of a kinetically detectable intermediate in the oxidation of hydroquinone and its monomethyl ether a t pH 1 even with stopped-flow methods. Our results are best interpreted in terms of second-order kinetics. However, this does not rule out the possibility that substrateperiodate complexes are intermediates in oxidation a t pH 1 if their formation rather than their decomposition to products is rate controlling.
PH Fig. 6.-Dependence of second-order rate constant on p H a t 25.0: 0, experimental points for hydroquinone; A , experimental points for p-methoxyphenol. +
the agreement between calculated and observed values is good. Our results are depicted graphically in Fig. F, and they lead to the conclusion t h a t both the un-ionized and monoionized forms of periodate are active in oxidizing hydroquinone and p-methoxyphenol.
[CONTRIBUTION FROM
THE
+ IO, % OH
intermediate complex k,, > > kI
%
+ 1
+ IO3
0
Further evidence concerning the mechanism of the periodate oxidation of aromatic diols and their mono ethers is being actively sought in our laboratory.
DEPARTMEXTS OF XEMATOLOGY AND CHEMISTRY, UNIVERSITY OF CALIFORNIA, RIVERSIDE,CALIF]
The Reduction of Multiple Bonds by Low-Valent Transition Metal Ions. The Homogeneous Reduction of Acetylenes by Chromous Sulfate BY C. E. CASTROA N D R. D. STEPHENS RECEIVED MAY1, 1964 T h e homogeneous reduction of acetylenes by chromous sulfate in water or aqueous dimethylformarnide a t room temperature yields trans-olefins in high yields T h e stoichiometry, stereospecificity, kinetics, and reactivities of the acetylenes toward Cr f Z are in accord with a mechanism which involves a rate-determining attack of Cr -2 upon a 1 : 1 acetylene-Cr T 2 complex.
Introduction Low-valent transition metal species are present in Ziegler-Natta polymerization catalysts' and active sites in these systems have been located a t a transition metal center. * Moreover, transition metal complexes are well known to play a key role in many enzymatic transformations in which a valence change of the metal species O C C U ~ S . ~Yet. much is to be learned about the
nature of the interaction of low valent ions with organic structures in less complicated environments. As part of a study of the homogeneous reduction of organic molecules by transition metal ions we have found the acetylenic bond to be one of the most effective multiply bonded structures to accomplish the oxidation of Cr(I1) t o C r ( I I I ) . 4 The reduction of acetylene to ethylene by an am-
(1) hl. L . Cooper a n d J. B. Rose, J . Chem. Soc., 795 (1959); C . Beerman and H . Bestian, A a g e w . Chein.. 71, 61R (1959). ( 2 ) W. L. Carrick. F. J. Karol, G. J Karapinka, and J. J. S m i t h , J . A m . Ckem. Soc., 82, 1502 (1960); W 1,. Carrick, el ~ l . t,b i d . , 82, 5319 (1960), el seq.
(3) J. S. Fruton and S. Simonds, "General Biochemistry," John Wiley and Sons, I n c . , New York. N. Y . , 1958, Chapter 11, 12, and 13. (4) This work has been reported in p a r t C E. Castro and I< D . Stephens, 143nd h-ational Sleeting of t h e American Chemical Society, Atlantic C i t y , K.J., Sept., 1962, Abstracts, p. 23Q.
