J . Am. Chem. Sor. 1989, 111 , 949-956 continued at room temperature for 0.8 h at which time 'H NMR analysis showed 87% consumption of enol silane. The reaction mixture was diluted with pentane (10 mL) and extracted with saturated brine (8 mL). The aqueous layer was washed twice with pentane ( I O mL). The combined organic layers were dried over Na2S04(1.5 g), filtered, and concentrated in vacuo. The pale yellow oil was purified by passing the mixture through a 1-in.plug of grade 111 basic alumina with hexane as eluant. A colorless oil, 86 mg (74% yield), was isolated. Anal. Calcd for C17H2802Si:C, 69.81;H, 9.64. Found: C, 69.86;H, 9.61. The extent of crossover was determined by 'H NMR analysis. A sample of recovered TMS aldol 14 (25.4mg, 0.087mmol) was dissolved in CDCI, (0.5 mL), and the spectrum was run on a Bruker AM-500spectrometer. To the solution was added two successive aliquots (10and 15 pL) of 0.14 M TMS aldol-d, (4.1 mg in 0.10mL CDCI,). The first aliquot corresponded to addition of 1.6% deuterium-labeled aldol, while after the second addition 4.0% was present. Within the limit of detections, a conservative estimate of 2.Ou(F:) parameters refined 316 R 0.037 0.040 Rw 1.04 esd of obsd of unit weight convergence, largest shift 0.00 u high peak in final diff map 0.27(5) e-/,&' low peak in final diff map -0.29(5) e-/8,'
of T H F . About 0.034 mL (0.6 mmol) of Br2 was added to the second flask. The solution in the second flask was stirred for 3 h before it was transferred to the first flask which was cooled to -78 "C. The reaction mixture was allowed to warm to room temperature, and stirring was continued for 30 h until all the CpCoC4Ph4BH was consumed as shown by TLC. More of the A1Br3/Br2 mixture (total of 2 mmol) was added to the reaction mixture during the course of the reaction. The reaction mixture was filtered through a syringe fitted with a glass frit, the solvent was pumped out completely, and the residue was dried for 1 h under vacuum. About 20 mL of toluene and 10 mL of diethyl ether were added to the flask. The flask was shaken gently to dissolve all the solid. A yellow compound precipitated after the flask was stored in the freezer for 1 h. After removing the solution, the yellow precipitate was washed twice with hexanes and dried for 1 h under vacuum. The product decomposes on silica gel. The final yield of CpCoC4Ph4B-Brwas 70%: ' H N M R (C6D6. 25 " c ) 6 6.6-7.7 (m, 20 H , Ph), 6 4.49 (S, 5 €3, Cp); "B N M R (CD,C(O)CD,, 25 "C) 6 17; MS, P+ = m/e 572, P - 3' = 0.22, P - 2' = 0.95, P - 1' = 0.54, P* = 1.0, P + 1' = 0.34, P 2' = 0.07 (calcd 0.22, 0.96, 0.55, 1.0, 0.34, 0.06). Reaction of CpCo(C4Ph4BBr) with PhMgBr. A solution of 0.034 mmol of CpCoC,Ph,BBr plus 5 mL of E t 2 0 was placed in a 25-mL Schlenk flask and cooled to 0 OC. PhMgBr (0.3 mmol, 0.1 mL, 3 M) was syringed in, and the solution was refluxed for 4 h. A small funnel with a frit was used to separate a white