439
Organometallics 1986, 5, 439-442
Oxidative Addition of Alkyl Halides in the Presence of Alkenes and the Rate of Addition of an Alkyl Radical to a Platinum(I1) Complex Patrick K. Monaghan and Richard J. Puddephatt” Department of Chemistry, University of Western Ontario, London, Ontario, Canada N6A 587 Received July 16, 1985
Dimethyl(1,lO-phenanthroline)platinum(II)reacts with isopropyl or tert-butyl iodide, RI, in the presence of alkenes [CH,=CHX where X = CN, CHO, or C(=O)Me] to give the new functionally substituted organoplatinum(1V)complexes [PtIMe,(CHXCH,R)(phen)] in high yield. The complexes were characterized by NMR and IR spectroscopy and by elemental analysis. The reactions occur by a free radical chain reaction, in which the radical Ro reacts with CH,=CHX to give RCH2CHXo,which then reacts with [PtMe,(phen)] to give [PtMe2(CHXCH2R)(phen)lo; iodine atom abstraction from RI by this platinum(II1) radical completes the cycle. A competition experiment, in which radicals Roreact with either [PtMe,(phen)] or CH,=CHCN, gives an approximate rate constant for addition of the i-Pro radical to [PtMe,(phen)] of lo7 M-‘ s-l , one of the first rate constants for radical addition to a diamagnetic transition-metal complex. A second competition experiment shows that the relative rate of iodine abstraction from t-BuI and i-PrI by the platinum(II1) radical is 1.7 f 0.2. It is suggested that this radical trapping technique will have general use in identifying the free radical mechanism of oxidative addition.
Introduction The complex [PtMe2(l,10-phenanthroline)]l (1) is very reactive toward oxidative addition of alkyl halides, and studies of reactivity and mechanism in such reactions have been The reaction of i-PrI with 1 in the presence of air gave not only the oxidative addition product 2 but also complexes 3 and This reaction was shown to occur primarily by a free radical chain mechanism, and complex 3 was formed by the sequence of reactions 1-3.5*6
Table I. Products Formed by Reaction of [PtMe2(phen)] with i - P r I (0.4 M) a n d CH,=CHCN in Acetone at 20 O C 1O3[PtMez(Phen)], M 4.2 3.2 2.1 4.2 4.2 4.2 4.2 4.2
4.435
i-Pr
+ O2
i-PrOOO+ [PtMes(phen)]
-
-
i-PrOOO
(1)
[PtMe2(OO-i-Pr)(phen)]0 (2)
-
[PtMe2(OO-i-Pr)(phen)]o+ i-PrI [PtMe,I(OO-i-Pr)(phen)]
+ i-Pro (3)
According to this mechanism, it should be possible to “insert” not only oxygen but also other unsaturated molecules A=B during the oxidative addition, provided that they trap the primary i-Pro radical efficiently and that the radical i-Pr-A-Bo then adds to platinum rather than reacting with more A=B or undergoing other reactions. In this paper, the results of reactions in which the unsaturated reagents are the activated alkenes CH,=CHX, where X = CN, CHO, or C(Me)O, are described. A preliminary account of parts of this work has been p u b l i ~ h e d . ~
Results and Discussion Synthesis of New Complexes. In the absence of oxygen, complex 1 in acetone solution reacted with i-PrI in diffuse daylight to give 2 in pure form.4 When the reactions were carried out in the presence of excess alkene CH,=CHX, the new products 5a-c were formed in high yield. The concentration of alkene was important in op-
a
product ratio x = 5a/2 2.4 3.8 4.7 7.6 4.6 2.5 1.7 1.0
[CHP= CHCN], M 0.12 0.12 0.12 0.40 0.20 0.10 0.06 0.02
0276-733318612305-0439$0l.50/0
23.8 19.7 24.3 25.0 20.7 19.0 16.8 9.5
k , / k z = [CH2=CHCN]/0.5[PtMez(phen)],,.
