J. Am. Chem. SOC.1984, 106, 5505-5512 aqueous solution that is better represented by Ni"*--02- dipolar formalism. The interaction of high-spin Ni(I1) with triplet 0, resembles the biological O2 uptake system of heme-containing high-spin Fe(II), although the resulting 1:l O2 adducts are paramagnetic (S = 1) with N i and diamagnetic with heme. Attaching an ethyl or benzyl substituent to the macrocycle ring enhances the reversibility of the O2 adduct formation. That metal-bound superoxide is a reactive oxygen species is a hypothesis serves as an appropriate of long tan ding.^' The present Ni"-; example, since the Ni-bound O2 is activated to the degree that it can attack benzene to yield phenol.48 W e believe a new 0, (47) Michelson, A. M.; McCord, J. M.; Fridovich, I. "Superoxide and Superoxide Dismutases"; Academic Press: London, 1977;p 77. (48) We have recently proved that the phenol oxygen is entirely and directly derived from 02:Kimura, E.; Machida, R. J . Chem. Soc., Chem. Commun. 1984, 499.
5505
chemistry will evolve out of the macrocyclic polyamine complexes of high-spin Ni(I1). Registry No. 2, 91327-96-7; 5, 63972-28-1; 5.5HBr, 91328-06-2; 7, 76201-28-0;8,91327-97-8; 9, 91327-98-9; 10, 91327-99-0;11, 9132800-6;12, 91328-01-7; 13, 91328-02-8; 15, 91328-03-9; 17, 91328-04-0; 19, 91328-05-1;Nil1-1, 78737-53-8;Nil1-2, 91328-07-3;NilI-3, 9138459-7;Nil1-4, 64616-26-8; Nil1-5,91328-08-4; Nil1-6,91 328-09-5; NiII-7, 80400-19-7; Nil1-8, 91328-10-8; Nil1-9,91328-11-9;Ni"-lO, 80389-72-6; NiIl-11, 80389-73-7;Ni"-12, 91328-12-0;Nil1-13,9 1328-13-1;Nil1-14, 77321-28-9;Ni"-15, 91328-14-2; Nil1-16, 91328-15-3; Nil1-17, 9132816-4;NilL-18,90751-78-3;Nil[-19, 91 328-17-5;Nil1-20, 91 328-18-6; Nil1-21,91328-19-7;Ni"'-lO, 821 35-48-6;Cull-7, 80386-21-6;CUI''-7, 91328-20-0;13-(4-(carbobenzyloxyamino)butyl)-1,4,8,11 -tetraazacyclotetradecane-l2,14-dione,91327-94-5;13-(3-~yanopropyl)-l,4,8,11 -tetraazacyclotetradecane-12,14-dione,63972-23-6;1,4,10,13-tetraaza-7thiotridecane-5,9-dione,91327-95-6;1,4,10,13-tetraaza-7-thiotridecane, 80042-28-0;1,4,10,13-tetraazatridecane,3551 3-91-8;imidazole, 28832-4.
Mechanism of Acetylene and Olefin Insertion into Palladium-Carbon CT Bonds Edward G. Samsel and Jack R. Norton* Contribution from the Department of Chemistry, Colorado State University, Fort Collins, Colorado 80.523. Received January 10, 1984
Abstract: The intramolecular acetylene insertion reactions of C1L,PdCO2(CH2),C=CCH3 (la, L = Ph3P, n = 2; lb, L = p-tol,P, n = 2; 2, L = Ph3P, n = 3) and the intramolecular olefin insertion reaction of CIL2PdC02CH2CH2CH=CH2(3, L = Ph3P) have been investigated. The acetylene insertion reactions give stable vinyl complexes 5a, 5b, and 6; the olefin insertion reaction gives an unsaturated lactone by @hydrogen elimination from the initially formed insertion product. Kinetic and 31P NMR studies show that, as predicted by Thorn and Hoffmann, the reactions proceed by a four-coordinate mechanism, with the triple or double bond displacing a phosphine ligand in a rapidly maintained equilibrium prior to insertion. the triple bond in 2, with the longer carbon chain, is more easily coordinated than that in l a but inserts less rapidly after coordination.
