Mechanism of the Molybdenum-Mediated Cadogan Reaction

University of Groningen

Mechanism of the Molybdenum-Mediated Cadogan Reaction

Castiñeira Reis, Marta; Marín-Luna, Marta; Silva López, Carlos; Faza, Olalla Nieto

ACS Omega DOI:

10.1021/acsomega.8b01278

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Castiñeira Reis, M., Marín-Luna, M., Silva López, C., & Faza, O. N. (2018). Mechanism of the Molybdenum-Mediated Cadogan Reaction. ACS Omega, 3(6), 7019-7026.

https://doi.org/10.1021/acsomega.8b01278

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Mechanism of the Molybdenum-Mediated Cadogan Reaction

Marta Castiñeira Reis,

Marta Marín-Luna,

Carlos Silva López,

*

and Olalla Nieto Faza

*

†_{Departamento de Quı́mica Orgánica, Universidade de Vigo, Campus Lagoas-Marcosende, 36310 Vigo, Spain} ‡_{Departamento de Quı́mica Orgánica, Universidade de Vigo, Campus As Lagoas, 32004 Orense, Spain}

*

ABSTRACT: Oxygen atom transfer reactions are receiving increasing attention because they bring about paramount transformations in the current biomass processing industry. Significant efforts have therefore been made lately in the development of efficient and scalable methods to deoxygenate organic compounds. One recent alternative involves the modification of the Cadogan reaction in which a Mo(VI) core catalyzes the reduction of o-nitrostyrene derivatives to indoles in the presence of PPh₃. We have used density functional

theory calculations to perform a comprehensive mechanistic study on this transformation, in which wefind two clearly defined stages: an associative path from the nitro to the nitroso compound, characterized by the reduction of the catalyst in the first step, and a peculiar mechanism involving oxazaphosphiridine and nitrene intermediates leading to an indole product, where the metal catalyst does not participate.

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Heteroatom transfer reactions (particularly those involving oxygen) have captivated the scientific community for a long time.1−5 Traditionally, most of the interest surrounding this kind of oxygen transfer has pivoted around the development of reactions oxygenating hydrocarbons (epoxydations, dihydrox-ylations, etc.), with the main goal of increasing the functionalization of simple petrol-derived building blocks.6−10 The interest in the generation of renewable feedstocks for the chemical industry has prompted attention to the reverse procedure, deoxygenation of oxygen-rich biomass sub-(products), to produce highly reduced building blocks, analogous to those obtained from petrol sources.11−17

In the development of oxygen transfer reactions, nature has provided us with a model: metalloenzymes.18−26Notable key examples of these enzymes are the high-valent manganese species implicated in the photosynthetic water splitting,27,28or cytochrome P-450 as the responsible agent for the metabolism of steroids.29 Among these metalloenzymes, molybdenum is found in different types of oxotransferases sharing a common active center that consists of one Mo(VI) atom with at least one oxo ligand.30,31Synthetic molybdenum systems mimicking the chemical behavior of these oxotransferases have been reported, the vast majority involving penta- and hexa-coordinated molybdenum complexes featuring sulphurated bidentate ligands.25,32,33 There is limited information on systems in which the coordination sphere is reduced to four ligands. In all these systems, the [MoO2]2+active core has been only taken into account considering its oxidizing power and more scarcely as a Lewis acid.34,35On the basis of this, we have recently published a theoretical study on the deoxygenation of dimethylsulfoxide catalyzed by MoO2Cl2 that showcases the unusual behavior of the molybdenum core as a Lewis acid in

the initial step of the mechanism36 diverging from previous studies on similar reactions.14,23

In 2007, Sanz et. al. reported a promising new synthetic method for the preparation of complex organic molecules bearing an indole,37 a scaffold with a strong relevance to medicinal and pharmaceutical chemistry.38−40This procedure is becoming very popular to prepare indole derivatives41−43 and it involves the modification of the classical Cadogan reaction in which a MoO2Cl2(dmf)2 complex catalyzes the reduction of o-nitrostyrene derivatives in the presence of PPh₃ under thermal conditions (see Figure 1).44,45 Sanz et. al.

proposed that molybdenum only partially catalyzes the overall process exerting its catalytic activity during the reduction of the nitro to nitroso group. The subsequent reductive cyclization process furnishing the indole is considered to evolve without the participation of the catalyst.