timizing yields. With low concentrations of alkene considerable amounts of 2 were formed (Table I) whereas, with very high concentrations of alkene, some polymer was formed. This polymerization limited the range of substituents, X, in the alkenes CH,=CHX, which could be accommodated. For example, styrene and methyl acrylate or methacrylate gave large amounts of polymer and it was not possible to isolate complexes of structure 5. j-7
i-Pr
I
I
I
I
I
2
3 H XC H2R
~~
(1)Chaudhury, N.;Puddephatt, R. J. J. Organomet. Chem. 1975,84, 105. (2)Jawad, J. K.;Puddephatt, R. J. J . Organomet. Chem. 1976,117, 297;J . Chem. SOC.,Dalton Trans. 1977,1466. (3)Monaghan, P. K.;Puddephatt, R. J. Inorg. Chim. Acta 1983,76, L237. (4)Ferguson, G.;Parvez, M.; Monaghan, P. K.; Puddephatt, R. J. J . Chem. Soc., Chem. Commun. 1983,267;Organometallics, in press. (5) Monaghan, P. K.; Puddephatt, R. J. Organometallics 1983,2,1698. (6)Hill, R.H.; Puddephatt, R. J. J . Am. Chem. SOC.1985,107,1218.
k,lkPa
Me
i 5a, X a C N : R = 1-Pr b. X = CHO: R = i - P r
C(=O)Me: R = / - P r d. X = C N : R * t - B u e, X 1 CHO: R = t - B u C. X =
0 1986 American Chemical Society
4
440
Organometallics, Vol. 5, No. 3, 1986
Monaghan and P u d d e p h a t t
Table 11. 'H NMR Data for Complexes [PtIMe2(CH,(X)CH,H,Rl(N-N)]n PtMe R He, H,, H, com p 1ex 6 2J(PtH)/Hz 6 J/Hz 6 J/Hz [PtIMe,(CH(CN)CH,-t-Pr)(phen)]* 1.68 71 0.40 (CHJ 6 [3J(HH)] 2.26 (H,) 13 [3J(H,H,)] 175 72.5 0.70 (CHJ 6 [,J(HH)] 4 ['J(H,H,)l 1.43 (CHI 6 [3J(HH)] 92 [2J(PtH)] 0.08 (H,) 4 [2J(H&)1 13 [*J(HpH)I 0.65 (H,) 11 [V(H,H)] [ PtIMe2(CH(CHO)CHZ-~-Pr}(phen)]' 1.82 71 0.49 (CH,) 6 [,J(HH)] 2.95 (H,) 1 .88 72 0.64 (CH3) 6 [3J(HH)] 1.17 (CH)d [PtIMez[CH(COMe)CHz-i-Pr)(phen)]e 1.74 70 0.52 (CH,) 6 [3J(HH)] 2.94 (He) 1.85 70.5 0.67 (CH3) 6 [,J(HH)] [PtIMe,(CH(CN)CH,-t-Bu)(phen)]f 1.68 70.5 0.67 (CH,) 3.25 (He) 2 [3H(H,H,)l 1.76 71.0 12.5 [3J(H,Hs)] 96 ['J(PtH,)] 0.23 (HB) 13 [*J(HpH,)] 0.83 (H,) 48 [3J(PtH)] [PtIMe2(CH(CHO)CHz-t-BuJ(phen)]g 1.81 71.5 1.76 71.5 0.57 (CH,) 2.86 (H,)
-
'"-N = phen; X = CN, CHO, or C(=O)Me; R = i-Pr or t-Bu. bSolvent, CDCI,. Spectrum run on Bruker AM250 instrument. cSolvent, CDCI,; 6 8.76 [m, 3J(HH) = 3, 3J(PtH) = 10 Hz, CHO]. dTentative assignment, due to poor resolution. eSolvent, CDCI,; 6 1.40 [s, COMe]. /Solvent, CD2CI2. #Solvent, CDzCIz;6 8.55 [m, CHO]. Table 111. 