Organometallics 1995, 14, 4525-4534
4525
Oxidative Addition of Catecholborane to IrX(CO)(dppe) Complexes Brian P. Cleary and Richard Eisenberg" Department of Chemistry, University of Rochester, Rochester, New York 14627 Received February 1, 1995@ The oxidative addition of catecholborane (1,3,2-benzodioxaborole) to the IdI) cis-phosphine complexes IrX(CO)(dppe)(X = Br, I, H, B O Z C ~ Hdppe ~ ; = 1,2-bis(diphenylphosphino)ethane) has been found to proceed stereoselectively under kinetic control. Of the four complexes that can be formed by cis oxidative addition of the B-H bond to IrBr(CO)(dppe),the one having hydride trans to phosphorus and boron trans to bromide is formed in >99% yield while addition to IrI(CO)(dppe) occurs similarly but with only 90% stereoselectivity. Isomerization of the initially formed diastereomers to thermodynamically less stable diastereomers which display hydride trans to halide and boron trans to phosphorus occurs after 1 day. The mechanism for the isomerism for X = Br has been determined to proceed via reductive eliminationloxidative addition processes. The oxidative addition reactions of catecholborane to [IrH(CO)(dppe)l and [IdB02C6H4)(CO)(dppe)l,generated from 1rHdCO)(dppe) and IrHz(BOzCsH4)(CO)(dppe),respectively, also occur in a cis fashion with 99% stereoselectivity yielding IdIII) products with hydride trans to CO and boron trans to phosphorus. The observed stereoselectivity for catecholborane addition to IrX(CO)(dppe) (X = Br, I) is reversed with respect to addition to IrX(CO)(dppe) (X = H, BO&&) and is accounted for by electronic factors involving the nbasicity of the halide ligands.
Introduction The activation of small molecules via oxidative addition at d8 metal centers is of fundamental importance in homogeneous hydrogenation, hydrosilation, and hydroformylation catalysis.1,2 Recently, there has been a renewed interest surrounding the chemistry of transition metal-catalyzed olefin hydroborations and, in particular, those reactions catalyzed by Rh(1) ~pecies.~-ll For example, RhCl(PPh& efficiently catalyzes hydroboration reactions between olefins and catecholborane (1,3,2-benzodioxaborole) at ambient temperatures, while the uncatalyzed reactions proceed only at temperatures at or above 70 0C.12J3 In addition to rate enhancement, transition metal catalysts can also alter the chemo-, regio-, and enantioselectivities of organic products formed during hydroboration relative to the uncatalyzed
r e a ~ t i o n . ~ , ~ JMannig ~ - ~ * and Noth have postulated a mechanism to account for olefin hydroborations catalyzed by RhCl(PPh3)3.l3 In this mechanism, hydroboration is initiated by oxidative addition of the B-H bond of catecholboraneto [RhCl(PPh3)21(generated via PPh3 loss from RhCl(PPh3)s)t o yield a Rh(II1) boryl hydride complex that coordinates olefin after dissociation of PPh3 (eq 1). Hydride migration, possibly promoted by phosphine ligation, then produces a Rh(II1) alkyl boryl intermediate from which B-C reductive elimination occurs to form the organic product and the catalyst [RhCl(PPh&] as in eq 2. R
H
Cl
@Abstractpublished in Advance ACS Abstracts, August 15, 1995. (1) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.; John Wiley and Sons, Inc.: New York, 1988; p 1455. (2) Collman. J. P.: Hegedus. L. S.: Norton. J. R.: Finke. R. G. Principles and Applications o f Organotrunsition Metal Chemistry; University Science Books: Mill Valley, CA, 1987. (3) Burgess, K.; Ohlmeyer, M. J. Chem. Rev. 1991,91, 1179-1191. (4) Hoveyda, A. H.; Evans, D. A.; Fu, G. C. Chem. Rev. 1993, 93, ~
~~~~
H
~~
/R
U
1307-1370.
(5) Westcott, S. A,; Blom, H. P.; Marder, T. B.; Baker, R. T. J.Am. Chem. SOC.1992,114, 8863-8869. (6) Burgess, K.; van der Donk, W. A.; Westcott, S. A.; Marder, T. B.; Baker, R. T.; Calabrese, J. C. J.Am. Chem. SOC.1992,114,93509359. (7)Evans, D. A.; Fu, G. C.; Anderson, B. A. J . Am. Chem. SOC.1992, 114, 6679-6685. (8) Evans, D. A.; Fu, G. C.; Hoveyda, A. H. J.Am. Chem. SOC.1992, 114, 6671-6679. (9) Rufing, C. J. Aldichim. Acta 1989, 22, 80-81. (10) Westcott. S. A.: Marder. T. B.: Baker, R. T. Orpanometallics 1993,12,975-979. (11)Brown, J. M.; Lloyd-Jones, G. C. J.Am. Chem. SOC.1994,116, 866-878. (12) Brown, H. C.; Gupta, S. K. J.Am. Chem. SOC.1971,93,18161818. (13) Mannig, D.; Noth, H. Angew. Chem., Int. Ed. Engl. 1986,878879. '
r
i
R
(14) Matsumoto, Y.; Naito, M.; Uozumi, Y.; Hayashi, T. J. Chem. Soc., Chem. Commun. 1993,1468-1469. (15)Sato, M.; Miyaura, N.; Suzuki, A. Tetrahedron Lett. 1990, 31, 231-234. (16)Zhang, J.; Lou, B.; Guo, G.; Dai, L. J. Org. Chem. 1991, 56, 1670-1672. (17) Kocienski, P.; Jarowicki, R; Marczak, S. Synthesis 1991,11911200. (18)Brands, R M. J.; Kende, A. S. Tetrahedron Lett. 1992, 33, 5887-5890.
