A New Alkene Carbon-Hydrogen Bond Activation Reaction: Facile and

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Organometallics 1995, 14, 898-911

A New Alkene Carbon-Hydrogen Bond Activation Reaction: Facile and Stereospecific Vinylic Deprotonation of the Chiral Cationic Rhenium Alkene Complexes [(q5-C5H5)Re(NO) (PPh3)(HzC=CHR)]+BFdTang-Sheng Peng and J. A. Gladysz" Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 Received November 7, 1994@ Alkene complexes [(g5-C5H5)Re(NO)(PPh3)(H2C=CHR)I+BF4(1;R = a, CH3; b, CH2CH2CH3; c, CH(CH&; d, H; e, CsH5; f, C(CH3h; ?96:4 RS,SR/RR,SS Re,C configurational diastereomers) and t-BuO-K+ react in THF to give alkenyl complexes (g5-C,H5)Re(NO)(PPh3)(CH=CHR) (2; 83-93% after workup). Allylbenzene complex l g (R = CH2CsHd gives a 89:ll mixture (91%)of 2g and allyl complex (g5-C5H5)Re(NO)(PPh3)(CH2CH=CHR) (3g). The 2g:3g ratio decreases when t-BuOH solvent or (RR,SS)-lgis used-conditions that also give 2a,b/3a,b mixtures. NMR experiments show that (RS,SR)-and (RR,SS)-lgive (E)and (Z)-2,respectively. However, the latter equilibrate (Keq> (99-82):( 98%.22 The configuration at rhenium, which corresponds t o retention, was assigned on the basis of the commonly observed cor-

relation with the sign of the optical rotation in this series of compounds,lg as well as the mechanistic implausibility of any pathway involving inversion. 3. Vinylic vs Allylic Deprotonation. We previously reported differing results in similar reactions utilizing tert-butyl alcohol solutions of t-BuO-K+ (0.75 M).13 Specifically, the hexafluorophosphate salts of propene complex la, allylbenzene complex [(r5-C5H5)R~(NO)(PP~~)(H~C=CHCH~C~HF,)I+BF~(lg), and isobutene complex (q5-C5H5)Re(NO)(PPh3)(H2C=C(CH3)z)I+BF4-(lh)gave the corresponding a-allyl complexes 3 (84%, 70%, and 73% after workup).23 We sought to reconcile this dichotomy24and define any influence of structure or solvent upon selectivity. Thus, the allylbenzene complex l g (955 RS,SRI RR,SS)5aib was dissolved in THF and treated with t-BuO-K+ in t-BuOH (0.75 M) at -80 "C (Scheme 7).16 A 31PNMR spectrum (room temperature) showed that the major product was indeed the trans-cinnamyl complex (E)-(r5-C5H5)Re(NO)(PPh3)(CH~CH=CHC~5) ((E)3g; 64%).13 However, the vinylic deprotonation product (E)-(y5-C5H5)Re(NO)(PPh3)(CH=CHCHzCsH5) ((E)-2g; 20%)17 was also present, along with four minor b y p r o d ~ c t s .Workup ~ ~ ~ ~ ~gave ~ a 21:79 (E)-2g/(E)-3g mixture (88%).23This establishes, in the absence of any product equilibration (below),the existence of competing deprotonation pathways. (22) Ramsden, J. A.; Gamer, C. M.; Gladysz, J. A. Organometallics 1991, 10, 1631. Retention times (95:5 hexaned2-propanol, 1.5 mL/ min): (-)-(E)-(R)-2e,7.9 min; (+)-(E)-(S)-2e,12.8 min. (23) In all preparations of allyl complexes 3 to date, only trans isomers have been detected. The unidentifiedminor byproduds formed in certain reactions may include cis isomers. (24)The 'H and '3C NMR spectra of the original sample of 3a prepared from the hexafluorophosphate salt of la (ca. 67:33 RS,SR/ RR,SS; C&C1 solvent)13showed ca. 10%of the propenyl complex (E)2a. Comparable amounts of (E)-2gwould have been removed in the workup procedure used for (E)-3g. (25) 31PNMR data (ppm, reaction solvent unless noted): (a) 21.8, 25.4,27.6,27.2,24.9,19.2 ((E)-2Bl(E)-3gmyproducts), 20:64:4:45:2;(b) 23.6, 25.9, 26.3, 25.6, 24.9 (C6D6, (E)-2alSalbyproducts),68:17:6:4:4; (c) 22.8, 26.0, 27.7, 27.2, 25.8, 18.7 (C&, (E)-2b/(E)-3bmyproducts), 61:12:6:10:5:7;(d) 24.7, 26.2, 12.5, 6.7 ((2)-2a/3a/byproducts),79:3:9: 9; (e) 23.6, 24.7, 33.5, 12.5, -13.8 ((E)-2al(Z)-2albyproducts),32:28: 10:23:7;(0 23.6,24.7,26.2,12.2,11.8,6.7 ((E)-2al(Z)-2al3amyproducts), 38:12:4:25:17:4.

902 Organometallics, Vol. 14,No. 2, 1995

Peng and Gladysz

Scheme 7. Vinylic vs Allylic Deprotonation of Allylbenzene Complex l g as a Function of Solvent"

t-BuO K+

Scheme 8. Vinylic vs Allylic Deprotonation of Alkene Complexes 1 as a Function of Allylic Substituenta 0.75 M t-BuO K+ in 1-BuOH

*

-80 "C to room temperature

de+

/-\,

c

-80 "C to room temperature

RRHC

(RS,Sfl-l in THF (3% RR,SS)

@

de ' \ ON PPh3

I

+

de

ON /

I

\

PPh3

ON'

solvent for 1-BuO K+

solvent for l g

yield (%)

(€)-2g/(€)-3g

t-BuOH

THF t-BuOH THF

88 90 91

21:79 397 89:ll

f-BuOH THF a

See text and ref 16 for reaction conditions,

a b

c

a

R'

R"

H CH2CH3 CH3 CsH5

H H CH3 H

yield (%)

2/3

72' 986 97 08

80:20 84:16 >99:99:99:99.5:0.5.