13C('HJNMR Data for Complexes [PtIMe2(C,H(X)C,H2R)(N-N)]a Pt-Me _____ R c,, c, X LJ(PtC)/ complex 6 Hz 6 J(PtC)/Hz 6 J(PtC)/Hz 6 J(PtC)/Hz [PtIMe,[CH(CN)CH,-i-Pr)- -4.36 638 27.46 (CHI 60.9 (,Jptc) 11.35 (C,) 681.3 (lJptc) (phen)]* 23.37 (CH,) -4.69 634 20.82 (CH3) 41.31 (C,) 32.5 ('Jptc) [PtIMe,(CH(COMe)CH,-i- -2.57 672 27.98 (CH) 70.3 (,JptC) 44.58 (C,) 583.5 ( l J p t C ) 209.39 (C=O) PrKphen)]' 21.75 (CHJ -4.93 660 23.62 (CH3) 40.18 (C,) 34.7 ('Jp,c) 31.34 (CHZCO) 66.6 (,Jptc) 43.94 (C,) 553.6 ('Jptc) 202.32 50 ('JP~c) [PtIMe,(CH(CHO)CH,-i-3.81 647.9 27.8 (CH) Pr}(phen)lc -6.24 638.9 21.57 (CH3) 37.49 (C,) 33.3 (2Jptc) 23.45 (CH3) [PtIMe2(CH2(CHO)CH,-t- -3.47 650.5 30.74 IC(CH,),J 42.74 (C,) 543 ('Jptc) 201.81 60.2 ('Jptc) Bul(phen)l' -5.44 652.0 29.64 (CH3) 42.17 (C,) 34.8 ( ' J p t ~ ) ~
a
R = i-Pr or t-Bu; X = CN, CHO, or C(=O)Me. *Solvent. CDCI,.
Solvent, CD'CI,.
Reaction of t-BuI with complex 1 gave only 4,4 but, in the presence of alkene CH,CHX, the complexes 5d and 5e could be prepared without difficulty. In this case, the reactions occurred rapidly even in the dark and dichloromethane was the preferred solvent. Characterization of Complexes. Elemental analyses were consistent with the structures 5 , and the major problem in characterization was to determine the structure of the added radical. The radicals RCH2CHXo,R = i-Pr or t-Bu, can exist in two resonance forms either of which could be trapped by the platinum center of complex 1. This is illustrated for the acrolein-derived radical in eq 4. RCH2C0H-CH=O
c*
RCH2CH=CH-Oo
(4)
Trialkylboranes, R,B, apparently trap the oxygen-centered radical to give RCH2CH=CHOBR2? but the spectroscopic data for complexes 5 show conclusively that platinum traps the carbon-centered radical. The IR spectra for 5a, 5b, and 5c give typical stretching vibrations for v(C=N) or u(C=O) at 2200, 1650,and 1665 cm-', respectively, and the NMR spectra provided conclusive evidence for these structures. A complete assignment of the 'H and 13CNMR (7) Kabalka, G. W.; Brown, H. C.; Suzuki, A,; Honma, S.; Arase, A,; Ioth, M.J . Am. Chem. SOC.1970, 92, 710.
2
PPm
1
0
Figure 1. 'H NMR spectrum (400 MHz) of complex 5a. The peak labeled with an asterisk is due t o water impurity.
spectra of 5a was made by recording the 'H (250 and 400 MHz), 13C('HJ,and 13C INEPT spectra and by a two-dimensional heteronuclear lH--13Cchemical shift correlated experiment. Part of the 'H NMR spectrum is shown in Figure 1, along with the Newman projection of the pro-
Organometallics, Vol. 5, No. 3, 1986 441
Oxidative Addition of Alkyl Halides
Scheme 11"
Scheme I
"Pt = [PtMe2(phen)] (R = t-Bu or LPr).
"Pt = [PtMe2(Phen)l.