Q276-7333195/2314-4525$09.QQlQ 0 1995 American Chemical Society
4526 Organometallics, Vol. 14,No.10, 1995
Cleary and Eisenberg
H-H H Accompanying the efforts aimed a t expanding the breadth of synthetic methodology of transition metalcatalyzed hydroborations are reports which have emphasized the characterization and structure determi0 nation of metal-boryl complexes synthesized from stoichiometric oxidative addition reactions with R&H19-29 kinetic and the estimation of transition metal-boryl covalent X-kLP’ (3) bond energies.30 However, little attention has been paid oc’ H2\ to the effects that specific ligands impart on the stereochemistry of products formed by the cis-oxidative additions of boranes to four coordinate transition metal compounds. In order t o extend studies elucidating the 0” extent and basis of kinetically controlled diastereoselectivity in B-H oxidative addition reactions, we have B thermodynamic examined the reactip ChemistryJetween catecholborane and IrX(CO)(P P), where P P = 1,2-bis(diphenAlthough both oxidative addition products of eq 3 form, y1phosphino)ethane(dppe) and (2S,3S)-bis(diphenylphosonly the dihydride product corresponding to pathway i phinolbutane (chiraphos) and X = Br, I, and H, in a is generated initially. Slow conversion of this isomer manner which complements previous reports from our to an equilibrium distribution of products that is mainly laboratory (vide infra) describing the kinetic stereosethe other isomer of eq 3 reveals the kinetic control of lectivities of hydrogen, silane, and hydrogen halide the oxidative addition reaction. The origin of the kinetic oxidative additions to IrX(CO)(dppe)(X = C1, Br, I, CN) selectivity is electronic in nature and arises from the complexes.31-34 relative propensity of mutually trans ligands in IrXWe have demonstrated previously that, for concerted (CO)(dppe)t o bend away from the incoming hydrogen cis-oxidative additions to IrX(CO)(dppe) complexes, a molecule upon oxidative addition. Qualitatively, apdirect comparison of the electronic and steric effects of proach A is favored over B as trans CO and P ligands the X and CO ligands can be made because the phospreferentially bend away from the approaching H2 phine donors are constrained to be in mutually cis molecule. The basis of the diastereoselectivity results positions and will therefore exert the same influence for from the carbonyl ligand being able to stabilize the each of the two stereochemical pathways of a d d i t i ~ n . ~ ~ ,transition ~~ state through the removal of electron density For the concerted oxidative addition of hydrogen to IrXfrom the Ir d,z orbital, thereby reducing the repulsive (CO)(dppe),two pathways can be followed which lead 4e- interaction between d,z and ab(Hz). to different diastereomers as shown in eq 3.32 For For the concerted oxidative addition of unsymmetrical addition along pathway i, hydrogen approaches IrX(C0)Y-H bonds t o IrX(CO)(dppe) complexes, a similar (dppe) over the OC-Ir-P axis (A) yielding an Ir(II1) stereochemical analysis can be applied with the adproduct with one hydride ligand trans to CO and the ditional consideration of the regiochemistry of the Y-H other trans to phosphorus while, for addition along addition.33 As shown in eq 4 for the cis oxidative pathway ii, hydrogen approaches IrX(CO)(dppe)over the e H X-Ir-P axis (B)forming an Ir(II1) product with one hydride trans to X and the other trans to phosphorus.
+
(19) Gilbert, K. B.; Boocock, S. K.; Shore, S. G. In Comprehensive Organometallic Chemistry: The Synthesis, Reactions and Structures of Organometallic Compounds; Wilkinson, S. G., Ed.; Pergamon Press: Oxford, U.K.,1982; Vol. 6. (20) Schmid, G. Angew. Chem., Int. Ed. Engl. 1970,9, 819-916. (21)Hartwig, J. F.; De Gala, S. R. J.Am. Chem. SOC.1994, 116, 3661-3662. (22) Hartwig, J. F.; Huber, S. J . Am. Chem. SOC.1993, 115, 49084909. (23) Westcott, S. A.; Marder, T. B.; Baker, R. T.; Calabrese, J. C. Can. J. Chem. 1993, 71, 930-936. (24) Nguyen, P.; Blom, H. P.;Westcott, S. A.; Taylor, N. J.; Marder, T. B. J . Am. Chem. SOC.1993,115,9329-9330. (25) Baker, R. T.; Calabrese, J. C.; Westcott, S. A.; Nguyen, P.; Marder, T. B. J. Am. Chem. SOC.1993,115, 4367-4368. (26) Westcott, S.A.; Taylor, N. J.;Marder, T. B.; Baker, R. T.; Jones, N. J.; Calabrese, J. C. J. Chem. SOC.,Chem. Commun. 1991, 304305. (27) Baker, R. T.; Overnall, D. W.; Calabrese, J. C.; Westcott, S. A.; Taylor, N. J.; Williams, I. D.; Marder, T. B. J.Am. Chem. Soc. 1990, 112, 9399-9400. (28)Baker, R. T.; Overnall, D. W.; Harlow, R. L.; Westcott, S. A.; Taylor, N . J.; Marder, T. B. Organometallics 1990,9, 3028-3030. (29) Knorr, J. R.; Merola,J. S. Organometallics 1990,9,3008-3010. (30) Rablen, P. R.; Hartwig, J. F.; Nolan, S. P. J . Am. Chem. SOC. 1994,116, 4121-4122. (31) Johnson, C. E.; Fisher, B. J.; Eisenberg, R. J.Am. Chem. SOC. 1983,105, 7772-7774. (32)Johnson, C. E.; Eisenberg, R. J. Am. Chem. Soc. 1985, 107, 3148-3160. (33) Johnson, C. E.; Eisenberg, R. J . Am. Chem. SOC.1985, 107, 6531-6540. (34) Kunin, A. J.; Farid, R.; Johnson, C. E.; Eisenberg, R. J . Am. Chem. SOC.1985,107, 5315-5317.
y X7IrL?
OC
+
/ HB
oc
C
i-HP
i-HC
0
\
(4)
X
X
ii-HP
ii-HX
addition of catecholborane to IrX(CO)(dppe),the asymmetry of the substrate B-H bond increases the number of diastereomers that can be formed from two to four. The identity of these diastereomers can be designated according to the pathway (i or ii) and the nature of the ligand which is trans to hydride in the product. For example, for oxidative addition along the OC-Ir-P axis (pathway i), when hydride is trans to P, the diastereomer is designated as i-HP, and when hydride is trans to CO, the diastereomer is indicated as i-HC. We will follow this manner of designation in the present paper. In concerted oxidative addition reactions, diastereomers
Organometallics, Vol.14,No.10, 1995 4527
IrX(CO)(dppe) Complexes
Table 1. lH N M R Spectroscopic Data for Complexes 1-1W G(1r-H)
complex
IrH(BOzCsH4)Br(CO)(dpee), 1
-7.52
IrH(BOzC6H4)Br(CO)(dppe),2
-16.18
G(CH2 of dppe)
2 J ~ - ~
dd, 128.5, 16.9
2.67 (4H)
dd, 15.7,8.1
I~H(BOZC~H~)I(C O ) (3~ P P ~ ) , ..