+

Scheme 11. Re-(C=C) Conformational Equilibria and Deprotonation of Isobutylene Complex l h

ON

ON

de ON'

2h

Ih

Ill

w CH2Clp

I 'PPha

3h

50:50

0

H C

I

XI

X

(a01h)

(seih)

+

of the type LX B:- were required for vinylic deprotonation, l h would be less likely to give an alkenyl complex. Thus, a THF solution of l h and t-BuO-K+/THF were combined in an NMR tube at -80 "C. A 31P NMR spectrum (room temperature) showed the clean formation of a 89:11 mixture of the known alkenyl complex (r5-CsHs)Re(NO)(PPh3)(CH=C(CH3)2) (2h)l' and methylallyl complex (q5-CsH5)Re(NO)(PPh3)(CHzC(CH3)=CH2) (3h).13An analogous reaction using a CH2C12 solution of l h gave a 50:50 2W3h mixture. Workups gave 89:ll and 5050 2W3h mixtures in 9497% yields. Interestingly, the 2W3h ratios match the averages of the 2 d 3 a ratios obtained from the diastereomeric propene complexes (RS,SR)-la and (RR,SS)l a in THF or CH2Clz (Scheme 9). Hence, there is no special impediment to vinylic deprotonation with lh. 6. Additions of t-BuO-K+ to Cyclopentadienyl Ligands. The reaction of propene complex l a (RS,SRI RR,SS 96:4) and t-BuO-K+ITHF was monitored by 31P NMR at -80 "C under the conditions of Scheme 3. Within a few minutes, the resonance of la was replaced by those of (E)-2a and a new species (23.7, 22.9 ppm, 68:32). When the sample was warmed to room temperature, only (E)-2a remained (23.4 ppm). The isomeric complexes (Z)-2a and 3a give resonances downfield of that of (E)-2a (24.2, 25.7 ppm). Thus, the 22.9 ppm resonance was ascribed to an intermediate. Analogous reactions were conducted in CDzClz with 96:4 and 8:92 mixtures of (RS,SR)-and (RR,SS)-la. In this solvent, the intermediate formed in 98% yields, as 96:4 and 8:92 mixtures of isomers (31PNMR: 21.5,19.4 ppm). lH and 13C NMR spectra were recorded at -60 "C (Experimental Section). Importantly, the cyclopentadienyl IH and 13C resonances of l a had been replaced by more complex signals. New tert-butoxy lH and 13C

A New Alkene C-H Bond Activation Reaction

Scheme 12. Addition of t-BuO-K+to Cyclopentadienyl Ligands EBuO ICc in THF CDpCI2

BF; (RS,SR)-l a

(RS,SR)-5a

or

or (RR,SS)-51

(RR,SS)-la

\

+

de ON'

I

de

1

' \

ON

'PPh3

PPh3

3e

2e

CBuO 'K in THF CD2C12

-80 "C to room temperature

L

room

J

k ON'

/c

H 'C'

I

I

warming rate.31 Attempts to isolate 5a at low temperatures were unsuccessful. We sought to determine whether 5a was on the reaction coordinate connecting la and 2a, or was simply due to a nonproductive but kinetically rapid equilibrium. In the former case, two 1,5-hydride shifts could isomerize the tert-butoxy group from an ex0 to an endo position,32allowing close proximity to the vinylic proton to be abstracted. In the latter case, the tert-butoxy group would dissociate and then externally attack the same vinylic proton. However, our inability to isolate Sa hampered approaches to resolving this issue. Thus, the pentamethylcyclopentadienylstyrene complex [(v5-C6Me5)Re(NO)(PP~~HzC=CHCsH5)1+BF4(leMe5)33 was studied. Although this compound might undergo tert-butoxide addition t o the cyclopentadienyl ligand, there would not be a precedented low-energy pathway for accessing an endo isomer. Thus, formation of the corresponding styrenyl complex (v5-C5Me5)Re(NO)(PPh3)(CH=CHC6H5)(2e-Me5)would indicate that vinylic deprotonation need not be mediated by the cyclopentadienyl ligand. Indeed, a preparative reaction of (RS,SR)-le-Mes and t-BuO-K+ITHF in CHzClz gave (E)-2e-Mes in 99% yield. A similar reaction was conducted in CDzClz (Scheme 12), and NMR spectra were recorded at room temperature. Multiple pentamethylcyclopentadienyl-derivedlH and 13Cresonances, and a 31P resonance plausible for addition product 5e-Me5 (16.5 ppm, br; 91%), were present. After 3 h, only (E)2e-Mes and small amounts of PPh3 remained (96:4).

Discussion (RS,SR)-Se-Mes

(RS,SR)-1e-Mes

Organometallics, Vol.14,No.2, 1995 905

'PPh3

H '

1. Additional Background. Before analyzing the mechanism of vinylic deprotonation of alkene complexes 1, we briefly summarize some related examples and previous studies of allylic deprotonation. First, the cyclopentene complexes [(7;15-CsH4R)Re(NO)(PPh~)m (CH=CH(CHz)3)1+BF4(7; R = H, CH3) and t-BuO-K+ cleanly react in THF to give the corresponding cyclopentenyl complexes 8 , as shown in Scheme 13 (top).6a The analogous allene complex is similarly transformed t o an allenyl c ~ m p l e x .However, ~ cyclohexene, cycloheptene, and cyclooctene complexes of I give mixtures of alkenyl and allyl complexes.6a Angelici has found that sulfur-ligated thiophene complexes of I react with KOH (and other bases) to give neutral 2-thienyl complexes, as depicted by 9 and 10 in Scheme 13 (middle).34 When SCH protons are absent, 3-thienyl complexes form. Accordingly, mechanisms involving initial isomerization t o C=C ligated v2thiophene complexes such as 11 have been proposed. The latter are spectroscopically observable in closely