posed dominant conformer. The chiral carbon, bound to platinum, leads to nonequivalence of the Me2Pt, CH2, and CHMe2groups. The assignments are given in Figure 1and, for the less intense signals due to H,, Hp, H,, and H,, were confirmed by the 2-D experiment. The spectra are not consistent with the possible N-bonded isomer, the most unambiguous evidence being the observation of 'J(PtC*) = 681 Hz. The complete spectral parameters are listed in Tables I1 and 111. The very different vicinal couplings ?J(H,H,) = 13 Hz and ,J(H,H,) = 4 Hz (Figure 1, H, signal) are expected (from the Karplus equation) if the conformation shown in Figure 1is dominant. Assignment of the lH and 13C NMR spectra of 5b-e was straightforward by comparison with the spectra of 5a. The Mechanism of Reaction. The reactions described above, like those in the absence of occur by a free radical chain reaction. This has been confirmed by showing that trace amounts of the free radical scavenger 4-methoxyphenol inhibit the reaction of 1 with i-PrI and CH2=CHCN in diffuse light.8 The mechanism is shown in Scheme I, and this scheme also shows how a competition experiment, in which the primary i-Pro radicals can be trapped by alkene or attack complex 1 directly to yield 5a or 2, respectively, can be set up. According to this mechanism, decreasing the concentration of 1 or increasing the concentration of alkene CH,=CHX should increase the product ratio 5a/2. The data in Table I show that this is indeed observed. The product ratio was determined by integration of the resonances due to the methyl protons of the isopropyl groups in the 'H NMR spectra. Quantitatively, the product ratio, x = 5a/2 = k2[CH2= CHCN]/k,[l], from which k1/k2 = [CH,=CHCN]/x[l]. The individual values of k,/k2 found in this way are listed in Table I and give a mean value of kl/k2 = 20 f 6.9 An approximate value for k2 can be estimated. Extrapolation of kinetic data for the reaction Eto CH2= CHCN at high temperatures in the gas phase gives k 2 = 2 X lo5 L mol-l s-l a t 20 'C.l0 It must then be assumed that i-Proreacts at the same rate as Eto and that the rates are equal for acetone solution and the gas phase. Very recently, the rate of reaction of the 5-hexenyl radical with acrylonitrile has been determined directly at 20 'C in solution in CH2C12and CH3CN/CH3CO2Hl1and the second-order rate constant was found to be k2 = 5 X lo5 L mol-' s-l; this is considered a better approximation for the
+
(8)Typical kinetic data were given in the preliminary communication as supplementary materiaL5 (9) A graph of [CH,=CHCN] vs. r [ l ] gives a straight line of slope = 26 f 1, r = 0.992, but this does not pass through the origin. It is not clear if this is due to a systematic error in integration of NMR signals or to some other effect. As can be seen from Table I, fairly consistent values of k J k , are obtained except at the lowest concentration of CH,=CHCN used. Since [l]decreases from [llOto zero during the reaction, the value used in calculating k l / k , is 0.5[110. (10) Abell, P. I. Compr. Chem. Kinet. 1976, 18, Chapter 3. (11) Giese, B.; Kretzschmar, G. Chem. Ber. 1984, 117, 3160.
+
rate of the reaction i-Pro CH,=CHCN in acetone solution at 20 OC.12 Hence a value of k, = 1 X lo7 L mol-' s-l is calculated under the experimental conditions. This is best regarded only as an order of magnitude estimate of kl, but it is significant as one of the first estimates of a rate of free radical addition to a diamagnetic transition metal c0mp1ex.l~ A similar value has been estimated for addition of the isopropyl radical to [IrC1(CO)(PMe3)2], and rate constants of similar magnitude have been determined for SH2 reactions of main-group metal alkyls, such as trialkylboranes, in which free radical addition to the metal center may be rate determining.14 For this electron-rich platinum(I1) system the platinum(II1) radical [PtMe2-iPr(phen)lodoes not undergo dissociation of an alkyl radical to give an SH2process and regenerate platinum(I1) but instead abstracts an iodine atom from i-PrI to give the stable platinum(1V)complexes. It is this ability to undergo two-electron oxidation which makes the chemistry of these platinum(I1) complexes so different from boron(II1). The reactions with t-BuI to give 5d or 5e occur by the same mechanism and are faster because of the greater ease of cleavage of the tertiary alkyl-iodide bond. This was confirmed by a study of the competition experiment of Scheme 11. Here a mixture of 1 with excess a~rylonitrile'~ competes for a mixture of t-BuI