Reaction of Fischer Alkynyl Carbene Complexes with Fluorenone

Publication Date (Web): June 22, 2010 ... The reaction of Fischer carbene complexes with imines to yield ... Nucleophilic attack of the imine to alkyn...
0 downloads 0 Views 3MB Size
Download PDF
Organometallics 2010, 29, 3117–3124 DOI: 10.1021/om100219h

3117

Reaction of Fischer Alkynyl Carbene Complexes with Fluorenone Imines: Mechanistic Studies Laura Rivado-Casas, Pedro J. Campos, and Diego Sampedro* Departamento de Quı´mica, Universidad de La Rioja, Grupo de Sı´ntesis Quı´mica de La Rioja, Unidad Asociada al CSIC, Madre de Dios, 51, E-26006, Logro~ no, Spain Received March 22, 2010

The reaction of Fischer carbene complexes with imines to yield fluorenylidene-pyrroline molecular switches has been explored. A combined experimental and theoretical study was carried out to understand the reaction mechanism. Experimental and theoretical data are in good agreement, which allows delimiting the reaction outcome. Nucleophilic attack of the imine to alkyne is the first step in the reaction path and the one with a higher energy barrier. The most stable intermediate was isolated from the reaction mixture, thus supporting the proposed mechanism. The different reaction outcome when using fluorenone or benzophenone imines was investigated with diverse imine moieties. The effect of substitution in both imine and carbene complexes was also explored in order to increase the reaction’s scope.

Introduction Fischer carbene complexes have shown in the last decades an impressive potential in synthesis due to their extremely versatile behavior.1-6 The extensive chemistry of this kind of organometallic reagents turns them into a very useful component of the synthetic toolbox. Among the most explored transformations, cycloaddition reactions consist of a valuable route toward different kinds of cycles of three, four, five, and six members.7-10 This versatility allows the synthesis of complex cyclic structures in a few steps and generally in good yields. We have recently reported11 the use of Fischer carbene complexes in the synthesis of a new family of photochemical switches based in the retinal chromophore: N-alkylated fluorenylidene-pyrroline (NAFP) switches. These compounds are readily available after methylation of the neutral species formed by reaction of different Fischer carbene complexes with imines (Scheme 1). In that paper it was shown that fast

Scheme 1. Synthesis of Fluorenylidene-Pyrroline Molecular Switches

photochemical isomerization can be achieved through irradiation of pure samples of neutral or cationic forms. The effect of substitution, solvent, methylation, and light source was investigated in order to control the switch behavior prior to its use in practical applications. In contrast with most switches based on overcrowded alkenes, the NAFP switches presented can use visible light to rotate. This constitutes a clear improvement over previous results and makes retinal-based molecular switches interesting candidates for technological applications. Although the first synthesis of similar compounds was reported by Aumman,12 no further efforts to explore the scope, limitations, and mechanism of the reaction have appeared in the literature. In our previous paper,11 it is made clear that the reaction path leading from the starting material to the isolated products is far from trivial. The scope of the method was explored in order to obtain information about the mechanism of this transformation. However, the usual complexity of mechanisms involving Fischer carbene complexes prevented us from suggesting a comprehensive reaction path. Even more, diverse reaction outcomes have been described when different R,β-unsaturated imines were used.13 Thus, the value of the reaction products prompted us to carry out a complete

*To whom correspondence should be addressed. E-mail: diego. [email protected]. (1) Barluenga, J.; Santamarı´ a, J.; Tomas, M. Chem. Rev. 2004, 104, 2259. (2) Barluenga, J.; Suarez-Sobrino, A. L.; Tomas, M.; Garcı´ aGranda, S.; Santiago-Garcı´ a, R. J. Am. Chem. Soc. 2001, 123, 10494. (3) Sierra, M. A. Chem. Rev. 2000, 100, 3591. (4) D€ otz, K. H.; Tomuschat, P. Chem. Soc. Rev. 1999, 28, 187. (5) D€ orwald, F. Z. Carbenes in Organic Chemistry; Wiley-VCH: Weinheim, 1999. (6) Wulff, W. D. In Comprehensive Organometallic Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon Press: Oxford, 1995; Vol. 12, p 469. (7) Barluenga, J.; Vicente, R.; L opez, L. A.; Rubio, E.; Tomas, M.;  Alvarez-R ua, C. J. Am. Chem. Soc. 2004, 126, 470. (8) Barluenga, J.; Martı´ nez, S.; Suarez-Sobrino, A. L.; Tomas, M. J. Am. Chem. Soc. 2001, 123, 11113. (9) Barluenga, J.; Tomas, M.; Rubio, E.; L opez-Pelegrin, J. A.; Garcı´ a-Granda, S.; Priede, M. P. J. Am. Chem. Soc. 1999, 121, 3065. (10) Fr€ uhauf, H. W. Chem. Rev. 1997, 97, 523. (11) Rivado-Casas, L.; Sampedro, D.; Campos, P. J.; Fusi, S.; Zanirato, V.; Olivucci, M. J. Org. Chem. 2009, 74, 4666.