precipitate from the orange solution. An orange solid precipitated out after the solution was concentrated. The yield of CpCoC,Ph4B-Ph is 53%: ' H N M R (C6D6,25 "C) 6 6.7-7.9 (m, 25 H, Ph), 6 4.52 (s, 5 H , Cp); IH N M R (CD2C12, 25 "C) 6 6.8-7.9 (m, 25 H , Ph), 6 4.83 (s, 5 H , Cp); IlB N M R (C6D,CD3, 25 "C) 6 16.5; MS, P' = m/e 568.1775 (calcd 568.1772), P - I' = 0.22, P' = 1.0, P 1' = 0.42, P 2' = 0.087. Reaction of CpCo(C4Ph4BH)with n-BuLi. CpCoC4Ph4BH (0.13 mmol) was placed in a 25-mL round flask and dissolved in 4 mL of toluene. BuLi (0.32 mmol, 0.2 mL, 1.6 M) was syringed into the solution which was at 0 "C. The color changed from orange to greenish yellow immediately when BuLi was injected into the flask. The solution was stirred for 5 min and then by separated in CTLC by using toluene. The first band (orange) was collected. The yield of CpCoC4Ph,B-Bu is 55%: ' H N M R (C6D6, 25 "C) 6 6.8-8.0 (m, 20 H, Ph), 6 4.55 ( s , 5 H, Cp), 6 1.05 (t, 3 H, CHI), 6 1.40 (m, 2 H, CH,), 6 1.62 (m, 2 H, CH2), 6 1.80
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Table 111. Positional Parameters atom X Y Co 0.20222 (2) 0.17992 (1) 0.1502 (1) C1 0.3028 (2) 0.1189 (1) C2 0.2228 (2) 0,10224 (9) c3 0.2002 i 2 j 0.1219 (1) C4 0.2658 (2) C6 0.3449 (2) 0.1720 (1) 0.2239 (1) C7 0.3678 (2) C8 0.4106 (2) 0.2427 (1) 0.2105 (2) C9 0.4330 (2) 0.1590 (2) C10 0.4119 (2) 0.1398 (1) C11 0.3687 (2) 0.1029 (1) C12 0.1743 (2) 0.1388 (1) C13 0.1420 (2) 0.1233 (1) C14 0.0987 (2) 0.0718 (2) C15 0.0876 (3) 0.0355 (1) C16 0.1198 (2) 0.0509 (1) C17 0.1629 (2) 0.0674 (1) C18 0.1230 (2) 0.0774 (1) C19 0.0374 (2) 0.0438 (1) C20 -0.0326 (2) -0.0004 ( I ) C21 -0.0170 (3) -0.0112 (1) C22 0.0672 (3) 0.0224 (1) C23 0.1387 (2) 0.1157 (1) C24 0.2605 (2) 0.1095 (1) C25 0.3400 (2) C26 0.3394 (3) 0.1043 (2) C27 0.2603 (3) 0.1065 (2) C28 0.1810 (2) 0.1134 (1) C29 0.1810 (2) 0.1172 ( I ) 0.2487 (1) C30 0.1605 (2) 0.2181 (1) C31 0.0860 (2) 0.2075 (1) C32 0.0907 (2) 0.2315 (1) C33 0.1674 (3) c34 0.2111 (2) 0.2572 ( I ) B 0.3361 (2) 0.1520 (1)
B(A2)' 2.779 (6) 2.80 (5) 2.78 (6) 2.77 (5) 2.94 (6) 3.10 (6) 3.96 (7) 4.85 (8) 5.22 (9) 5.17 (9) 4.21 (7) 3.07 (6) 3.94 (7) 5.00 (9) 5.34 (9) 5.05 (8) 4.18 (7) 3.01 (6) 3.81 (7) 4.71 (8) 5.75 (9) 5.9 (1) 4.52 (8) 3.20 (6) 4.56 (8) 6.1 (1) 5.7 (1) 4.68 (8) 3.81 (7) 4.41 (7) 5.17 (9) 5.77 (9) 5.59 (9) 4.70 (8) 3.07 (7)
Z
0.72040 (3) 0.8113 ( 2 ) ' 0.8148 (2) 0.7109 (2) 0.6392 (2) 0.9049 (2) 0.9126 (3) 0.9991 (3) 1.0787 (3) 1.0733 (3) 0.9861 (3) 0.9099 (2) 0.9808 (3) 1.0700 (3) 1.0902 (3) 1.0215 (3) 0.9319 (3) 0.6889 (2) 0.7243 (3) 0.7063 (3) 0.6505 (3) 0.6157 (3) 0.6344 (3) 0.5249 (2) 0.4701 (3) 0.3633 (3) 0.3087 (3) 0.3613 (3) 0.4682 (3) 0.7810 (3) 0.7534 (3) 0.6494 (3) 0.6097 (3) 0.6907 (3) 0.6996 (3)
"Anisotropically refined atoms are given in the form of the isotropic equivalent displacement parameter defined as (4/3)*[a2*B(l,l) b2*B(2,2) c2*B(3,3) ab(cos -y)*B(1,2) ac(cos @)*B(1,3) bc(cos a)*B(2,3)].