The insertion of carbon-carbon multiple bonds into metalcarbon bonds has traditionally been assumed to be a key step in many important reactions in homogeneous catalysis. For example, the catalytic trimerization1s2and (in some cases) the carboalkoxylation3 of acetylenes are believed to involve the insertion of triple bonds into metal-carbon u bonds; the catalytic a r y l a t i ~ n , ~ oligomerization,' and (again, in some cases) carboalk~xylation~ of olefins have been said to involve the insertion of double bonds into metal-carbon u bonds. In view of the importance of these catalytic reactions and of the fact that alternative mechanisms not involving insertion have recently been put forward for some of them (e.g., for ethylene and propylene polymerization6), considerable effort has been devoted to the search for stoichiometric systems in which such insertions can be directly observed and investigated. Watson has reportedSa the formation of an isobutyl (1) Maitlis, P. M. Acr. Chem. Res. 1976, 9, 93. (2) Vollhardt, K. P. C. Acc. Chem. Res. 1977, 10, I . (3) Mullen, A. In "New Syntheses with Carbon Monoxide"; Falbe, J., Ed.; Springer-Verlag: New York, 1980;Chapter 3 and references therein. (4)Heck, R. F. Acr. Chem. Res. 1979, 12, 146 and references therein. ( 5 ) (a) Watson, P. L.; J . Am. Chem. SOC.1982, 104, 337. Watson, P. L.; Roe, D. C. J . Am. Chem. SOC.1982, 104, 6471 and references therein. (b) Soto, J.; Steigerwald, M. L.; Grubbs, R. H. J . Am. Chem. SOC.1982, 104, 4479 and references therein. (6) (a) Ivin, R. J.; Rooney, J. J.; Stewart, C. D.; Green, M. L. H.; Mahtab, R. J . Chem. SOC.,Chem. Commun. 1978,604. (b) Turner, H. W.; Schrock, R. R.; Fellmann, J. D.; Holmes, S. J. J . Am. Chem. SOC.1983,105,4942and references therein.
0002-786318411506-5505$01.50/0
complex from the insertion of propylene into the Lu-CH, bond of (CSMe5),LuCH3;Stone,7Alt? and Bergman and co-workers+" have reported the formation of vinyl complexes from the insertion of ~ n a c t i v a t e d ' acetylenes ~~'~ into metal-carbon u bonds. Many of the catalytic reactions cited above involve planar complexes of d8 metals such as Pd(I1) and Pt(I1). Although the direct observation (uncomplicated by subsequent reactions) of the insertion of a free olefin or unactivated acetylene into a Pd-C or Pt-C bond has not been r e p ~ r t e d ,the ' ~ insertion of olefins and acetylenes into Pd-H and Pt-H bonds have been extensively ~ t u d i e d . ~ ' , Thorn '~ and Hoffmann have carried out a detailed (7) Davidson, J. L.;Green, M.; Nyathi, J. 2.; Scott, C.; Stone, F. G. A.; Welch, A. J.; Woodward, P. J . Chem. SOC.,Chem. Commun. 1976, 714. (8) Alt, H. G. J . Organomet. Chem. 1977, 127, 349. Alt, H. G.; Schwarzle, J. A. Ibid. 1978, 155, C65. Ah, H. G. Z . Naturforsch., E: Anorg. Chem. Org. Chem. 1977, 328, 1139. (9)Tremont, S. J.: Bergman, R. G. J . Organomet. Chem. 1977, 140, C12. (10) Huggins, J. M.;Bergman, R. G. J . Am. Chem. SOC.1981,103, 3002. (11) Watson, P.L.;Bergman, R. G. J . Am. Chem. SOC.1979, 101, 2055. (12) As has been pointed out previously,I0considerably more examples are known where the acetylene is activated by aryl, fluoro, carboalkoxy, or other electron-withdrawing substituents. Some of these examples are listed in ref 13.