-8.38
dd, 126.5, 17.0
2.67 ( l H ) , 2.49 ( l H ) , 1.94 (iH), 1.68 (1H) 2.73 (4H)
IrH(BOzCsH4)I(CO)(dppe),4
-9.47
t, 17.5
2.10 (2H), 1.75 (2H)
IrH(BOzCsH4)I(CO)(dppe),5 IrHz(BOzCsH4)(CO)(dppe),6
-14.29 -9.12, -10.04
dd, 15.9, 8.3 dd, 114.8, 12.6; t, 18.8
2.68 (3H), 1.78 (1H) 2.19 (2H), 1.95 (2H)
IrH(BOzCsH4)z(CO)(dppe),7
-9.08 -7.20 -7.65 -8.19 -8.33 -16.11 -16.28
t, 18.0 dd, 122.4, 14.4 dd, 127.0, 13.8 dd, 20.5, 13.2 dd, 23.3, 15.3 dd, 18.8, 5.9 dd, 12.9, 10.6
2.13 (2H), 1.89 (2H) e e
IrH(BOzC6H4)Br(CO)(chiraphos), 8ad IrH(BOzC&)Br(CO)(chiraophos), 8bd IrH(BOzC&)Br(CO)(chiraphos), gad IrH(BOzC6H4)Br(CO)(chiraphos),9bd IrH(BOzC6H4)Br(CO)(chiraphos),load IrH(BOzC,&)Br(CO)(chiraphos), lobd
e e
G(o-Ph of dppe)b 7.87 (2H), 7.69 (2H), 7.48 (2H), 7.28 (2H) 7.78 (2H), 7.68 (2H), 7.63 (2H), 7.42 (2H) 7.73 (2H). 7.59 (2H). 7.46 (2H), 7.31 (2H) 7.95 (2H), 7.83 (2H), 7.71 (2H), 7.32 (2H) 7.51 (2H), 7.11 (2H)C 7.87 (2H), 7.77 (2H), 7.69 (2H), 7.48 (2H) 7.76 (4H), 7.58 (4H) f
f
f f
h g h g a 'H NMR (400.13 MHz) spectra are reported in ppm downfield of tetramethylsilane in C6D6 unless otherwise noted. 2 J ~ - pare reported in Hz. The o-phenyl resonances of dppe are generally downfield of the corresponding meta and para resonances. The remaining two 0-Ph dppe resonances were not located due to overlap with the Ph resonances of 4. Spectra obtained in toluene-&. e Seven resonances were observed for the chiraphos methylene protons for complexes 8 and 9 but were not assigned. f Six overlapping resonances are observed between 8.25 and 7.40 ppm attributed to o-Ph protons of chiraphos. Chiraphos methylene protons not assigned. Chiraphos 0-Ph protons not assigned.
i-HP and i-HC correlate respectively with B-H approaches to IrX(CO)(dppe)that are shown as C and D, whereas diastereomers ii-HP and ii-HX arise from approaches E and F, respectively.
Ob
OG
E
F
In the present paper we report the results of a systematic study of the oxidative addition of catecholborane to the Ir(1) cis-phosphine complexes IrX(CO)(dppe) and IrBr(CO)(chiraphos)complexes.
hydride chemical shift indicative of hydride-trans-tohalide will occur. Moreover, the orientation of the catecholboryl ligand will be established through the influence of the boron nucleus (spin 3/2, 80.4% natural abundance; spin 3,19.6% natural abundance) on the lH, 13C, or 31P resonance of the hydride, carbonyl, or phosphine ligand trans to it since the resonance of the spin l/2 nucleus in that position will display significant broadening due to quadrupolar coupling.35 Oxidative Addition of CeH402BH to IrBr(C0)(dppe). Facile Stereochemical Isomerization of IrH(BOzC&LdBr(CO)(dppe). A new six-coordinate IrYdppe) boryl-hydride complex (1) is obtained by treating a benzene solution of IrBr(CO)(dppe) with 5 equiv of CsK02BH (catecholborane)under N:! as shown in eq 5. After precipitation with hexanes and recrystallization from CHzCldhexanes, the analytically pure product is isolated in 81% yield as an air sensitive, cream colored powder.
Results and Discussion All of the catecholborane oxidative addition reactions with IrX(CO)(dppe)and IrBr(CO)(chiraphos) and subsequent reaction chemistry of the Ir(II1) products were monitored by infrared and NMR spectroscopies. The stereochemistries of the six-coordinate Ir(II1) diphosphine oxidative addition products were established unambiguously by determining the 'H, 31P, and 13C chemical shifts and coupling constants of the hydride, carbonyl, and phosphine ligand nuclei of the Ir(II1) products. For the cis addition of catecholborane to the MI)(P P) complexes, four different stereoisomers can be envisaged in principle as shown in eq 4. "he resonances of the hydride ligand in stereoisomers i-HP and ii-HP will be similar and appear as a doublet of doublets with p (110-130 Hz) and a smaller a large trans 2 J ~ -coupling cis 2 J ~ - p(5-20 Hz). For the two stereoisomers i-HC and ii-HX, only smaller cis 2 J ~ - couplings p will be seen, and for ii-HX only, a distinctive upfield shift in the
Br
1
Complex 1 has been characterized spectroscopically (Tables 1 and 2 list spectroscopic data for all new complexes). The solution infrared spectrum of 1 in benzene exhibits a single terminal YCO a t 2052 cm-l while the l3C(lH} NMR spectrum shows the carbonyl ligand as a doublet of doublets ( 2 J c -=~ 120.6,5.1 Hz) at 175.5 ppm, where, on the basis of the very different 2J~-p values, CO resides trans and cis to the two dppe phosphorus nuclei. The 31P{1H} NMR spectrum of 1 displays two doublets a t 34.12 ppm and 20.90 ppm (35) Harris, R. K. Nuclear Magnetic Resonance Spectroscopy;Longman Scientific and Technical: Essex, U.K., 1986.
