Baldwin-Type Rules for Metal-Controlled Intramolecular Migratory

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Baldwin-Type Rules for Metal-Controlled Intramolecular Migratory Insertions. A Computational Study of Ni, Pd, and Pt Case Béla Fiser,†,‡,§ Juan M. Cuerva,*,∥ and Enrique Gómez-Bengoa*,† †

Department of Organic Chemistry I, University of the Basque Country (UPV-EHU), Donostia−San Sebastián, 20018, Spain Institute of Chemistry, University of Miskolc, Miskolc-Egyetemváros, 3515, Hungary § Ferenc Rákóczi II, Transcarpathian Hungarian Institute, Beregszász, Transcarpathia, 90200, Ukraine ∥ Department of Organic Chemistry, University of Granada, Granada, 18071, Spain ‡

S Supporting Information *

ABSTRACT: Transition metal (Ni, Pd, Pt) promoted carbocyclizations of alkenes and alkynes have been computationally investigated to rationalize the easiness or difficulty of such processes depending on the size of the forming ring. Special emphasis has been placed on measuring the activation barriers and the exo versus endo selectivities of the processes. The study leads us to propose some qualitative rules for metal controlled ring closures, noting some discrepancies with the classical Baldwin’s rules for nonmetallic cyclizations. Also, some unexpected results were found, like the extremely low activation barriers for 3-exo-dig and 3-exo-trig closures, contrasting with the scarcity of such experimental procedures, which can be computationally interpreted by the reversibility and endergonicity of cyclopropane/cyclopropene ring formation in the presence of these metals. The consistency of the present rules has been further checked by introduction of selected structural variations in the substrates.

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group 10 elements (Ni, Pd, Pt) with different chain lengths.5 In all cases, the coordination sphere of the metal was saturated with phosphine ligands, PH3 for simplicity, and also PMe3 or PPh3 for validation purposes (Scheme 1). Within this context, carbometalations can be viewed as a special case of electrophilepromoted nucleophilic cyclizations, in which the electrophilic center, the metal, is strongly interacting with the carbanion. For that reason, we have also calculated as a control the corresponding parameters of related carbolithiations (Figure

INTRODUCTION Baldwin’s rules for ring closure, based on the favored trajectories between reacting species, have been of tremendous utility since they were first published in 1976,1 becoming one of the most inspiring and influential concepts in organic synthetic chemistry in the last four decades. In his seminal paper, Baldwin also introduced the well-known nomenclature for cyclizations, which is still of general use in many organic reactions involving anions, cations, and radicals. The rules, empirical in nature, have been shown to have quite general applicability, although some of the predictions have found exceptions and limitations, leading to later corrections.2 Also, in the case of the nucleophilic addition, they were originally limited to first row elements, as a consequence of the larger atomic radii of heavier atoms, which can bypass the geometric restraints on disfavored ring closures. However, the rules, or at least the nomenclature, have been assumed to be applicable to transition metal controlled reactions,3 especially in the context of the Pdcatalyzed Heck-type reactions.4 However, a systematic mechanistic study (computed activation energies) of their validity and selectivity in these metal-including systems has not been reported.

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Scheme 1. Computed Carbometalation System and Its Structural Variations

RESULTS AND DISCUSSION

For that reason and to expand the undoubted utility of these rules, we have computationally studied a set of intramolecular carbometalations of alkenes and alkynes of alkyl metals of © XXXX American Chemical Society

Received: November 7, 2017

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DOI: 10.1021/acs.organomet.7b00812 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics 1), to compare them with those previously computed2b−g and our own data (Figure 1) on pure anionic reactions (absence of