(13) (a) Clark, H.C.; Jablonski, C. R.; von Werner, K. J . Organomet. Chem. 1974,82, C51. (b) Clark, H. C.: Puddephatt, R. J. Inorg. Chem. 1970, 9, 2670. (c) Clark, H. C.; von Werner, K. J . Orgunomet. Chem. 1975, 101, 347. (d) Davies, B. W.; Payne, N. C. J . Organomet. Chem. 1975, 102, 245. (14)Examples involving Ni-C bonds are reported in ref 9 and I O .
0 1984 American Chemical Society
5506 J . A m . Chem. Soc., Vol. 106, No. 19, 1984
Samsel and Norton
theoretical analysis of the reaction of ethylene with trans(H,P)2Pt(H)C1.17 They found that the ground state of the five-coordinate complex A could not be readily transformed into H
H
CI A
CI 0
the configuration B (with coplanar ethylene and hydride ligands) as required for insertion; in contrast, they found that the perpendicular ethylene in a four-coordinate complex C could easily +
L-Pt-11
,."J-
=
H'
+
,.a*
Results Analogues lb-4 of l a were easily prepared from PdL4 or PdL, and the appropriate chloroformate. On heating, the acetylenes lb and 2 underwent smooth insertion (reactions 2 and 3, analogous to reaction 1) to give vinyl complexes 5b and 6. The shorter chain
L
0
L-Pt--//
\/ C
H' C
D
OCH&H&ECCH3
I I CI
lb
(P-tol)3P-Pd-P(p-to1)3
I CI
0
\/ C
OCH CH CH C=CCH3 2
Fh3P-bd-PPh3
I
I
(1)
I
CI
CI
la
5a
offer two significant advantages: (1) they are kinetically simpler than intermolecular ones (where the M-C bond, the inserting (15) Particularly well-known examples include the following: (a) Chatt, J.; Coffey, R. S . ; Thompson, D. T. J . Chem. SOC.A 1968, 190. (b) Cramer, R.; Lindsey, R. V. J . Am. Chem. SOC.1966, 88, 3534. (c) Clark, H. C.; Kurosawa, H. Inorg. Chem. 1972, I I , 1275; J. Chem. SOC.,Chem. Commun. 1971,975. (d) Clark, H. C.; Jablonski, C. R.; Wong, C. S. Znorg. Chem. 1975, 14, 1332. (e) Clark, H. C.; Jablonski, C.; Halpern, J.; Mantovani, A.; Weil, T. A. Inorg. Chem. 1974, 13, 1541. ( f ) Clark, H. C.; Jablonski, C. R. Inorg. Chem. 1974, 13, 2213. (g) Clark, H. C.; Wong, C. S . J . Am. Chem. SOC. 1974, 96, 7213. (h) Bracker, G.; Pregosin, P. S . ; Venanzi, L. M. Angew. Chem. I n f . Ed. 1975, 14, 563. (i) Clark, H. C.; Wong, C. S . J . Organomet. Chem. 1975, 92, C31. Clark, H. C.; Fiess, P. L.; Wong, C. S. Can. J . Chem. 1977,55, 177. (j) Clark, H. C.; Milne, C. R. J. Urgunomet. Chem. 1978,161, 51. (k) Attig, T. G.; Clark, H. C.; Wong, C. S. Can. J . Chem. 1977,55, 189. (16) Ros, R.; Michelin, R. A,; Bataillard, R.; Roulet, R. J . Orgunomet. Chem. 1979, 165, 107. (17) Thorn, D. L.; Hoffmann, R. J . Am. Chem. SOC.1978, 100, 2079. (18) Albright, T. A.; Hoffmann, R.; Thibeault, J. C.; Thorn, D. L. J . Am. Chem. SOC.1979, 101, 3801. (19) An in-plane coordinated styrene complex of Pt(I1) has just been reported: Miki, K.; Kai, Y.; Kasai, N.; Kurosawa, H. J . Am. Chem. SOC. 1983, 105, 2482. (20) (a) Murray, T. F.; Norton, J. R. J . Am. Chem. Soc. 1979,101,4107.