4528 Organometallics, Vol. 14, No. 10, 1995
Cleary and Eisenberg
Table 2. Infrared, S1P{lH}, and lsC{lH} Spectroscopic Data for Complexes 1-1W complex IrH(BO&sH4)Br(CO)(dppe), 1 IrH(BOzC6H4)Br(CO)(dppe),2 IrH(BOzC6H4)I(CO)(dppe), 3 IrH(B02C6H4)I(CO)(dppe),4 IrH(BO2CsH4)I(CO)(dppe),5 IrHz(BOzCsH4)(CO)(dppe),6 IrH(BO2CsH4)2(CO)(dppe),7 IrH(BO2C6H4)Br(CO)(chiraphos),8ab IrH(BO2C6H4)Br(CO)(chiraphos), 8bb IrH(BO2C6H4)Br(CO)(chiraphos), 9ab IrH(BO2C6H4)Br(CO)(chiraphos),9bb IrH(BO2CsH4)Br(CO)(chiraphos),loab IrH(BO2C6H4)Br(CO)(chiraphos),lobb
(cm-’) 2052
YCO
2049 1984 1995
6(P) (2JP-P) 34.1,20.9 (d, 7.3) 25.0 (d, 9.0), 24.0 (broad, W ~ = A 60 Hz) 27.5, 14.7 (d, 7.4) 29.5 (d, 2.5), 7.1 (broad, w112 = 37 Hz) 20.4 (d, 9.4), 17.7 (broad, 0112 = 53 Hz) 28.12 (AB quartet) 26.1 (broad, 0112 = 40 Hz) 30.5, 19.3 (d, 12.2) 29.8,23.9 (d, 13.0) 37.1, (d, 3.81, 22.0 (broad, w112 = 71 Hz) 32.9 (d, 4.91, 18.7 (broad, 0112 = 56 Hz) 20.2 (d, 17.11, 27.8 (broad, w112 = 55 Hz)O 31.45 (d, 18.6), 12.5 (broad, 01/2 = 58 Hz)
&CO) (2Jc-P)
175.5 (dd, 121, 5) 174.3 (dd, 120, 6) 174.3 (dd, 119, 5) 178.3 (t, 5) 174.6 (dd, 117, 7) 173.9 (dd, 120,6) 176.7 (t,3) 176.3 (dd, 5, 3)
a Infrared spectra are reported in benzene solution. 31P{1H}(161.98 MHz) and 13C{lH} (161.98 MHz) N M R spectra are reported in ppm down field of external 85% phosphoric acid and tetramethylsilane in C6D6 solution unless otherwise noted. 2Jp-p and 2Jc-p are reported in Hz. b Spectra recorded in toluene-&.
(2Jp-p
= 7.3 Hz) consistent with two inequivalent cis-
phosphorus nuclei. In the ‘H NMR spectrum, the hydride ligand resonates at -7.52 ppm as a doublet of doublets ( 2 J ~ - p= 128.5,16.9 Hz) with the larger 2 J ~ - p coupling establishing that the hydride is trans to one dppe phosphorus nucleus. Also in the ‘H NMR spectrum, four sets of dppe o-phenyl protons are observed at 7.87, 7.69, 7.48, and 7.28 ppm. From these results, 1 is assigned unambiguously as IrH(BOzCsH4)Br(CO)(dppe) with hydride and carbonyl ligands trans to dppe phosphine donors. An equilibrium between 1 and a new hydridecontaining complex (2) is established in benzene after 24 h, eq 6. In the lH NMR spectrum, the hydride
f
-0.2
.
Temperature Dependence of K,, for the Equilibrium Between 1 and 2
....
~OSC
.......................
0.00285 0.00295 0.00305 0.00315 0.00325 0.00335 1/T (K-‘)
Figure 1. Van’t Hoff plot for the equilibrium of 1 and 2.
resonance of 2 appears a t -16.18 ppm as a doublet of doublets ( 2 J ~ - p= 15.7, 8.1 Hz). Since the hydride resonance of 2 shows a characteristic upfield chemical shift and two small 2 J ~ - pcouplings, it is assigned t o be trans to bromide and cis to two inequivalent phosphorus nuclei. Confirmation of this arrangement is obtained from both 31Pand 13CNMR spectroscopy. The 31P{lH} NMR spectrum shows two phosphorus resonances at 25.0 and 24.0 ppm with the former appearing as a doublet P J p - p = 9 Hz) and the latter as a broad singlet (w1/2 = 60 Hz) due t o trans coupling t o boron, while the l3C(lH} NMR spectrum shows the carbonyl ligand at 174.28 ppm as a doublet of doublets ( 2 J c - p = 120,6 Hz) with 2 J ~ - pvalues establishing that it is both trans and cis to the phosphorus nuclei of dppe. While in theory 2 can form by the direct oxidative addition of catecholborane to IrBr(CO)(dppe) via pathway ii with B-H orientation leading to the HX diastereomer (eq 41, the fact that 1 is formed with ~ 9 9 % stereoselectivity indicates that the catecholborane oxidative addition to IrBr(CO)(dppe)proceeds under kinetic control via pathway i with HP orientation and that this reaction route is favored over the pathway leading to ii-HX by 22.7 kcal mol-l.
At 298 K, Keq for the equilibrium depicted in eq 6 is 0.62 which corresponds to a free energy difference between 1 and 2 of 0.29 kcal mol-’ and establishes that 1 is slightly more stable than 2 in addition to being the kinetically preferred isomer. A plot of Keq vs 1/T (Figure 1)is linear over the temperature range 298-350 K and yields AH = 1.04 f 0.04 kcal mol-l and AS = 2.5 f 0.1 eu. Isomerization Mechanism for the Interconversion of 1 to 2. A kinetic study of the isomerization of IrH(BOzCsH4)Br(CO>(dppe) from 1 to 2 in has been carried out using lH NMR spectroscopy t o monitor the progress of the reaction. Relative amounts of 1 and 2 were determined by recording the integrated intensities of the hydride resonances for both complexes. At 338 K and early reaction times, the isomerization follows first-order kinetics. Two possible mechanisms t o account for the observed first-order kinetics are (1) a simple intramolecular rearrangement and (2) a two-step process involving catecholborane reductive elimination/ oxidative addition as shown in eq 7. For the mechanism in eq 7 to satisfy first-order kinetics, the rate of reductive elimination of catecholborane must be significantly faster than the rate of isomerization. Since 1 is obtained uniquely from the addition of catecholborane t o IrBr(CO)(dppe) with greater than 99% stereoselectivity, the ratio of the oxidative addition rate constants, K-llkg, must be greater than or equal to 100. If the ratedetermining step for the isomerization is assumed to be catecholborane oxidative addition leading to 2, the
Organometallics, Vol. 14, No. 10, 1995 4529
IrX(CO)(dppe) Complexes
intramolecular rearrangement for eq 6 cannot be unambiguously eliminated, it seems improbable given the facility of B-H reductive elimination and oxidative addition involving 1. Our conclusion is in accord with results reported previously for isomerization via a reductive eliminatiordoxidative addition mechanism in related IrlI1(dppe) system^.^^,^^ The approach to equilibrium for the conversion of 1 t o 2 was monitored by 'H NMR spectroscopy between 333 and 348 K over the entire course of the isomerization and yielded linear plots of ln(([lIo - [lIJ([ll [l]J)vs time as shown in Figure 3. The full integrated rate equation for this isomerization which allows for reductive elimination from 2 is shown as eq 9 and is
Q O y O
steady state approximation can be applied to IrX(C0)(dppe) at early reaction times when k-2[21 is negligible (reaction times prior to -40% conversion). The firstorder rate expression describing the disappearance of 1(eq 8) is thus obtained. Given the requirement that k-1Ik2 L 100, the rate expression requires that kl > lookob,, where kobs is the observed rate constant for the isomerization.