resonances were present, but shifted propene ligand lH and 13Cresonances remained. These data indicate the formation of isomeric, configurationally stable y4-cyclopentadienyl complexes (v4-C5H5O-t-Bu)Re(NO)(PPh3)(H2C-CHCH3) (Sa), derived from attack of t-BuO-K+ upon the cyclopentadienyl ligand of l a (Scheme 12). (31) One sample of the 96:4 mixture was slowly warmed in an NMR probe, and a -80 "C bath was abruptly removed from another. The Many properties of Sa matched those of the related gave a 69:23:8 (E)-2d(Z)-2a/3a mixture and the second a 98:2 y4-cyclopentadienecomplex (v4-C5HsCC13)Re(NO>(PPh3)- first (E)-2d3a mixture (ambient probe temperatures). Identical experi(PPhC12),30for which an ex0 relationship of the rhenium ments with the 8:92 mixture gave 21:14:65 (E)-2d(Z)-2d3aand 3:97 (E)-2d3a mixtures. and CCl3 moiety has been established crystallographi(32) For examples of 1,5-hydrideshifts in v4-C&R ligands, see: (a) cally. An analogous geometry was assumed for 5a. Both Merrifield, J. H.; Gladysz, J. A. Organometallics 1983, 2, 782 and references therein. (b) Colomer, E.; Corriu, R. J. P.; Vioux, A. J. isomers of 5a cleanly converted to 2d3a mixtures below Organomet. Chem. 1984,267, 107. 0 "C. However, the 2 d 3 a ratios depended upon the (33)Peng, T.-S.;Winter, C. H.; Gladysz, J. A. Inorg. Chem. 1994, (30) Buhro, W. E.; Arif, A. M.; Gladysz, J. A. Inorg. Chem. 1989, 28, 3837.

33, 2534. (34) Robertson, M. J.;White, C. J.;Angelici, R. J. J.Am. Chem. SOC. 1994, 116, 5190.

906 Organometallics, Vol. 14, No. 2, 1995

Peng and Gladysz

Scheme 13. Additional Deprotonation Reactions &R

&R 1-BuO- K’ in THF

4

ON’

‘PPh3

THFsolvent (R = H, Me)

* ON’

0 8

7

@I Re

ON’

Y

KOH

‘PPh3

~

CH30H solvent

BF;

10

9 possibly via:

ON’

”i”

‘PPh3

Figure 1. Partial Crystal Structures of the Cyclopentene Complex 7-Me (top) and the Isopropylethene Complex (RS,SR)-IC (bottom).

11

Scheme 14. Mechanism for Allylic Deprotonation of Alkene Complexes

E:‘&

H

XI1

Xlll

related systems and should undergo vinylic deprotonation similarly to 1 and 7. Reactions of alkene complexes of the iron Lewis acid [($-CsHs>Fe(CO)2]+ and triethylamine have been extensively studied by Rosenblum.12 Only allylic deprotonations are observed, and stereochemical data require transition states in which the carbon-hydrogen and M-(C=C) bonds are a n t i ~ e r i p l a n a r . This ~ ~ is illustrated in XI1 (Scheme 14),utilizing a metallacyclopropane resonance form to emphasize the similarity with common organic anti eliminations. Hence, one approach to rationalizing allylic vs vinylic deprotonation selectivity is to analyze whether some alkenes can accommodate this stereoelectronic requirement better than others. For example, the propene complex l a should have no extraordinary barrier to achieving the geometry in (35)(a) Amine bases are usually not strong enough to deprotonate alkene complexes of I.5c For example, la and NEt3 do not react in THF (12 h, room temperature). Thus, deprotonation selectivities cannot be measured under conditions analogous to Rosenblum’s.

XII-especially in view of the ready allylic deprotonation of allylbenzene complex lg. Similarly, the crystal structure of cyclopentene complex 7-Me is shown in Figure 1(top).6a Two allylic protons (H3, H7) are clearly prealigned for abstraction as in XII. However, both (RS,SR)-laand 7 give exclusive vinylic deprotonation under the conditions of Scheme 3, indicating the availability of even lower energy transition states. Nonetheless, the transition-state model XI1 does account for the trends in Scheme 8. As noted above, vinylic deprotonation increases as the number of alkyl substituents in the allylic position increases (IC> l b > la). Sterically, an alkyl group would be more likely to be anti to the metal in XI1 than a hydrogen, disfavoring allylic deprotonation. Indeed, as shown in Figure 1(bottom),(RS,SR)-lccrystallizes with a methyl group ( C 5 ) anti to rhenium.5b In contrast, the phenyl substituent in l g promotes allylic deprotonation-an electronic effect often observed in mbond-formingeliminations. However, in the absence of such factors, allylic substituents disfavor allylic deprotonation. 2. Mechanism of Vinylic Deprotonation. The above data exclude a variety of mechanisms for the vinylic deprotonation of 1 and provide evidence against others. To summarize, deprotonation (1)is irreversible and rate-determining, (2) occurs with retention a t rhenium, (3) is regiospecific for the =CH2 terminus, (4) is stereospecific,with rhenium replacing only one of the diastereotopic =CH2 protons, as controlled by the configuration at rhenium (Hs from (RS)-or (RR)-l), and (5) does not involve PPh3 dissociation or initial isomerization of 1 to an alkylidene complex, as represented by the RS,SR diastereomer series in Scheme 15 (12 and 4; bottom). Within these constraints, several types of preequilib-

A New Alkene C-H Bond Activation Reaction

Organometallics, Vol. 14,No. 2, 1995 907

Scheme 15. Mechanisms for Vinylic Deprotonation of Alkene Complexes (RS,SR)-l favored:

xv excluded or disfavored intermediates:

0 ON’ H\

-I

P-C\,

R

BF;

12

A

BF;

BF;

1

14

13

de ON’

0

I ‘PPhg BF;

5

15

aO(RS,SFI)-l

rium steps remain viable. For example, a reversible intramolecular oxidative addition involving the reactive vinylic carbon-hydrogen bond might occur, giving an alkenyl hydride complex (13; Scheme 15). Alternatively, a related “a-bond”complex could form (14; Scheme 15). Either species would plausibly react with base to give the alkenyl complex 2. However, a detailed study of the mechanism of equilibration of (RS,SR)-and (RR,SS)-l indicates that such species (which allow exchange of the C-C enantioface bound t o rhenium) can be accessed only a t temperatures of 1 9 5 0C.5c Other preequilibrium steps are more difficult t o exclude. As discussed above, the vinylic deprotonation of pentamethylcyclopentadienylstyrene complex (RS,$R)le-Me5 (Scheme 12, bottom) provides good evidence against intermediates derived from ex0 tert-butoxide additions t o cyclopentadienyl ligands (5). A related possibility would involve initial tert-butoxide attack at the alkene ligand =CHR terminus to give an alkyl (15, complex (r5-C5H5)Re(NO)(PPh3)(CH2CH(R)O-t-Bu) Scheme 15). Similar additions have in fact been observed with methoxide ion, as well as carbon nucleop h i l e ~ . ~However, ~ - ~ ~ in no case has such an addition product been found t o eliminate alcohol to give an alkenyl complex. In our view, neither concerted four-center eliminations nor E2 steps involving a second equiva(36)Peng, T.-S. Unpublished results, University of Utah.