and i-PrI. The selectivity for formation of 5d or 5a is governed by the relative rates of iodine atom abstraction by the radical [PtMe2(CH(CN)CH,R)(phen)]O from t-BuI or i-PrI, and hence the product ratio 5d/5a = k,[t-BuI]/kb[i-PrI]. Experimental data were in good agreement with this relationship and gave the ratio of rate constants k,/kb = 1.7 f 0.2. The kinetics of reaction of i-PrI with complex 1 in the presence of different concentrations of acrylonitrile were studied. The rates were measured by spectrophotometry, as described earlier for reactions in the absence of alkene.4 As expected for a radical chain reaction the kinetics were complex, but the rates were not much affected by the presence or absence of acrylonitrile. The olefin therefore does not appear to affect the chain length to any significant extent, in marked contrast to the behavior of oxygen described previ~usly.~ The reactions described here occur remarkably cleanly due to a favorable combination of rate constants. Thus, the electron-rich isopropyl or tert-butyl radicals are trapped efficiently by the electrophilic alkenes to give the relatively electron-poor radicals RCH2CHXo. The rate constant for attack by this radical on more alkene (12) The rate of radical attack on simple alkenes is not much different for primary and secondary alkyl radicals, so this approximation should be reasonably good.'O (13) Note that a very similar value has since been determined by an independent method.6 (14)Labinger, J. A.; Osborn, J. A.; Coville, N. J. Inorg. Chem. 1980, 19, 3236. Davies, A. G.; Roberts, B. P. In "Free Radicals"; Kochi, J. K., Ed.; Wiley: New York, 1973; Chapter 10. (:5) So that essentially none of complex 2 is formed under the experimental conditions.
442 Organometallics, Vol. 5, No. 3, 1986
ultimately to give polymers is then much lower than the rate of attack at the nucleophilic platinum center of 1 (note that the propagation rate constant for polymerization of acrylonitrile is ca. lo2 at 20 "C in DMF solvent,16 much lower than the rate of addition of the 5-hexenyl radical to acrylonitrile of 5 X lo5 L mol-' s-l). Hence complexes 5 are formed cleanly with the alkenes.
Conclusions The alkene trapping technique developed here has been shown to be useful for synthesizing unusual functionally substituted organometallic compounds. It is also useful as a test for a free radical mechanism of oxidative addition. Thus the alkene inserted compounds are not formed in reactions of 1 with primary alkyl iodides, such as Me1 or EtI, or with the secondary alkyl bromide i-PrBr, all of which are thought to undergo oxidative addition by the SN2mechani~m.~,~ The method is clearly capable of being extended to oxidative additions of other metal complexes, though complications can be expected when the metal complexes react with the alkenes used here. If the complexes 5 underwent reductive elimination of RCH,CHXI, a catalytic cycle for addition of RI to CH2= CHX would be completed. It is possible that a mechanism, as in Schemes I and 11, with the additional reductive elimination step could account for the photochemical addition of alkyl halides to acrylonitrile and related alkenes catalyzed by CuC1/PBu3. A similar mechanism, but with an initial two-electron oxidative addition step rather than the free radical mechanism established here, was proposed for this reaction." Experimental Section 13Cand 'H NMR spectra were recorded by using Varian XL200 and XLl00 spectrometers, respectively. IR spectra were recorded on a Beckman 4250 instrument. UV-visible spectra were recorded by using a Hewlett-Packard 8450A diode array spectrophotometer. All alkenes were distilled under vacuum before use. Elemental analysis was carried out by Guelph Chemical Laboratories, Ontario. The solvents used were CH2Cl, and acetone, and they were deoxygenated, before use, by means of several freeze/pump/thaw cycles, and all reactions were performed a t ambient temperature and in diffuse daylight unless otherwise stated. t-BuI was purified by shaking in contact with a saturated aqueous solution of Na2S203 and drying over anhydrous MgSO,. [PtIMe,{CH(CHO)CH,-i-Pr)(phen)]. T o a solution of [PtMe,(phen)] (0.06 g) in deoxygenated acetone (20 mL) was added acrolein (2 mL) followed by i-PrI (1 mL). The reaction was carried out under N2. The solution turned yellow after 15 min, the solvent removed under vacuum, and the solid residue (16) Eastmond, G. C. Compr. Chem. Kinet. 1976,14a, 232. (17) Mitani, M.; Kato, I.; Koyama, K. J . Am. Chem. SOC.1983, 105, 6719.