In light of the potential of this reaction in the synthesis of bio-active compounds41,43 and its mechanistic uncertain-ties,46−50 we decided to study the mechanism of the Mo-mediated deoxygenation of ortho-nitrostyrene in the presence Received: June 7, 2018

Accepted: June 13, 2018

Published: June 28, 2018

Figure 1. Conversion of a general o-nitrostyrene scaffold into its indole derivative via a molybdenum-catalyzed reduction.

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of a phosphine as a reducing agent.37,51 The possible initial roles of [MoO2]2+ as a Lewis acid and as an oxidizing agent have been explored, resulting infive alternative reaction paths. The subsequent transformation of the transient nitroso derivative into the final indole has also been analyzed and several mechanistic questions on this second part of the mechanism have been addressed. In our recent work, we have found that similar chemistry operates through multisurface pathways, involving spin-crossing events along the reaction mechanisms.14,36We have, therefore, taken particular account of the spin multiplicity of the stationary points along the studied reaction mechanisms and when changes in spin were present, the minimum energy crossing points (MECP) on the seam formed between the crossing potential energy surfaces were computed to obtain a qualitative picture of the probability of such forbidden process to take place (for a quantitative picture spin−orbit coupled density functional theory should be considered).52

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Different mechanisms can be envisaged for the molybdenum-catalyzed monodeoxygenation of the nitro group in the presence of PMe₃. We first followed the classical approach for the oxygen atom transfer (OAT) reaction followed in nature,23,53that is: the reduction of the catalyst as thefirst step, followed by the generation of a coordination vacancy on the metal center and a subsequent coordination of the substrate.

The reduction of the molybdenum catalyst is usually proposed to take place through the nucleophilic attack of PMe3on theπMo−O* orbital in the Mo(VI) catalyst 1, leading to the formation of Mo(IV) complex 2 (see Figure 2).36,54−57 The P−O distance found in 2 (d_P−O= 1.56 Å) is very similar to that found in isolated phosphine oxide compounds (dP−O= 1.52 Å),58−60indicating a weak interaction between the metal

and OPMe₃. Our calculations show that the triplet state in this complex (2) is 7.3 kcal/mol more stable than the singlet, suggesting that a switch between the singlet and triplet potential energy surfaces may occur along the reaction path during the evolution from 1 to 2. In this sense, we have located a MECP near 2 (seeFigure 2) analogous to that described for the Mo(VI)-mediated reduction of sulfoxides with phos-phine.36

The initial reduction of the catalyst therefore involves a barrier of 23.6 kcal/mol and a late post-transitional spin flip process. The ulterior release of the OPMe3ligand trough TS2−3 (ΔG‡ = 7.7 kcal/mol) results in the formation of a trigonal complex, 3 (5.7 kcal/mol), which features an extra coordination vacancy prompted to be filled by nitrostyrene with the formation of complex 4 (through a barrier of 10.8 kcal/mol). This energetically uphill path leading to 4, further evolves toward the more stable chelate 5 via a facile coordination of the dangling oxygen atom of the nitro group to the metal center (seeFigure 2). The difference in energy between the triplet and singlet in 5 is here reduced to just 0.13 kcal/mol from 2.27 in 4, indicating that a crossing between the singlet and triplet surfaces may lie nearby. Then, an oxygen atom transfer reaction from the nitro group to the molybdenum center takes place through an N−O bond cleavage (TS₅₋₆) involving an activation barrier of 19.3 kcal/ mol. The ground state of this transition state is singlet, confirming that the spin switch has occurred before the transition state. A low-energy crossing point could actually be located between 5 and TS₅₋₆(seeFigure 2). In the last step of this catalytic cycle, the departure of the reduced nitroso ligand allows the recovery of the catalyst through TS6−1 in an exergonic process (−24.7 kcal/mol).