REDUCTION OF ACETYLENES BY CHROMOUS SULFATE
Oct. 20, 1964
4359
moniacal solution of chromous sulfate was first reportedAa early in this century. Subsequently, the acetylene to ethylene conversion has been accomplished in acidic media with chromous ~ h l o r i d e . ~ ~ - ~ Similarly, propiolic acidje and monosodium acetylenedicarboxylateAfare reported to oxidize chromous chlo8 4ride solutions. The heterogeneous reduction of phenylpropiolic acid by chromous hydroxide to cis- and Initlol conc trans-cinnamic acid6 and phenylpropionic acidsC has fHC+C-CH20HI 0576 M been noted, as has the reduction of enynes’ to dienes of (NH,CIO,) = 045 (HCI0.J = 005 the vitamin A series with this reagent. (CrS0.J = 00247 Results 5 IO 15 20 25 30 35 4 0 45 5 0 5 5 60 Stoichiometry and Stereochemistry-The homogeneT i m e (min). ous reduction of acetylenes (0.1-0.3 M ) by chromous Figure 1. sulfate (0.4-0.7 Jf C r + 2 )proceeds readily a t room temperature in water (pH 4) or aqueous dimethylformtrans stereochemistry of these reductions accords with amide under a nitrogen atmosphere. Internal acetythat reported for the reduction of dideuterioacety1enejd lenes are converted in a stereospecific manner to transand, by analogy with this case, for propyne-ldg by olefins (1) aqueous acidic chromous chloride. lo R H It is noteworthy t h a t in water the spectrum of the \ / 2H’ + 2CriZ + RC=CR’ +2CrC3 + C=C (1) Cr(II1) product (A,,, 409, 575 mp) corresponds to that of the aquo ion C r ( H 2 0 ) 6 f 3 . However, when the reacH/ \R, tion is run in dimethylformamide- water the product The results of the reduction of a variety of acetylenes spectrum corresponds to t h a t of the inner sulfate comare reported in Table I. No other organic products plex” C r ( S 0 4 ) + (A,,, 585 mp) in this solvent. The spectra and extinction coefficients are given in the ExTABLE I perimental section (Table IV). PRODUCTS OF THE REDUCTION O F ACETYLENES B Y CHROMOUS Reactivity.-Qualitatively the reactivity of acetySULFATE IN WATER lenes (-0.2 M) toward chromous sulfate (-0.5 M ) can Yield, Product 70 Reactant be grouped as (a) very reactive (instantaneous reaction HC=CCHzOH CHz=CHCHzOH 89 essentially complete in 5-15 min. a t room temperature) HC=CPha CHz=CHPh 94 (propargyl alcohol, 1-butyn-3-01, acetylenedicarboxylic HOzC H acid, phenylpropiolic acid, and 1,4-dihydroxybutyne-2); \ / (b) moderately reactive (reaction complete in 2-3 hr.) 94 HOzCC-CCOzH (2-butyne-1-01, phenylacetylene, hexyne-1) ; (c) slow H COzH (1 day for completion) (2-carboxydipheiiylacetylene) ; Ph (d) inert (no appreciable reaction in 1 day) (diphenyl\ /H acetylene, 2-amino-, 4-amino-, 2-methoxy-, 4-methoxy-, 91 PhCECCOzH 4-hydroxy-, 4-~arboxydiphenlacetylene,pentyne-2, and H /c=c\
x
-
COzH H
CH3
\ c=c/ / \
H HO-CHz
\
HO-CHZC=CCH~-OH
84
CHzOH H
/
2,5,5-trimethyl-3-hexyn-2-01). Thus, in the absence of hydroxylic substituents terminal acetylenes are more reactive than internal ones. On the contrary, the sequence illustrates the importance HOCHZC-CCH~OH
92 HC=CnBu
H H
CHz-OH
-
HCECCHZOH
>
CHaCrCCHzOH CH3 CHs
I >> C H ~ C = C C H Z C H , ,CHsCC=C&CHa I I OH
Ph
>
CH3
were detected, and the yields reflect the efficiency of the work-up procedure. Isomerization of cis- or trunsolefins did not occur under reaction conditions.* The
