Kinetics and Mechanism of the Reductive Elimination of Cyclic

Nov 30, 1994 - Facultad de Ciencias Quimicas, Universidad Complutense, 28040 Madrid, Spain ... 24.3 ± 0.2 Real mol, and AS* = ... Tumas, W.; Wheeler,...
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Organometallics 1995, 14, 2039-2046

2039

Kinetics and Mechanism of the Reductive Elimination of Cyclic Titanocene Iminoacyls Juan Campora and Stephen L. Buchwald" Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

Enrique Gutierrez-Puebla" and Angeles Monge Instituto de Ciencia de Materiales, Sede D, CSIC, Serrano 113,28006 Madrid, and Facultad de Ciencias Quimicas, Universidad Complutense, 28040 Madrid, Spain Received November 30,1994@

1,l-Dicyclopentadienyltitanaindan(1) reacts with isocyanides (RNC) to give cyclic iminoacyl complexes 2 (R = t-Bu) and 5a-e (R = p-RC6H.4, R = EtzN, MeO, Me, H, and C1) where the isocyanide has inserted into the Ti-CH2 bond. The X-ray crystal structure of 2 has been determined. Complex 2 crystallizes in a moiioclinic space group P21 with a = 14.353(1), b = 8.556(2), c = 15.652(2) A; ,8 = 88.95(5); and 2 = 4. Compounds 2 and 5a-e decompose in solution with formation of imines and paramagnetic Ti species. The decomposition follows first-order kinetics, with AG* = 24.5 f 0.2 Kcal mol, AW = 24.3 f 0.2 Kcal mol, and AS*= -0.4 f 2 eu in the case of 2. The rate of imine elimination depends on the nature of the substituent on the nitrogen atom, higher rates are observed for electron-withdrawing substituents (Hammet = +1.59 f 0.04 for Sa-e), and the rate is little affected by the solvent or by the presence of PMe3. These results are consistent with a concerted reductive elimination mechanism t h a t initially leads t o a n unstable +imine complex which subsequently converts to the observed products. Scheme 1

Introduction Reductive elimination reactions of metal bis(cr-hydrocarbyl) derivatives have a central role in many stoichiometric and catalytic processes of interest in organic synthesis,l since they lead to the formation of new carbon-carbon bonds. Examples of these reactions are known for virtually all transition metals.2 Late transition metal dialkyl or metallacyclic complexes frequently decompose by reductive e l i m i n a t i ~ n . For ~ their early transition metal counterparts, however, processes such as hydrogen abstraction from the ligand^^^-^ or metalcarbon bond homolysis are more c ~ m m o n . ~Thus, ~-~ while reductive coupling of aryl ligands is well docu, ~ analogous process on mented for group 10 b i a r y l ~the biaryl zirconocene derivatives can only be induced under photochemical and a radical decomposition seems t o be preferred when diaryltitanocenes are Abstract published in Advance ACS Abstracts, February 1, 1995. (1)(a) Davies, S. Organotransition Metal Chemistry: Applications to Organic Synthesis; Pergamon Press: Oxford, 1982. (b) The Chemistry of the Metal-Carbon Bond; Hartley, F. R., Patai, S., Eds.; J. Wiley & Sons: Chichester, 1985;Vol. 3 (Carbon-Carbon bond formation using Organometallic Compounds). (2)Collman, P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry, 2nd ed.; University Science Books: Mill Valley, CA, 1987. (3)(a) Yamamoto, A. Organotransition Metal Chemistry. Fundamental Concepts and Applications; J. Wiley & Sons: Toronto, 1986. (b) Menvin, P. K.; Schanbel, R. C.; Koola, J. D.; Roddick, D. H. Organometallics 1992,11, 2972 and references therein. (4)(a) Erker, G.;Kropp, K. J . A m . Chem. SOC.1979,101,3659.(b) Kropp, K.;Erker, G. Organometallics 1982,I, 1246. (c) Buchwald, S. L.; Watson, B. T. J . A m . Chem. SOC.1986,108, 7411. (d) Rausch, M. D.; Boon, W. H.; Mintz, E. A. J . Organomet. Chem. 1978,160,81.(e) Tumas, W.; Wheeler, D. R.; Grubbs, R. H. J . A m . Chem. SOC.1987, 109, 6182. (0 Erker, G. J . Organomet. Chem. 1977, 134, 189. (g) Buchwald, S. L.; Nielsen, R. B. J . A m . Chem. SOC.1988,110,3171 and references therein. (h) Burk, M. J.; Tumas, W.; Ward, M. D.; Wheeler, D. R. J . A m . Chem. SOC.1990,112,6133. @

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i r ~ - a d i a t e d . ~One-electron ~,~ oxidation of titanacyclobutanes also induces Ti-C bond homolysis, leading to cyclopropane^.^^ Yet, reductive elimination is still a feasible process involving complexes of group 4 a-hydrocarbyl derivatives. The most commonly observed carbon-carbon reductive elimination process involving complexes of group 4 metals is the coupling of an alkyl and an acyl ligand t o yield a ketone, free or complexed to the metallocene m ~ i e t y .The ~ formation of a strong metal-oxygen bond in the intermediate ketone complex (e.g. Scheme 1)has been invoked t o explain the occurrence of this kind of process. More recently, examples of related alkyl-iminoacyl couplings have also been observed.6 These coupling reactions are valuable tools ( 5 ) (a) Fachinetti, G.; Floriani, C. J . Chem. SOC.,Chem. Commun. 1972,654. (b) Wilson, M. E.; McDermott, J. X.; Whitesides, G. M. J . A m . Chem. SOC.1976,98,6529.(c) Erker, G.; Kropp, K. J . Organomet. Chem. 1980,194,45.(d) Erker, G. Acc. Chem. Res. 1984,17,103.(e) Erker, G.; Rosenfeldt, F. J . Organomet. Chem. 1982,224, 29. (0 Negishi, E.; Holmes, S.J.; Tour, J. M.; Miller, J. A.; Cederbaum, F. E.; Swanson, D. R.; Takahashi, T. J . A m . Chem. SOC.1989,111,3336. (g) Grossman, R. B.; Buchwald, S. L. J . Org. Chem. 1992,57, 5803. (h) Fang, Q.Ph.D. dissertation, Massachusetts Institute of Technology, 1990. (i) Cardin, D. J.; Lappert, M. F.; Raston, C. L. Chemistry of Organo-Zirconium and -Hafnium Compounds; Ellis Horwood: Chichester, 1986;pp 304-306.

0 1995 American Chemical Society

Campora et al.

2040 Organometallics, Vol. 14, No. 4, 1995

Table 1. Selected Bond Distances (A) and Angles (deg) of Complex 2

c11

Ti-NI Ti-C1 Ti-C5 NI-CI NI-C10 C13

I/

c3

c4

WC9

ca

Figure 1. Crystal structure of compound 2. for ring formation, in view of the ready availability of metallacyclic compounds of group 4 metals,' although, in some instances, the carbonylation-reductive elimination sequence with titanium or zirconium metallacycles is complicated by side r e a ~ t i o n s . ~ ~ , ~ We are currently investigating the chemistry and possible synthetic applications of titanaindans and other related metallacycles.8 As a result of this research, we have discovered that the titanium metallacycles of the type 2, which can be readily prepared by reaction of dicyclopentadienyltitanaindan (1) with a suitable isocyanide, cleanly undergo reductive elimination, providing the corresponding imines as the only observed organic product. In order to gain a deeper understanding of the reductive elimination reaction in these iminoacyls and the related acyl complexes, we have studied this process in detail. The results of this study are reported herein.