lent of base have obvious kinetic driving forces that would render them rapid below room t e m ~ e r a t u r e . ~ ~ We also disfavor, on the basis of the data in Scheme 11 and other con~iderations,2~~ mechanisms involving the less stable ac Re-(C-C) conformers. From this analysis, we arrive at the two pathways at the top of Scheme 15, both of which feature initial and ratedetermining carbon-hydrogen bond cleavage. The first is concerted, involving the transition state XTV. The second is stepwise, involving a carbanion or zwitterion XV that rapidly rearranges to an alkenyl complex. The concerted transition state XIV could also have considerable carbanion character. Concerning the transition state XIV, it is obvious from the crystal structures in Figure 1 that the carbonhydrogen and rhenium-carbon bonds being broken do not have an antiperiplanar relationship. This contrasts with the transition state XI1 for allylic deprotonation. Thus, there should be more stabilization from the developing C=C n-bond in the latter. At the same time, there is no obvious feature that would stabilize the carbanionic intermediate XV in the stepwise mechanism. The fragment I lacks low-lying acceptor orbitals that can interact with potential n-donor ligands.40 Hence, we are presently unable to formulate an intuitive rationale for the lower transition state energy of vinylic depr~tonation.~~ However, it should be stressed that vinylic deprotonation gives the thermodynamically more stable product. It follows as a corollary that the vinylic protons of 1 are thermodynamically more acidic than the allylic protons. Isomeric propargyl, allenyl, and propynyl complexes show a similar energetic relationship, as illustrated in Scheme In both series of compounds, stability tracks the hybridization of the ligating carbon (sp > sp2 > sp3). The ease of vinylic deprotonation is also more reassuring in light of the conceptually related process in eq i of Scheme 16. Cationic ethyne complexes XVI undergo analogous rapid reactions with alkoxide bases t o give neutral ethynyl complexes XVII.42143On the basis of available data, there appear to be few mechanistic options other than analogs of those in Scheme 15.42c Importantly, the vinylic deprotonation mechanisms in Scheme 15 have abundant precedent in organic (37)Ghazy, T.;&ne-Maguire, L. A. P.; Do, IC J . Organomet. Chem. 1990,390,91. (38)Peng, T.-S.; Gladysz, J. A. Tetrahedron Lett. 1990,31,4417; J. Chem. SOC.,Dalton Trans., submitted for publication. (39)Methoxide and carbon nucleophiles add to the -CHR terminus . ~order ~ ~ ~to~account for the of 1 from a direction anti to r h e n i ~ m In product stereochemistry in Scheme 5 , a syn elimination of t-BuOH from 15 would be required. (40)(a) Czech, P. T.; Gladysz, J. A.; Fenske, R. F. Organometallics 1989,8,1806.(b) Several properties of amido complexes of I illustrate this point: Dewey, M. A.; Knight, D. A,; Arif, A. M.; Gladysz, J. A. Chem. Ber. 1992,125,815. (41)However, other types of cationic complexes of I can be deprotonated at low temperature to charge-separated, zwitterionic, or ylidic intermediates: (a) Crocco, G. L.; Lee, K. E.; Gladysz, J. A. Organometallics 1990,9,2819. (b) Cagle, P. C.; Arif, A. M.; Gladysz, J. A. J. Am. Chem. SOC.1994,116,3655. (42)(a)Kowalczyk, J.J.;Arif, A. M.; Gladysz, J. A.Organometallics 1991, 10, 1079. (b) Ramsden, J. A.; Weng, W.; Gladysz, J. A. Organometallics 1992,11, 3635. ( c ) For terminal alkyne complexes of I, eq i of Scheme 16 is faster than isomerization to vinylidene complexes, which give analogous deprotonation products. (43)Some representative examples: (a) Appel, M.; Heidrich, J.; Beck, W. Chem. Ber. 1987,120,1087. (b) Nicklas, P. N.; Selegue, J. P.; Young, B. A. Organometallics 1988,7,2248. ( c ) Akita, M.; Terada, M.; Oyama, S.; Moro-oka, Y. Organometallics 1990, 9, 816. (d) Lomprey, J. R.; Selegue, J. P. J. Am. Chem. SOC.1992,114,5518.

Peng and Gladysz

908 Organometallics, Vol. 14,No. 2, 1995

Scheme 16. A Series of sp, sp2,and sp3 Carbon-Hydrogen Bond Activation Reactions (1)

XVI

XVll

(ii)

(iii) XXll

XXI

chemistry.44 Three-membered heterocycles often react with strong bases to give ring-opened alkenyl derivatives. Scheme 13 (bottom) shows an example involving chiral thiirane S-oxides, with stereochemical features reminiscent of those in Scheme 6.44a,bOxiranes undergo similar reactions, as well as eliminations analogous to the allylic deprotonation in Scheme 14.44cRing-opening reactions of cyclopropyl carbanions are also well-known and are generally stereospecific due to orbital symmetry control.44d 3. Other Selectivity Issues. The vinylic deprotonation of 1 is under kinetic control. As noted above, t-BuO-K+ removes only one of the two =CH2 protons, as controlled by the rhenium configuration (Hs in I1 and 111, Scheme 1). The thermodynamically more acidic proton will always be that which gives the more stable trans-alkenyl complex (E)-2. Hence, the less acidic protons are abstracted from the less stable diastereomers (RR,SS)-1,which give cis-alkenyl complexes (2)2.

At our present level of understanding, there is no obvious electronic basis for an enhancement of the kinetic acidities of the Hs protons in I1 or 111. We therefore suggest that the selectivity is steric in origin. The interstice between the PPh3 and nitrosyl ligands is much more congested than that between the PPh3 and cyclopentadienyl ligand^.^^",^^ Also, in all crystal structures t o date,5a,b,6a,b,7,8a the alkene ligands rotate 9-23’ counterclockwisefrom the idealized conformations in I1 and 111. This moves Hs somewhat further from the PPh3 ligand than HR, facilitating attack by base as in VI11 (Scheme 11). In preliminary experiments, we have (44) (a) Schwan, A. L.; Pippert, M. F.; Pham, H. H.; Roche, M. R. J . Chem. SOC.,Chem. Commun. 1993,1312. (b) Refvik, M. D.; Froese, R. D. J.; Goddard, J . D.; Pham, H. H.; Pippert, M. F.; Schwan, A. L. J. A m . Chem. SOC.,in press. We thank Professor Schwan for a preprint. (c) Crandall, J. K.; Apparu, M. O g .React. 1983,29, 345. (d) Boche, G.; Walborsky, H. M. In The Chemistry of The Cyclopropyl Group; Rappoport, Z., Ed.; Wiley: New York, 1987;part 1, pp 788794.