Using the energetic span model,61 we found that the rate determining stationary points of this catalytic cycle are 2 (as the turnover determining intermediate) and TS3−4 (as the turnover determining transition state) with an energy span of 28.0 kcal/mol. This value is in fair agreement with the thermal conditions required in this reaction (seeFigure 1).

The uphill energy values found for the tricoordinated Mo(IV) species 3 and the region around it, led us to reconsider the reactivity of complex 2. This complex, which still displays a coordination vacancy, could behave as a Lewis acid, thus, opening the door to alternative mechanisms, as depicted in Figure 3. The Lewis acid reactivity of polyoxomolybdenum(VI) compounds in redox processes is not usually taken into account, although there is some literature on this topic.34−36,62,63In this context, we found it relevant to also explore the possibility of the catalyst behaving as a Lewis acid before (complex 1 inFigure 2) and after its reduction to Mo(IV) has taken place (complex 2 inFigure 2). Figure 3, therefore, contains the complex network of reaction pathways available for both complexes when they react as Lewis acids. The top section (blue) highlights the chemistry in the Mo(VI) state, whereas the bottom section (yellow) contains the chemistry in the Mo(IV) state. A manifold of steps connecting both regions via PMe3 addition or OPMe3 release has also been considered. The alternative pathways starting from Mo(VI) complex 1 (seeFigure 3, blue region) can be classified in terms of how late the reduction of molybdenum occurs along the path. Hence, how late they merge with the Mo(IV) path (yellow section inFigure 3).

Thefirst step, common to all these paths, involves the direct coordination of nitrostyrene to the metal center, yielding 11 Figure 2. Enzymatic-type path: proposed mechanism for the

molybdenum-catalyzed OAT reaction between Ar-NO2 and PMe3,

considering the reduction of the catalyst as thefirst step. Red color indicates structures with triplet ground states, whereas black color is used for structures in the singlet state. Computations were performed at the UB3PW91/Def2-SVPD (polarizable continuum model (PCM), toluene) theoretical level. Gibbs free energies are reported in kcal/mol (1 atm and 298 K), relative to 1 + PMe3+ Ar-NO2.

ACS Omega Article

DOI:10.1021/acsomega.8b01278 ACS Omega 2018, 3, 7019−7026

(11.0 kcal/mol). The long Mo−ONO bond distance, 2.54 Å, and the invariance of the bond length between the oxygen and nitrogen atoms d_O−NO= 1.23 Å with respect to that of the free nitro-system suggest a weak coordination between this ligand and the [MoO₂]2+_{core. This may be the result of the strong} trans effect caused by the oxo ligand.64a Starting from intermediate 11, we found three different reduction paths:

1. Path A: this path avoids altogether merging with the Mo(IV) mechanism. The reduction step occurs via a direct attack of PMe3 on an oxygen of the nitro group (instead of attacking an oxo ligand on the metal center), which leads to intermediate 13 (Figure 3). This intermediate can then evolve toward 6 through the release of OPMe3 with an almost barrierless transition state (0.6 kcal/mol). It could also be proposed that complex 13 releases the organic ligand, thus, connecting with the energetically more demanding noncatalyzed process (see the Supporting Information (SI) for more details). Once 6 has been formed, it evolves through a low-lying transition state (ΔG‡ = 4.4 kcal/mol) to the aryl nitroso compound recovering the active catalyst, 1. 2. Path B: the nitro ligand chelates the metal core through TS₁₁₋₁₂ with a very low barrier (0.6 kcal/mol) yielding 12. This complex is subsequently reduced to 8 in the presence of PMe3, thus, entering the Mo(IV) pathway. This reduction step is however energetically very demanding (32.0 kcal/mol).

3. Path C: PMe₃attacks theπ* MoO bond trans to the nitro ligand in 11 through TS11−7 (ΔG‡ = 21.2 kcal/

mol) and enters the Mo(IV) pathway (Figure 3, in yellow) with the formation of complex 7. The significant decrease of the Mo−ONO distance (from 2.54 Å in 11 to 2.00 Å in 7) and the increase of the O−NO distance (1.27 Å) with respect to the free ligand are associated with the impact of the softer trans effect created by the phosphine oxide. The selectivity found in TS11−7can be rationalized on the grounds of the steric hindrance imparted by the nitro ligand and the presumed lower apicophilicity of the oxo ligand.65,66 (See the SI for further discussion on the stereoselectivity of the described processes.)