Results Treatment of diethyl ether solutions of red dicyclopentadienyltitanaindan (1)with tert-butyl isocyanide at room temperature causes an immediate color change to pale yellow. Concentration and cooling of the reaction mixture affords yellow crystals of the iminoacyl product 2 in high yield (eq 1). The 'H NMR spectrum of 2 t-Bu

displays a single resonance at 6 0.91 that integrates for nine hydrogens, indicating that only 1equiv of isocya(6) (a) Durfee, L.D.; Fanwick, P. E.; Rothwell, I. P. Organometallics 1990,9, 75. (b) Chamberlain, L. R.; Staffey, B. D.; Rothwell, I. P. Polyhedron 1984,8, 351. (c) Hessen, B.; Blenkers, J.; Teuben, J. H.; Helgesson, G.; Jagner, S. Organometallics 1989,8, 830. (d) Berk, S. C.; Grossman, R. B.; Buchwald, S. L. J . A m . Chem. SOC.1993,115, 4912. (e) Davis, J . M.; Whitby, J . M.; Jexa-Chamiec, A. Tetrahedron Lett. 1992,33,5655. (7) (a) Buchwald, S. L.; Nielsen, R. B. Chem. Reu. 1988,88, 1047. (b) RajanBabu, T. V.; Nugent, W. A.; Taber, D. F.; Fagan, P. J. J . A m . Chem. SOC.1988,111, 7128. (c) Knight, K. S.; Waymouth, R. M. J . A m . Chem. SOC.1991,113,6268 and references therein. (8) Campora, J.; Buchwald, S. L. Organometallics 1993,12,4182.

C I -Ti-C5 NI-Ti-CS NI -Ti-Cl Ti-CI-C2

2.148(2) 2.056(2) 2.229(2) I .24 1(3) 1.496(3)

c1-c2 C2-C3 c3-c4 c4-c5

77.8(1)

C1-N1-CIO NI-CI-C2 Cp I -Ti-Cp2 Ti-N1-CIO

1 I l.7(1)

34.3(1) 143.8(2)

I .483(3) I .S21(4) 1.520(3) I .410(3)

131.4(2) 139.1(2) 131.7(1) 1S9.7( 1)

nide has been incorporated. An absorption at 1725 cm-' in the IR spectrum and a signal for an imino carbon atom at 6 219.2 ppm in the 13C NMR spectrum are - ~1,9 ~ 'H characteristic of the iminoacyl g r o ~ p . l ~In NMR signals for the methylene groups appear at 6 3.23 (t,2H) and 6 1.45 (t,2H). The corresponding methylene groups in 2 give rise to signals at 6 2.76 (t, 2H) and 6 2.53 (t, 2H). The chemical shifts for the corresponding methylene 13C NMR resonances for 2 (6 37.4 and 58.6, respectively) are also very similar to those for 1 (6 38.8 and 60.1). These data do not allow the unambiguous demonstration of the regiochemistry of the isocyanide insertion in 1. This was determined by treatment of compound 2 with methanesulfonic acid, followed by 30% aqueous hydrogen peroxide to afford the amide 3 in 85% isolated yield. This result proves that the isocyanide insertion has taken place into the Ti-CH2 bond (eq 2). 1-Bu

We have also addressed the question of whether the iminoacyl group coordinates to the metal in a dihapto (q2)or monohapto (VI) fashion. Although the spectroscopic features of the iminoacyl fragment can be used to ascertain the coordination mode within a particular class of compounds,loathere is not a generally applicable set of criterialOJ' to unambiguously make this determination. Therefore, an X-ray structure determination was undertaken in order to complete the characterization of 2. Figure 1 shows an ORTEP diagram for 2, which clearly indicates that the iminoacyl ligand is bound in a q2 fashion. The Ti-C, Ti-N, and C=N distances (Table 1)in 2 are very similar to those found The for [CpzTi(q2-C(=NBut)Me)(C~NBut)l+[BP~l-.11 cyclic nature of complex 2 forces the nitrogen group to adopt a "N-outside" coordination mode, which is disfavored in open-chain metallocene acyl and iminoacyl ~omp1exes.l~ When a benzene solution of 2 is allowed t o stand a t room temperature for 2 days or is gently heated, its color changes to dark brown. From these solutions it is possible to isolate imine 4. If this transformation is monitored by lH NMR, the gradual disappearance of ( 9 )Erker, G.; Korek, U.; Petrenz, P.; Rheingold, A. L. J . Orgunomet. Chem. 1991,421,215. (10)Carmona, E.; Palma, P.; Paneque, M.; Poveda, M. L. Organometallics 1990,9 , 583. (11)Bochmann, M.; Wilson, L. M.; Hursthouse, M. B.; Short, R. Organometallics 1987,6 , 2556. (12) Durfee, L. D.; Rothwell, I. P. Chem. Reu. 1988,88, 1059. (13) Tatsumi, K.;Nakamura, A.; Hofmann, P.: Stauffert. P.: Hoffmann, R. J . Am. Chem. SOC.1985,107,4440.

Reductive Elimination of Cyclic Titanocene Iminoacyls Table 2. Crvstal and Refinement Data formula M, crystal system space group (1.

A

h, 8, c.

A

P

v, A'

Z F(OO0)

e (calcd), g cm-j temp, "C

p , cm-' cryst dimens, mm diffractometer radiation scan technique data collected unique data unique data (0Z 3u((1) R(int), 92 std rflns variables RF. 92 RWF,8 (unit weights) average shifderror

TiC23H27N1 365.37 monoclinic P2,lc 14.353(I ) 8.556(2) 15.652(2) 88.95(5) 192 I .8(5) 4 776 I .26 21 4.43 0.2 x 0.2 x 0.2 Enraf-Nonius CAD4 graphite-monochromatic Mo K a (1= 0.7 1069 A) R128 1 < 300 e; (-20,0,0) to (20~2.22) 5580 4072 1.o 3/78 rflns 222 3.8 4.2 0.008

Table 3. Atomic Parameters for TiCzH2,N" atom Ti NI C1 C2 C3 C4 C5 C6 C7 C8 C9 CIO Cl1 C12 C13 C21 C22 C23 C24 C25 C31 C32 C33 C34 C35

Xlu

Ylh

uc

Ut,

0.29399(2) 0.27993(12) 0.20900(15) 0.1 1907(18) 0.10739(19) 0.10973(15) 0.17410(14) 0.16741(17) 0.10495(18) 0.04296(18) 0.04539(17) 0.30448(17) 0.35369(24) 0.22824(22) 0.39759(19) 0.20688(19) 0.29990(21) 0.35571(18) 0.29748( 19) 0.20602(17) 0.36288(23) 0.42336(27) 0.45657(19) 0.41917(25) 0.36219(20)