Table 1. Computed Activation Energies for Pd(II)Controlled Carbometallations

trigonal 3-exo 4-exo 5-exo 6-exo

2.6 22.6 11.9 15.6

44.3 26.7 16.1 18.4

digonal 4-endo 5-endo 6-endo 7-endo

3-exo 4-exo 5-exo 6-exo

3.6 18.0 13.2 21.2

62.9 40.1 25.9 24.2

4-endo 5-endo 6-endo 7-endo

With respect to the less favored endo cyclization mode, the most favored processes are represented by the larger 7-endo-trig (18.4 kcal/mol)10 and 6-endo-trig (16.1 kcal/mol),11 which should be feasible in general, whereas 5-endo-trig (26.7 kcal/ mol) could also be possible in certain circumstances. Although 5-endo-trig processes were regarded as disfavored in the original Baldwin’s rules, our finding is in line with the recent studies by Alabugin on the related anionic systems and with a small number of experimental reports on Pd-catalyzed 5-endo-trig cyclizations.12 In contrast, the energy dramatically increases for the smallest rings, 4-endo-trig and 4- and 5-endo dig, becoming prohibited processes (over 40 kcal/mol), in agreement with the absolute lack of experimental examples in the literature. The most surprising data in Table 1 correspond to the 3-exoprocesses. In fact, the lowest activation energy (2.6 kcal/mol) was computed for the 3-exo-trig cyclization. This extremely low energy value suggests that cyclopropane ring formation should be a spontaneous and straightforward reaction, in notorious contradiction with the scarcity of such experimental processes in the literature.13 Even in the case of the rare cyclobutane/ cyclobutene ring formation (4-exo-trig and -dig reactions),14 the computed values are low (22 and 18 kcal/mol). The most dramatic example was found for the 3-exo-dig mode, whose activation barrier amounts to only 3.6 kcal/mol, while at the same time has only a few precedents in the literature.2b We can also note here the discrepancy with the original Baldwin’s predictions, which regarded 3- and 4-exo-dig processes as forbidden. As a plausible explanation to these intriguing phenomena, we found that 3- and 4-exo processes are endergonic, and thus, highly reversible (Figure 3, top).15 The forming 3- and 4membered palladacycles are kinetically unstable; thus, these reactions must be coupled with a subsequent exergonic fast reaction (i.e., β-hydride elimination) in order to lead them to completion (Figure 3, bottom).7,13 Previously, similar conclusions were drawn by Alabugin et al.2b,c in related systems. The difficulty of designing such a strategy for alkyne substrates might be the reason for its nonoccurrence. Remarkably, competitive 3-exo versus 5-exo cyclizations have been described in the literature,16 where 3-exo-trig is sometimes preferred, in agreement with our computational data.17 Nickel. Next, we checked the reactivity of the smaller Ni atom derivatives (Table 2, Figure 4). There are two sets of data worth mentioning. First, the activation barriers are consistently lower than the corresponding ones for Pd, while at the same time similar trends of reactivity are maintained. For example, all exocycles can be easily formed (0.5 to 16.7 kcal/mol). The

Figure 1. Selected transition states: anionic cyclizations (middle) and carbolithiations (edges). Corresponding barrier heights (kcal/mol) and attacking angles in the forming cycles are also shown. Green check, favorable; red “x”, unfavorable.

any metal). In fact, carbolithiations can be considered as more realistic model systems than pure anionic cyclizations. As a result, exo cyclizations are always highly favored over the corresponding (n + 1)-endo mode when lithium atom is present, with the differences being very large for small ring sizes (3-exo versus 4-endo, > 30 kcal/mol) and moderate as the number of atoms in the ring increases (6-exo versus 7-endo, ca. 10 kcal/mol). The cases where an increase in the activation energy is noted from the pure anion to the organolithium derivative can be rationalized by the lack of stabilization of the pure anionic starting material. Thus, the inclusion of the lithium atom decreases the energy of the starting point, increasing the activation energy. The computed data clearly show that all exo cyclizations are attainable and consistently under 20 kcal/mol,6 while endo types are over 20 kcal/mol, becoming prohibitive for the 4-endosystem (53.7 kcal/mol). These data, including the similarities in the activation energies computed in our case for trig- and dig-type reactions (Figure 1) are in qualitative agreement with Alabugin’s predictions.2b,d Regardless of the type of approximation (endo/exo) or the alkene/alkyne character of the reacting bond, there seems to be an ideal angle of around 115−125° where activation energies are minimal. Larger angles have not been found, while the prohibitive activation energies appear when due to structural restrictions, the attacking angles are decreased below 90° (4endo situations). In general, the presence of lithium widens the attacking angle by ca. 10° relative to the naked anion situation. Palladium. Next, we computed the carbometalations involving transition metals. In the case of Pd, some general trends could be noted after careful inspection of the data in Table 1. For example, the exo cyclizations are always feasible, in both digonal and trigonal cases, showing energies lower than 23 kcal/mol in every case (Table 1). Also, the exo closures are much lower in energy than the corresponding (n + 1)-endo modes, as expected from the many known examples of Pdmediated alkyl carbopalladations that fall into 5-,7,8 and 6-exo9 cyclization modes. Finally, double bonds (trigonal) are predicted to be more reactive than triple bonds (digonal) in general, with the sole exception of the 4-exo case. B


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Organometallics

Figure 4. Computed structures of selected transition states, their attacking angles, and activation energies for Ni(II)-controlled cyclizations.

Figure 2. Computed structures of selected transition states, their attacking angles, and activation energies for Pd(II)-controlled cyclizations.