(b) Murray, T. F.; Samsel, E. G.; Varma, V.; Norton, J. R. J . Am. Chem. SOC.1981, 103, 7520. (21) There have been previous
2
2
-
-
I
I Ph3P-Pd--PPh3
toluene or diglyme
I CI
I
Ph3P-Pd-PPh3
I
of intramolecular insertion reactions as a function of the size of ring formed, but there have been no quantitative kinetic analyses of such systems. (22) Heck, R. F. J . Am. Chem. SOC.1963, 85, 3116.
(3)
CI 6
2
I
I
(2)
5b
OCHzCHzCrCCH3
\/
Ph3P-Pd-PPh3
a or diqlyme
(p-tol),P-Pd--P(p-tol),
rotate to give the coplanar complex D and insertion (the normal preference for a perpendicular orientation is apparently the result of steric and not electronic factor^'^^'^). They therefore proposed that the insertion of olefins into the Pt-H bonds of planar complexes proceeded via a four-coordinate intermediate (with the olefin replacing a ligand and achieving a coordination site cis to the hydride) rather than a five-coordinate one (with no ligand loss prior to coordination and insertion of the olefin). Thorn and Hoffmann also suggested that their results should extend to acetylenes and to Pd--C and Pt-C u bonds and thus that olefin and acetylene insertions into Pd-C and Pt-C u bonds should aso prefer four-coordinate mechanisms over five-coordinate ones. We found ourselves in an ideal position to test the latter proposal. In the course of our studies on the mechanism of the cyclocarbonylation of acetylenic alcohols to methylene lactones,2° we found that the triple bond in 1 inserted into its Pd-C bond to give the vinyl complex 5a. Such intramolecular21q22 insertions 0
multiple bond, and the dissociable ligand all belong to separate molecular species); (2) they restrict the geometries possible at various stages of the insertion reaction and thus permit inference of the nature of intermediates from the effect of chain length on the individual rate constants. We have therefore examined, with and without added free phosphine, the kinetics of reaction 1 and of related reactions with carbon chains of different lengths, different ligands, and double instead of triple bonds.
double bond in 3 apparently underwent insertion but with immediate @-hydrogenelimination from the resulting palladium alkyl to give a-methylene-y-butyrolactone; the Pd(PPh,), either disproportionated to palladium metal and Pd(PPh,), or, in the presence of excess PPh,, went entirely to Pd(PPh&. The longer chain double bond in 4 underwent insertion very slowly if at all; at 130 OC 4 decomposed slowly to give a complex mixture of unidentified products, among which no a-methylene-6-valerolactone could be detected. The kinetics of reaction 1 were determined by monitoring the disappearance of the carbonyl band (1665 cm-*) of the initial carboalkoxy complex l a and the appearance of the carbonyl band (1 730 crn-') of the vinylic product 5a. The kinetics of reactions 2-4 were determined by monitoring only the disappearance of the
Ph,P-Pd-.-
PPh3
toluene
I
CI
3 C(Ph3P),PdI 0
\C/
+
HCI
+
A
x - x o
(4)
OCHzCHzCH2CH=CHz
I I CI
130 'C
Ph3P-Pd-PPh,
toluln.-
decomposition without formation of identiflable insertion product
4
(5)
Acetylene and Olefin Insertion into Pd-C
u
J . Am. Chem. SOC..Vol. 106, No. 19, 1984 5507
Bonds
.I
t!I
320-
280-
240-
200-
:I
qc0
,'.
00
e':
0"
,io
33
,o"t';i'
2 0 3 0,
A i 33
;il;>a
?l"
LO
o; .