0.37505(4) 0.53660(20) 0.45484(26) 0.42746(33) 0.25133(35) 0.16489(27) 0.19694(25) 0.09798(29) -0.02442(32) -0.05284(34) 0.04172(34) 0.66153(27) 0.58563(36) 0.78550(34) 0.73378(32) 0.57174(30) 0.62365(31) 0.52109(35) 0.40424(32) 0.43655(31) 0.18109(46) 0.30510(44) 0.31741(41) 0.19822(48) 0.11596(33)

0.79171(2) 0.89619(10) 0.88951(13) 0.93534(16) 0.94302(16) 0.85812(14) 0.79123(13) 0.71974(16) 0.71372(18) 0.77970(20) 0.85068(18) 0.95879( 14) 1.04653(16) 0.95903(22) 0.93147(17) 0.71466(16) 0.71849(15) 0.67163(16) 0.63863( 14) 0.66459( 14) 0.88678(21) 0.88305(27) 0.80122(30) 0.75490(20) 0.80752(25)

301(1) 356(5) 397(6) 552(8) 566(9) 418(7) 368(6) 487(8) 568(9) 616(10) 562(9) 449(7) 695(11) 708(20) 558(9) 517(8) 563(9) 556(9) 518(8)

472(7) 791(13) 921(25) 826(22) 819(13) 691(19)

'(Coordinates and thermal parameters are defined as Ueq = 'I&&:i(/tju,*aj*u,a, x IO4.

the signals for 2 occurs, concomitant with the appearance of new resonances for 4 (eq 3). No new cyclopen-

2

4

tadienyl signals are observed; any "titanocene" species which are presumably formed could not be is01ated.l~ (14)Bercaw, J. E.; Marvich, R. H.; Bell, L. G.; Britzinger, H. H. J . Am. Chem. SOC.1972, 94, 1219.

Organometallics, Vol. 14, No. 4, 1995 2041 If the reaction is run in the presence of MezSZ, however, CpzTi(SMe)2l5is formed in high yield, without affecting the rate of the elimination reaction. In order to study the rate of this reductive elimination reaction, compound 2 was generated in situ in a n NMR tube, and its conversion to 4 was monitored by 'H NMR. The reaction follows first-order kinetics for > 3 half-lives. Additionally, the rate constant does not change appreciably when the initial concentration of iminoacyl complex was varied from 0.07 to 0.3 M (in benzene, k = (1.1f 0.1) x s-l a t 65.7 "C). The solvent dependence of this transformation has also been investigated. A small solvent effect was found when the reaction rate was measured in four different solvents (benzene, chloroform, tetrahydrofuran, and acetonitrile) at 65 "C; the elimination being somewhat slower in polar solvents (k(benzene) = (1.14 f 0.02) x s-l, Macetonitrile) = (4.48 f 0.05) x s-l, see Experimental Section for details). To ascertain whether the transformation of 2 to 4 was an example of a ligand-induced reductive e l i m i n a t i ~ n , ~ ~ , ~ ~ the reaction was run in the presence of added PMe3. A small decrease in the rate was observed in the presence of this ligand. For example, the rate constant for the reaction in benzene at 59.7 "C was (4.5 f 0.1) x s-l. Addition of a 2- or a 5-fold excess of PMe3 had almost no effect, and a 10-fold excess (ca. 1.43 M) caused a very small rate decrease (k = (3.2 f 0.1) x s-l). Thus, there is little or no effect in the reductive elimination process by added ligand in this system. Figure 2a shows one set of measurements of the temperature dependence of the elimination rate of compound 2. Activation parameters deduced from an Eyring plot (Figure 2b, rates averaged from three runs) are AG* = 24.5 f 0.2 KcaVmol, @ = 24.3 f 0.2 Kcall mol, and AS* = -0.4 f 2 eu. Rate measurements were made a t temperatures from 311.5 K (38.5 "C) to 354 K (81 "C). Our interpretation of these results will be discussed in a later section of this paper. As previously mentioned, the reductive elimination of some bis(a-hydrocarbyl) group 4 derivatives can be photochemically induced. Since all the kinetic measurements described herein were performed in the dark, in a n NMR probe, we decided to qualitatively determine whether there is any influence of light on the reductive elimination of 2. Two NMR tubes containing samples of a solution of 2 in benzene were placed in a water bath at 20 "C. One of these was covered with aluminum foil and the other exposed to the light of a sun lamp. In both cases the progress of the reaction was monitored by lH NMR. The rate of reductive elimination observed for the sample exposed t o the light ( t l i z ca. 11 h) was approximately twice that of the foil-covered sample ( t l l z ca. 20 h). This shows that, under these conditions, photochemical and thermal elimination reactions proceed a t roughly comparable rates. In order to evaluate the influence of electronic factors, we have prepared and spectroscopically characterized a series of iminoacyl complexes (Sa-e) derived from the reaction of 1with five different para-substituted phenyl isocyanides (Scheme 2). The spectroscopic features of these complexes are similar to those of 2 (see Experimental Section), except for the chemical shifts of the 13C resonances of one of the methylene groups. Thus, (15)Abel, E. W.; Jenkins, C. R. J . Organomet. Chem. 1968,14,285.

Campora et al.

2042 Organometallics, Vol. 14, No. 4, 1995 4

I

1 /

1

/ 354.0K/

344.0K

n so

p = + 1.59 f 0.04 0 00

3

2 -0 7 5

-0 25

0 00

0 25

Figure 3. Hammett plot of the rates of reductive elimination of 5a-e.

31 1.5K 0 0

1000

2000

3000

41

Time (rec)

7

i,

-7 0.0028

0.0029

0.0030

0.0031

0.0032

0.0033

Figure 2. Reaction rates for the reductive elimination of

3 at different temperatures (a) and the resulting Eyring plot (b).

cp*TB

Scheme 2=

p-RCsH4NC_

1

1 t "CpZTi'

5a-t a

so

(T

1

e,

-0

"Q,

ea-e

Legend: a,R = Et2N; b,R = MeO; c, R = Me; d, R = H; = C1.

R

while the signal for one methylene carbon appears a t 37 ppm in 2 and in Sa-e, the other methylene signal shifts from 58.6 ppm in 2 to ca. 29 ppm in 5a-e. Although all other features, including the chemical shifts of the NMR signals of the phenylene group remain unchanged on passing from 2 to 5a-e, we chose to confirm the regiochemistry of aryl isocyanide insertion. Therefore, the structure of one of these compounds, 5b, was unequivocally established by a procedure analogous t o the one used for 2. In this manner, N44 methox-