(12) Aumann, R.; Yu, Z.; Fr€ ohlich, R.; Zippel, F. Eur. J. Inorg. Chem. 1998, 11, 1623. (13) Aumann, R.; Yu, Z.; Fr€ ohlich, R. J. Organomet. Chem. 1997, 549, 311.

r 2010 American Chemical Society

Published on Web 06/22/2010

pubs.acs.org/Organometallics

3118

Organometallics, Vol. 29, No. 14, 2010

Rivado-Casas et al.

Scheme 2. Mechanism Proposed by Aumann et al.

theoretical investigation on the reaction mechanism. Apart from the inherent interest of this study, a deep knowledge of the transformations involved could help us to understand the limitations of the method. Furthermore, with this information at hand we will be able to design the starting materials that could subsequently lead to new molecular motors and switches. The first try to explain the formation of these compounds was made by Aumann et al. (Scheme 2).12 This mechanistic proposal is far too simple to allow any useful prediction to be made. However, the intermediate proposed (structure A in Scheme 2) was isolated from the reaction mixture. This could be used as a benchmark in our own reaction mechanism, as this compound should be a stable intermediate in the reaction course. Previous experimental work showed11 some of the features the starting materials should have in order to successfully yield the desired compounds. The imine moiety seems to be limited to fluorenone derivatives. Indanone and benzophenone imines have also been tried with no success. However the amine part seems to be less sensitive to change, and several substituents can be introduced. Although not necessary for the switch structure, the only requirement is the presence of a CH subunit directly bonded to the iminic nitrogen atom. This H atom must migrate in any point along the reaction mechanism. Regarding the Fischer carbene complexes, modification of the metal, the alkoxy, and alkynyl substituents has been carried out, showing the versatility of the synthetic route. As a complementary approach, we decided to carry out theoretical research on the subject in order to unveil the reaction mechanism. Doing so, we would be able to tune the chemical behavior of these compounds through combined theoretical and experimental work. Theoretical calculations of transition metal complexes are now a well-established tool for the study of organometallic species.14 Especially important are density functional theory (DFT) methods, due to their expediency, which makes them viable for the study of large-size molecules at a fraction of the time required for post-Hartree-Fock (HF) calculations. An even more important advantage is that in most cases the expectation values derived from approximate DFT are better in line with experiment than results obtained from HF calculations. In the particular case of group VI carbene complexes, DFT (14) Niu, S.; Hall, M. B. Chem. Rev. 2000, 100, 353.

methods have shown a remarkable agreement with the experimental results, as they have been able to reproduce experimental results with considerable accuracy.15-17 We have also applied this methodology to the study of the photochemical cyclopentannulation of imine-carbene complexes with alkynes to make up for the difficulties in the experimental characterization of the intermediates involved.18,19

Computational Details All calculations were carried out using the Gaussian 03 program package.20 Since DFT methods combine the importance of including electron correlation effects and the possibility of dealing with large systems, ground-state molecular geometries were optimized within the nonlocal density approximation (NLDA), including Becke’s21 nonlocal exchange corrections as well as Perdew’s22 inhomogeneous gradient corrections for correlation. For C, O, N, and H, the standard split-valence 6-31G* basis set23 was employed. For the tungsten atom, we used the Hay-Wadt effective core potential24 with the minimal basis set split to [441/2111/21]. Geometry was fully optimized without any symmetry constraint for all model compounds. Optimized structures were characterized as minima or saddle points by frequency calculations, which also € (15) Tafipolsky, M.; Scherer, W.; Ofele, K.; Artus, G.; Pedersen, B.; Herrmann, W. A.; McGrady, G. S. J. Am. Chem. Soc. 2002, 124, 5865. (16) Torrent, M.; Duran, M.; Sola, M. J. Chem. Soc., Chem. Commun. 1998, 999. (17) Ziegler, T.; Autschbach, J. Chem. Rev. 2005, 105, 2695. (18) Campos, P. J.; Sampedro, D.; Rodrı´ guez, M. A. J. Org. Chem. 2003, 68, 4674. (19) Sampedro, D.; Caro, M.; Rodriguez, M. A.; Campos, P. J. J. Org. Chem. 2005, 70, 6705. (20) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 03; Revision C.02 ed.; Gaussian Inc.: Pittsburgh, PA, 2003. (21) Becke, A. D. Phys. Rev. 1988, A38, 3098. (22) Perdew, J. P. Phys. Rev. 1986, B34. (23) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (24) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299.