Monaghan and Puddephatt redissolved in CHzClz (3 mL). The product was recovered by precipitation using pentane (15 mL): yield 70%; mp 197 "C. Anal. Calcd for C2,H2,N210Pt: C, 38.0; H , 4.0; N, 4.4. Found: C, 38.0; H , 3.9; N, 4.5. Similarly were prepared [PtIMe2(CH(COMe)CH,-i-Pr)(phen)] [Anal. Cnlcd for C23H27N210Pt:C, 39.0; H, 4.2; N, 4.4. Found: C, 39.0; H, 4.1; N, 4.3.1 and [PtIMe,(CH(CN)CH,-i-Pr)(phen)] [Anal. Calcd for CzzH2,N3IPt: C, 38.2; H. 3.8; N, 6.7. Found: C, 37.6; H , 3.7; N , 6.4.1. [PtIMez{CH(CN)CH,-t-BuJ(phen)]. T o a solution of [PtMez(phen)] (0.09 g) in deoxygenated CH,Cl, (10 mL) was added acrylonitrile (2.5 mL) followed by freshly purified t-BuI (0.2 mL). The reaction was performed under N, an d in the dark. After 1 h the solvent was reduced in volume ( 5 mL), and the product precipitated as a pale yellow solid by the addition of pentane (20 mL): yield 78%; mp 246 "C. Anal. Calcd for CZ3Hz6N3IPt:C, 39.1; H, 4.0; N , 6.5. Found: C, 38.0; H, 3.6; N, 6.0. In a similar way was prepared [PtIMe2(CH(CHO)CH,-tBuJ(phen)],Anal. Calcd for C23H27N210Pt:C, 38.5; H, 4.2; N, 4.3. Found: C, 37.6; H, 3.9; N, 4.2. Competition Experiments. Acrylonitrile (1.15 mL) was added to a solution of [PtMez(phen)] (0.035 g) in deoxygenated CH,Cl, (25 mL). T o this was added a mixture of i-PrI and t-BuI (1.0 mL, 1:1 ratio by volume). The reaction was carried out under N,, and the product was recovered by evaporation of the solvent followed by pentane washing of the residue. The product ratio was determined by integration of the 'H NMR spectra. The procedure was repeated by varying only the relative amounts of i-PrI and t-BuI. A similar method but using a constant [i-PrI] and varying [CH,=CHCN] was used in the competition experiments of Scheme I. Kinetic Studies. To a solution of [PtMez(phen)](0.008 g) in acetone (35.0 mL) was added acrylonitrile (1.0 mL), and the sample was deoxygenated. The solution was kept under nitrogen. Working in the dark, a portion of the solution (5.0 mL) was transferred to a 1-cm quartz cuvette which was then sealed with a serum cap. Nitrogen gas was bubbled through the sample, and i-PrI (1.0 mL) was added. The cuvette was placed in the cell compartment of the UV-visible spectrophotometer. The decay of the band a t 473 nm was monitored with time. A plot of log (A, - A,) vs. time was made. This procedure was repeated several times by using the same concentrations of [PtMe,(phen)] and i-PrI but varying that of CH,CH(CN). Effect of Radical Inhibitor. T o a solution of [PtMez(phen)] (0.004 g) in degassed acetone (20.0 mL) was added distilled CHzCH(CN) (0.8 mL). A portion was placed in a 1-cm quartz cuvette and, with use of the same procedure as in the last section, i-PrI (0.1 mL) was added. The decay of the band a t 473 nm was monitored. This was done in diffuse light. The experiment was repeated by using CH,CH(CN) containing the radical scavenger 4-methoxyphenol (approximately 170).
Acknowledgment. We thank NSERC (Canada) for financial support. Registry No. 5a, 87338-36-1; 5b, 87338-37-2; 5c, 87350-68-3; 5d, 87338-38-3;5e, 87338-39-4; [PtMez(phen)],52594-55-5;i-PrI, 75-30-9; CH,=CHC(=O)Me, 78-94-4; CH,=CHCN, 107-13-1; t-BuI, 558-17-8; acrolein, 107-02-8.