The energy spans in these three mechanisms are 32.2, 32.0, and 30.9 kcal/mol for paths A, B, and C, respectively. These values are too high for these mechanisms to be competitive with the enzymatic-type mechanism provided inFigure 2(28.0 kcal/mol).

If we consider that the action of molybdenum as a Lewis acid takes place after reduction of complex 1 to 2, thefirst step would be the complexation of a nitrostyrene molecule to 2, leading to the formation of complex 7 (−8.1 kcal/mol, see Figure 3). The ground state for complex 2 is a triplet, whereas 7shows near degeneracy between the singlet and triplet states (the latter lying only 0.6 kcal/mol above the former). A spin flip area should therefore be located near 7. Indeed, a minimum energy crossing point between the two surfaces, which qualitatively determines the activation barrier, can be located so close to 7 that it is isoenergetic with this minimum at the 0.1 kcal/mol precision level commonly employed in Figure 3.Lewis acid-type path: available reaction pathways for the molybdenum-catalyzed OAT reaction between Ar-NO2and PMe3considering

the Lewis acid character of the catalyst. Mo(VI) pathways in blue (top) and Mo(IV) paths in yellow (bottom) with the connecting manifold between the two alternatives as vertical arrows on a blank background. Entry/exit points to the traditional mechanism illustrated inFigure 2and are indicated as boldface structures (red color for triplets and black color for singlets as inFigure 2). Computations were performed at the UB3PW91/ Def2-SVPD (PCM, toluene) theoretical level. Gibbs free energies in kcal/mol (1 atm and 298 K) relative to 1 + PMe3+ Ar-NO2are shown.

density functional theory calculations (and also in most experimental kinetic measurements). Once complex 7 is obtained, the nitro ligand chelates easily, through a barrier of 3.5 kcal/mol, leading to octahedral complex 8 (−15.4 kcal/ mol). At this point, intermediate 8 can release a molecule of OPMe₃ to furnish complex 5 (thus, connecting this path to that shown inFigure 2), but it can also suffer from (2 + 2) cycloreversion through TS₈₋₉by which the initial nitro group loses an oxygen atom (ΔG‡ = 23.8 kcal/mol) leading to 9. This last alternative is not only kinetically but also thermodynamically preferred, because 9 is a rather stable intermediate compared with 5 (−27.8 vs −5.1 kcal/mol, for 9 and 5, respectively, seeFigure 3). Complex 9 can then evolve either through cleaving OPMe3, which yields 6 (−21.9 kcal/ mol) or through cleavage of the nitrosoaromatic ligand, yielding 10 (−31.3 kcal/mol), the latter being clearly favored. Finally, the departure of the OPMe3 ligand in 10 through TS₁₀₋₁recovers the initial catalyst which can start a new cycle. This last reaction pathway involving the initial reduction of the complex and proceeding via 9 has an energy span of 23.8 kcal/mol, and hence, is more favorable than either of the alternatives considered above, including the classical mecha-nism illustrated in Figure 2. These results suggest that the classical pathway, involving a dissociative mechanism and occurring via severely deficient species in terms of coordination number, should be replaced by an associative mechanism,first involving a reduction step and then the addition of nitrostyrene in a Lewis acid/base step.

Once the deoxygenation of ortho-nitrostyrene is completed, the reduction and concomitant cyclization of the nitro-sostyrene derivative to furnish the final indole is expected, nevertheless, the mechanism is far from being well-known. For this second step, we have discarded the participation of the metal as a catalyst taking into account previous experimental results.67Cadogan described how 2-nitrosobiphenyl and other derivatives can be reduced to the corresponding carbazoles in the presence of phosphines or phosphites very rapidly even under mild conditions (a few minutes at 0−5 °C). This is indicative of low activation barriers in the absence of the catalyst. Our own exploratory calculations agreed with these results.b Some studies in the literature suggest a tandem cyclization/reduction process in which the indole is trans-formedfirst into an N-oxo-indole derivative and ultimately into a hydroxyindole.41,68,69 However, the high reaction barriers computed for the reduction of the former and the absence of a complete description for the reduction of the latter led us to discard these alternatives (see theSIfor further discussion of these pathways) and to consider the approach described for the Cadogan−Sundberg reaction.70