yphenyl)-3-phenylpropionamide(7)was formed from 5b. On the basis of the spectroscopic similarity of the series 5a-e and the analogous IR and I3C features of the iminoacyl group of 5a-e and 2, we conclude that all these cyclic iminoacyl complexes must be structurally similar. The rates of the reductive elimination reactions of the (N-ary1imino)acylcomplexes are appreciably faster than those for 2. This instability prevented the isolation of these compounds in an analytically pure form. They were, however, all characterized by 'H and 13C NMR and IR spectroscopy. Fortunately, the reaction of 1 with the corresponding isocyanides is still fast as compared to the reductive elimination of Sa-e. Samples of paleyellow 5a-e of purity higher than 90% (as deduced from their 'H NMR spectra using ferrocene as a n internal standard) can be prepared by reacting cold (-40 "C) solutions of 1 in diethyl ether with the isocyanide, collecting the precipitated compound and washing it with cold hexane. The general features of the decomposition of 5a-e are similar to those seen for 2. In all cases they react by a process which exhibits first-order kinetics for a t least 3 half-lives. No significant differences were found when the decomposition of 5c was monitored starting with different initial concentrations (0.14, 0.28, and 0.7 MI. Although the formation of the imine is apparently not quantitative in benzene-& (presumably due t o complexation or reaction with some paramagnetic titanium species), quantitative yields of imine, as determined by lH NMR, were observed to form when the reaction was carried out in CDzClz or CD3CN, even when no new 'H NMR signals attributable to the cyclopentadienyl group of diamagnetic titanium products could be observed. For this reason, CDzClz was the solvent used for quantitative runs. When 5c was allowed to decompose in benzene-& in the presence of Me&, quantitative production of CpzTi(SMe)z and the imine was observed. Figure 3 is a Hammett plot of the elimination rate (averaged over duplicate runs) of 5a-e at 23.5 "C. As can be seen, the transformation is slowed by electron-releasing substituents (5a,b)and accelerated by electron-withdrawing substituents (5e). A e value of +1.59 f 0.04 was found. Discussion Reductive elimination appears to be a relatively disfavored process for early transition metal bis(ahydrocarbyls).2b However, reductive elimination reactions of early transition metal acyl-alkyl or iminoacylalkyl complexes are more ~ o m m o n . Nearly ~ , ~ all known examples of well-characterized acyl and iminoacyl de-

Reductive Elimination of Cyclic Titanocene Iminoacyls

Organometallics, Vol. 14, No. 4, 1995 2043

Scheme 3”

r

P Lqi.$j %y‘

R

11

a Legend: path A, “in-pl,ane” reductive elimination to yl-imine complex; path B, “in-plane” reductive elimination to y2-imine complex (note: n-system of the imine is orthogonal to the LUMO of the titanocene fragment); path C, reductive elimination via nucleophilic attack.

rivatives of group 4 and other early transition metals display q2-typecoordination.12 Therefore, we have first considered whether the special reactivity associated with this ligand combination could partially explain the facile elimination that occurs in acyl- or iminoacylalkyl complexes. It is known that the product of the elimination from a number of titanium and zirconium acyl- and iminoacyl-hydrocarbyl derivatives is the corresponding k e t ~ n e ~ or~imine6a-c -~ either free or coordinated t o the metal (Scheme 1). On this basis, Erker has proposed that, in the case of the acyl derivatives, the stability gained by the formation of a strong metal-oxygen bond may overcome the unfavorable energetic contribution of the reduction of the metallic center by two units.5d It should be noted that in this process only a formal reduction of the zirconium center takes place, since these ketone and imine complexes are better described as metalla~xiranes~j or metallaaziridines,6J6bthan as n-complexes. We have not been able to detect imine complexes during the decomposition of 2 or 5a-e even when the reaction was run in the presence of ligands (PMe3)16or trapping agents (MeCN, MeOH), probably due to the low stability of such complexes. In contrast, reductive elimination of aryloxide-supported (iminoacy1)alkyltitaniumcomplexes has been observed by Rothwell,6a-dand Whitby has reported the generation and in situ trapping of zirconocene complexes of imines formed by reductive elimination processes .6e Formation of a n q2-ketone ligand from Zr-acyl complexes has been shown to be promoted by aluminum alkyls.17 Grubbs has attributed this to the increase in the electrophilic character of the carbon atom induced upon coordination of the acyl oxygen to aluminum, which facilitates the migration of the alkyl to the acyl group. In this case the carbon-carbon bond-forming (16) (a) Buchwald, S. L.; Wannamaker, M. W.; Watson, B. T. J.A m . Chem. SOC. 1989, 111, 776. (b) Buchwald, S. L.; Watson, B. T.; Wannamaker, M. W.; Dewan, J. C. J.Am. Chem. SOC.1989,111,4486. ( c )Coles, R. J.; Whitby, R. J.; Blagg, J. Synlett 1989, 4486. (d) Coles, N.; Harris, M. C. J.; Whitby, R. J.; Blagg, J. Organometallics 1994, 13, 190. (17) Waymouth, R. M.; Grubbs, R. H. Organometallics 1988,7,1631.

process can best be described as a nucleophilic attack of the methyl group at the carbon atom of the electrophilic carbonyl. Here, the intermediate 8 should be flexible enough to allow for attack to occur in a stereoelectronically favorable manner. l9

6 0

a In contrast to 8, the rigidly planar structure of complexes 2 and 5a-e renders a n analogous attack stereoelectronically unfavorable (Scheme 3, path C). A concerted, “in-plane”,reductive elimination appears to more feasible. Such a process is consistent with the nearzero value for AS*. Although the meaning of AS* values is often unclear, the simplicity of this system, in particular the absence of ligand or solvent binding processes (i.e., association or dissociation) makes the interpretation of this number for AS* less ambiguous. Thus, we can surmise that there is a small degree of structural change on going from the ground state to the transition state (Scheme 3, path A) in these reactions of 2 and 5a-e. Since the ligand is generated in a position in which the n-(C=N) orbitals of the imine are orthogonal to the LUMO of the Cp2Ti fragment, intermediate 9 should be of high energy. As a consequence, we postulate that the initial product of the reductive elimination should be an ql-imine complex (10). The fate of 10 depends on the conditions under which it is generated: it could either react (e.g., with Me2S2) or rearrange to the more stable complex 11. (18)While there is no effect on the reaction rate by the addition of ligands such as PMe3 (2 and 5a-e are 18-electron complexes), the electronically less saturated species (ArO)2Ti(v2-C(=N-t-Bu)CH2Ph)(CH2Ph) undergoes a ligand-induced reductive elimination to give (A~O)~T~(I~~-C(=N-~-B~)(CH~P~)~)(L).~~ In the absence of added ligand, no reductive elimination is seen. (19) Deslongschamps, P. Stereoelectronic Effects in Organic Chemistry; Pergamon: Oxford, 1983.

Campora et al.

2044 Organometallics, Vol. 14, No. 4, 1995 Since a nucleophilic attack on the iminoacyl carbon is not likely in our system, the accelerating effect of electron-withdrawing substituents is not easily explicable on the basis of the increase of the electrophilic character of the iminoacyl carbon. In order to explain the observed reaction rates, it is necessary to consider both ground-state and transition-state effects. In the ground state, X = electron-donating substituent should be stabilizing, causing enhanced interaction of the nitrogen of the y2-iminoacyl to the titanium center. Thus, the order of stability would be 5a > 5b > 6c > 5d > 5e. Therefore, ground-state effects should tend to accelerate the rate of reductive elimination when electron-withdrawing substituents are present. In the transition state, there are two opposing effects. Electron-withdrawing substituents can stabilize the nascent Ti(I1) center by decreasing the electron density of the complex. Thus, an enhanced reaction rate would be expected. In contrast, the 14-electron CpzTi fragment is being stabilized by the +imine ligand,18 and for this reason, better electron-donor substituents should be stabilizing and contribute to lower the energy of the transition state. These two factors oppose one to another and might be expected to cancel each other to some extent. If this is the case, ground-state effects would be expected to dominate.