of accessible coorGination sites on the acetylene. Kinetics.-The kinetics were studied by employing varying high (20-fold excess) initial concentrations of substrates. A typical pseudo-second-order plot of 1 / ( C r f 2 ) v s . time is depicted in Fig. 1. In all cases good linearity was obtained through 90yc completion. With stoichiometric ratios of reactants, third-order
(5) (a) M. Berthelot, A n n . chim. fihys., [41 9 , 401 (1909); (b) W. Traube a n d W. Passarge, Eer., 49, 1692 (1916); ( c ) W. I. Patterson and V . d u V i g n e a u d , J . Bioi. Chem., 123, 127 (1938); (d) J . E . Douglas and B. S. Rabinovitch, J . A m . Chem. Soc., 74, 2486 (19.52); (e) R S. Bottei, A n a l . C h e m . A d a , 30, 6 (19641, (f) R S. Bottei a n d S H. F u r m a n , A n a l . Chem., 27, 1183 (1955). (6) E. O t t a n d V . Barth, Ber., 67, 1672 (1934). (7) O r t h o Pharmaceutical C o r p . . British P a t e n t 740,851 (1955). (8) T h e reduction of acetylenes by Cr-2 c a n be cleanly controlled to stop a t t h e olefin. Hou-ever, contrary to recent reports [ K . D. Kopple, J . A m . Chem. Soc., 84, 1586 (1962) 1, olefins bearing electron-withdrawing
substituents can be reduced a t room t e m p e r a t u r e u n d e r t h e s a m e (acidic) reaction conditions to t h e alkane. Our studies of the slower reduction of olefins will be reported separately. (9) B. S. Rabinovitch a n d F. S. Looney, ibid.,76, 2652 (1953) (10) I n t h e case of reactive c a r b o n y l conjugated acetylenes, acidic solutions of t h i s reagent allow t h e possibility of reduction b y a nonstereospecific path involving HCI a d d i t i o n followed by Cr(I1) cleavage of t h e carbon-halogen bond. T h e reductive dimerization of 1,l-diphenylethylene is such a case: C. E . Castro, i b i d . , 83, 3262 (1961). (11) Detailed spectra of these solutions were recently published S . B. Fogel. J. M. J . Tai, a n d J. Yarborough. ibid., 84, 1145 (1962).
85
0
Run in 2 : 1 DMF-HzO
C. E. CASTRO AND R . D . STEPHENS
4360
plots of l / ( C r + * ) *vs. time were nicely linear. The disappearance of Cr(I1) was followed by a titrimetric procedure.’* I n separate runs the appearance of Cr(111)was determined spectrophotometrically a t 590 mk. The third-order rate expression (2) was cleanly obeyed.
(2)
rate = k3(Cr+2)2(acetylene)
Vol. 86
Discussion A mechanism that is most consistent with our observations is: the rapid and reversible formation of a 1 : 1 acetylene-chromous complex ( 3 ) followed by the ratedetermining attack of chromous on the chromousacetylene complex (5)
A summary of the kinetic data appears in Table 11. The very mild temperature dependence of k3 for the reduction of propargyl alcohol in water is portrayed in Table 111.
R
H
TABLE I1 OF ACETYLENES B Y CHROMOUS RATESOF REDUCTION SULFATE AT 22’ kad X 10%.
H +,* Acetylene
Solvento
mole/l.
pc
l.s/rnolel/ sec.
% 5 f
0.50 0 5 1 2 T .50 5 1 32 S 50 5 0 37 s 50 5 13 0 S 05 5 12 6 S 10-4 13 7 S 5 1 . 1 0 1 10 23 7 S 0 55 0 55 1 7 s 0 55 0 55 0 028 S Volume ratios. * Perchloric acid added. Ionic strength adjusted with NH4CI04. R a t e constants are the average of at least 3 runs; precision was within 5%. E T = titrimetric, S = spectrophotometric. HCECCH~OH HCECCHZOH HC=CCHzOH HCECCH~OH HC=CCHzOH HCECCHZOH HCECCHZOH CH~C=CCHZOH PhC-CH
Hz0 Hz0 Dz0 111 DMF-Hz0 1 : l DMF-H20 1:1 DMF-Hz0 1 : 1 DMF-H20 2 : 1 DMF-Hz0 2: 1 DMF-Hz0
TABLE I11 TEMPERATURE DEPENDENCE OF kB FOR THE REDUCTION O F PROPARGYL ALCOHOLBY CHROMOUS SULFATE I N WATER WITH ( H + ) = 0.53 AI’ = IONICSTRENGTH O C .
ka X 101, 1.2/molel/sec.