20
i i j 0;
Figure 1. Plot of -In ( A - A , ) vs. time for reaction 2 at 130 OC in diglyme with [lb] = 0.013 M and [(p-t~l)~P] = 0.016 M. Shaded points are omitted from least-squares straight line. Table I. Observed Rate Constants for Insertion Reactions in the Presence of Added Phosphine at 130 OC in Diglyme reactant," added phosphine reaction M lL1addcd. 10Skobsd,b s-i [Lltotai~c 1 la (0.014) Ph3P (0.0180) 66 (1) 0.0187 (4) 0.0268 (3) 51.5 (1) 1 la (0.014) Ph3P (0.0264) 1 la (0.015) Ph3P (0.0369) 37.9 (6) 0.0373 (2) 29.9 (1) 0.0465 (2) 1 la (0.014) Ph,P (0.0462) 0.0640 (1) 21.5 (4) 1 la (0.013) Ph3P (0.0638) 0.020 (1) 50.9 (7) 2 l b (0.013) (p-tol),P (0.0164) 0.032 (1) 31.7 (8) 2 lb (0.013) (p-tol),P (0.0295) 0.0595 (6) 19.1 (2) 2 l b (0.013) (p-tol),P (0.0583) 13.8 (1) 0.0906 (5) 2 lb (0.014) (p-tOl),P (0.0894) 7.6 (1) 0.0177 (6) 3 2 (0.013) Ph3P (0.0146) 3 2 (0.01 1) Ph3P (0.0201) 5.55 (5) 0.0223 (5) 4.21 (3) 0.0308 (4) 3 2 (0.014) Ph3P (0.0289) PhjP (0.0437) 3.00 (4) 0.0448 (3) 3 2 (0.012) 105 (4) unknown 4 3 (0.016) Ph,P (0.00947) 4 3 (0.016) 89 (1) unknown Ph3P (0.0137) 4 3 (0.014) Ph3P (0.0280) 60.3 (5) unknown 4 3 (0.015) 33.4 (3) unknown Ph3P (0.0559) a Initial concentration from zero-time absorbance. Numbers in parentheses are the standard deviations in the least significant figure. 'After correction of [LIadddby the mean value of [L]dis (from eq 16, with K estimated from Figure 2 and eq 15) during the reaction. carbonyl bands (1665 cm-') of the starting materials lb, 2, and 3, although the product 5b of reaction 2 was formed in quantitative yield. The product 6 of reaction 3 decomposed slowly at 130 "C, the highest temperature at which rate measurements were made; however, the yield of 6 was over 90% during the initial 20% of the reaction. Similarly, polymerization of a-methylene-ybutyrolactone, the product of reaction 4, was apparently rapid at 130 OC, but it was formed in high yield at early reaction times and low conversions. The fact that the products of reactions 3 and 4 are not present in quantitative yield at the end of the reaction is thus the result of a consecutive reaction (product decomposition) rather than a competitive side reaction, and the rate constants obtained from starting material disappearance are those of reactions 3 and 4. Diglyme was used as solvent for the kinetic runs because of its high boiling point and absence of IR absorptions in the range of interest; it has little affinity for Pd(II), and rate constants for reaction 1 obtained in it differed by only 30% from the less precise ones obtained in toluene. In preliminary experiments reaction 1 showed apparent firstorder behavior and marked inhibition by added free ligand: at 90 OC 0.5 equiv of Ph3P decreased the rate by a factor of about 50. However, in the absence of added free Ph3P, the apparent first-order rate constants varied somewhat with initial concentration: at 87.6 OC kapparent increased from 4.4 X s-' at [lalo = 0.012 M to 5.8 X IO4 s-] at [lalo = 0.0074 M. It thus became clear that neither reaction 1 nor any of the reactions being studied
n
v,
v v)
160-
0 Y
I
x
120-
9 80
/
I /
O
0
Y
,
2b
' do ' 6 b ' lo3X [ L] ADDED (M)
d0
do
Figure 2. Dependence of l/kObsdfor various reactants upon [L] added in diglyme at 130 OC: (U) la: (0)lb; (0) 2; (0)3. Where not shown, standard deviations are smaller than the size of the symbols. Table 11. Temperature Dependence of Rate Constants (from the Linear Region of Plots of In ( A - A , ) ) for Reaction 1 in Diglyme with No Added PhosuhineQ T , OC 105k,bsd,bS-' T, O C 105kObsd:s-1 80.2 87.6 95.1
22.8 (2) 44.1 (8) 105 (2)
100.4 108.3
165 (2) 307 (7)