(45)(a) Davies, S. G.; Dordor-Hedgecock, I. M.; Sutton, K. H.; Whittaker, M. J . A m . Chem. SOC.1987,109, 5711. (b) Mackie, S. C.; Baird, M. C.Organometallics 1992,11, 3712.

not observed vinylic deprotonation when the & position of I1 is blocked-such as in a trans-/3-methylstyrene adduct .6c,36 Marked solvent effects upon vinyliclallylic deprotonation selectivities are apparent in Schemes 7 and 9. Importantly, t-BuO-K+ forms a 1:l solvent complex and other hydrogen-bonded aggregates in ~ - B u O H Thus, .~~ the thermodynamic basicity is lower than in THF or DMSO. Accordingly, reactions of alkene complexes 1 and t-BuO-K+ are much slower in t-BuOH than in THF.lGCOften, a less reactive base will give increased selectivity for a more stable product. However, t-BuOH solvent instead favors the less stable allyl complexes 3. Hence, we suggest that t-BuO-K+ has a much greater effective bulk in t-BuOH, enhancing selectivity for the sterically more remote allylic deprotonation site. Also, somewhat more allylic deprotonation is observed in CH2C12 than in THF. Although there could be many reasons for this trend, we suspect that it is associated with the greater stabilities of (or kinetic preference for) tert-butoxide addition products 5 in CH2C12. Thus, regardless of mechanism, the effective temperature at which deprotonation occurs in CHzCl2 would be higher. Indeed, reactions in CH2C12 involving different warming rates give differing 213 ratios, indicating a temperature dependence upon ~ e l e c t i v i t y . ~ ~ It is also apparent in Scheme 9 that the less stable RR,SS diastereomers of propene and allylbenzene complexes la,g give more allylic deprotonation than the RS,SR diastereomers. Perhaps this reflects some transition-state destabilization associated with the formation of the less stable cis-alkenyl complexes (21-2. However, due to the proximity of the cyclopentadienyl ligand and =CHR substituent in (RR,SS)-lg,it should at the same time be more difficult to attain the stereoelectronically most favorable transition state XI1 for allylic deprotonation. 4. Implications for Carbon-Hydrogen Bond Activation. Scheme 3 provides an exceptionally mild protocol for the metalation of vinylic carbon-hydrogen bonds in simple unactivated alkenes. Oxidative additions of free alkenes to metal complexes commonly require heating, although reactions can be rapid when coordinatively unsaturated intermediates are photochemically generated in low-temperature matrices.1,2 Also, some comparable metalations have been effected with alkyllithium or -potassium reagents.47 However, allylic protons are usually preferentially abstracted. In this context, the allylic protons of most alkenes exhibit slightly greater kinetic and thermodynamic acidities than the vinylic protons.48 Regardless, these acidities (pKcScm143)are dramatically enhanced upon coordination t o the cationic Lewis acid I-becoming even greater than that of t-BuOH (pKa(H20) ca. 19).46 It is instructive to consider the three conceptually related reactions in Scheme 16. The first two involve (46) Pearson, D. E.; Buehler, C . A. Chem. Rev. 1974,74, 45. (47) (a)Brandsma, L.; Verkruijsse, H. D.; Schade, C.; Schleyer, P. v. R. J. Chem. SOC.,Chem. Commun. 1986,260. (b) Brandsma, L.;

Verkruijsse, H. D. Preparative Polar Organometallic Chemistry; Springer-Verlag: New York, 1987; Vol. 1, Chapter 111. (c) Brandsma, L. Preparative Polar Organometallic Chemistry; Springer-Verlag: New York. 1990:Vol. 2. Chauter 11. (48) (a) Boerth, D. W.-; Streitwieser, A., Jr. J.Am. Chem. SOC.1981, 103, 6443. (b) Streitwieser, A,, Jr.; Boerth, D. W. J . A m . Chem. SOC. 1978,100, 755.

A New Alkene C-H Bond Activation Reaction sp and sp2carbon-hydrogen bond activation. As noted above, the former has been observed p r e v i o ~ s l y , 4and ~?~~ this study provides the first explicit demonstration of the latter. By analogy, a complementary mode of sp3 carbon-hydrogen bond activation should exist. This could, as illustrated in eq iii of Scheme 16, involve a ''0 bond" complex of either a carbon-carbon (XX)or carbon-hydrogen (XXI) linkage. Although there is more precedent for the latter, either would be expected t o undergo facile deprotonation to an alkyl complex. Indeed, comparable p a t h w a y s h a v e been proposed for sp3 carbon-hydrogen bond activation reactions involving electrophilic Pd(II), R ( I I ) , and Hg(I1) species.49 Hence, there a p p e a r s to be a continuum of closely related mechanisms that can be applied to the activation of carbon-hydrogen bonds of any hybridization level. M a n y metal-catalyzed transformations of feedstock chemicals involve basic additives or sites on heterogeneous supports. The preceding analysis suggests heretofore unappreciated roles for these components, at least in carbon-hydrogen bond-breaking reactions. Indeed, it should not be difficult t o a p p e n d a noncoordinating Bransted base t o the rhenium f r a g m e n t I and effect vinylic deprotonation in the absence of an exogenous agent.50 However, prospects for applications in homogeneous catalysis would be even f u r t h e r enhanced by the demonstration of analogous reactivity with coordinatively unsaturated metal complexes. In conclusion, this work has suggested n e w mechanism-based approaches t o catalytic reactions of important commodity chemicals. The development and application of these concepts is under active investigation.