Two experiments were performed t o test the notion that isomerization between 1 and 2 proceeds via catecholborane reductive elimination from 1and subsequent oxidative addition to give the resulting IrH(B02CsH4)Br(CO)(dppe)product, ii-HX. In the first, the reaction between a benzene solution of IrD(BOzCsH4)Br(CO)(dppe) ( 1 4 1 ) and 10 equiv of catecholborane-do at 25 "C was monitored using lH NMR spectroscopy. Formation of l-do was observed to be complete after 3 min and occurs without any observation of 2 over ca. 5 h. In the second, the reaction between an equilibrium mixture of IrD(BOzCsH4)Br(CO)(dppe) (1-dl and 2-dd and catecholborane-do at 55 "C was monitored by lH NMR spectroscopy. As shown in Figure 2, lH NMR spectra of the hydride region for both isomers obtained during the reaction show that hydride incorporation into 1-dl is facile and complete before the first scan can be acquired ( 100kobs is satisfied and that all of the kinetic data are consistent with the reductive eliminationloxidative addition mechanism shown in eq 7. While simple
ln(([lI, - [1le)N11- [1IeN = koblt = ((k1k2+ k-,k-,)/(k2 k-,))t (9)
+
derived in the Appendix. From the measured values of the equilibrium constant Keq and the first-order rate constant kohl for the approach t o equilibrium that are given in Table 3, it is possible to extract additional information about the rate constants of eq 7. Specifically, on the basis of the observed diastereoselectivity of catecholborane oxidative addition that leads to the relationship k-1 L lOOkz, the isomerization rate constant hobs' can be approximated as in eq 10, where Keq hob:
= (Keq
+ 1)k-z
(10)
is simply (klkz)/(k-lk-2). With experimental determination of both Keq and k o b i , we can obtain values for the reductive elimination rate constant k-2 from eq 11 k-2
= kobi/(Keq
+ 1)
(11)
a t different temperatures, and these values are given in Table 3. In addition, a lower bound for the reductive elimination rate constant k l can be calculated based on eq 12 which arises from expression of I(eq in terms of the kinetic parameters of eq 7 and the relationship that k-1 L 100kz. These values are also given in Table 3.
k'
'10OKeqk-,
(12)
The temperature dependence of k-2 allowed for the calculation of the kinetic activation parameters for the k-2 step. A linear plot of In k-dT vs 1/T was obtained and is shown in Figure 4. For the k-2 step, AlF = 28.4 4~1.4 kcal mol-' and AS* = 7.6 f 4.1 eu. A similar temperature dependence for the lower limit of 121 was also obtained, yielding AlF = 29.9 k 1.4 kcal mol-l.
I^-fldw---
4254 min min
A
IIII
24min 13min
9min 6 min
p 3min 0 min
I PPm
I
I -7.6
I
I -8.0
1
I PPm
I
1
-16.2
+
1
I
1
-16.6
Figure 2. lH NMR spectra for the reaction of (1-dl + 2 4 ) catecholborane-do. The hydride for 1 is shown at -7.52 ppm as a doublet of doublets ( ~ J H= -P 128.5, 16.9Hz).The hydride for 2 is shown at -16.18 ppm as a doublet of doublets ( 2 J ~ - p= 15.7, 8.1 Hz).
4530 Organometallics, Vol. 14,No. 10,1995
Cleary and Eisenberg Eyring Plot for the Reductive Elimination of Catecholborane
Approach to Equilbrium Between 1 and 2
From 2. -13
h
8
T
3
z
C
5000
0
loo00
.... ..
Time (seconds)
Figure 3. Plots for the approach to equilibrium for the isomerization of 1 to 2, where n = 333 K, u = 338 K, 1 = 343 K, and s = 348 K.
....
. ..: . . . . .:, :. ..:. ;. . . . . I 0.00285 0.00288 0.00291 0.00294 0.00297 0.003 0.00303
2oooo
15000
1/T( K ’ )
Figure 4. Eyring plot for the reductive elimination of catecholborane from 2, where n = k-2 step.
Table 3. Equilibrium and Rate Constants for the Interconversion of 1 to t
AAG’ 22.7 kcal md-’
temp
(K) 333 338 343 348
Ke,
0.739 f 0.006 0.742 f 0.006 0.773 f 0.007 0.809f 0.006
104kOb,(s-Y 103kl(9-1) 1.17 & 0.01 24.96 f 0.2 2.42 f 0.03 >10.3 f 0.4 5.26 f 0.09 >22.9 f 1.3 7.7 f 0.1 >34.5 f 1.9
1 0 5 ( ~ - 1~) 6.7f 0.2 13.9 f 0.4 29.7f 0.9 42.6 f 1.2
Reaction conditions: [l]= 0.024M in C6D6. Values obtained from least squares fit of lines from plots of ln([l]o - [11,)/([11[1],)vs time. k& = ( k l k z k-lk-z)/(kz + 12-1).