Article

Organometallics, Vol. 29, No. 14, 2010

3119

Scheme 3. Nucleophilic Attacka

a

Distances in angstroms, angles in degrees, free energies in kcal/mol relative to 1.

allowed obtaining the ZPE and thermal corrections. The Fischer carbene complex was calculated without any simplification. Although both chromium and tungsten carbene complexes have proven their utility in these reactions, we used the tungsten complex despite the bigger computational cost as far as better experimental results were obtained. The imine moiety was simplified in some calculations. When exploring the effect of the imine structure, complete benzophenone and fluorenone structures were used. In this case, we also checked the influence of diffuse functions by using the 6-31þG* basis set.

Results and Discussion Several examples of reactions between alkyne carbene complexes and imines have appeared in the literature.13,25 All of these reactions yield products of formal addition to the alkyne triple bond. This is not the case in the reaction under study, as the products obtained are far more complex, although some common features can be found. In particular, the whole reaction cascade seems to start in every case by nucleophilic attack of the nitrogen lone pair to the alkyne β-carbon atom. This is not surprising, as this atom is in the most electrophilic point in Fischer carbene complexes.4-6 Thus, we started the theoretical exploration of the reaction mechanism by computing the nucleophilic attack of the imine moiety to the alkyne (Scheme 3). (25) Funke, F.; Duetsch, M.; Stein, F.; Noltenmeyer, M.; de Meijere, A. Chem. Ber. 1994, 127, 911.

As shown in Scheme 3, two different possibilities for the nucleophilic attack were explored. The only difference is the relative approach between the two molecules, and both routes share most of the geometrical parameters. Interaction between the imine N atom and the alkyne β C atom is shown by the distance in the transition structures (2.020 A˚ in 2-TS and 2.060 A˚ in 3-TS), while the C-C triple bond lengthens (1.239 A˚ in 1, 1.279 A˚ in 2-TS, and 1.271 A˚ in 3-TS) and the alkyne C-C-C angle decreases (174° in 1, 138° in 2-TS, and 144° in 3-TS). Although both routes imply relatively high activation free energies (24.4 and 20.6 kcal/mol), both should be accessible under our reaction conditions (90-100 °C). The difference of ca. 4 kcal/mol in the free energy value for the two transition structures is related to the higher steric hindrance of the two cycles close together in 2-TS. Two different compounds (2 and 3 in Scheme 3) are formed through these two routes, but it is worth noting that these structures represent different conformations, and thus they can easily equilibrate through single-bond rotation along the newly formed C-N bond. These two conformations differ in the relative disposition of the imine moiety. While in 3 the imine part faces the metal atom with its coordination sphere, in 2 this interaction is minimized. This causes 2 to be 4 kcal/ mol more stable than 3. However, due to this different orientation, only 3 will be able to proceed because the reacting parts are close enough. Thus, the two different routes shown in Scheme 3 have to converge, as only 3 can proceed along the