"Initial [la] = 0.012 M. bNumbers in parentheses are the standard deviations in the least significant figure. CAccordingto eq 15, under these conditions /cobs,, = k2. was truly first order. Closer examination of plots of I n ( A - A,) vs. time (shown in Figure 1 for reaction 2 at 130 O C with 1.3 equiv of added phosphine) showed initial nonlinear behavior when the amount of added phosphine was small or zero; for reactions 1-3, the initial rate was slower than that found after significant amounts of starting material had been converted to product. No such nonlinear behavior was observed when the amount of added free phosphine was substantial. The rate constants (kobsd)observed for reactions 1-4 at 130 "C in the presence of various amounts of added phosphine are given in Table I. As shown in Figure 2, plots of l/kobsd vs. [LIadded are linear, implying that phosphine inhibition obeys eq 6. Other plots (such as kobsd vs. 1/[Lladdd) which, if linear, would suggest other equations for phosphine inhibition, are instead severely curved.
Temperature Dependence. In the absence of added free phosphine, reaction 1 occurred too quickly for its rate to be
Samsel and Norton
5508 J . Am. Chem. SOC.,Vol. 106, No. 19, 1984 Table 111. lip NMR Line Widths in Diglyme T, "C [la], M [Ph,P], M Av1,2(1a)L7 Avli2(Ph3P)u 7.1 100 0 0.023 100 0.015 0 9.5 100 90 90
0.015 0.045 0.045
30.8 88.1 152.3
0.023 0.090 0.180
Scheme I
39.4 88.0 100.1
1
11
"In Hz. measurable at 130 OC. Rate constants, given in Table 11, were therefore obtained at lower temperatures from the linear portion of plots of In ( A - A , ) vs. time (Le., after the initial nonlinear behavior had ceased). 3 1 p NMR Investigation of ligand Dissociation and Exchange. As will be discussed below, the nonlinear behavior observed early in reactions in which little or no phosphine had been added suggested the operation of rapid dissociative equilibria. 31P(1H) NMR was the obvious method for independently investigating phosphine ligand dissociation, association, and exchange in these systems. The following experiments were performed: (1) In diglyme at 100 O C , complex l a (6 20.2) was smoothly converted to 5a (6 25.6). In the presence of 1.5 equiv of PPh, the reaction rate decreased tenfold, but no resonances other than those of la, 5a, and PPh3 were observed. The addition of PPh3 increased the line width of the l a resonance, and the addition of l a to a solution of PPh, increased the line width of the resonance of the latter, to an extent (Table 111) indicating a slow associative exchange process between free PPh, and the coordinated PPh, in l a . (2) In diglyme at room temperature, a mixture of l a (6 19.6) and l b (6 17.7) showed an additional signal at an intermediate chemical shift (6 18.6) within the time required for spectrum acquisition. The new signal was a closely spaced (8 Hz or 0.1 ppm) pair of lines consistent with the central portion of the AB pattern expected for the mixed phosphine complex 7. (The large 2Jppexpected24for a trans complex such as 7 makes the outer peaks of the AB pattern unobservably small.) 0
la
+
lb
\/ C
-
fa't' 25
.
OCH2CH2C=CCH3
I I CI
( p -tol),P-Pd-PPh,
c
J
5
Room-temperature 31PN M R spectra showed only l a and Sa in one tube only l b and 5b in the other. After the solutions were mixed, the product 7 of phosphine scrambling between l a and l b was observed. In contrast there was no initial evidence for 10, the mixed phosphine complex of the insertion products 5a and 5b. Although a small signal that may have been due to 10 was observed at long reaction times, it was clear that phosphine exchange among product molecules was far slower than among molecules of the starting material.
