O–CN Bond Cleavage of Cyanates by a Transition-Metal Complex

Communication pubs.acs.org/Organometallics

O−CN Bond Cleavage of Cyanates by a Transition-Metal Complex Kozo Fukumoto,† AbdelRahman A. Dahy,‡,§ Tsukuru Oya,∥ Kazumasa Hayasaka,∥ Masumi Itazaki,∥ Nobuaki Koga,§ and Hiroshi Nakazawa*,∥ †

Department of General, Kobe City College of Technology, Kobe 651-2194, Japan Department of Chemistry, Faculty of Science, Assiut University, Assiut 71516, Egypt § Graduate School of Information Science, Nagoya University, Nagoya 464-8601, Japan ∥ Department of Chemistry, Graduate School of Science, Osaka City University, Osaka 558-8585, Japan ‡

S Supporting Information *

ABSTRACT: O−CN bond cleavage of cyanates (ROCN) has been achieved at room temperature in the reaction of ROCN with a methyl Fe, Mo, or W complex. A mechanistic investigation involving DFT calculations revealed that silyl migration from Mo to the CN nitrogen gave an N-silylated η2-imidato Mo complex. This intermediate analogue was isolated and characterized by X-ray analysis. Catalytic O−CN bond cleavage was achieved using Cp(CO)3MoMe under thermal conditions.

S

elective bond cleavage and selective bond formation are among the most important subjects in the field of sustainable chemistry. The selective cleavage of a weak bond is relatively feasible, whereas that of a strong bond with a weak bond remaining intact is generally quite difficult. The use of a transition-metal complex is one of the most appropriate approaches1 to achieve selective strong bond cleavage, because a transition metal may attach itself to a special part of the molecule through coordination, which in turn affects the electronic properties of a strong bond. This eventually leads to strong bond cleavage. Recently, we reported that iron and molybdenum complexes can cleave C−CN bonds2 in organonitriles and N−CN bonds3 in cyanamides, respectively, even though both C−CN and N−CN bonds are strong and hence difficult to cleave. In both cases, the key steps for achieving E−CN bond cleavage (E = C, N) are the same; these steps are shown in Scheme 1. The cyano group coordinates in an η1

reaction (SiMI reaction), may be applied to achieve the cleavage of the E−CN bond where E is an element other than C and N. Cyanate (ROCN) gained our attention because it is formally constructed by replacing the E in ECN with RO and it is considered to be a suitable target for carrying out the SiMI reaction. Cyanate exhibited two types of reactions:4 (i) an addition reaction to the CN triple bond (eq 1) and (ii)

nucleophilic substitution to cleave an R−O bond (eq 2). However, only a few examples involving RO−CN bond cleavage have been reported.4f,h,5 The RO−CN bond is considered to be strong and is not readily broken. For example, the X-ray structure of 2-chloro-5-cyanato-1,3-dimethylbenzene shows that the bond length of O−CN is 1.27 Å, which lies between that of a normal O−C single bond (1.43 Å) and that of an OC double bond (1.20 Å).6 This paper reports the first example of catalytic O−CN bond cleavage in ROCN. A solution of an equimolar amount of isopropyl cyanate, Et3SiH, and Cp(CO)2FeMe or Cp(CO)3MoMe (1) in toluene was irradiated with a 400 W medium-pressure mercury arc lamp at room temperature (Table 1, entries 1 and 3). The 1H NMR spectra of the product showed the formation of Et3SiCN in both cases. These results show that O−CN bond cleavage was achieved in this system, even though the yields of Et3SiCN were low.

Scheme 1. Reaction Sequences of E−CN Bond Cleavage

fashion to a 16e complex bearing a silyl group, and the complex is converted into an η2 complex. Then, the silyl migration from the metal center to the nitrile nitrogen atom occurs. Thus, an M−C−N three-membered-ring complex is formed. This migration is followed by E−C bond cleavage in the coordination sphere. The bond cleavage gives a silyl isocyanide complex with an E ligand. In the reaction sequences, the silyl migration triggers the cleavage of a strong C−E bond. This type of reaction, that is, a silyl-migration-induced © 2012 American Chemical Society