e- k
*’+
e
26.5
e
2
a
+
However, since the actual values of k1 are not known, this value for AH+ should be viewed as an upper limit and the value of AS* is not strictly obtainable. On the basis of all of these results, a reaction coordinate diagram for the isomerization of 1 to 2 at 298 K can be constructed as shown in Figure 5. Oxidative Addition of Caa02BH to IrI(C0)(dppe). The reaction of IrI(CO)(dppe)with catecholbolane leads initially to the formation of two diastereomers of IrH(B02C6HdBr(CO)(dppe)(3 and 4) in a 9:l ratio, respectively. The assignment of structure 3 (shown in eq 13)is based on the magnitude of 2 J ~ - pfor
Y
-
1
3
2
4
the hydride ligand and 2 J c - p for the carbonyl group in the lH and l3C(lH} NMR spectra respectively (see Tables 1 and 2) and by analogy with the observed spectroscopic data and structural assignment made for complex 1. In the lH NMR spectrum of complex 4, the hydride ligand appears as a pseudotriplet at -9.47 ppm, and on the basis of a 2 J ~ - pof 17.5 Hz and its chemical shift, the hydride ligand is assigned as cis t o both
Figure 5. Reaction profile for the equilibrium between 1 and 2 at 298 K. phosphorus nuclei and trans to CO. The 31P(1H}NMR spectrum for 4 shows two phosphorus signals a t 29.5 and 7.1 ppm with the former as a doublet due t o cisphosphorus coupling and the latter exhibiting significant broadening (w1/2 = 37 Hz) due to trans coupling to boron. Thus, the oxidative addition of catecholborane to IrI(CO)(dppe)shows a 90% stereoselectivity with the formation of 3 favored over 4 by 1.3 kcal mol-l. After 1 day at ambient temperature, a new hydride complex (5) is observed t o form a t the expense of both 3 and 4. The hydride resonance of 5 appears at -14.29 ppm in the lH NMR spectrum as a doublet of doublets with two small cis-phosphorus couplings. As was observed for 2, the hydride resonance of 5 shows a characteristic upfield chemical shift which indicates that it resides trans t o iodide. The 31P(1H}NMR spectrum of 5 displays two resonances: a doublet a t 20.4 ppm that shows a typical cis 2 J p - ~of 9.4 Hz and a broad signal at 17.7 ppm with w1/z = 40 Hz for phosphorus trans to boron. On the basis of the spectroscopy and comparison to 2, the coordination geometry of 5 is assigned to that shown in eq 14. Oxidative Addition of CsH402BH to [IrH(CO)(dppe)l and [Ir(B02Cfi)(CO)(dppe)l. The 4-coordinate complex [IrH(CO)(dppe)l has been proposed to be generated photolytically and thermally as an intermediate during reactions of IrHdCOXdppe)with DZand ethylene yielding IrDzH(CO)(dppe) and I~(CZH&T$ CzHd(CO)(dppe),r e s p e c t i ~ e l y . In ~ ~order ? ~ ~ to study the stereoselectivity of borane addition to [IrH(CO)(dppe)l, ~
(36) Deutsch, P. P.; Eisenberg, R. J.Am. Chem. Soc. 1990,112,714-
721.
ZrX(CO)(dppe) Complexes
Organometallics, Vol. 14,No. 10,1995 4531
H
i:7 0
a reaction between IrH~(CO)(dppe) and catecholborane was carried out. When a benzene solution of IrHs(C0)(dppe) was photolyzed (hv > 300 nm) or heated to 60 "C in the presence of 1 equiv of catecholborane, a new IdIII) product (6) was formed after ca. 2 h. Examination of the lH NMR spectrum of 6 shows two hydride resonances, a doublet of doublets a t -9.12 ppm ( 2 J ~ - p = 114.8,12.6 Hz) and a triplet at -10.04 ppm ( 2 J ~ - p= 18.8 Hz), indicating that the former is trans and cis to dppe phosphorus nuclei while the latter is cis to both dppe phosphorus atoms. Four sets of o-phenyl dppe lH resonances are assignable while the remaining dppe mand p-phenyl lH resonances overlap with resonances attributed to the catecholate moiety of the boryl ligand. Terminal CO coordination is established by a single vco a t 1984 and by 13C{lH} NMR spectroscopy which shows the carbonyl carbon as a triplet a t 178.3 ppm C2Jc-p = 5 Hz), thereby fixing its orientation as cis to both dppe phosphorus nuclei. In the 31P{1H}NMR spectrum, an AB quartet is observed for the two dppe phosphorus nuclei. On the basis of the spectroscopic data, 6 is assigned as IrHz(BOzC6H4)(CO)(dppe). Although no quadrupolar coupling to boron is observed in any of the NMR spectra, the geometry of 6 is unambiguously assigned as that shown in eq 15 from the coupling constants of the hydride, carbonyl, and phosphine ligands and the fact that the catecholate protons are observed in the lH NMR spectrum indicating boryl coordination.
the 31P{1H}NMR spectrum shows one broad resonance a t 26.1 ppm (a112 = 40 Hz) confirming that both phosphorus nuclei of dppe are trans to boron. On the basis of these results, complex 7 is assigned as the bis(boryl) derivative IrH(BOzCsH4)2(CO)(dppe)with the C, structure shown in eq 16. Oxidative Addition of C a 0 2 B H to IrBr(C0)(chiraphos). As described above, the oxidative addition of catecholborane to IrBr(CO)(dppe)proceeds under kinetic control with initial formation of a single isomer (1) that subsequently equilibrates with a less stable isomer (2). In analyzing the stereochemistry of concerted oxidative addition reactions by eqs 3 and 4,we have noted previously that substrate approaches from either side of the square planar IdI) dppe complex are equivalent and that the oxidative addition reaction thus generates a racemic mixture of Ir(II1) products. Through the use of a chiral diphosphine, the degeneracy of substrate approach from either side of the square planar complex is removed, leading to possible diastereoselectivity in the oxidative addition reaction. Indeed, such diastereoselectivity in both H2 and silane oxidative additions to the chiral complex IrBr(CO)(chiraphos)has been observed and reported p r e v i o ~ s l y .In ~ ~view of the kinetic stereoselectivity of catecholborane oxidative addition t o IrBr(CO)(dppe) (eq l),a series of experiments were run with IrBr(CO)(chiraphos) in order to assess possible diastereoselectivity in the oxidative addition process. However, instead of observing any diastereoselectivity, we found a surprising absence of kinetic selectivity. As shown in eq 17, IrBr(CO)(chiraphos)reacts with 5 equiv of catecholborane in toluene-& at -30 "C to
H
I
8a and 8b C 0
6
A reaction at 60 "C, or by irradiation (hv > 300 nm), between a benzene solution of 6 and 5 equiv of catecholborane yields a new complex (7) after ca. 2 h (eq 16). The lH NMR spectrum confirms the presence of the hydride as a triplet a t -9.08 ppm with a small cisphosphorus coupling constant at 18.0 Hz. Terminal CO coordination is established from vco at 1995 cm-l while
0
9a and 9b
yield two sets of diastereomers @a,band 9a,b)in a 1:l:
4532 Organometallics, Vol. 14, No. 10,1995
1:0.5 ratio as determined by NMR spectroscopy (see Tables 1 and 2). Diastereomers 8a,b display hydride signals in the lH NhrIR spectrum a t -7.20 and -7.65 ppm, respectively, with large trans 2 J ~ - pand terminal CO resonances in the l3CC1H} NMR spectrum a t 174.6 and 173.9 ppm, respectively, with large trans 2 J ~ - p . When the solution is allowed to react for 1 day at 25 "C, an equilibrium is observed to form between the diastereomeric pairs 8 and 9 and a new set of diastereomers 10a,b, as shown in eq 18. The conversion of diastereomers 8 and 9 to a set of diastereomers with hydride trans to bromide is similar to that observed for the dppe analogs in the equilibrium between 1 and 2 (eq 6). 8a + 8b
+ 9a + 9b
-
Br
H
10a and 10b