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(t, 2 H , CH2); "B N M R (C,H,CH,, 25 "C) 6 19.5 ppm; MS, P' = m/e 548.2085 (calcd 548.2085), P - 1' = 0.24, P' = 1.0, P 1' = 0.33, P 2+ = 0.10. Reaction of CpCo(C4Ph4BBr)with KOH. A 20-mL Schlenk flask containing 0.026 mmol of CpCoC4Ph4BBr and 6 mL of toluene was cooled to -78 OC, and 0.03 mmol of KOH powder was added. The solution was allowed to warm up to room temperature and was stirred overnight. About 3 more equiv of KOH were placed in the tube, and the solution was stirred for 2 more h till all the starting material was consumed. CTLC with toluene followed by CH2C12was used to separate the products. The yellow band which eluted with CH2CI2was collected. The yield of CpCo(C4Ph4B-OH) is about 50%: 'H N M R (C6D6,25 "C) 6 6.8-8.0 (m, 20 H, Ph), 6 4.52 (s, 5 H, Cp), 6 3.30 (s, 1 H, O H ) ; "B N M R (C6H,CH,, 25 "C) 6 23 ppm; MS, P+ = m/e 508.1404 (calcd 508.1408), P - 1' = 0.23, P' = 1.0, P + 1' = 0.35, P 2' = 0.06; IR (CsF, toluene) ~ ( 0 - H )3625 cm-', v(B-0) 1595 cm-'; UV-vis (toluene) 420 sh, 520 nM. Reaction of CpCoC4Ph4B-OHwith MeOH and Me'SiCI. An excess of MeOH was added to a Schlenk flask containing about 0.01 mmol of CpCoC4Ph,B-OH in toluene. After stirring at room temperature for 48 h, the product was separated from unreacted starting material with a short silica gel column by using toluene, and the orange band eluting first was collected. The yield of CpCoC,Ph4B-OMe was about 50% [IH N M R (C6D6, 25 "C) 6 6.8-8.0 (m, 20 H, Ph), 6 4.67 (S, 5 H, Cp), 6 3.75 (s, 3 H , CHI); MS, P' = m/e 522.1568 (calcd 522.1565), P - 1' = 0.23, P+ = 1.0, P 1' = 0.35, P 2' = 0.07]. A similar sample of CpCoC,Ph,B-OH was cooled to -78 "C before adding 0.1 mL of Me,SiCI and 0.1 mL of Et,N. After warming to room temperature, the mixture was stirred for 10 h monitoring the reaction progress by TLC. The first orange band eluting with toluene from a short silica gel column was collected. The yield of CpCoC4Ph4B-OSiMe' was about 50%: ' H N M R (C6D6, 25 "C) 6 6.8-8.0 (m, 20 H, Ph), 6 4.61 (S, 5 H, Cp), 6 2.33 (s, 9 H, CH,); MS, P' = m/e 580.1803 (calcd 580.1804), P = 1' = 0.22, P+ = 1.0, P 1+ = 0.43, P 2' = 0.12, P 3 + = 0.03. Crystal Structure of 2. An orange, regular parallelepiped crystal of 2 having approximate dimensions of 0.30 X 0.36 X 0.38 mm was mounted in a glass capillary in a random orientation. Cell constants and
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956
J . A m . Chem. SOC. 1989, 111, 956-962
an orientation matrix for data collection were obtained from least-squares refinement, by using the setting angles of 25 reflections in the range 12' < 1281 < 15', yielding the cell parameters given in Table 11. From the systematic absences of hkO, h = 2n; hOl, I = 2n; Okl, k = 2n and subsequent least-squares refinement, the space group was determined to be Pbca (no. 61). Moving-crystal moving-counter background counts were made by scanning an additional 25% above and below this range. Thus the ratio of peak counting time to background counting time was 2:l. The counter aperture was 3.0 mm wide by 4.0 mm high. The diameter of the incident beam collimator was 1.5 mm, and the crystal to detector distance was 21 cm. A total of 12 777 reflections (+h,+k,+/-I) were