3120

Organometallics, Vol. 29, No. 14, 2010 Scheme 4. Ring Closing

reaction path. 2’s only options are to lead to 3 or give back the starting materials. The next step in the reaction path corresponds to a ringclosing to yield a product of formal [2þ2] cycloaddition between the imine and alkyne moieties (Scheme 4). Related cycloaddition products have also appeared in the literature.13 Slight distortion in 3, which also has the adequate disposition of atoms, leads to some interaction between the imine C atom and alkyne R C atom (2.427 A˚ in 4-TS). This interaction also reflects a lengthening of the imine CdN bond (1.388 A˚ in 3 and 1.408 A˚ in 4-TS) and shortening of the bond formed in the previous step between the N atom and the alkyne β C atom (1.378 A˚ in 3 and 1.368 A˚ in 4-TS). These geometrical changes lead to the formation of 4, where the four-membered-ring formation is complete. This step implies a small barrier (15.3 kcal/mol above the starting materials, 1.2 kcal/mol above 3) and leads to the formation of the stable intermediate 4 (-6.4 kcal/mol). Although the computed energy for 4 shows the relative stability of this intermediate, the presence of an azetine moiety and the zwitterionic nature of this compound suggest the evolution to a different structure through ring-opening (Scheme 5). In fact, the breaking of the single C-N bond leads to the formation of a new Fischer carbene complex without the strain caused by the cycle. The main geometric changes leading from 4 to 5 are related to the readjustment of the electronic density caused by the ring-opening: CdC (1.368 A˚ in 4, 1.341 A˚ in 5-TS, and 1.288 A˚ in 5) and CdN (1.580 A˚ in 4, 1.469 A˚ in 5-TS, and 1.378 A˚ in 5) double bonds are now localized and connected by a C-C single bond (1.425 A˚ in 4, 1.444 A˚ in 5-TS, and 1.533 A˚ in 5). Also the metal fragment is affected, because the Fischer carbene complex structure is recovered, as shown by the W-C bond shortening (2.251 A˚ in 4, 2.224 A˚ in 5-TS, and 2.174 A˚ in 5). Besides, ring-opening also causes some general reorganization, allowing the five-membered ring and phenyl and isopropyl substituents to drift away. As a result, 5 shows a clear stabilization with a free energy value of -7.4 kcal/mol. In fact, 5 has the typical structure of an alkenyl carbene complex, and its structure is analogous to the intermediate located by Aumann et al. when they first reported the reaction.12 This fact clearly supports the proposed mechanism. Thus, the reaction steps studied by now lead to the formation of a new Fischer carbene complex through a sequential formal cycloaddition of an imine to an alkynyl complex and ring-opening.

Rivado-Casas et al. Scheme 5. Ring-Opening

From a synthetic point of view, it could be possible to avoid all the previous steps through synthesis of a carbene complex with a structure similar to 5. This could allow obtaining switches of structure similar to those already found in those cases where the reaction of alkynyl Fischer carbenes and imines (1) does not prove successful. Several examples of unsuccessful reactions were found when exploring the scope of this reaction.11 Some features of the final products are also present in 5, i.e., the fluorene moiety and the CdC double bond connecting the two cycles. One of the cycles still has to be formed, and this would need some kind of interaction between the carbene C atom and the imine moiety. This kind of interaction is clearly impossible to achieve in 5, as these two parts of the molecule are in opposite places. Thus, some kind of conformational change should take place before formation of the products can happen. Twisting along one C-C single bond could easily place these reacting points closer (Scheme 6). As expected, no significant changes in the structures were found and the conformational equilibrium should be easily achieved, as an energy barrier of only 6.4 kcal/mol from 5 was found. Thus, 6 and 5 will be in equilibrium, although 5 will be the main isomer. This agrees with the fact that only 5 could be isolated. As only 6 can proceed through the reaction path, the equilibrium will be displaced as 6 is consumed. After this conformational change, the carbene atom and isopropyl substituent are placed in the right position to interact and subsequently lead to ring-closing. Ring formation will be produced by a two-step sequence of hydride transfer followed by carbon-carbon bond formation and loss of the metal fragment. This two-step process has been previously invoked in Fischer carbene complex cyclizations (Scheme 7).26 The transition structure for this step shows that the hydride transfer is almost complete (H-Ccarbene 1.180 A˚, H-Cimine 1.720 A˚). Some other changes come with the transfer, such as CdN double-bond formation (1.471 A˚ in 6, 1.354 A˚ in 7-TS, and 1.315 A˚ in 7) and W-C bond lengthening (2.197 A˚ in 6, 2.419 A˚ in 7-TS, and 2.864 A˚ in 7). The energy barrier for this step is as high as 18.7 kcal/mol, and 7 has an energy of 5.8 kcal/ mol above the reactants. This suggests a rapid progression to the next intermediate through C-C bond formation and ringclosing. In fact, a compound analogous to 7 has been previously (26) Barluenga, J.; Rodriguez, F.; Vadecard, J.; Bendix, M.; Fa~ nanas, F. J.; L opez-Ortiz, F. J. Am. Chem. Soc. 1996, 118, 6090.

