Strong bond cleavage promoted by silyl group migration in a coordination sphere

H. Nakazawa

Department of Chemistry, Graduate School of Science, Osaka City University, Sumiyoshi-ku, Japan

Abstract

C-CN bond cleavage in organonitriles was attained in the photoreaction with HSiEt3 in the presence of a catalytic amount of (r|5-C5H5)Fe(CO)2Me. The reaction sequence was proposed, where silyl migration from Fe to N in the coordinating nitrile is a key step. DFT calculations proposed a silyl migration with small activation energy of 4.0 kcal/mol and subsequent C-CN bond cleavage on the Fe coordination sphere. N-CN bond cleavage in cyanamide (R2NCN) was also attained by a transition metal catalyst. A catalytic cycle was proposed where a silyl migration from a transition metal to the nitrile nitrogen is involved. An N-silylated |2-amidino complex was isolated, which was shown to be an intermediate in the catalytic pathway.

Keywords: C-C bond cleavage, C-N bond cleavage, transition metal catalysis, silyl migration.

1 Introduction

Investigations of selective bond cleavage as well as selective bond formation of chemical compounds is important from the viewpoint of atom efficiency, low environmental load, and sustainable chemistry. Weak bond cleavage is not so difficult, whereas strong bond cleavage with keeping weak bonds intact is quite difficult. Therefore, selective strong bond cleavage is a challenging topic in chemistry. One of the promising ways to overcome the difficulties is using a transition metal catalyst.

As the carbon-carbon bond is relatively unreactive, C-C bond cleavage is an area of considerable current interest [1]. In particular, C-C bond activation in

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acetonitrile is difficult because it has rather strong C-C bond energy (133 kcal mol-1) compared with alkane C-C bond energy (ca. 83 kcal mol-1).

Transition metal complexes have been used to attain C-C bond cleavage of nitrile. They mainly involve Group 10 transition metal triads [2-10]. In addition, one example for Mo [11], Co [12], U [13], and two examples of Cu [14, 15] have been reported. These reactions show stoichiometric C-C bond cleavage. In contrast, a few catalytic reactions involving Ni [16] and Pd [17] complexes have been reported. For these examples, it is proposed or clearly shown that a direct oxidative addition of a C-CN bond toward an electronically unsaturated transition metal fragment takes place to give an alkyl(aryl)-cyano complex (eq. (1)).

This article introduces a new reaction pathway for organonitrile C-C bond cleavage where silyl group migration from a transition metal to nitrile nitrogen in the coordination sphere is a key step [18-20]. This article also report silyl assisted N-CN bond cleavage of cyanamide [21].

2 Stoichiometric C-CN bond cleavage of acetonitrile by a silyl-iron complex

Photoreaction with a 400-W medium pressure mercury arc lamp (pyrex filtered) of a THF solution containing CpFe(CO)2(SiMe3), MeCN, and 2 equiv of P(NMeCH2)2(OMe) (L) generated CpFe(CO)L(SiMe3) (25% yield), CpFe(CO)L(Me) (6% yield), CpFeL2(Me) (15% yield), CpFeL2(CN) (19% yield), and Me3SiCN (66% yield) (eq. (2)). When CD3CN was used in place of CH3CN, the corresponding CD3-iron complexes (CpFe(CO)L(CD3) and CpFeL2(CD3)) were obtained. These results obviously show that C-C bond in acetonitrile is cleaved in this reaction. In order to check whether the silyl group on the iron plays an important role to cleave the C-C bond in acetonitrile, CpFe(CO)2Me, CpFe(COMGeMe3), and CpFe(COMSnMe3) were used in place of CpFe(CO)2(SiMe3). No C-CN bond cleavage reaction took place, indicating that the silyl ligand on the iron is inevitable to cleave C-CN bond in acetonitrile [18].

in THF

0CL/ SiMej oCL/ CHj NCHj

in THF

Me^SiCN

Similar silyl-assisted nitrile C-C bond cleavage was reported independently by Bergman, Brookhart and co-workers using Rh [22] and Ir [23] complexes. Later, Hashimoto et al. [24] and Tobiso et al. [25] and their co-workers reported this type of silyl-assisted nitrile C-C bond cleavage.