Received: December 20, 2011 Published: January 24, 2012 787

dx.doi.org/10.1021/om201257h | Organometallics 2012, 31, 787−790

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Table 1. iPr−OCN Bond Cleavage with Methyl Complexes

complex H. The oxidative addition of Et3SiH to H results in the formation of complex I. ROSiEt3 may be eliminated from I to produce J. The oxidative addition of Et3SiH to J results in the formation of K, and the reductive elimination of H2 regenerates A, thus completing the catalytic cycle. The oxidative addition/reductive elimination sequences shown in this cycle, such as the sequence from H to J and that from J to A, can be explained by considering σ-bond metathesis pathways. On the basis of theoretical calculations, it is proposed that σ-bond metathesis results in the formation of I′ instead of I and K′ and K″ instead of K (vide infra). According to the catalytic cycle shown in Scheme 2, CH4, ROSiEt3, and H2 are expected to be produced. The reaction of Cp(CO)3MoMe, 10 equiv of p-MeC6H4OCN and 200 equiv of Et3SiH was conducted in an NMR tube, and the reaction mixture was subjected to 1H NMR measurement. The presence of CH4 was observed. The presence of p-MeC6H4OSiEt3 and Et3SiCN in similar amounts was also observed (the TONs of p-MeC6H4OSiEt3 and Et3SiCN were 5 and 6, respectively). In Scheme 2, H reacts with Et3SiH to give J and ROSiEt3. However, the reaction of H with Et3SiH may also give ROH and A. The ROH thus formed may react with Et3SiH present in this system to form ROSiEt3 and H2 in the presence of a Mo catalyst. The reaction of p-MeC6H4OH with Et3SiH was carried out in the presence of 1 as a catalyst in order to determine which reaction was more favorable (eq 4). The reaction conditions

complex + iPrO−CN + Et3SiH ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Et3SiCN in toluene

entry 1 2 3 4 5 6 7

complex Cp(CO)2FeMe Cp(CO)2FeMe Cp(CO)3MoMe (1) Cp(CO)3MoMe (1) (C5Me5)(CO)3MoMe Cp(CO)3WMe (C5Me5)(CO)3WMe

conditions b

hν 70 °Cc hνb 100 °Cc 100 °Cc 100 °Cc 100 °Cc

time/h

yield/%a

12 12 24 24 24 24 24

16 trace 35 44 trace 4 trace

a

Calculated from the isolated (Et3Si)2O. bPhotoirradiation at room temperature. cThermal reaction under room light.

O−CN bond cleavage was also observed in thermal reactions (entries 2 and 4). A comparison of entries 4−7 shows that Cp(CO)3MoMe has higher activity than (C5Me5)(CO)3MoMe and the corresponding W complexes. The results of catalytic reactions (10 mol % of 1 was used on the basis of the concentration of ROCN) are given in eq 3.

were the same as those given in eq 3. p-MeC6H4OSiEt3 was obtained, and the TON was 4.1. Under the conditions given in eq 3, for which p-MeC6H4OH was not detected after the reaction, the same silyl ether was obtained with a TON of 5.6. This observation indicates that p-MeC6H4OH was not accumulated in this catalytic system. Therefore, even if p-MeC6H4OH is formed under the reaction conditions, the amount might be considerably lower than 10 equiv toward the Mo catalyst. Nevertheless, this system showed a greater TON (5.6) than that (4.1) in eq 4, where the amount of p-MeC6H4OH is 10 equiv toward the Mo catalyst. Therefore, we believe that the formation of ROSiEt3 from H and Et3SiH is preferable to that of ROH. Density functional theory (DFT) calculations were performed to verify the feasibility of the catalytic cycle shown in Scheme 2.7 The profiles of the potential energy corrected by zero-point energy (ZPE) and that of the Gibbs free energy (in italics) for the catalytic cycle are shown in Figures 1 and 2. The optimized structures are shown in Figures S1 and S2 in the