Article

Organometallics, Vol. 29, No. 14, 2010

3121

Scheme 6. Conformational Change

Figure 1. Energy profile. Scheme 7. Hydride Transfer

Scheme 8. Ring-Closing

proposed26 as a transition structure and not as a stable compound itself. However, our theoretical data show that this structure is indeed an intermediate in the two-step process of cyclization, although quite unstable. To complete the ring process, C-C bond formation must take place (Scheme 8.) The cyclization step will take place easily after 7 is formed, as a barrier of only 1.5 kcal/mol was obtained from calculations. The transition structure 8-TS is easily reached by slight deformation of the molecule approaching the two C atoms involved in the C-C bond formation. Although in 8-TS the distance between these two atoms is still 2.711 A˚, the effect of these interactions also appears in the terminal CdN bond (1.315 A˚ in 7, 1.329 A˚ in 8-TS, and 1.469 A˚ in 8) and W-C distance (2.864 A˚ in 7 and 2.962 A˚ in 8-TS). As can be seen, C-C bond formation and metal fragment loss take place simultaneously and the isolated product finally is formed. The metal fragment is still weakly bonded to the product, but it is easily removed under reaction conditions or during workup. Clearly this final step will be fast due to the small barrier and the formation of the most stable compound along the reaction path 13.4 kcal/mol below the reactants. Figure 1 shows the energetic profile of the reaction. Several conclusions can be drawn from the data shown in Figure 1. Formation of the final products is a favorable process, as 8 is far more stable than the reactants. The most difficult step is the first one, the nucleophilic attack to the alkyne by the imine, although the barrier could be surmounted under the reaction conditions (90-100 °C). After the 3 is

formed, the rest of the path should be easier to cover. Furthermore, the more stable intermediate 5 corresponds with the structure isolated by Aumann et al.12 6 is only a different, more unstable conformation, so it could not be isolated, and 4, also an stable intermediate, will be quite hard to isolate due to its zwitterionic character. All these features agree well with the experimental results.11 Once the general mechanism for the formation of the products under study was explored, we tried to understand the different reaction outcome found when imines with similar structures were used. Specifically, we were able to synthesize the target compounds using fluorenone imines but not when using benzophenone imines. Thus, we carried out theoretical calculations of the first reaction steps using in these cases complete models of fluorenone and benzophenone imines (Figure 2). Several differences can be found for the calculated paths. Every structure has a higher relative energy in the case of the benzophenone imine. Transition structures for the nucleophilic attack show similar free energies (10-TS 1.2 kcal/mol higher), but a bigger difference was found in the ring-closing step. In this case, benzophenone structure 11-TS is more than 5 kcal/mol higher in energy. Besides, intermediate 11 is only 0.5 kcal/mol more stable than the reactants, while in the fluorenone imine reaction intermediate 14 is clearly more stable than the reactants (5.4 kcal/mol). The reason behind this could be the higher steric hindrance in the transition structures and intermediates in the benzophenone reaction

3122

Organometallics, Vol. 29, No. 14, 2010

Rivado-Casas et al.

Figure 2. Energy profile for the first reaction steps of benzophenone and fluorenone imines.

caused by the phenyl rings. While in the fluorenone moiety they are forced into a planar disposition, in the benzophenone moiety they are free to rotate, and the space requirement is higher, as the two phenyl rings are rotated by ca. 30 degrees. Although our theoretical results qualitatively confirm the experimental outcome found, the difference between both paths (1.2 kcal/mol for the rate-determining steps) seems too small to account for the different reactivity of fluorenone and benzophenone imines. Due to the extended π-systems of the molecules and, also, the changes in the charge distribution along the path, we recomputed the first steps in Figure 2 with a basis set with diffuse functions included.27 Using 6-31þG*, the free energy for 10-TS increased to 22.8 kcal/mol, while the free energy found for 13TS was 20.0 kcal/mol. Thus, the energy difference changed from 1.2 kcal/mol to 2.8 by including diffuse functions. Although this energy barrier still seems too small to completely justify the reactivity, it emphasizes the different behavior of the imines. Several experiments were carried out in order to determinate whether the benzophenone imine and the carbene complex were able to yield the switch structure. Different reaction conditions were used (including temperature in the range between rt and 110 °C and diverse reactants ratios) with no success. The reaction was followed by 1H NMR; in all cases no signals due to any of the stable intermediates or products were found. The reaction outcome was always Fischer carbene complex decomposition and imine recovery after several days. Next, we aimed for the study of some structural modifications in the benzophenone imine that could lead to the (27) We thank the referee for this suggestion.

Figure 3. Energy profile for the first reaction steps of substituted benzophenone imines.

successful formation of the desired products. In order to do this, we explored the first two reaction steps using electron-rich and electron-poor benzophenone imines. We used in this part of the study imines with six electron-withdrawing (NO2) or electron-donating (OH) substituents to maximize the effect (Figure 3).