3 Theoretical approach

In order to elucidate the role of a silyl group in the iron complex toward C-C bond activation of acetonitrile, DFT calculations were performed. It is expected that at first the irradiation of the coordinatively saturated CpFe(CO)2(SiMe3) will lead to the dissociation of one CO ligand to form 16e species CpFe(CO)(SiMe3) (A). The direct C-C bond oxidative addition of acetonitrile to the 16e species is less likely to happen, because this reaction requires a very high activation barrier of ca. 53 kcal/mol. Several DFT calculations could find a plausible reaction path which is shown in Figure 1 [18].

Dft Reaction Profile
Figure 1: Energy profiles of the reaction of CpFe(CO)(SiMe3) with MeCN (kcal.mol).

The reaction of A with MeCN forms B with the end-on coordination of acetonitrile with releasing 23.5 kcal/mol. Then, B is converted into the CN n-coordinated species C. This process takes place through a transition state (TS-1) with the activation energy of 14.8 kcal/mol. In C, the migration of the silyl group from Fe to nitrile nitrogen is possible. The calculations showed that this requires only a small activation energy of 4.0 kcal/mol (C to TS-2). From TS-2, a somewhat stable imino complex with the coordination of a nitrogen lone pair to the Fe atom, D, is formed. If the methyl group of the imino ligand migrates to the iron atom, the methyl iron complex (E) could be formed. It has been found that a distortion in D that breaks the Fe-N bond leads to the transition state TS-3. This process requires an activation energy of 15.0 kcal/mol. The product of this step, E, is 5.9 kcal/mol more stable than D, which is presumably due to the presence of a strong Fe-C bond.

The reaction profile in Scheme 1 well explains the rupture of the acetonitrile C-C bond by the Fe(II) complex. The highest activation barrier of 15 kcal/mol is well within the reach of a feasible chemical reaction. Once E is formed, the formation of CpFe(CO)LMe can be easily achieved by a silylisocyanide/L exchange reaction. The trimethylsilyl isocyanide dissociated from E may isomerize to trimethylsilyl cyanide. Theoretical calculation suggests that the cyanide is more stable than the isocyanide. This is in accord with the experimental fact that trimethylsilyl cyanide is thermodynamically more favorable [26].

4 Catalytic C-CN bond cleavage of acetonitrile

In order to establish a catalytic cycle, photoreaction of a THF solution containing an equimolar amount of Et3SiH and MeCN in the presence of a catalytic amount of CpFe(CO)2(SiMe3) (2 mol%) was examined. The main Si-containing product was Et3SiCN. The yield based on Et3SiH used was 45% (TON = 23). The results show the C-C bond in MeCN is cleaved catalytically.

Table 1: Catalytic activities of various iron complexes.3 Et3SiH + H-jC-CN -catalyst » Et3SiCN + CH4

Entry Catalyst TONb

Table 1: Catalytic activities of various iron complexes.3 Et3SiH + H-jC-CN -catalyst » Et3SiCN + CH4

Entry Catalyst TONb

1

Cp(CO)2Fe(SiMe3)

22.5

2

Cp(CO)2Fe(benzyl)

21.3

3

Cp(CO)2FeMe

36.0

4

Cp(CO)2FeCl

0.6

5

Cp(CO)2FeI

2.0

6

(C5MeH4)(CO)2FeMe

32.5

7

(C5Me4H)(CO)2FeMe

5.7

8

(C5Me5)(CO)2FeMe

4.3

9

(C5H4SiMe3)(CO)2FeMe

26.4

10

(C5H4SiMe3)(CO)2FeI

2.0

11

(C5H4I)(CO)2FeMe

12.6

12

(C5H4{P(O)(OMe)2})(CO)2FeMe

9.8

a Reactions were carried out at room temperature for 24 h under photoirradiation by using catallyst (0.52 mmol), Et3SiH (26.00 mmol), and THF (5.00 mL) in aoetonitrile (13.6 mL, 260.00 mmol). b Determined by GC with toluene as internal a Reactions were carried out at room temperature for 24 h under photoirradiation by using catallyst (0.52 mmol), Et3SiH (26.00 mmol), and THF (5.00 mL) in aoetonitrile (13.6 mL, 260.00 mmol). b Determined by GC with toluene as internal

Next, the effect of a ligand at the Fe center on the catalytic activity was examined. For CpFe(CO)2R the methyl complex showed better activity than the silyl, benzyl, halo complexes. For (C5R5)Fe(CO)2Me introduction of substituents into the Cp ring reduced the catalytic activity. therefore, CpFe(CO)2Me showed the best catalytic activity among complexes in Table 1. The catalytic activities of the related methyl complexes of Mo and W, CpM(CO)3Me (M = Mo, W), were sluggish.