Experimental Sectione1 Deprotonation of [(tl5-C5H5)Re(NO)(PPhs)(H2C=CHCHs)]+BF4- (la). A. A 5 mm NMR tube was charged with l a (16.8 mg, 0.025 mmol; 96:4 RS,SR/RR,SS)5aand THF (0.8 mL), capped with a septum, and cooled to -80 "C. Then t-BuO-K+/THF (1.0 M, 0.035 mL, 0.035 mmol) was added, and the tube was shaken and warmed to room temperature. The yellow-brown solution turned orange. A 31PNMR spectrum showed one resonance (23.4 ppm). Solvent was removed under oil-pump vacuum. The residue was extracted with ether (2 mL, under Nz), and hexane (10 mL) was added. Solvent was removed under oil-pump vacuum to give (E)-(r5-C5H5)Re(NO)(PPh3)(CH=CHCH3)((E)-2a;12.4 mg, 0.021 mmol, 85%)17,53354 as an orange powder. B. Complex l a (16.8 mg, 0.025 mmol; 96:4 RS,SR/RR,SS), THF (1.0 mL), and t-BuO-K+/t-BuOH(0.75 M, 0.040 mL, 0.030 mmol) were combined in a procedure analogous to A. An (49) Periana, R. A.; Taube, D. J.; Evitt, E. R.; Loffler, D. G.; Wentrcek, P. R.; Voss, G.; Masuda, T. Science 1993, 259, 340 and references therein. (50) For a demonstration of this strategy in arene carbon-hydrogen bond activation, see: Cordone, R.; Taube, H. J . Am. Chem. SOC. 1987, 109,8101. (51)(a) General procedures were identical with those in a previous paper.5b (b) Solvent or reagent data: t-BuOH, distilled from Mg/12;@ toluene, heptane, hexane, and ether, distilled from Na; THF, distilled from Wbenzophenone; CeD6, CH2C12, CDzC12, CDC13, and TMEDA, vacuum transferred or distilled from CaH2; acetone-de, distilled from 4A molecular sieves; t-BuO-K+/t-BuOH, prepared from t-BuO-K+ powder (Aldrich) and t-BuOH; t-BuO-K+PTHF, t-BuOD ('98% D), PPh3d15, HBF40Et2, n-BuLi, and (CH3)2C-CHCHzCl,used as received from Aldrich. (c) NMR spectra were recorded on 300 MHz spectrometers a t ambient probe temperatures unless noted and referenced to residual CeDbH, CHDC12, or CHC13 (lH, 6 7.15, 5.32, 7.261, C&, CDzCl2, or CDC13 (13C, 128.0, 53.8, 77.0 ppm), and external 85% H4P04 (31P,0.00 ppm); all coupling constants (J)are in Hz. (52) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of Laboratory Chemicals, 2nd ed.; Pergamon: New York, 1980; p 146.