These results show that 1 can serve as a catalyst precursor to cleave O−CN bonds. Several solvents were examined, and toluene was found to be the best solvent. An aryl group that donates electrons and is less sterically hindered is considered to be a suitable substituent for the RO−CN bond cleavage in this system. Scheme 2 shows a proposed catalytic cycle for the reaction of 1 with Et3SiH and ROCN, which is based on the catalytic cycles of C−CN and N−CN bond cleavage.2,3 The 16e species A formed from Cp(CO)3MoMe reacts with ROCN to give either B or C. Complex C lie outside the catalytic cycle. Complex B isomerizes to complex D with η2-CN coordination. The silyl group in D migrates from Mo to the nitrile nitrogen to give the N-silylated η2-imidato complex E. The dissociation of the nitrogen from the Mo center and the coordination of the cyanate oxygen form the Mo−C−O three-membered-ring complex F. This rearrangement activates the O−C bond to give G. The dissociation of silyl isocyanide gives the 16e alkoxy Scheme 2. Proposed Catalytic Cycle

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almost planar (with a dihedral angle of 179.9°). On the other hand, as expected the Si−N bond formation weakens the C−N bond. The C−N bond distance is 1.272 Å in Em, whereas it is 1.166 Å in free methyl cyanate. As shown in Figure 1, Em is the most stable intermediate formed before the completion of the O−CN bond cleavage. The nitrogen atom dissociation in Em leads to the formation of another intermediate Fm with a threemembered-ring structure in which the oxygen atom lone pair coordinates to the Mo atom. The activation energy of this rearrangement is 26.6 kcal/mol, which is the largest in the catalytic cycle. The O−CN bond distance in Fm is calculated to be 1.452 Å. This shows that the coordination of the oxygen atom activates the O−CN bond, so that the O−CN bond cleavage is realized with a low activation energy of 1.5 kcal/mol, thereby resulting in the formation of the intermediate Gm. The orbital interaction that is important at TS4 is similar to that discussed in the study of acetonitrile C−CN bond cleavage.2a The profile of the free energy is qualitatively unchanged from that of the ZPE-corrected potential energy except for the coordination of methyl cyanate, where the entropy term has a significant contribution. Next, silyl isocyanide must rearrange itself as silyl cyanide. Our previous study showed that, after silyl isocyanide dissociates from the central atom, this rearrangement occurs with an activation energy of 23.9 kcal/mol.2a In order to complete the catalytic cycle, the silyl complex must be regenerated from the methoxy complex Hm produced by the silyl isocyanide dissociation. The energy profiles for the reactions are shown in Figure 2. The replacement of the methoxy group is carried out by the σ-metathesis of Hm with Me3SiH to form Jm via I′m and that of Jm with Me3SiH to form Am via K′m and K″m. In the former step, MeOSiMe3 is formed as an intermediate, and the latter step yields H2. Although we made several attempts to locate the oxidative addition product Cp(CO)2Mo(OMe)(SiMe3)(H) (Im), our attempts failed; this result indicates that oxidative addition could not be adopted. The activation energies for these two σ-metathesis reactions are 9.1 and 5.2 kcal/mol, respectively. Even after taking the entropy contribution into account, the free energies of activation are 12.3 and 7.4 kcal/mol, respectively; these values indicate that both reactions are easily activated. We also performed the calculations for the σmetathesis pathway that leads to MeOH + Am. The calculations showed that this path is less favorable than the path shown in Figure 2. The activation energy of this path (Figure S3) is 20.8 kcal/mol higher than that of the path shown in Figure 2. The bond formation between the negatively charged H atom in hydrosilane and the oxygen lone pair is difficult. As a consequence, the calculations show that the catalytic cycle proposed in Scheme 2 is feasible. In order to obtain experimental evidence supporting the catalytic cycle, we attempted to isolate E, which is one of the intermediates in the cycle. The isolation was unsuccessful in the reaction of 1 with iPrOCN. In contrast, the photoirradiation of a solution containing (C5Me5)(CO)3W(SiPh3) and iPrOCN in toluene at room temperature for 1 h generated 2 quantitatively according to NMR measurements in 91% isolated yield. Complex 3 having an Si(p-Tol)3 group was also isolated in a similar manner (Figure 3). The η2-imidato complex 3, which was formed for the first time, was confirmed using X-ray analysis (see the Supporting Information). Both the 1H and the 13 C NMR spectra show that the solid-state structure is retained in solution.

Figure 1. Energy profile for the cleavage of the C−O bond of cyanate through insertion into the Fe−Si bond. The depicted values are ZPEcorrected potential energies and the Gibbs free energies (in italics) in kcal/mol relative to Am + methyl cyanate.