In this approach the proposed order of the steps is inverted with respect to the tandem cyclization/reduction and a deoxygenation reaction is followed by a cyclization step (Figure 4). We propose that the reducing agent, PMe3, initially attacks the nitrogen atom of 14, through a barrier of just 13.1 kcal/mol, leading to 19, which can further evolve toward oxazaphosphiridine 20. The N−P bond in 20 is subsequently cleaved, allowing the formation of a nitrene intermediate, 22, via the loss of OPMe3.

All our efforts to locate TS20−21were unfruitful, although the chemistry associated seems relatively simple (cleaving a single N−P bond). To explore the potential energy surface in greater detail in this region, we decided to apply the nudged elastic band (NEB) method. This method provides the lowest energy

pathway connecting two minima and it thus offers an approximated value for the activation energy associated with this bond cleavage.71 With this method (see the SI), we estimated the reaction barrier to be about 11 kcal/mol and found why a transition state could not be located with standard algorithms.c In this step the P−N bond cleavage is strongly coupled with a twist of the aromatic ring, as in an fouetté en tournant pirouette, whose ends rotated 180° with respect to its initial position (Figure 5). Then 21 releases a molecule of

OPMe₃ providing a nitrene intermediate which can undergo cyclization. For the transformation of 14 into 22, we also explored the direct attack of the reducing agent on the oxygen atom on the nitroso group but this resulted in a non-competitive path (see the SI,Figure S2).

Cadogan proposed that the cyclization step from nitrene 22 could occur on the most stable triplet surface.67,70,72,73 We confirmed that the triplet state is indeed the ground state of the nitrene, lying 9.3 kcal/mol lower than the singlet state, but found that the barrier for cyclization in this spin state is incompatible with the low thermal requirements of this reduction/cyclization tandem (0−5 °C).67If, on the contrary, Figure 4. Proposed mechanisms for the transformation of nitroso compound 14 into indole 24. Computations were performed at the UB3PW91/Def2-SVPD theoretical level. Gibbs free energies in kcal/ mol (1 atm and 298 K) relative to 14 + PMe3are shown.

Figure 5.Strong coupled motion in the P−N bond cleavage of 20 (See theWeb enhanced objectfor motion picture). Ballerina picture reproduced with permission from GFYCAT.com (unknown artist).

ACS Omega Article

DOI:10.1021/acsomega.8b01278 ACS Omega 2018, 3, 7019−7026

the reaction is considered to proceed adiabatically, nitrene 22 would evolve through cyclization via transition state TS22−23in the singlet state, with an associated activation barrier of only 5.8 kcal/mol. This small activation energy is not only compatible with the experimental conditions, but also with the nitrene being such afleeting intermediate so as to prevent it from relaxing electronically to the triplet state due to the spin forbidden nature of this process.

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In summary, we have disclosed here a detailed theoretical study on the reductive cyclization of ortho-nitrostyrene to furnish indole catalyzed by MoO2Cl2 and with the partic-ipation of a trialkyl-phosphine molecule as a reductant. Two series of mechanistic alternatives have been considered for the monodeoxygenation of the nitro to nitroso group mainly diverging on the role of the metal complex in the initial steps of the mechanism: as a redox agent or as Lewis acid. Our computations predicted that the most favorable mechanistic alternative is that in which the molybdenum(VI) catalyst acts as a redox agent rather than a Lewis acid in thefirst step and it immediately switches to a Lewis acid role for the second step. The catalyst is thus acting in a manner similar to that proposed for molybdoenzymes, but we have found that an associative mechanism is more favorable than the commonly proposed dissociative path. The subsequent conversion of the nitroso substrate to the final indole system occurs preferentially following a reduction/cyclization tandem process. In this second step the reductant is involved in a nucleophilic attack on the NO bond leading to an unusual oxazaphosphiridine intermediate. These results, in agreement with the previous hypothesis based on the experimental observations, reveal that the participation of a PMe3molecule is a requirement in both sections of the mechanism (the nitro reduction and the reduction/cyclization tandem process) but molybdenum seems to be only necessary for the reduction of the initial nitro compound.