Organometallics, Vol. 14, No. 2, 1995 909 identical workup gave a mixture of (E)-2a,(q5-C5H5)Re(NO)(PPh3)(CHzCH=CHz) (3a),18and three byproducts (12.4 mg, 0.018 mmol, 72%).25b C. Complex l a (16.8 mg, 0.025 mmol; 96:4 RS,SR/RR,SS), CHzClz (0.8mL), and t-BuO-K+/THF (1.0 M,0.025 mL, 0.025 mmol) were combined in a procedure analogous to A. An identical workup gave a (E)-2a/3amixture (12.2 mg, 0.021 mmol, 84%; 98:2). D. Complex la (16.8 mg, 0.025 mmol; 6:94 RS,SR/ RR,SS),loaTHF (0.8 mL), and t-BuO-K+/THF (1.0 M, 0.025 mL, 0.025 mmol) were combined in a procedure analogous to A. An identical workup gave a (E)-2a/(Z)-2a/3a mixture (12.6 mg, 0.022 mmol, 86%; 41:41:18). E. Complex l a (16.8 mg, 0.025 mmol; 6:94 RS,SR/RR,SS), CHzClz (0.8mL), and t-BuO-K+/THF (1.0 M,0.025 mL, 0.025 mmol) were combined in a procedure analogous t o A. An identical workup gave a (E)-2a/3a53754 mixture (14.4 mg, 0.025 mmol, 99%; 3:97). F. Complex l a (16.8 mg, 0.025 mmol; 96:4 RS,SR/RR,SS), CHzClz (1.0 mL),t-BuOD (0.024mL, 0.25 m o l ) , and t-BuO-K+/ THF (1.0 M, 0.013 mL, 0.013 mmol) were combined in a procedure analogous to A. An identical workup gave (E)-2a (5.4 mg, 0.009 mmol, 37%). The residue left after ether extraction was dissolved in CHzCldacetone (90:10 v/v) and filtered through a pipet containing silica (3 cm). This gave a 95:5 ldla-dl mixture (5.0 mg, 0.007 mmol, 30%; 96:4 RS,SR/ RR,SS),as assayed by MS (all label in the [(q5-C5H5)Re(NO)(PPh3)]+ion).55a,56a Deprotonation of [(?5-CaHa)Re(NO)(PP~)(H~C=CHCH~ CHzC&)]+BFr- (lb). The following reactions were conducted analogously to reaction A of la. A. Complex l b (17.5 mg, 0.025 mmol; 96:4 RS,SR/RR,SS),S" THF (0.8mL), and t-BuO-K+/THF (1.0 M,0.035 mL, 0.035 (53) The ,lP, 'H, and 13C NMR spectra were identical with those for an authentic sample. (54) Unless noted, some NMR data for the following compounds have been reported earlier. Many chemical shifts of isomeric alkenyl and allvl comdexes are similar. and sDectra in new solvents were needed to assignments. (a) (E)-2a {CeDe): 'H NMR (6) 7.96 (ddm, JHH 16.5, JHP 3.4, Ha), 7.60-7.10 (m, PPhd, 5.45 (ddq, JHH 6.0, 16.5, JHP 2.0, Hp), 4.68 ( 8 , C5H5), 2.13 (dd, JHH 6.0, JHF1.3, CH3); 13C{1H)NMR (ppm) 136.9 (Jcp 52.5, i-Ph), 134.0 (d, JCP10.5, 0-Ph), 132.3 (d, JCP 4.4, Cp), 129.9 ( ~ , p - P h )128.2 , (d, Jcp 11.3, m-Ph), 123.3 (d, JCP 11.9, ca), 90.9 (s,C5H5),25.7 ( 8 , CH3);31P{1H1NMR (ppm) 23.5 (s). (b) (2)2a (CeDs, partial): 'H NMR (6) 7.75 (ddm, JHH 11.5,JHP 3.4, Ha), 6.72 (ddq, JHH 6.5, 11.5, JHP 3.1, Hg), 4.68 (s,C5H& 2.41 (dd, JHH 6.5, JHP 1.4, CH,); 13C{1H}NMR (ppm) 125.2 (d, Jcp 11.3, Ca), 90.5 (s, C5H5), 21.4 (s,CH3); 3lP{'H} NMR (ppm) 24.4 (s). (c) 3a (CeDe): 'H NMR (6) 7.55-6.95 (m, PPhs), 6.68 (m, Hp), 4.74 (dm, JHH 16.8, H,z), 4.67 (dd, J H H9.9, 2.8, H E ) , 4.56 (s, C5H5), 3.33 (m, Ha), 2.65 (ddd, JHH 10.4, 10.4, JHP2.2, I&,); l3C{lH) (ppm) 153.0 (s, Cg), 137.2 (Jcp 51.0, i-Ph), 133.9 (d, Jcp 10.4,o-Ph), 130.0 (S,p-Ph), 128.4 (d, Jcp 8.8, m-Ph), 103.0 (s,Cy),90.6 (s,C5H5), -4.8 (d, JCP 4.1, Ca); 31P{1H}NMR ( P P ~25.7 ) 11.2, JHP 7.0, Ha), ( 8 ) . (d) (Z)-2b (CDCl3): 'H NMR (6) 7.65 (dd, JHH 7.50-7.30 (m, PPhs), 6.07 (dddd, JHH 6.7, 6.7, 11.2, JHP 2.8, Hg), 5.01 (s, C5H5),2.24 (m, Hy),1.38 (qt, JHH 7.3, 7.3, Ha), 0.93 (t, JHH 7.3, H6); '3CI'HI (DDm) 140.3 (d. JPP2.2. CR).136.0 (Jrp 52.4. z-Ph). 133.7 (d. Jcp'lO.5, OlPh), 129.9 (Sip-Ph), 128:1(d, Jcp lO.-l,m-Ph), 123.5 (d, Jcp 10.7, Ca), 90.5 (6,C5H51, 37.3 (s, H,),23.6 (s,Ha), 14.3 (s, Ha); 31P{1H) (ppm) 21.9 (9). (e) (E)-3b(C~DB, partial; new compound): 'H NMR ( 6 ) 5.17 (ddd, J m 17.6,6.9,6.9,HJ, 4.62 (C5H5);13C{lH)NMR (ppm) 143.5 (d, Jcp 3.5, Cp), 121.6 ( 8 , Cy),89.7 (s,C5H5), -6.9 (d, JCP 4.5, Ca); 31P{'H} NMR (ppm) 26.0 (s). (0 (2)-2g (C6D6, partial): 'H ( 6 )8.04 (dd, J m 11.2, JHP 7.0, Ha), 6.76 (dddd, JHH 11.2, 7.0, 7.0,JHP 3.0, Hp), 4.66 (s,C5H5), 4.22 (dd, JHH 15.3,7.0,H,), 4.04 (dd, JHH 15.3, 7.0, H,,); 13C{'H} (ppm) 139.2 (s, i-CPh), 129.3 (6, CPh), 128.2 (s, CPh), 125.3 (s, CPh); 137.5 (br s, Cp), 125.6 (d, J c p 10.7, C d , 90.5 (s, C5H5), 42.5 (6, Cy);3lP{lH} (ppm) 23.8 (9). (55) Mass spectra of deuterated and undeuterated samples were recorded under identical conditions. Deuterium levels were calculated with the program "Matrix" (D. A. Chrisope, IBM). (a) la-d, M S P a 588, 6.32%;587, 34.58%;586, 100%;585, 23.24%; 584, 56.20%;583, 2.14%;546,4.47%;545,24.08%;544,81.34%;543,16.95%;542,47.95%; l a MS:56" 588, 4.39%; 587, 29.83%; 586, 100%; 585, 20.03%; 584, 60.17%;583, 1.86%;546,3.59%;545,22.73%;544,86.82%;543,15.53%; 542,50.24%. (b)(E)-2ednMS:m 650,1.34%;649,31.14%;648,88.71%; 647, 100%;646, 54.27%;645, 45.93%;644, 2.30%. (E)-2e MS:56b650, 0.95%;649,8.46%;648, 45.25%;647, 100%;646, 29.98%;645,55.70%; 644, 2.26%. (56) Conditions (mlz, relative intensity, lE7Re):(a) (+)-FAB, 5 kv, Ar, 3-nitrobenzyl alcohoVCHC13 matrix; (b) EI, 17 eV.