5 Catalytic C-CN bond cleavage of organonitriles other than acetonitrile

To see the scope and limitation of catalytic activity of CpFe(CO)2Me for C-CN bond cleavage, reactions with several organonitriles other than acetonitrile were examined. Table 2 summarizes the results. Propionitrile (EtCN) was converted into Et3SiCN in 73% yield (TON: 18.2) (Table 2, entry 1). Isobutyronitrile ('PrCN) and malononitrile (NCCH2CN) were poorly converted (Table 2, entries 2, 3). However, succinonitrile (NCCH2CH2CN) was converted as much as propionitrile (65% yield, TON = 16.3; Table 2, entry 4).

Organonitriles in entries 5-10 were resistant to the C-CN bond cleavage in the reaction conditions (Table 2, entries 5-11). These results indicate that an electron-withdrawing, bulky, or coordination-feasible substituent on a carbon adjacent to the CN group is unfavorable for the C-CN bond cleavage.

Table 2: R-CN bond cleavage reactions of organonitriles.3

a Catalyst (0.20 mmol), Et3SiH (5.00 mmol), THF (0.40 mL), nitrile compound (50.00 mmol). b Nitrile (2.50 mmol), THF (1.30 mL). c Catalyst (0.20 mmol), Et3SiH (5.00 mmol), THF (8.00 mL), nitrile (5.00 mmol). d Determined by GC with toluene as internal standard.

a Catalyst (0.20 mmol), Et3SiH (5.00 mmol), THF (0.40 mL), nitrile compound (50.00 mmol). b Nitrile (2.50 mmol), THF (1.30 mL). c Catalyst (0.20 mmol), Et3SiH (5.00 mmol), THF (8.00 mL), nitrile (5.00 mmol). d Determined by GC with toluene as internal standard.

For aryl nitriles, the C-CN bonds were cleaved with TONs of about 10 (Table 2, entries 12-15). The TON for phthalonitrile is slightly lower (Table 2, entry 13) than that for benzonitrile, which may be due to the presence of an electron-withdrawing group in the ortho position. The TON for p-methoxybenzonitrile is slightly higher than for benxonitrile (Table 2, entry 16), which may be due to the electron-releasing OMe group. The C-CN bond was hardly cleaved for pentafluorobenzonitrile (table 2, entry 17), possibly due to strongly electronegative nature of F.

6 Consideration of catalytic cycle

A plausible catalytic cycle is shown in Figure 2 for the reaction of MeCN with Et3SiH in the presence of CpFe(CO)2Me as a catalyst precursor. One CO ligand in the precursor is released by photolysis to give CpFe(CO)Me, which reacts with Et3SiH to give CpFe(CO)(Me)(H)(SiEt3). This process seems plausible because oxidative addition of an Si-H bond to a 16e-species of Fe was reported to give, for example, CpFe(CO)(H)(SiEt3)2 [27]. The subsequent reductive elimination of CH4 yields CpFe(CO)(SiEt3). As SiEt3Me was not observed at all in this system, reductive elimination of CH4 from CpFe(CO)Me(H)(SiEt3) seems to precede that of Et3SiMe. The 16e species thus formed reacts with MeCN to give ultimately CpFe(CO)(Me)(^1-CNSiEt3). The reaction sequences have been demonstrated theoretically (Figure 1) [18]. Dissociation of Et3SiNC generates CpFe(CO)Me to complete the catalytic cycle. The released Et3SiNC isomerizes to thermodynamically stable Et3SiCN.

Geographie Carte Maine Loire
Figure 2: Proposed catalytic cycle.

According to the catalytic cycle shown in Figure 2, CpFe(CO)(SiEt3) can react with both MeCN and Et3SiH. The former reaction seems dominant, but the latter is not negligible. If the reaction of CpFe(CO)(SiEt3) with Et3SiH is suppressed, the catalytic cycle is expected to work more effectively. Thus the reaction on changing the molar ratio of Et3SiH and MeCN was examined. Reaction of Et3SiH with a 10-fold molar excess of MeCN in the presence of 0.83 mol% CpFe(CO)2Me under photolysis for 24 h at 50°C produced 99% yield of Et3SiCN base on Et3SiH (TON: 118). The TON increased with the photoirradiation time (48 h: 156, 96 h: 197). The highest TON (251) was obtained when Et3SiH and a 10-fold molar excess of MeCN were photolyzed for

1 week at 50°C in the presence of 0.2 mol% of CpFe(CO)2Me. Compared with Pd [17] and Rh [22] systems, this catalytic system is very effective.