collected, of which 5735 were unique and not systematically absent. As a check on crystal and electronic stability, four representative reflections were measured every 60 m of X-ray exposure. A total loss in intensity of 5.0% was observed and a linear decay correction was applied. The correction factors on I ranged from 1.000 to 1.026 with an average value of 1.011. Lorentz and polarization corrections were applied to the data. The linear absorption coefficient is 7.0 cm-l for Mo K ( a ) radiation. $ scans of five reflections with 8 between 4.45 and 14.66' showed small variations in intensity. No absorption correction was made, the largest error left untreated thereby being approximately 2% on I. Intensities of equivalent reflections were averaged. The agreement factors for the averaging of the 5269 observed and accepted reflections was 2.7% based on intensity and 2.4% based on F, (Eleven reflections were rejected from the averaging statistics because their intensities differed significantly from their average. This was due to large ratios of [/ut.) The structure was solved by direct methods by using the data up to sin e/x = 0.478 (0.4 of a copper sphere). Using 319 reflections (minimum E of 1.50) and 12 655 relationships, a total of 21 phase sets were produced. The cobalt ion was located from an E-map prepared from the phase set with probability statistics: absolute figure of merit = 1.32, residual = 14.45, and J.0 = 2.291. No other atoms were discerned in this E-map, but the cobalt position was confirmed by a Patterson map. The remaining atoms were located in succeeding difference Fourier syntheses. Hydrogen atom positions were calculated and then added to the structure factor calculations, but their positions were not refined. The position of the hydrogen atom bonding to the boron was taken from a persistent difference peak. The structure was refined in full-matrix least squares where the function minimized was xw(lFol - lFc1)2,and the weight w is defined as: w = 4F,2/u2(F,2) = 1/u2(Fo). The standard deviation on
intensities, u(F,2), is defined as d(F,,) = [S2*(C+ R2B) + @F,2)2]/Lp2, where S is the scan rate, C is the total integrated peak count, Lp is the Lorentz-polarization factor, and the parameter p is a factor introduced to downweight intense reflections. Here p was set to 0.040. Scattering factors were taken from Cromer and Waber.46 Anomalous dispersion effects were included in Fc;47 the values for Af'and A Y were those of C r ~ m e r .Only ~ ~ the 3106 reflections having intensities greater than 2.0 times their standard deviation were used in the refinements. The final cycle of refinement included 316 variable parameters and converged with unweighted and weighted agreement factors of R , = x l F o - Fcl/ CIF,,I = 0.037 and R2 = [ x w ( F o - Fc)2/~wF,2]o.5= 0.040. The standard deviation of an observation of unit weight was 1.04. There were no correlation coefficients greater than 0.50. The final difference Fourier map was judged to be featureless. Plots of xw(lFol - lFc1)2 vs IF,], reflection order in data collection, sin (e)/X, and various classes of indices showed no unusual trends. Final positional parameters are given in Table 111, and thermal parameters appear in the Supplementary Material.