Article

Organometallics, Vol. 29, No. 14, 2010

3123

Scheme 9. Electron-Rich Benzophenone Imine

Scheme 10. Alkynyl Fischer Amino-Carbene Complexes

The effect of the substituent’s electronic nature (although maximized by the presence of multiple substituents) is dramatic. The nucleophilic attack step is far more easy using electron-rich benzophenone derivatives (16-TS, barrier of 10.3 kcal/mol) than electron-poor imines (19-TS, 38.2 kcal/ mol) or even unsubstituted benzophenone (10-TS, 20.0 kcal/ mol). The increase in the energy barrier using electron-poor imines will lead to slow reaction rates and eventually low reaction yields due to competition of the Fischer carbene complex decomposition reaction. This effect has already been noticed when using fluorenones with different substitution.11 However, the use of electron-rich imines also leads to an increase in the energy barrier for the ring-closing step (17-TS, 24.5 kcal/mol) compared to the unsubstituted analogue (11TS, 23.9 kcal/mol), thus suggesting that excessive electron density could be counterproductive to successfully yield the desired products. To test these conclusions, we carried out some complementary experimental work on substituted benzophenone imines. Using a standard protocol, we synthesized an electron-rich imine from a benzophenone with donor groups28 and tried its reactivity toward the carbene complex (Scheme 9). Several experiments were carried out at different temperatures (from rt to 70 °C) and followed by 1H NMR for days. In all cases no signals corresponding to stable intermediates or products were found. Finally, we decided to explore the use of Fischer aminocarbene complexes in the synthesis of analogues of the fluorenylidene-pyrroline molecular switches. By using these organometallic species we tested the influence of the carbene carbon atom substituent in the reaction outcome as well as included a linkage point in the molecule. This will be necessary to incorporate these switches into a bigger molecular framework. We synthesized several alkynyl Fischer amino-carbene complexes29 (28) Zhou, C.; Larock, R. C. J. Org. Chem. 2006, 71, 3551. (29) Aumann, R. Chem. Ber. 1993, 126, 2325.

Figure 4. Energy profile for the first reaction steps of aminocarbene complexes with fluorenone imines.

and tested them in our optimized reaction conditions. The reaction was tried at different temperatures and reactant ratios. In all cases, the carbene complex decomposed and only the imine was recovered. In order to explain the lack of reactivity of Fischer aminocarbene complexes, we computed the first steps in the reaction mechanism for the reaction of phenylacetylenyl(isopropylamine) carbene pentacarbonyl-tungsten with fluorenone isopropyl imine (Figure 4). As can be seen, every structure in Figure 4 (two transition structures 22-TS and 23-TS and the first intermediate 22) is higher in energy than the analogues with alkoxy carbene complexes (Figure 2). Changing the ethoxy substituent for the amino group leads to a destabilization of the species involved, at least in the first steps in the reaction mechanism. Thus, the use of amino-carbene complexes will not yield the analogue switches, as both the nucleophilic attack of the imine (23.4 kcal/mol) and the cyclization steps (23.6 kcal/mol) have higher energy barriers. In this case, competing Fischer carbene complex decomposition takes place predominantly. The higher donating ability of the amino group in the carbene complex seems responsible for the higher energy of 22-TS, as it would

3124

Organometallics, Vol. 29, No. 14, 2010

make the alkynyl moiety less electrophilic. The same reason could explain the higher free energy of 22 as compared to 13.

Conclusions A complete mechanistic study of the reaction between Fischer alkynyl carbene complexes and imines to yield potential molecular switches and motors has been carried out by means of theoretical calculations. The mechanism suggested by computational data agrees well with the experimental findings. Nucleophilic attack of the imine to alkyne is the first step in the reaction path and the one with a higher energy barrier. The most stable intermediate was isolated from the reaction mixture, thus supporting the proposed mechanism.

Rivado-Casas et al.

The different reaction outcome when using fluorenone or benzophenone imines was explored with diverse imine moieties. The effect of substitution in both imine and carbene complexes was also explored in order to increase the reaction’s scope. A complementary experimental study was carried out to confirm the computational data.

Acknowledgment. We thank the Spanish MEC (CTQ2007-64197) for financial support. L.R. thanks the Comunidad Aut onoma de La Rioja for her fellowship. Supporting Information Available: Cartesian coordinates for the geometries discussed in the text. This material is available free of charge via the Internet at http://pubs.acs.org.