CI
10
Discussion The mere observation of inhibition by added phosphine (L) in reactions like 1-5 does not distinguish between four-coordinate and five-coordinate mechanisms. If a reaction occurs by a four-coordinate mechanism, addition of free L will push an equilibrium such as 10 to the left, decrease the concentration of a four-coordinate intermediate such as 11, and decrease the rate
(7)
7
(3) A mixture of 8a and 8b (analogues of l a and l b without triple bonds) in diglyme at room temperature also showed an additional signal at an intermediate chemical shift. This new signal was also a closely spaced pair of lines, as expected for the trans mixed phosphine complex 9. C02Et Ph P
'-1
I
Pd-PPh,
Y2Et
+
(p-tol),P-Pd-P(p-tol)j
I
CI
CI
ea
8b
fort, 25 *C
-2
1
11
of the reaction. (It will be shown later that the intermediate in this reaction is in fact 11.) If a reaction occurs by a five-coordinate mechanism, free L may compete with the triple bond for the fifth coordination site, tying up the starting material as an unproductive trisphosphine complex such as 12, decreasing the concentration of a five-coordinate insertion intermediate such as 13, and decreasing the rate of the reaction.
C02E t
I
(p-tolI3P-Pd-PPhj
(8) I
I CI
9
(4) A solution of the insertion product 5a was formed in situ by heating a solution of l a until insertion was 60-70% complete; similarly, a solution of the p-tolylphosphine-containing insertion product 5b was formed in situ by heating a solution of lb.
I
CI
CI
12
1
CI
13
(23) Tolman, C.A.; Seidel, W . C.; Gerlach, D. H. J . Am. Chem. SOC. 1972, 94, 2669.
(24) Verkade, J. G. Coord. Chem. Reo. 1972-1973, 9, 1.
It is clear that the first explanation (the operation of an equilibrium like eq 10) is correct for reaction 1 and by implication
Acetylene and Olefin Insertion into Pd-C
u
J . A m . Chem. SOC.,Vol. 106, No. 19, 1984 5 5 ~ 9
Bonds
Scheme I1
Table IV. Rate and Equilibrium Constants for Acetylene Insertion Reactions a t 130 O C in Diglyme reaction reactant 103k,,a.b s - ~ IO~K,Q M. ~ 7 (3) [20 (2)lC 2.0 (9) la 0.89 ( 5 ) 16.4 ( 9 ) lb 3 2 0.301 (2) 4.40 (2) "Calculated by fitting I/kohd and [L],,,,, to eq I 5 as described in ref 34. bNumbers in parentheses are standard deviations in the least significant figure. 'Value in brackets obtained by extrapolation of the data in Table 11 to 130 O C . 1
2
I
CI
1
I
CI
I
CI
5
for all the reactions under study. An equilibrium like (1 1) can at most decrease the initial rate by a factor of 2 when half an equivalent of L is added and thus cannot explain the observed inhibition of reaction 1 by a factor of 50. Furthermore, the absence of any 3'P N M R signals other than those of l a , free PPh,, and the product 5a, when reaction 1 is carried out in the presence of added PPh,, rules out the presence of a five-coordinate complex such as 12 in quantities sufficient to cause significant i n h i b i t i ~ n . ~ ~ The observation of the associative exchange of free L with the phosphine ligand on l a implies that 12 can be formed, but only as a short-lived intermediate. There are, however, two different ways in which a four-coordinate mechanism can lead to a rate law consistent with eq 6. In Scheme I (which obeys the rate law in eq 12 and 13), loss of
The kinetic alternatives are similar to those which arise for the X- inhibition of the cis/trans isomerization of PtL,RX in methano1,28 where both a steady-state approximation in PtL,R(MeOH)+ and the assumption that it is formed in a rapid solvolysis equilibrium lead to rate laws of the same form. Neither situation is a special case of the other; both are special cases of the general situation
+
where k, must be