Comments on the Mechanism of Catecholborane Oxidative Addition. To rationalize the results obtained here for catecholborane oxidative additions to IrX(CO)(dppe)complexes, it is necessary to review the results and conclusions made for hydrogen, silane, and hydrogen halide (HX') oxidative additions to IrX(C0)(dppe) c o m p l e ~ e s . ~ The ~ , cis ~ ~ oxidative addition of H2 and R3SiH to IrX(CO)(dppe)proceeds stereospecifically along pathway i (yielding the product i-HC for silane addition) (eq 19) while HX' reacts oppositely along
Cleary and Eisenberg
for silane orientation leading to product i-HC is probably steric in nature, involving minimization of nonbonded interactions between the silyl and dppe phenyl groups. While Ha and R3SiH approach the metal center as nucleophiles, the hydrogen halides approach IrX(C0)(dppe) in aprotic media as electrophiles. Therefore, interactions that retain or enhance electron density at the metal center will favor addition along that pathway. Pathway ii is preferred for HX' additions because bending of the X-Ir-P axis effects an antibonding interaction between an occupied pz orbital of X and the dz2orbital of Ir, thus enhancing the ability of Ir to donate electrons to the incoming e l e c t r ~ p h i l e .Preference ~~ for HX' orientation leading to the ii-HX diastereomer is likely to be steric in nature involving minimization of nonbonded contacts. Catecholborane oxidative addition to IrX(CO)(dppe) complexes (X = Br, I) resembles HX' additions since the initially formed products, 1 (>99%) and 3 (go%), correspond to addition via pathway ii although the relative orientation of the B-H bond is reversed in both instances to that of the H-X' bond. This result implies that catecholborane approaches the metal center as an electrophile in accord with the view that the vacant B pz orbital can overlap with the filled dz2 of Ir (G).
+
G
Y = H, R3Si
X = CI, Br, I
X = Br, I X' = Br, I
OC
-
iL? lr
X
pathway ii with HX' orientation yielding product ii-HX (eq 20). Since both H2 and R3SiH add to IrX(CO)(dppe) complexes as nucleophiles, the additions occur over the OC-Ir-P axis because n*co is able to stabilize the developing transition state by removal of electron density from the Ir dz2 orbital, thus reducing the repulsive 4e- interaction between the filled dz2and ab(H2 and R3Si-H) orbitals. For R3SiH addition, preference
Rearrangement of 1 to 2 and of 3 to 5 occurs via catecholborane reductive elimination and subsequent oxidative addition along pathway ii, with the regiochemistry of B-H addition reversed such that the B-H orientation is the same as that observed during HX' oxidative additions. For catecholborane oxidative addition to [IrH(CO)and for the minor (dppe)] and [Ir(BO2C~H4)(CO)(dppe)l, isomer 4 (10%) of catecholborane addition to IrI(C0)(dppe), diastereomers are obtained corresponding to addition along pathway i with B-H orientation so as to yield product i-HC. The formation of these diastereomers is similar to the products obtained from H2 and R3SiH additions to IrX(CO)(dppe)via pathway i with addition over the OC-Ir-P axis of the four-coordinate complex. The change in kinetic selectivity for catecholborane addition to [IrH(CO)(dppe)land [Ir(B02CsH4)(CO)(dppe)]relative to that seen for IrBr(CO)(dppe)may arise from the fact that, with the former compounds, the ligand interaction with Ir dz2 that increases the complex's basicity upon bending is absent. Thus the addition proceeds more in accord with the factors that control nucleophilic additions of H2 and silanes. The regiochemistry of the addition to give i-HC products ~~
~
(37) Sargent, A. L.; Hall, M. B.; Guest, M. F. J. Am. Chem. SOC. 1992,114, 517-522.
IrX(CO)(dppe) Complexes
rather than i-HP products is apparently determined by minimization of nonbounded interactions. The observation that oxidative addition of catecholborane to IrBr(CO)(chiraphos) leads to the formation of both ii-HP and i-HC diastereomers in nearly equal proportions is, a t this time, very puzzling. While we expected the chiraphos ligand to impart subtle preferences upon the facial diastereoselectivity of the oxidative addition,34it was not expected to influence the stereoselectivity of the addition in terms of pathway ii us i relative to that seen with IrBr(CO)(dppe). The initially formed pairs of diastereomers, however, indicate the absence of kinetic selectivity (pathway ii us pathway i) as well as facial diastereoselectivity. On the basis of these results, we conclude that a fine balance between steric and electronic factors influences the stereoselectivity and diastereoselectivity of catech2lborane oxidative addition reactions to IrX(CO)(P P) complexes. Additional studies with other systematically varied complexes and computational studies are necessary to l l l y comprehend the factors governing borane additions to Ir(1) centers.
Experimental Section Reactions and sample preparations were performed in a nitrogen filled glovebox or under the appropriate gas using a high-vacuum line or Schlenk line. All solvents were reagent grade or better and were dried and degassed prior to use by accepted methods. Catecholborane-d~~~ and the complexes IrBr(CO)(dppe) and IrI(CO)(dppe),31I r H 3 ( C O ) ( d ~ p e )and ,~~ IrBr(CO)(~hiraphos)~~ were prepared according to literature procedures. The identities of all complexes are determined solely by spectroscopic methods. Microanalysis has also been included for 1. NMR samples were prepared using resealable NMR tubes fitted with J Young Teflon valves (Brunfeldt) and high-vacuum line adapters. 'H, 13C,and 31PNMR spectra were recorded at 400.13, 100.62, and 161.98 MHz, respectively, on a Bruker AMX 400 NMR spectrometer. Temperature control was achieved using a B-VT 1000 variable-temperature unit, a Cuconstantan temperature sensor, and calibration for high and low temperatures using ethylene glycol and methanol standards, respectively. 'H NMR chemical shifts are reported in ppm downfield of tetramethylsilane but measured from residual 'H signal in the deuterated solvents. 13CNMR spectra are reported in ppm downfield of tetramethylsilane and referenced t o a known carbon signal in the solvent. 31PNMR spectra are reported in ppm downfield of an external 85% solution of phosphoric acid. Benzene-& (MSD) and toluened8 (Cambridge) were dried and distilled from purple solutions of sodium benzophenone ketyl. Methylene chloride-& (Cambridge) was dried and distilled from a calcium hydride suspension. Solution and KBr (Aldrich) mull infrared spectra were recorded on a Matteson 6020 Galaxy FT infrared spectrometer. Elemental analysis was performed by Desert Analytics Laboratory, Tucson, Az. IrH(BOzC&)Br(CO)(dppe), 1. To a 50 mL benzene solution of IrBr(CO)(dppe)(75 mg, 0.107 mmol) was added 5 equiv of catecholborane (57 pL, 0.535 mmol). The solution was stirred for 10 min after which the volume was reduced to 15 mL. The addition of 50 mL of hexanes induced precipitation of the crude product. Recrystallization from methylene chloride and hexanes yielded the analytically pure 1in 81% yield. See Tables 1 and 2 for spectroscopic characterization. Anal. Calcd for C33HzsBBrIr03Pz: C, 48.43; H, 3.57. Found: C, 48.19; H, 3.51. (38)Newsom, H.C.;Woods, W. G.Inorg. Chem. 1968,7,177-178. (39)Fisher, B.J.;Eisenberg, R.Organometallics 1983,2,764-767.