Figure 2. Energy profile for generation of Cp(CO)2Mo(SiMe3) (Am) from Cp(CO)2Mo(OMe)(CNSiMe3) (Gm). The depicted values are ZPE-corrected potential energies and the Gibbs free energies (in italics) in kcal/mol relative to Am + methyl cyanate + 2 Me3SiH. The energy of Me3SiCN was added to those of Hm to Am.

Supporting Information. Unless explicitly stated otherwise, the ZPE-corrected energies are used in the discussion. Similar to the case for the Cp(CO)FeSiMe32a 16e complex, Am participates in an agostic interaction of the C−H bond with the Mo atom. The calculations show that the reaction first passes through the cyanate complex Bm with an η 1 coordination. The cyanate coordination saturates the Mo atom to result in a relatively large binding energy of 16.1 kcal/mol; however, it breaks the agostic interaction. Cm, with the coordination of cyanate oxygen, was calculated to be 18.7 kcal/mol less stable than Bm. For the reaction to proceed, Bm rearranges to the η2 complex Dm.8 The activation energy for this step is 10.9 kcal/mol. The O−CN bond distances are 1.286 and 1.326 Å in Bm and Dm, respectively, whereas the distance in free MeO− CN is 1.295 Å at the present level of calculation. The O−CN bond is not considerably affected by the coordination to the Mo atom. The next step in the sequence is the migration of the silyl group to the nitrile nitrogen, which requires a low activation energy of only 0.5 kcal/mol. The Si···N distance of 2.728 Å in Dm is relatively short. This short distance suggests that, in Dm, the weak attractive interaction has already occurred. This step results in Em having an η2-imidato fragment with the coordination of a nitrogen lone pair to the Mo atom. The O−CN bond distance of 1.324 Å in Em shows that the O−CN bond has not yet been activated and that the double-bond character is retained. In fact, the O−C−N−Mo fragment is 789

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Education, Culture, Sports, Science and Technology and by a Grant-in-Aid for Young Scientists (B) (No. 23750067) from the Japan Society for the Promotion of Science (JSPS). A.A.D. and N.K. were supported by CREST, JST.

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Figure 3. Synthesis of η2-imidato complexes and ORTEP drawing of 3.

Complex 2 was subjected to a thermal reaction in C6D6 at reflux temperature for 24 h, and the formation of Ph3SiCN in 5% yield was confirmed. This yield is approximately equal to the 3% yield of Ph3SiCN in the thermal reaction of (C5Me5)(CO)3WMe with Ph3SiH and iPrOCN in C6D6 for 24 h. The results indicate that an N-silylated η2-imidato complex is an intermediate product in the O−CN bond cleavage of cyanate. In conclusion, the first catalytic cleavage of the inert O−CN bond in cyanates was achieved using molybdenum methyl complexes. A feasible catalytic cycle was proposed. This cycle involves the η2-CN coordination of ROCN, the silyl migration from Mo to nitrile nitrogen to give an N-silylated η2-imidato complex with an Mo−C−N three-membered ring, and the rearrangement of Mo−C−N to a Mo−C−O three-memberedring complex. DFT calculations supported the catalytic cycle and showed that the O−C bond was activated by the coordination of the cyanate oxygen to Mo, forming a Mo− C−O ring complex. One of the important intermediates in the catalytic cyclean N-silylated η2-imidato complexwas isolated and identified by X-ray analysis. This paper shows that a silyl-migration-induced reaction (SiMI reaction) in a coordination sphere is one of the key steps in catalytic O−CN bond cleavage, as has been shown for catalytic C−CN and N− CN bond cleavage. It can be said qualitatively from the comparison of E−CN (E = C, N, O) bond cleavage reactions that a C−CN bond is cleaved more effectively than N−CN and O−CN bonds, and the last two bonds are similarly cleaved in a SiMI reaction.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Text, tables, and figures giving detailed experimental procedures, characterization data of the products, and details of the calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Challenging Exploratory Research grant (No. 23655056) from the Ministry of 790

dx.doi.org/10.1021/om201257h | Organometallics 2012, 31, 787−790