Conclusions Cyclic titanocene iminoacyl complexes 2 and 5a-e readily undergo reductive elimination to yield the corresponding imines. The reaction rate is controlled by the nature of the substituents on the nitrogen atom, probably due to ground-state factors. In contrast with the reductive elimination in related Zr complexes induced by aluminum alkyls, we believe that the reaction takes place in a concerted manner, giving rise to a transient +imine complex which subsequently converts to the observed products. Stereoelectronic considerations lead us to believe that this result is relevant to the carbonylation of zircona and titanacycles to ket o n e ~ . Here ~,~~ an yl-ketone complex may be a transient intermediate prior t o the formation of a more stable r2 complex.

Experimental Section All manipulations were performed using either standard Schlenk techniques under argon or a Vacuum Atmospheres dry box under NP,unless stated otherwise. Argon used in the Schlenk work was purified by passage through columns of BASF-RS-11 (Chemalog) and Linde 4 A molecular sieves. Nuclear magnetic resonance (NMR) spectra were recorded on a Varian XL-300 spectrometer. Infrared (IR) spectra were recorded on a Mattson Cygnus Starlab 100 Fourier transform spectrometer. Only significant IR bands are reported. Electronimpact high-resolution mass determinations (HRMS) were recorded on a Finnegan MAT system 8200 spectrometer. Elemental analyses were performed by Desert Analytics, Tucson, AZ. Spinning plate chromatography was performed with a Harrison Research Chromatotron using 2 and 4 mm thick plates poured with Merck Kieselgel60 PF25A silica gel. Tetrahydrofuran, benzene, and diethyl ether were dried and deoxygenated by refluxing over sodiumhenzophenone ketyl followed by distillation under argon or nitrogen. Hexane was (20) Swanson, D.R.; Rousset, C. J.; Negishi, E.; Takahashi, T.; Seki, T.; Saburi, M.; Uchida, Y. J . Org. Chem. 1989,54, 3521.

deolefinated by standard procedures and then stored over CaH2. The deolefinated hexane thus obtained was dried and deoxygenated by refluxing over sodiumbenzophenone ketyl followed by distillation under nitrogen. Toluene was dried over sodium followed by distillation under nitrogen. Dichloromethane was dried by distillation from CaH2 under nitrogen. Deuterated solvents were purchased from Cambridge Isotopes Co. Benzene-& and THF-ds were vacuum transferred from sodiumhenzophenone ketyl. CDC13 and CDzCl2 were vacuum transferred from CaH2. CD3CN was vacuum transferred without further purification. tert-Butyl isocyanide was purchased from Aldrich and used as received. 1,l-Dicyclopentadienyl-1-titanaindane (1),9p (diethylamino)phenyl,p-methoxyphenyl, p-methylphenyl, phenyl, and p-chlorophenyl isocyanide were prepared according published procedures.21 Preparation of Iminoacyl2. A solution of compound 1 (282 mg, 1 mmol) in diethyl ether (30 mL) was treated with tert-butyl isocyanide (0.11 mL, 1 mmol). The color of the solution immediately changed from red to bright yellow. Concentration in vacuo to ca. 5-10 mL and cooling to -20 "C yielded 230 mg of yellow crystals of 2 which were collected by filtration and washed with cold hexane. The mother liquor was concentrated to ca. 3 mL and was mixed with an equal volume of hexane. This solution was cooled to -20 "C overnight, to yield an additional 50 mg of compound 2 which was isolated by filtration and washed with cold hexane. Overall yield, 280 mg (77%). IR (Nujol mull) 1725 cm-l (v(C=N)). 'H NMR (300 MHz, C6D6, 20 "C) 6 0.91 (s, 9H, C(CH3)3),2.53 (m, 2H, CHz), 2.76 (m, 2H, CHz), 5.26 (s, 10H, Cp), 7.18 (m, 2H, CH,,,,), 7.31 (m, l H , CH,,,,), 7.51 (m, l H , CH,,,). l3C {lH} (75 MHz, C&, 20 "c) 6 29.3 (C(CH3)3),29.6 (C(CH3)3),37.4 (CH2), 58.6 (CH21, 106.6 (Cp), 122.0, 123.1, 129.8, 144.4 (CHarom), 145.2, 175.7 (Cquat am,), 219.2 (C=N). Anal. Calcd for C23H27NTi: C, 75.61; H, 7.45. Found: C, 75.44; H, 7.35. Conversion of Iminoacyl 2 to N-tert-Butyl-3-phenylpropionamide (3). A solution of 1 (282 mg, 1 mmol) in 30 mL of THF was treated with t-BuNC (0.11 mL, 1 mmol) a t room temperature. After 5 min, MsOH (96 mg, 1 mmol) was added. The mixture was allowed t o stir for 5 min and 35% aqueous HzOz (0.25 mL) was added. After 10 min, a yellow precipitate had formed. The mixture was concentrated in vacuo and filtered through a pad of silica gel. Purification by spinning plate chromatography using a 4:6 diethyl ether/ pentane mixture yielded 144 mg (83%) of the title compound. IR (Nujol mull) 3311 (v (N-HI), 1639 (amide I), 1548 (amide 11). 'H NMR (300 MHz, CDC13 20 "C) d 1.24 (s, 9H, C(CH3)3), 2.34 (t, J = 7.8 Hz, 2H, CHz), 2.89 (t, J = 7.8 Hz, 2H, CHz), 13C {lH} (75 MHz, 5.05 (bs, l H , NH), 7.20 (m, 5H, CHaKom). CDC13, 20 "C) 6 29.1 (C(CH3)3),32.2 (CHz), 32.6 (CH2), 51.4 (C(CH&), 126.5 (CHarom), 128.8 (4 overlapping CH,,,,), 141.5 ,),,,, 171.8(C=O). HRMS calcd for C13H19NO 205.1466, (CqUat found 205.1465. Conversion of 2 to N-tert-Butyl-1-indanimine 4. A solution of compound 1 (601 mg, 1mmol) in THF (20 mL) was treated with tert-butyl isocyanide (0.24 mL, 2.13 mmol), and the resulting solution of 2 was heated at 60 "C for 2 h. The solvent was evaporated in vacuo and the residue treated with chloroform (10 mL). The resulting red solution was evaporated and the residue was extracted with 2 x 15 mL of hexane. The solution was filtrated and evaporated and the residue extracted again with hexane (2 x 15 mL) in order to eliminate as much titanium-containing solids as possible. After removal of the hexane, the residue was distilled using a Kugelrohr apparatus. The colorless liquid thus obtained was a mixture of 4 and its enamine tautomer 4' in an 2:l ratio. Yield, 213 mg (53%). IR (liquid film) 1715 (vC=N). Imine 4: 'H NMR (300 MHz, C6D6 20 " c ) d 1.42 ( 8 , 9H, C(CH3)3),2.78 (m, 2H, CHz), 3.35 (m, 2H, CHz), 7.2-7.5 (m, 3H, CH,,,,), 7.83 (d, J = (21)Ugi, I.; Meyr, P. Chem. Ber. 1960,93, 239.