Organometallics,
Vol.14,No.10,1995 4533
IrD(BOzC&dBr(CO)(dppe), l-d~.A similar procedure was used to prepare 141using IrBr(CO)(dppe) and catecholboRecrystallization from methylene chloride and hexanes afforded 1-dl in 78% yield. IrH(BOzCsHdI(CO)(dppe),3 and 4. In a nitrogen-filled glovebox, a resealable NMR tube was charged with IrI(C0)mmol) and 0.5 mL of a 8 x mM (dppe) (3 mg, 4 x C6D6 solution of catecholborane (1 equiv, 4 x mmol) delivered by a 0.5 mL syringe. The NMR tube was sealed and shaken to ensure complete mixing. After 5 min the colorless solution was analyzed by infrared and NMR spectroscopies (see Tables 1 and 2). IrHz(BOaC&)(CO)(dppe), 6. In a nitrogen-filled glovebox, a resealable NMR tube was charged with 8 mg of IrH3(CO)(dppe) (0.013 mmol) and 0.5 mL of a 0.026 mM CsD6 solution of catecholborane (1equiv, 0.013 mmol) delivered by a 0.5 mL syringe. The NMR tube was shaken t o to ensure complete mixing and either photolyzed (hv > 300 nm) at ambient temperature or warmed t o 60 "C for 2 h. The formation of 6 was confirmed by infrared and NMR spectroscopies (see Tables 1 and 2). IrH(BOzC&)z(CO)(dppe), 7. To an opened resealable NMR tube containing 6 in a nitrogen-filled glovebox was added 5 equiv (7 pL, 0.065 mmol) of catecholborane. The tube was sealed, shaken, and either photolyzed (hv > 300 nm) or heated a t 60 "C for 2 h. The formation of 7 was observed by infrared and NMR spectroscopies (see Tables 1and 2). Kinetic Studies of the Interconversion of 1to 2. In a nitrogen-filled glovebox, a resealable NMR tube was charged with 0.5 mL of a standard CsD6 solution of 1 (0.024 M) delivered by a 0.5 mL syringe, sealed, and inserted into a preheated NMR probe. The interconversion of 1 to 2 was monitored using 'H NMR spectroscopy by measuring the integrated intensities of the hydride ligands due t o 1(-7.52 ppm, dd, 2 J ~ -= p 128.5,16.9 Hz) and 2 (-16.18 ppm, dd, 2 J ~ - p = 15.7, 8.1 Hz). The reactions were monitored until equilibrium was reached in the temperature range 333-348 K. Plots of ln(([llo - [11,)/([11 - [l],)) vs t yielded straight lines with the slope equal to the first-order rate constant kob; (Kob; = (klkz k-lk-z)/(kz k-1); see Appendix). Incorporation of Catecholborane-dointo l-dl and 2 4 . To an open resealable NMR tube in a nitrogen-filled glovebox was added 10 mg (0.012 mmol) of 141and 0.5 mL of C6D6. The tube was sealed, removed from the glovebox, and heated overnight a t 75 "C to ensure an equilibrium solution was obtained. After the equilibrium was established, the tube was transferred into the glovebox where 10 equiv (13.0 mL, 0.12 mmol) of catecholborane-do was added to the tube. The tube was immediately removed from the box and placed into a preheated (55 "C) NMR probe. The incorporation of catecholborane-do into 1-dl and 241 was monitored by 'H NMR spectroscopy for 1 h.
+
+
Acknowledgment. We wish to thank the National Science Foundation (Grants CHE 89-09060and CHE 94-04991)for support of this work and the Johnson Matthey Co. Inc. for a generous loan of iridium trichloride. B.P.C. gratefully acknowledges Sherman Clarke, Bristol Myers-Squibb, and Arnold Weissberger Fellowships. Appendix In view of the fact that the dissociative isomerization mechanism shown in eq 7 is not routinely treated in standard kinetics references, we outline the mathematical treatment of the approach to equilibrium of this reaction. Since the studies were performed in the absence of excess borane, it is clear from the stoichiometry of eq 7 that the concentrations of IrBr(CO)(dppe) and catecholborane are equal. If we represent their
4534 Organometallics, Vol. 14,No. 10,1995 concentrations as [XI, then the differential rate law for the disappearance of 1 is given by eq A.l. Similarly, the rate law for the appearance of 2 is eq A.2 and for the time dependence of [XI is eq A.3.
Application of the steady state approximation for [XI yields eq A.4, and by substitution into eq A.l and
ln(([l10 - [lIeY([lI - [11,)/([11 - [lie)) = ((k1k2 + k-lk-J(k2 k-1))t = kobit (A.10) which may then be integrated to yield eq A.lO. A linear plot of ln(([lIo - [l]e)/([l] - [lie)) vs t then yields the first-order rate constant hobs’ = ((k1k2 + k-lk-z)/(kz f k-1)).
On the basis of the experimental results that indicate k - 1 ~ -k2 and the equilibrium constant in terms of the kinetic parameters of eq 7, we obtain the equation for estimating k-2 in terms of kohl and Keq, eq A.11. k-2
+
= kobi/(Keq 1)
(A.ll)
The lower limit value of k l is based on the fact that that k-1 > look2 which by substitution into Keq = klkd k-lk-2 leads to the inequality of eq A.12.
k, OM950084Y
’1O0Keqk-,
(A.12)