26.7 kcal/mol with Pd) or 7-endo-dig (8.4 versus 24.2 kcal/ mol), becoming feasible processes with Ni-complexes. A second significant aspect to remark is that the differences between the competing cyclization modes (n-exo versus n + 1endo) are much closer with nickel than with palladium. Even a reversal in reactivity is observed in some cases, like 6-endo-trig (8.2 kcal/mol) versus 5-exo-trig (9.3 kcal/mol), suggesting that its selectivity could be tuned depending on experimental conditions. Cramer et al.18 have nicely shown that by a proper choice of the ligand coordinated to the nickel atom, the regiodivergent experimental formation of either 5-exo or 6-endo cycles from alkenyl nickel intermediates is achievable from a single starting material, in qualitative agreement with our calculations (Figure 5). Other authors have recently described Ni-catalyzed 5-exodig19,20 or 6-exo-dig20 reactions of various substrates.

Figure 5. Cramer’s reversal of regioselectivity (5-exo or 6-endo) by a proper choice of ligand with Ni(cod)2.18

Figure 3. Computed activation and reaction energies for the 3-exo-trig process (top), and its synthesis implementation for cyclopropane ring formation (bottom; Bräse et al.).13a

Platinum. The reversal effect in reactivity is observed for the larger size atom of Pt (Table 3, Figure 6). While the reactivity trend is preserved, the activation energies are moderately higher

Table 2. Computed Activation Energies for Ni(II)Controlled Carbometallations

Table 3. Computed Activation Energies for Pt(II)Controlled Carbometallations


0.5 16.7 9.3 9.8

34.6 17.5 8.2 16.1



1.0 9.7 9.2 9.6

58.4 32.5 12.6 8.4



decrease in the reaction barriers positively affects some Pdborderline cases, like 5-endo-trig (17.7 kcal/mol with Ni versus C

9.8 27.5 18.9 21.6

52.1 33.9 25.5 26.9



8.7 23.7 20.2 19.7

64.7 44.8 31.8 21.2



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Organometallics

This is a clear indication that the structural requirements of the transition state are not dependent on the size of the transition metal but rather only on the nature of the triple/double bond and the length of the alkyl chain to be cyclized. Finally, after having compared the cyclizations of these simple model structures controlled by Ni, Pd and Pt-(PH3)2 complexes, we decided to investigate other related systems in order to extend the scope of the study and to measure the influence of some of the most typical structural factors present in the forming rings. The 5-exo- (ΔG = 11.9 kcal/mol, Figure 7) and 6-endo-trig (ΔG⧧ = 16.1 kcal/mol) carbopalladation

Figure 6. Computed structures of selected transition states, their attacking angles, and activation energies for Pt(II)-controlled cyclizations.

in all cases, around 5−10 kcal/mol with respect to Pd and 10 kcal/mol or more comparing to Ni. As a consequence, all endo modes become either borderline or forbidden, while exo modes are in general over 20 kcal/mol. This is probably one of the reasons for the scarcity of cyclization methods employing Platinum catalysis.21 The selectivity trends found for Pd are maintained with little changes for Pt. We then analyzed in more detail the structural features of the transition states, some of them responsible for the exo/endo selectivity,22 and the intriguing finding was that the difference between the exo and endo cyclization becomes lower or even reversed for the Ni(II) species. In general, the transition states are early according to the C1−C2 bond forming distance (>2 Å, Table 4), but at the same time, the M−C3 distance is in Table 4. Comparison of the Computed Distances and Activation Energies for 5-exo- and 6-endo-Trig Processes Figure 7. Calculation of the effect of different structural variations on the energies of 5-exo and 6-endo cyclizations.

5-exo-trig

reaction pair (I) was selected as a reference. Different types of structural modifications were introduced in the reference system, regarding the ligand, functional groups, and hybridization of the unsaturated system or type of metal. For example, the reactivity and selectivity are not sensitive to changes in the phophine, as electron richer PMe3 (II) or bulkier PPh3 (III) ligands exert a minimal effect on the energy values, validating the results obtained for PH3 as a general model. Introduction of methyl substituents at the terminal position of the double bond (IV) increases the activation energies, and interestingly, the steric effect is much more noticeable for the C---C(Me2) bond formation (29.5 kcal/mol, endo mode) than for the Pd--C(Me2) case (14.1 kcal/mol, exo). This significant difference means that Pd is able to overcome the steric effects more easily than carbon when forming a bond. The present model reaction does not seem to be sensitive to introduction of substituents in the chain, as the energies do not suffer any variation when the double methyl substitution is located at the contiguous position to palladium (V). Changing the hybridization type of the carbon bound to Pd from C(sp3)−Pd to C(sp2)−Pd (VI) decreases the activation energy of the exo mode to 7.8 kcal/mol but disfavors the formation of the 6-endo cycle (18.7 kcal/mol). The large ring strain introduced by the phenyl substitution in VII raises the activation barriers, especially for the endo mode,