Acknowledgment. W e a r e grateful for t h e support of t h e National Science Foundation a n d t h e Petroleum Research Fund, administered by t h e American Chemical Society, as well as t h e contributions of D. B. Palladino and J. Feilong to early and related studies, respectively. T h e support of the DAAD and the hospitality of Prof. Dr. W . Siebert a t Heidelberg where t h e seeds of these ideas were planted a r e also gratefully acknowledged. W e t h a n k Prof. T . A. Albright for information in advance of publication and appreciate t h e comments of a referee. Supplementary Material Available: Listing of thermal parameters a n d hydrogen a t o m positions a n d t h e packing d i a g r a m (6 pages); listing of observed a n d calculated structure factor amplitudes (29 pages). Ordering information is given on any current masthead page. (46) Cromer, D. T.; Waber, J. T. International Tables f o r X-ray Crysfallography;The Kynoch Press; Birmingham,GB, Vol. IV, 1974; Table 2.2B. (47) Ibers, J. A,; Hamilton, W. C. Acta Crystallogr. 1964, 17, 781. (48) Cromer, D. T.; Waber, J. T. International Tables f o r X-ray Crystallography; The Kynoch Press: Birmingham, GB, Vol. IV, 1974; Table 2.3.1.
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Rate Constants for the Reactions t-C4H9 DX i-C4H9D X (X = Br, I), 295 < T (K) < 384: Heat of Formation of the tert-Butyl Radical Wolfgang Muller-Markgraf, Michel J. Rossi,* and David M. Golden Contribution from t h e D e p a r t m e n t of Chemical Kinetics, Chemical Physics Laboratory, SRI International, M e n l o P a r k , California 9402.5. Received May 23, 1988
Abstract: Absolute values for the rate constants of the metathesis reactions of DX (X = Br, I) with tert-butyl, generated by 351-nm photolysis of 2,2'-azoisobutane, were determined in a low-pressure Knudsen cell reactor using the VLPQ (very low pressure photolysis) technique. For X = Br, the values are 10-8k (M-' s-') = 0.9 and 2.3, and for X = I the values are 2.1 and 3.1 a t 295 and 384 K. The latter are in good agreement with earlier measurements from this laboratory. The former values are lower by a factor of 50 compared to those recently reported a t 295 K and fail to show the negative activation energy found in that work. Correcting by 4 2 for the primary isotope effect and combining with the rate constants for the reverse reactions lead to AHt298(terr-butyl) (kcal mol-') = 9.2 f 0.5 in both cases. First-order heterogeneous loss of tert-butyl radicals on the Teflon-coated surface employed is small ( < 2 s-').
T h e importance of knowledge of thermochemical d a t a for tert-butyl radicals a n d other prototypical radicals (e.g., i-propyl, ethyl, a n d methyl) has been addressed by m a n y a u t h o r s in t h e T h e persisting discrepancies between d a t a derived from
different types of kinetic studies, e.g., from iodination/bromination
( I ) Russell, J. J.; Seetula, J. A.; Timonen, R. S.; Gutman, D.; Nava, D. J. J . Am. Chem. SOC.1988, 110, 3084. (2) Russell, J. J.; Seetula, J. A,; Gutman, D. J . A m . Chem. SOC.1988, 110, 3092.
(3) Rossi, M. J.; Golden, D. M. Int. J . Chem. Kinef. 1983, 15, 1283. (4) Rossi, M . J.; Golden, D. M. Int. J . Chem. Kinef. 1979, I ! , 969. ( 5 ) Choo, K. Y.; Beadle, P. C.; Piszkiewicz, L. W.; Golden, D. M . I n f . J . Chem. Kinet. 1976, 8, 45.
0002-7863/89/1511-0956$01.50/0
investigation^^^^@ R-H
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0 1989 American Chemical Society
X = Br, I