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Geometries of all the stationary points were fully optimized by using the UB3PW91 functional74,75with the Def2-SVPD basis set.76,77 The SDD(28, MWB) effective core potential associated with this basis set was used to describe the inner electrons of the Mo atom.78The effect of the solvent (toluene) was modeled using the polarizable continuum model (PCM)79 with the default parameters implemented in the Gaussian 09 package.80 The Orca program81 was used, at the same computational level, to obtain the minimum energy crossing points between triplet and singlet surfaces when a change in multiplicity is needed along the reaction path. In these cases, the conductor-like screening model82was applied to take into account the solvation effect. To check the accuracy of this approximation, we optimized some of the minima and checked their relative energies with both programs resulting in similar values, as expected. Harmonic analysis was used to establish the nature of all optimized structures as either minima or transition structures. For all stationary points, the stability of the wave function was also confirmed.

To reduce the computational cost associated with both the extra electrons and a larger conformational space, we are using the smaller trimethylphosphine as the reducing agent, instead of the actual triphenylphosphine used in the experiments. We

have checked that the reaction barrier for the rate determining step is not severely affected by this simplification: the reaction barrier obtained for TS₁₋₂with PPh₃is 34.11 kcal/mol and the reaction energy−7.83 kcal/mol at the UB3PW91/Def2-SVPP level in toluene.

Although the catalyst used in the experimental conditions is a hexacoordinated species (MoO₂Cl₂(dmf)₂, dmf = dimethyl-formamide), we have used the tetracoordinated MoO2Cl2 complex as the starting point of all our catalytic cycles. In previous work, with similar Mo(VI)-catalyzed oxygen transfer reactions, we analyzed the reactivity of MoO₂Cl₂(dmf)₂, MoO2Cl2(dmf), and MoO2Cl2 and found that the latter is the most active catalyst.36

To simplify the reaction schemes, we have chosen a color code: the structures and data colored in red are those for which the most stable spin state is the triplet state and the data corresponds to this same state. On the other hand, those structures and data colored in black are those for which the singlet state is the most stable state and the data corresponds to this same state.

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*

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsome-ga.8b01278.

Cartesian coordinates, electronic energies and the number of imaginary frequencies for all the structures reported in this work, additional schemes and discussion of alternative mechanisms that have been considered (PDF)

Strong coupled motion in the P−N bond cleavage of 20 (MPG)

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Corresponding Authors

*E-mail: carlos.silva@uvigo.es. Phone: +34 986813268 (C.S.L.).

*E-mail:faza@uvigo.es (O.N.F.). ORCID

Carlos Silva López: 0000-0003-4955-9844 Olalla Nieto Faza: 0000-0001-8754-1341

Notes

The authors declare no competingfinancial interest.

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The authors thank the Centro de Supercomputación de Galicia (CESGA) for the allocation of computational resources. M.C.R. is thankful to the Spanish Ministerio de Ministerio de Educación Cultura y Deporte for her FPU fellowship. M.M.-L. thanks the Xunta de Galicia for a post-doctoral research contract (ED481B 2016/166-0). This work has been funded by the Ministerio de Economi ́a, Industria y Competitividad (grant CTQ2016-75023-C2-2P) and the Xunta de Galicia (grant EM2014/040).

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a_{Despite being aware of the basic character of PMe}

3, we have omitted the study of those paths in which it is directly coordinated to the metal center due to the high barriers reported elsewhere.36As a consequence, only those routes in ACS Omega

which the nitro compound is in the inner coordination sphere of the metal, in thefirst step, have been explored.

b

We performed preliminary calculations considering the metal-catalyzed reduction of nitrosostyrene to indole and found that the catalyst had deleterious effects on the kinetics of this process (produced higher activation barriers).

c

The energy obtained from the application of NEB is electronic energy.

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