Reductive Elimination of Cyclic Titanocene Iminoacyls 7.4 Hz, CHarom). I3C {'H} (75 MHz, C6D6, 20 "C) 6 2.89 (CHz), 29.9 (C(CH3)3),30.1 ( C H 2 ) , 55.3 (C(CH&), 122.4 (CHarom), 125.0 (CHarom), 126.6 (CHarom),130.4 (CHarom), 141.9 (Cquat am,), 147.7 (Cquat 168.9 (C=N). Enamine 4': 'H NMR (300 MHz, CsDs 20 "C) 6 1.23 (s,9H, C(CH3)3), 3.4 (d, J = 2.0 Hz, 2H, CHz), 5.21 (t,J 2.0 Hz, l H , =CHI, 7.2-7.5 (m, 3H, CHarom). 13C {'H} (75 MHz, C6D6, 20 "C) 6 29.9 (C(CH&, 36.1 (CH21, 50.6 (C(CH3)3),98.2 (CHI, 115.8 (CHamm),123.5 (CHamm),124.6 ( m a m m ) , 125.4 (CHamm),142.0 (Cquat), 142.4 (Cquat), 143.3 (Cquat). HRMS M+ calcd for C13H19N 187.1360, found 187.1359. Preparation of 5a-e. A solution of 1 (282 mg, 1 mmol) in diethyl ether (20-30 mL), cooled to -40 "C, was treated with a solution of 1 mmol of the corresponding isocyanide in diethyl ether (10 mL). After stirring for 5-15 min at -40 "C, a yellow precipitate of the iminoacyl complex had formed. In the case of 5d, precipitation was accomplished by addition of 35 mL of hexane and cooling the resulting solution to -80 "C overnight. The compounds thus obtained have moderate thermal stability in the solid state, their color darkening upon standing at room temperature for several hours. The purity of the samples was higher than 90% as estimated by 'H NMR, using an internal standard of ferrocene. Yields: 5a, 323 mg, 71%; 5b, 212 mg, 51%; 512,260 mg, 65%; 5d, 196 mg, 20%; 5e, 260 mg, 62%. 5a: IR (Nujol mull) 1703 cm-I ( v (C=N)). H NMR (300 MHz, CD2C12, -20 "C) 6 1.20 (t,J = 7.0, 6H, NCHZCH~), 2.99 (m, 2H, CHd, 3.07 (m, 2H, CH2), 3.42 (9, J = 7.0, 4H, NCH2CH31, 5.42 (s, 10H, Cp), 6.75 (m, 2H, CHarom), 6.86 (m, 2H, CHarom), 7.50 (m, lH, CHarom),7.38 (m, 2H, CHarom), 7.46 (m, l H , CH,,,). 13C {'H} (75 MHz, CDC13, -20 "C) 6 12.2 (NCHzCHs), 28.8 (CHz), 37.2 (CH21, 44.2 (NCHZCH~), 106.0 (Cp), 110.1 (2 CH,,,,), 121.4 (CHarom), 122.1 (CHarom), 124.6 (2 CHarom), 127.2 (CHarom),130.0 (Cquat arom), 144.2 (CHarom),145.0 (Cquat arom), 146.1 (Cquat arom), 175.6 (Cquat arom), 218.8 (C=N). 5b: IR (Nujol mull) 1717 cm-' (v (C=N)). H NMR (300 MHz, CDzC12, -20 "C) 6 3.02 (m, 2H, CH2), 3.09 (m, 2H, CH2), 3.87 (s, 3H, ocH3),5.42 (s, 10H, Cp), 6.84 (m, lH, CHaro,), 7.05 (m, 2H, CH,,,,), 7.20 (m, lH, CH,,,,), 7.63 (m, lH, CHarom), 7.82 (m, 3H, CHarom).I3C {'H} (75 MHz, CDC13, -20 "C) 6 28.9 (CH2), 37.0 (CH2), 55.2 (OCH3), 106.0 (Cp), 114.1 (2 CH,,,), 121.4 (CHarom), 122.1 (CHarom), 124.1 (2, CH,,,,), 129.1 (CHarom), 135.1 (Cquat arom), 144.1 (CHarom), 144.7 (Cquat arom), 158.1 (Cquat arom), 175.1 (Cquat arom), 225.6 (C=N). 5c: IR (Nujol mull) 1712 cm-' ( v (C-N)); H NMR (300 MHz, CD2C12, -20 "C) 6 2.44 (s, 3H, CH3), 3.02 (m, 2H, CH2), 3.08 7.12 (m, (m, 2H, CHd, 5.45 (s, 10H, Cp), 6.87 (m, 2H, CH,,,), lH, CHaro,), 7.34 (m, 4H, CHaro,), 7.80 (m, lH, CHarom).I3C {'H} (75 MHz, CDC13, -20 "C) d 21.1 (CH3), 29.3 (CH2), 37.1 (CHz), 106.1 (Cp), 121.5 (CHarom),122.3 (CHarom),122.7 (2 CHarom), 129.4 (CHarom), 129.9 (2 CHarom), 137.2 (Cquat arom), 139.3 (Cquat arom), 144.2 (CHarom), 144.8 (Cquat arom), 175.2 (Cquat arom), 228.5 (C=N). 5 d IR (Nujol mull) 1712 cm-' ( v (C=N)). H NMR (300 MHz, CD2C12, -20 "C) 6 3.01 (m, 2H, CH2), 3.10 (m, 2H, CHz), 5.46 (s,10 H, Cp), 6.86 (m, 2H, CHarom), 7.11 (m, lH, CHarom), 7.39 (m, 4H, CH,,,,), 7.54 (m, 2H, CHarom).I3C {'H} (75 MHz, CDC13, -20 "C) 6 29.4 (CH2), 37.1 (CH21, 106.2 (Cp), 121.5 (CHarom), 122.4 (CHamm), 122.8 (2 CH,,,,), 127.2 (CHarom), 129.2 (CHarom), 129.4 (2 CHarom), 144.1 (CHarom), 141.7 (Cquat arom), 144.8 (Cquat arom), 175.8 (Cquat arom), 230.8 (C=N). 5e: IR (Nujol mull) 1699 cm-' (v (C=N)). 'H NMR (300 MHz, CD2C12, -20 "C) 6 3.00 (m, 2H, CHd, 3.06 (m, 2H, CHd, 5.45 (s,10 H, Cp), 6.85 (m, 2H, CH,,,,), 7.10 (m, lH, CHarom), 7.34 (m, 3H, CH,,,,), 7.50 (m, 2H, CHarom).13C {'H} (75 MHz, CDC13, -20 "C): 6 29.4 (CHz), 36.9 (CH2), 106.3 (Cp), 121.5 (CHarom),122.4 (CHaram), 123.9 (2 CH,,,,), 129.4 (2 CH,,,,), 141.8 (Cquat am,), 144.0 ( C i i a m m ) , 132.4 (CHamm),140.2 (Cquat a"), 144.5 (Cquat arom), 174.8 (Cquat am,), 233.0 (C=N). N-Aryl-1-indanimines(Aryl = p-(Diethy1amino)phenyl, 6a; p-Methoxyphenyl, 6b; p-Tolyl, 6c; Phenyl, 6d; p-Chlorophenyl,6e). A solution of 1 (282 mg, 1 mmol) in methylene chloride (20 mL) at -20 "C was treated with the