Method
0.3 18.3 18.3 21.4 41 6
0.88 1.4 1.4 1.3 1.2
T S T T T
Temp.,
Reduction with Cr(I1) EDTA.-The reduction of 2butyn- l-ol with chromous ethylenediaminetetraacetate in aqueous media a t pH 9-10 was examined to ascertain the influence of a highly coordinated Cr(I1) upon the course of the reaction. l3 Moreover, the large reductive capacity of this entity (Cr(I1) EDTA4-+ Cr(111) EDTA, EIl1= -1.4 v. vs. s.c.e.14)was desirable. The reaction proceeded only to 6y0 completion in 2 days. The product distribution indicated by gas chromatography was trans-crotyl alcohol 1-2%, czscrotyl alcohol -60%, 3-buten- l-ol -40%. This reaction was too slow for rate studies and consequently the more rapid reduction of propargyl alcohol by Cr(I1) EDTA was examined. This latter reduction proceeded with a rate independent of acetylene. The expression
( 3 ) was valid through 85% completion (12) C . E , Castro and W . C. K r a y , Jr., J . A m . Chem. Soc , 86, 2768 (1963). (13) Recent studies [ K . L. Pecsok, A n d . Chem., 36, 1995 (1963)l suggest a quinquedentate coordination of Cr+Z with E D T A ; W. C. E . Higginson, J . Chem. Soc., 2761 (1962). proposes such a binding for a variety of divalent cations. (14) R . L. Pecsok, L. D. Shields, and W. P Schaefer, J I n o v g . Chem., 3, 114 (19641, and references therein.
The 1 : 1 Complex.-The reactivity sequence noted above is not what would be expected for the reaction of acetylenes with nascent hydropen,jb.15but, rather, i t parallels the facilities with which the acetylenes might be expected to function as ligands for metal ions.16z17 T h a t coordination is essential to reaction is emphasized by the fact that o-carboxytolane is reduced by chromous whereas the para isomer is not. Furthermore, upon the reduction of the former bidentate ligand a transient red color is observed initially which might be attributed to the 1 : 1 complex.18 A similar red color is observed during the initial phase of the reduction of acetylenedicarboxylic acid. The rapid equilibrium (4) is in keeping with the rapid exchange rates of Cr (11) complexes.l 9 By similar considerations the Cr(I1) EDTA reduction of 2-butyne-1-01 would not be expected to proceed by the same mechanism. Indeed, the facts that the reaction is quite slow in spite of the very favorable thermodynamics’? and t h a t the trans stereochemistry is lost points up the necessity for “proper” coordination with the acetylene. Similarly, the markedly different kinetics of the reduction of propargyl alcohol by Cr(I1) EDTA and C r + * further emphasize the different course of these reductions. ro It would seem reasonable t h a t a quinquedentate Cr(I1) EDTA complex should be capable of binding an acetylene but not a water molecule simultaneously. Consequently, a transition state corresponding to I1 depicted below should be precluded in this milieu.21.”l” The Rate-Determining Step.-Certainly the second chromous ion should approach a chromous-acetylene complex (I) from the side opposite to that of the metal ion already there and a t a position most remote from it. Thus, electrostatic considerations and the stereospeci(15) T h e chromous titer of stock solutions is stable for months. (16) M.A. Bennett, Chem. Reo.. 62, 611 (1962). (17) Though not strictly a fair comparison, because of different coordination properties, the stabilities of Ag---acetylene complexes decrease with substitution of methyl groups o n the a-carbon. G. K . Helmkamp, Ii 1. Carter, and H. J. Lucas, J . A m . Chem Soc., 79, 1308 (1957). (18) If a Cr(II1)-acetylene complex should be present, i t must rapidly he reduced by C r + ? since even with stoichiometric ratios of reactants good third-order kinetic plots were obtained. Hence t h e C r ( I I 1 ) product is not inhibiting t h e reaction as might be expected if significant concentrations of a slow reacting Cr(II1)-acetylene complex were present. (19) D. R . Stranks and I