Peng and Gladysz

910 Organometallics, Vol. 14, No. 2, 1995

combined as in reaction A. An identical workup gave (E)-2e mmol) gave (r5-C5H5)Re(NO)(PPh3)(CH=CHCH2CH&H3) (2b; (58.1 mg, 0.090 mmol, 13.3 mg, 0.022 mmol, 87%; 91:9 E/Z)17153154 as an orange E. Complex (-)-(SR)-le(73.5 mg, 0.100 mmol, '98% powder. THF (2 mL), and t-BuO-K+/THF (1.0 M, 0.150 mL, 0.150 B. Complex (RR,SS)-lb (21.0 mg, 0.030 m m ~ l )THF , ~ ~(0.8 mmol) were combined as in reaction A. An identical workup mL), and t-BuO-K+/THF (1.0 M, 0.042 mL, 0.042 mmol) gave gave (-)-(E)-(R)-2e (52.2 mg, 0.081 mmol, 81%), [a1255~~ = -231 2b (13.6 mg, 0.022 mmol, 74%; 25:75 EIZ). Additional data: 5" (CHC13,c 0.49 mg/mL),58'98% ee (chiral HPLC).22 see text. F. Complex (RS,SR)-le(36.7 mg, 0.050 mmol), THF (2 mL), C. Complex l b (17.5 mg, 0.025 mmol; 96:4 RS,SRIRR,SS), PPh3-d15 (27.7 mg, 0.100 mmol), and t-BuO-K+/THF (1.0 M, THF (0.8 mL), and t-BuO-K+/t-BuOH (0.75 M, 0.037 mL, 0.028 0.070 mL, 0.070 mmol) were combined as in reaction A. An "01) gave a mixture of (E)-2b,(E)-(r5-C5H5)Re(NOXPP~)(CH2identical workup gave (E)-2e(30.0 mg, 0.046 mmol, 93%). MS: CH=CHCH2CH3) ((E)-3b),54e and four byproducts (15.0 mg, 56a 662 (M+-d& O%), 647 (M', 74%), 544 (M' - C~H7,100%). 0.025 mmol, 98%).25c (E)-2e(C&): 'H NMR (6)9.48 (dd, JHH 17.1, JHP 3.0, Ha), Deprotonation of [(r16-C5H5)Re(NO)(PPh3)(H2C=CHCH- 7.60-6.90 (m, PPhdCPh), 6.40 (dd, JHH 17.1, J ~ p 2 . 1 ,Hp), 4.71 (CH&)]+BF4- (IC).A. A Schlenk flask was charged with (s, C5H5);13C{1H}NMR (ppm) 143.0 ( s , i-CPh), 137.5 (br s, (RS,SR)-lc(35.0 mg, 0.050 m m ~ l )THF , ~ ~(2 mL), and a stirbar Cp), 136.4 (d, J c p 52.9, i-PPh), 135.4 (d, JCP 12.2, Ca), 133.8 (d, and cooled to -80 "C. Then t-BuO-K+ITHF (1.0 M, 0.070 mL, Jcp 10.4,o-PPh), 130.1 (s, p-PPh), 128.4 (d, JCP 10.3, m-PPh), 0.070 mmol) was added with stirring, and the cold bath was 127.7 (s, CPh), 124.8 (s, CPh), 124.0 (s, CPh), 91.6 (s, C5H5); removed. After 1 h, the solvent was removed under oil-pump 31P{1H}NMR (ppm) 21.4 (SI. (E)-2e (CD2C12, -25 "C, parvacuum. Workup as in reaction A of l a gave (r5-C5H5)Re(NO)tial): 'H NMR (6) 9.13 (dd, JHH 17.2, JHP 2.8, Ha), 7.60-6.70 (PPh3)(CH=CHCH(CH&) (2c;27.5 mg, 0.045 mmol, 90%; 97:3 17.2, JHP 2.0, Hp), 5.18 ( s , C5Hs); (m, PPhdCPh), 5.98 (dd, JHH EIZ) as an orange powder. Yellow needles crystallized from 31P{1H}(ppm) 19.9 ( s ) . (Z)-2e(CD2C12, -25 "C): 'H NMR (6) hexane (slow evaporation; 97:3 EIZ), mp 154-155 "C dec. 8.35 (dd, JHH 7.6, JHP 12.8, Ha), 7.75-7.10 (m, PPhdCPh), 7.00 Anal. Calcd for C2sH29NOPRe: C, 54.89; H, 4.77; N, 2.29. 7.6, JHP 0.9, Hp), 5.08 (s, C5H5); l3C{IH} NMR (ppm) (dd, JHH Found: C, 55.14; H, 4.86; N, 2.40. IR (cm-l, thin film): V N O 143.5 (s, i-CPh), 137.1 (br s, Cp), 135.2 (d, Jcp 52.8, i-PPh), 1635 VS. MS:56a613 (M+, loo%), 544 (M+ - C5H9, 89%). 133.6 (d, J c p 10.0, 0-PPh), 131.9 (d, Jcp 10.6, Ca), 130.2 (s, B. Complex (RS,SR)-lc(17.5 mg, 0.025 mmol), THF (2 mL), p-PPh), 128.3 (d, Jcp 9.8, m-PPh), 127.5 (s, CPh), 124.7 (s, and t-BuO-K+lt-BuOH (0.75 M, 0.040 mL, 0.030 mmol) were CPh), 124.4 ( s , CPh), 91.3 ( s , C5H5);31P{1H] NMR (ppm) 23.2 analogously reacted. An identical workup gave 2c (14.9 mg, (SI. 0.024 mmol, 97%; 97:3 EIZ). Deprotonation of led,. The following were conducted ( E 1 - 2 ~'H : NMR 7.96 (ddd, JHH 1.1, 16.5, JHP 2.9, Ha), 7.62analogously to reaction A of le. 6.95 (m, PPha), 5.38 (ddd, JHH 6.4, 16.5, JHP 2.2, Hp), 4.67 ( s , A. Complex (RSR,SRS)-le-dl (95% D, 73.6 mg, 0.100 C5H5), 2.50 (m, H?), 1.03 (d, JHH 6.6, CH3), 0.99 (d, JHH 6.6, m m ~ l )THF , ~ ~(2 mL), and t-BuO-K+/THF (1.0 M, 0.150 mL, CH3'); l3C{'H} NMR (ppm) 145.3 (br s, Cp), 137.0 (JCP 52.2, 0.150 mmol) gave (E)-2e-dl(95% D,5957.1 mg, 0.088 mmol, i-Ph), 134.0 (d, Jcp 10.3, 0-Ph), 129.9 (s, p-Ph), 128.3 (d, Jcp 88%). 10.0, m-Ph), 119.7 (d, Jcp 11.8, Ca), 91.2 (s,C5H5),38.0 (s, Cy), B. Complex (RRR,SSS)-le-dl (83% D, 18.4 mg, 0.025 24.2 ( s , CH3), 24.1 (s, CH3'); 31P{1H}NMR (ppm) 22.0 (s). (2)m m ~ l )THF , ~ ~(1 mL) and t-BuO-K+/THF (1.0 M, 0.035 mL, 2c (partial): 'H NMR (6)4.68 ( s , C5H5), 1.43 (d, JHH 6.6, CH3), 0.035 mmol) gave (E)-2e-d1(83% D,5914.1 mg, 0.022 mmol, 6.6, CH3'); 31P{1H}NMR ppm) 23.8 ( s ) . 1.33 (d, JHH 87%).21 Deprotonation of [(q6-C~Hs)Re(NO)(PPhs)(H~C=CH~)l+ C. Complex (RSS,SRR)-le-dl (97% D, 36.8 mg, 0.050 BF4- (Id). Complex Id (16.5 mg, 0.025 mmol),19THF (2 mL), IO^),^^ THF (1 mL) and t-BuO-K+lt-BuOH (0.50 M, 0.150 and t-BuO-K+/THF (1.0 M, 0.035 mL, 0.035 mmol) were mL, 0.075 mmol) gave (E)-2e(