Organometallics, Vol. 14, No. 4, 1995 2045 corresponding isocyanide (1 mmol). The mixture was allowed to stir a t room temperature for 14 h, the volume was reduced to ca. 1 mL, and the solution was eluted through a column of silica gel using a mixture of hexanehiethylamine (1:l). The solvent was removed, and the residue was purified by spinning plate chromatography using hexanefNEt3 mixtures as the eluent. Yields: 6a, 203 mg, 73%; 6b, 185 mg, 77%; 6c, 65 mg, 29%; 6d, 200 mg, 97%; 6e, 184 mg, 77%. 6a: IR (Nujol mull) 1636 cm-' (v (C-N)). 'H NMR (300 MHz, CDC13, 20 "C) 6 1.15 (t, J = 7.0, 6 H, NCHZCH~), 3.32 (m, 2H, CHd, 3.06 (m, 2H, CH21, 3.32 (9, J = 7.0, 4H, NCH2CHd, 6.71 (m, 2H, CH,,,,), 6.98 (m, 2H, CHarom), 7.35 (m, 2H, CHarom), 7.41 (m, 1H, CHarom), 7.93 (d, J = 7.6, lH, CHarom). I3C {'H} (75 MHz, CDC13,20 "C) 6 12.5 (NCH2CH3),28.3 ( C H 2 1 , 30.1 (CHd, 44.5 (NCH&H3),112.6 (2 CHamm), 122.2 (2 CHarom), 122.5 (CHarom), 125.4 (CHarom), 126.8 (CHarom), 131.1 (CHarom), 140.3 (Cquat arom), 140.5 (Cquat arom), 144.9 (Cquat mom), 149.4 (Cqut a"), 172.6 (C=N). HEMS M+ cdcd for CigHz&J2278.1783, found 278.1781. 6b: IR (Nujol mull) 1643 cm-' ( v (C=N)). 'H NMR (300 MHz, CDC13,20 "C) 6 2.68 (m, 2H, CH2),3.01 (m, 2H, CHz), 3.78 (s,3H, ocH3),6.90 (m, 4H, CHarom), 7.32 (m, 2H, CHarom), 7.40 (m, lH, CHarom), 7.94 (d, J = 7.4, lH, CHarom). {'H} (75 MHz, CDC13,20 "C) d 28.0 (CH2),29.5 (CH21, 55.2 (OCH3), 114.1 (2 CHarom), 121.1 (2 CHarom), 122.6 (CHarom), 125.5 ( m a r o m ) , 126.9 (CHarom), 131.5 (CHarom),139.6 (Cquat arom), 145.2 (Cquat arom), 150.0 (Cquat arom), 156.0 (Cquat arom), 174.6 (C=N). HRMS M+ calcd for C I ~ H I ~ N237.1154, O found 237.1152. 6c: IR (Nujol mull) 1652 cm-' ( v (C=N)). 'H NMR (300 MHz, CDC13, 20 "C) 6 2.35 (s, 3H, CH3), 2.68 (m, 2H, CH2), 3.01 (m, 2H, CH2),6.83 (m, 2H, CHarom), 7.15 (m, 2H, CHarom), 7.37 (m, 2H, CHaro,), 7.43 (d, J = 6.7, l H , CH,,,), 7.94 (d, J = 7.6, l H , CHaromL3C {'H} (75 MHz, CDC13, 20 "C) 6 20.9 (CH3), 28.1 (CH2), 29.4 (CH2), 119.7 (2 CHarom), 125.6 (CHarom), 127.1 (CHarom), 128.9 (CHarom),131.8 (2 overlapping CHarom), 131.8 (CHarom), 132.9 (Cquat arom), 139.5 (Cquat arom), 149.7 (Cquat mom), 150.3 (Cquatarom), 175.0 (C=N). HRMS M+ calcd for C16H1tiN 221.1204, found 221.1203. 6d: IR (Nujol mull) 1640 cm-' (v (C=N)). 'H NMR (300 MHz, CDC13, 20 "C) 6 2.67 (m, 2H, CH2), 3.06 (m, 2H, CH2), 6.92 (m, 2H, CHarom), 7.10 (m, lH, CH,,,), 7.36 (m, 4H, CHarom),7.47 (m, lH, CHarom), 7.94 (d, J = 8.0 Hz, l H , CHarom). I3C {'H} (75 MHz, CDC13,20 "C) 6 28.0 (CH2),29.2 (CH2),119.6 (2 CHarom), 122.8 (CHamm),123.1 (CHarom),125.6 (CHamm),127.0 (CHam,), 128.9 (2 CHaro,), 131.8 (CHarom), 174.9 (C=N). HRMS M+ calcd for C15H13N 207.1048, found 207.1047. 6e: IR (Nujol mull) 1652 cm-' ( v (C=N)). 'H NMR (300 MHz, CDC13, 20 "C) 6 2.64 (m, 2H, CHz), 3.07 (m, 2H, CHz), 6.85 (m, 2H, CH,,,,), 7.31 (m, 2H, CH,,,), 7.38 (m, lH, CHarom), 7.45 (m, 1H, CHarom), 7.92 (m, 1H, CHarom). 13C {'H} (75 MHz, CDC13, 20 "C): d 28.0 (CH2), 29.3 (CH2), 121.0 (2 CHarom), 122.8 (CHarom), 125.6 (CHarom), 127.0 (CHarom), 128.9 (2 CHarom), 132.0 (CHarom), 129.0 (Cquat arom), 139.1 (Cquat arom), 150.4 (Cquat 150.8 (Cquat 175.5 (C=N). HRMS M+ calcd for C15H12NC1 241.0658, found 241.0656. Conversion of 5b to N-(4-Methoxypenyl)-3-phenylpropionamide (7). A solution of 1 (400 mg, 1.4 mmol) in THF (30 mL) was cooled to -80 "C and treated with a solution of p-methoxyphenyl isocyanide (190 mg, 1.4 mmol) in THF (5 mL). The cooling bath was removed, and the mixture was allowed to warm to 0 "C. The resulting yellow mixture was cooled again to -80 "C and treated with 15 mL of a 0.1 M solution of HC1 in THF. The resulting mixture was allowed to warm to -5 "C, and 35% H202 (2 mL) was added. The mixture was stirred at room temperature for 30 min and was concentrated to approximately 5-10 mL. The solution was filtered through a pad of silica gel, which was washed with a 1:l mixture of hexane and ethyl acetate and the filtrate was concentrated in vacuo. The product was purified by spinning plate cromatography to yield 110 mg (0.43 mmol, 28%) of 7. IR (Nujol mull) 3295 (v(NH)),1616 (amide I), 1548 (amide 11).

