J . Am. Chem. SOC.1984,106, 3680-3681
3680
Mechanism of Carbon-Carbon Bond Formation in the Reaction of 1,2-Disubstituted Alkenes with a Cationic Bridging Methylidyne Iron Complex Charles P. Casey,* Mark W. Meszaros, Seth R. Marder, and Paul J. Fagant Department of Chemistry, University of Wisconsin Madison, Wisconsin 53706 Received January 3, I984 The diiron p-methylidyne complex [(C5H5)2(C0)2FeZ(pCO)(p-CH)]' PF6- (1) reacts with ethylene and with terminal alkenes by adding the methylidyne C-H bond across the carbon-carbon double bond.'.* This unprecedented hydrocarbation reaction was proposed to occur via a concerted process involving a transition state 2 in which the C-H bond and C=C bonds are parallel. The reaction of 1 with I-methylcyclohexene proceeds by an essentially different pathway to produce the p-vinyl complex 7
[cis-(CsHs)2(CO),Fe,(p-CO) (trans-p-q1,q2-CH=CHC(CH3)CH2CH,CH2CH2)]+PF6-(3). Formation of 3 was proposed to
,CP
Fe-Fe
1
jL M
H3CQ
D.
F L ',-'
eC--H CpiCOiFe- FeiCOiCp
involve initial generation of a carbocation intermediate 4 that undergoes carbon migration to give ring contracted product 3. The failure of 1-methylcyclohexene to undergo hydrocarbation may be due to a combination of steric inhibition of formation of the hydrocarbation transition state and of acceleration of a carbocation process by formation of an intermediate tertiary cation. The reaction of p-methylidyne complex 1 with cis- or trans2-butene, cyclohexene, or other 1,2-disubstituted alkenes led to the formation of mixtures of p-alkylidyne and p-alkenyl comp l e ~ e s . ~Unlike ,~ carbon migration product 3 seen in the reaction of 1 with 1-methylcyclohexene, the p-alkenyl complexes derived from 1,2-disubstituted alkenes resulted from a net hydrogen m i g r a t i ~ n .Initially ~ we proposed that these products were formed by competing hydrocarbation and carbocation pathways. However, we recently discovered that p-alkylidyne complexes with two carbon substituents on the carbon a to the carbyne center undergo rapid and reversible hydride shifts to form an equilibrium mixture of p-alkylidyne and p-alkenyl complexes! The rapid equilibration of the p-alkylidyne and p-alkenyl products of the reaction of 1 with 1,2-disubstituted alkenes is consistent with either the hydrccarbation or carbocation mechanism for the key carbon-carbon bond-forming step. Here we present two independent lines of evidence that establish that reactions of 1 with 1,2-disubstituted alkenes occurs via hydrocarbation followed by equilibration. The reaction of deuterated 1-d with cis-2-butene was studied to determine the kinetic product of the reaction. Initial hydrocarbation would place the deuterium label in the methylene unit of the ethyl group of 5 and rearrangement would leave the label ' N S F Postdoctoral Fellow 1981-1982. ( I ) Casey, C. P.; Fagan, P. J.; Miles, W. H. J . Am. Chem. Soc. 1982, 104, 1134- 1 136. (2) Casey, C. P.; Fagan, P. J. J . Am. Chem. SOC.1982, 104, 4950-4951. (3) Casey, C. P.; Fagan, P. J.; Miles, W. H.; Marder, S. R. J . Mol. Catd. 1983, 21, 173-188. (4) Casey, C. P.; Marder, S. R.; Fagan, P. J. J . Am. Chem. SOC.1983, 105, 7 197-7198.
0002-7863/84/ 1506-3680$01.50/0
in the methylene units of the ethyl groups of 6 and 7. In contrast,
1-d
1%
6-d
."k -
7- d
.rA .
8-d
Fe-
Fe
a carbocation process would lead to deuterium in the a-vinyl position of 6 and 7 and rearrangement to alkylidyne complex 5 would place the deuterium label at the a-methine center of 5. The reaction of 1-d with excess cis-2-butene in CH2C1, at 0 OC gave a 2.3:1.0:1.5 mixture of (cis-(CSH5),(CO),Fe2(p-CO)(p-CCH(CH3)CHDCH3))'PF6- ( 5 - 4 , { ~ i s - ( C ~ H , ) , ( C 0 ) ~ F e ~ ( p - C 0 ) ((E)-p-q',$-CH=C(CH3)CHDCH3)\+PF, (6-d), and (cis( C 5 H s) z ( C 0 ) z F e z ( ~ - C O(l ( Z ) - P - V ', q 2 - CH = C ( C H 3) CHDCH3)ifPF6-(7-d) in 75% isolated yield. Comparison of the N M R spectra of labeled and unlabeled material revealed that the deuterium label was located exclusively on the methylene carbons of the ethyl groups., In the 'H('H1 NMR, resonances were seen only at 6 2.55 and 1.48 for the CHDCH3 groups. The site of deuterium incorporation was confirmed by conversion of the mixture of 5-d, 6-d, and 7-d to the neutral vinylidene complex cis-(CsH5)2(CO),Fe,(p-CO)(p-C=C(CH,)CHDCH3) ( 8 - 4 (80% yield >90% dl by mass spectrometry5) by treatment with aqueous bicarbonate. Previously, we have shown that this reaction proceeds by deprotonation of the alkylidyne component in the rapidly equilibrating mixture of 5 , 6 , and 7.3 If the reaction of 1-d with cis-2-butene had proceeded by a carbocation intermediate, no deuterium would have been incorporated into vinylidene complex 8. Similarly, the reaction of 1-d with trans-2-butene gave a 2.3:1.0:1.5 ratio of 5-d/&-d/7-d in which all of the deuterium label was located in the methylene units of the ethyl groups.6 The reaction of 1-d with cyclohexene in CH2C1, at 0 OC gave a 1.4: 1 mixture of (cis-(C,H5),(CO),Fe2(p-CO)(p-C
, CHCHDCH2CH2CH2CH,)]+PF6(9-d) and (cis-(C5HS),, (CO),Fe,(p-CO)(p-q1,qz-CH=CCHDCH2CH2CH2CH2))+PF~ I
I
(10-4, in 74% yie1d.j Deprotonation of the mixture of 9 and 10 by treatment with aqueous bicarbonate gave vinylidene complex 1
c~~-(C~H~)~(CO),F~~(~-CO)(~-C=CCHDCH~CH~ ( 1 1 - 4 (77% yield, >89% d,'). In the 2H(1H)NMR'of 11-d,the only resonance observed was at 6 2.90 due to allylic deuterium. The reaction of 1-d with 1-methylcyclohexene gave the ringcontracted bridging vinyl complex 3-d5(80%) with a very different labeling pattern consistent with a carbocation mechanism. The only resonance seen in the 'H('H) N M R of 3-d is a singlet at 6 1 1.96 due to the vinyl deuterium. By studying the reaction of 1 with trans-2-butene at -60 OC, we have directly observed p-alkylidyne complex 5 as the kinetic product of the reaction. A mixture of 1 and trans-2-butene (12.4 "01) in 0.34 mL of CD2C12was stirred at -60 "C for 3 h and then analyzed by 'H N M R at -22 OC.' In addition to unreacted 1 (-50%), the 'H NMR indicated that alkylidyne complex 5 was the major addition product of the reaction (- 50%); no 6 or 7 (