6-endo-trig

Ni

Pd

Pt

2.15 2.10 2.16 1.94 9.3

2.11 2.34 2.42 2.08 11.9

2.07 2.39 2.48 2.09 18.9

C1−C2 M−C1 M−C2 M−C3 ΔG

Ni

Pd

Pt

2.13 2.01 1.99 1.99 8.2

2.15 2.26 2.24 2.14 16.1

2.10 2.31 2.28 2.13 25.5

every case much shorter than M−C1, meaning that the stabilization of the developing negative charge at C3 by the metal is more important than the stabilization of the prior existing one at C1. In this sense, the exo transition states can be considered as [2σ + 2π]-four membered cyclic structures (M− C2 largest), while the endo transition states resemble more closely the attack of a naked anion to a metal-activated alkene or alkyne (M−C1 largest), being particularly true in the case of the Ni species.23 The attacking angles (see Figures 2, 4, and 6, bottom, and Supporting Information) do not vary significantly between the three metals, Ni, Pd, and Pt, despite the large size differences among them. The maximum deviation found was 7° for the case of 7-endo-dig cyclization (103° for Pd, 96° for Ni). D


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Organometallics largely disfavored in this case. The 5-exo cycle formation would be still feasible for this substrate (13.2 kcal/mol). Also, a Rh(I) complex (VIII) as cyclization promoter24 would have similar energies to those of Pt(II), while the performance of a Ni(II) complex with NHC carbene ligand (IX) is similar to its phosphine analogue in Table 2.

to extend the applicability to different experimental scenarios. We hope that the present results could serve as a guide to explore not yet described cyclization processes based on the very accessible computed activation energies and the qualitative comparison of the preferences.

CONCLUSIONS An extension of the Baldwin’s rules was proposed which can be applied to group 10 alkyl metalations, in terms of relative favorable/unfavorable processes (Figure 8), showing a larger

The studied structures were optimized at DFT level by using the M0625 functional as implemented in Gaussian 09.26 Calculations were carried out by using the 6-31+G(d,p) basis set for C, H and P and Stuttgart/Dresden (SDD)27 effective core potential (ECP) basis set for Ni, Pd, Pt and Rh. Solvent effects were taken into account at the same levels of theory by applying the conductor-like polarizable continuum model (CPCM, solvent = dichloromethane).28 The solute cavity was constructed using radii from the UFF force field. The critical stationary points were characterized by frequency calculations in order to verify that they have the right number of imaginary frequencies, and the intrinsic reaction coordinates (IRC)29 were followed to verify the energy profiles connecting the key transition structures to the correct associated local minima. In some cases, the transition states were located using M062X/631+G(d,p)/SDD level of theory. Thereafter, the resulting structures were used in single-point calculations at the M06/6-31+g(d,p)/SDD level of theory. Then, these single-point energies were used to get the corresponding Gibbs free energies which are comparable with those coming from the above-mentioned one step M06/6-31+G(d,p)/SDD calculations.

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COMPUTATIONAL METHODS

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00812. Computational methods and tables of results (PDF) Cartesian coordinates for all the complexes (XYZ)

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Figure 8. Summary of favorable, unfavorable, and borderline cyclizations in the presence and absence of metal.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

and richer reactivity than their nontransition metal promoted counterparts. Some general reactivity trends are comparable with those of anionic, or radical cyclizations, like the ease of all exo cyclizations, difficulty of the endo processes in small cycles (4- and 5-), and the lower activation energy for n-exo ring formation than for (n + 1)-endo cases. These effects are in agreement with the abundance of exo cyclizations in the literature and with the lack of 4-,5-endo-dig and trig cyclizations. It is worth noting the surprising extremely low activation energy for 3-exo processes, contrasting with the scarcity of experimental examples. The calculations revealed that despite the kinetic preference this disagreement might be due to the endergonic character and the high reversibility of these processes, which only can lead to completion if it is coupled with some other subsequent irreversible step. These findings are also in agreement with previously reported ones in classical anionic and radical cyclizations.2 In general, the exo/endo energy gap is lower for the Nipromoted processes in agreement with some described Nicatalyzed regio-divergent cyclizations, becoming largest for the Pt case. The analysis of the structural features show quite different geometries and coordination patterns for exo versus endo transition states. Finally, some structural variations were introduced in the calculations to further validate the conclusions of the study and

ORCID

Enrique Gómez-Bengoa: 0000-0002-8753-3760 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS B.F. thanks the European Commission for a Marie Curie fellowship (FP7-PEOPLE-2012-ITN-316379) and the support by the New National Excellence Program of the Ministry of Human Capacities (HU) under the UNKP-17-4-I-ME/17 project. This research was also supported by the European Union and the Hungarian State, cofinanced by the European Regional Development Fund in the framework of the GINOP2.3.4-15-2016-00004 project, aimed to promote the cooperation between the higher education and the industry. We also thank IZO-SGI SGIker of UPV/EHU for their technical and human support.

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REFERENCES

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