2046 Organometallics, Vol. 14, No. 4, 1995 Table 4 substitutent

a

k( l), s-I

P-ChHj-NEt? -0.72 (1.13 f O . O 1 ) (4.36 i0.03) p-C6Hj-OMe -0.27 -0.17 (7.8 f 0.1) x p-ChHj-Me 0 (12.2 i0.2) x ChHs 0.23 (26.2 f 0.6) x P-C~HJ-CI

k(2). s-' x IO-' x IO-4

IO-' lo-'

(1.21 iO.01) x 101-' (3.95 i 0.03) x IO-' (7.9 & 0.1) x (12.8 f 0.3) x (23.4 0.6) x IO-'

*

'H NMR (300 MHz, CDC13, 20 "C) 6 2.59 (t, J = 8.0 Hz, 2H, CH2), 3.01 (t,J = 8.0 Hz, 2H, CHz), 3.54 (s, 3H, OCH3), 6.777.31 (m, 10H, CH,,,, and NH). l3C{IH} (75 MHz, CDC13, 20 "C) b 31.6 (CHz), 39.1 (CHz), 55.4 (OCH3), 114.4 (2 CH,,,,), 121.9 (2 CH,,,,), 126.2 (CHarOm), 128.3 (2 CH,,,,), 128.5 (2 CHmom), 130.8 (Cquat amm), 140.7 (Cquat arom), 156.3 (Cquat arom), 170.3 (C-0). HRMS calcd for C16H17N02 255.12592, found 255.1268. X-Ray Structure Determination of 2. A crystal was mounted on the diffractometer and the cell dimensions were refined by the least-squares fitting the 0 values of 25 reflections. The crystal data and the parameters used during the collection and refinement of the diffraction data are summarized in Table 2. Three reflections were monitored periodically during data collection and revealed no crystal decomposition. The intensities were corrected for the Lorentz and polarization effects. Scattering factors for neutral atoms and anomalous dispersion correction for the Ti atom was taken from the International Tables for X-Ray Crystallography.22 The structure was solved by Patterson and Fourier methods. An empirical absorption correction was applied at the end of the isotropic temperature factors and positions for the H atoms.23a The calculations were carried out with X-RAY80.23b General Procedure for the Determination of the Rate of Reductive Elimination of Compound 2. Samples of 2 at the specified concentrations containing an internal standard of ferrocene were prepared in situ from solutions of 1 prepared by dissolving this compound and ferrocene (equimolecular amounts) in 0.7 mL of the deuterated solvent (C& unless otherwise indicated). The solutions of 1 were placed in 5 mm NMR tubes and were treated with a equimolecular amount of tert-butyl isocyanide. Conversion of 1 to 2 was clean and quantitative under these conditions. A sample was placed in a NMR probe a t a temperature determined by using a sample of neat methanol, and the progress of the reaction monitored by 'H NMR. The rate was determined from the ratio of the cyclopentadienyl signal of 2 to the signal of ferrocene. Determination of the Rate of Reductive Elimination of 2 at Different Concentrations. Two sets of solutions of 2 at a concentration 0.07, 0.14, and 0.29 M, containing ferrocene as an internal standard were prepared as described before. The elimination rate measurements were carried at 69 "C. The rate constant was found to be the same in all cases within the experimental error (k = (1.1 f 0.2) x s-l). Determination of the Activation Parameters for the Reductive Elimination of 2. The elimination rate of samples of 2 at a concentration of 0.14 M was determined three times at each temperature, and the average values of the rate (22) International Tables for X-Ray Crystallography; Kynoch Press: Birmingham, U.K., 1974. (23) ( a ) Stewart, J. H. The X-RAY80 System; Computer Science Center, University of Maryland: College Park, MD, 1985. (b) Walker, N.; Stuart, D. Acta Crystallogr. 1983,A39, 158.

Campora et al. constants obtained from these measurements were used in the calculation of the activation parameters. Averaged rate s-l; T = 334.6 constants: T = 311.5 K, k = (4.2 f 0.5) x K, k = (6.5 f 0.8) x s-l; T = 344.0 K, k = (2.1 k 0.1) x s-l. s-l; T = 354.0 K k = (5.2 f 0.3) x Determination of the Rate of Reductive Elimination of 2 in Different Solvents. The rate of reductive elimination was measured at 338 K for two sets of samples of 2 of concentration 0.14 M prepared as described in CsDs, CDC13, THF-de, and CD3CN. C&: k(1) = (1.14 f 0.02) x s-l, k(2) = (9.0 f 0.3) x s-l. CDC13: k(1) = (8.8 f 0.2) x s-l, k(2) = (7.6 k 0.3) x s-l. THF-de: k(1) = (7.5 f 0.4) x s-l, k(2) = (6.0 f 0.2) x s-l. CD3CN: k(1) = (4.48 f 0.05) 10-4 s-1, ~ 2 = )(4.7 0.1) 10-4 s-1. Determination of the Rate of Reductive Elimination of 2 in the Presence of Trimethylphosphineand Dimethyl Disulfide. Samples of compound 2 (0.14 M), prepared as described, were treated with trimethylphosphine (20, 50, and 100 pL), and the rate of reductive elimination reaction was determined at 333 K for each concentration of trimethylphosphine (0.28, 0.71, and 1.43 M, respectively). The rate in the s-l; two first cases (k([PMe31=0.28M) = (4.9 k 0.1)x k([PMesl=O.7lM) = (4.6 k 0.1) x s-l) was very similar to the value found in the absence of phosphine ( h = (4.5 & 0.1) x s-l). A small decrease of the rate (k([PMe3]=1.43M)= (3.2 0.1) x s-l) was observed in the last case. By following a similar procedure, a solution of 2 was treated with 17 pL (0.2 mmol) of dimethyl disulfide and the reaction rate determined at the same temperature (k = (4.1 5 0.1) x

*

S-1).

Determination of Hammett e for Compounds Sa-e. The corresponding compound (0.1 mmol) and ferrocene (18mg, 0.1 mmol) were placed in a 5 mm NMR tube. The tube was cooled at -80 "C and CDzClz (0.7 mL) was slowly added. The mixture was shaken for several seconds and quickly returned to the cooling bath. This operation was repeated until a clear solution was obtained. The sample was then stored in a bath between -10 and -20 "C. Samples were allowed to stand 5 min in a NMR probe at 23.5 "C before the starting rate measurement, in order to stabilize the temperature of the sample. It was previously determined that the temperature of a methanol standard is equilibrated under the same conditions. The value of the rate constants (Table 4) were averaged from two sets of measurements. The values of the u constants was obtained from ref 24.

Acknowledgment. We thank the National Science Foundation for support of this work. S.L.B. acknowledges additional support as an Alfred P. Sloan Fellow and a Camille & Henry Dreyfus Teacher-Scholar. J.C. thanks the Ministerio de Educaci6n y Ciencia (Spain) and the Fulbright Foundation for a postdoctoral fellowship. We acknowledge helpful discussions with Professors Robert H. Grubbs and Christopher Cummins as well as B. P. Warner and C. A. Willoughby. SupplementaryMaterial Available: Tables of complete bond distances and angles, H atom coordinates, and thermal parameters (7 pages). Ordering information is given on any current masthead page. OM940916C (24) Hansch, C.; Leo, A,; Taft, W. R. Chem. Reu. 1991,91,165.