7 N-CN bond cleavage of cyanamides

New reaction pattern of C-CN bond cleavage of organonitriles promoted by a silyl-iron complex has been described above. The essence of the reaction mechanism is silyl migration from Fe to the nitrogen in nitrile which is coordinated through C=N rc-bond. This migration causes C-CN bond cleavage. The replacement of the R group in RCN (organonitrile) by an NR2 group yields cyanamide described as R2NCN. Therefore, there is a possibility of N-CN bond cleavage of cyanamide in the reaction with a transition metal complex bearing a silyl ligand.

The R2N-CN bond is known to be strong and not broken readily. For example, the N-CN bond length in Me^-C^C^N-CN is reported to be 1.331 A, which lies just between that of a normal N-C single bond (1.47 A) and that of an N=C double bond (1.27 A) [28]. The von Braun reaction is the only reaction known to date to cleave R2N-CN bond. However, it requires harsh reaction conditions (strong acid or base conditions). Reactions of cyanamides with a transition metal complex bearing a silyl ligand were examined both in hopes of N-CN bond cleavage and with worrying about coordination of cyanamide to the 16e Fe species, Cp(CO)Fe(SiEt3), through a lone pair of electrons on the amino nitrogen causing disappearance of the activity of the iron complex toward R2N-CN bond cleavage.

Table 3: Photoreaction of cyanamides with CpFe(CO)2(SiEt3).

R2N-CN + CpFe(CO>2(SiEt3> to|uene Et3siCN

Entry Substrates Time/h Yield (%) a

1 Me2NCN 12 51

2 "Hex2NCN 12 30

3 N-cyanopiperidine 12 41

4 N-cyanomorpholine 12 32

5 N-cyanopyrrolidine 12 26

6 H2NCN 12 20b

7 Me2N(BH3)CN 24 14

8 Me2N(BF3)CN 24 0

9 C5H10N(BH3)CN 24 18

a Yield of Et3SiCN obtained by GC. b In 1,2-dichloroethane

A solution of an equimolar amount of dimethylcyanamide and CpFe(CO)2(SiEt3) (1) in toluene was photoirradiated at room temperature for 12 h. The :H NMR spectrum and the GC analysis of the reaction mixture showed formation of Et3SiCN. The yield was 51% (Table 3, entry 1), showing that the

Me2N-CN bond cleavage could be attained at room temperature using a silyl-iron complex.

Results of reactions with other cyanamides are presented in Table 3. Although the yields of Et3SiCN are less than 50%, these N-CN bonds are cleaved (entries 2-6). The reaction of H2NCN is noteworthy (entry 6). The H2N-CN bond has a double bond character because H2N-CN (cyanamide) is a tautomer of HN=C=NH (carbodiimide). Therefore H2N-CN bond is stronger than other R2N-CN. The first H2N-CN bond cleavage is attainable in these reaction conditions, although the efficiency remains insufficient.

Derivation of cyanamide into the borane adduct at the amino nitrogen, R2N(BX3)CN (X = H, F) [29], might engender more effective R2N-CN bond cleavage because of masking of the lone pair electrons on the amino nitrogen. The results (entries 7-9) showed that the introduction of borane into cyanamide did not facilitate R2N-CN bond cleavage; instead, it reduced the activity, presumably because of steric hindrance.

Reaction sequences resembling those in Figure 2 are expected for the reaction of 1 with cyanamide. Isolation of the N-silylated ^2-amidino iron intermediate was attempted, but the reaction of 1 with Me2NCN was unsuccessful. However, reactions with Me2NCN of (C5R5)Fe(CO)(py)(SiR'R"2) (py = pyridine), considered as a synthon of a 16e complex (C5R5)Fe(CO)(SiR'R"2), led to isolation of N-silylated ^2-amidino iron complexes (eq. (3)). Heating a solution containing 2 and Me2NCN in benzene at 50 °C for 10 h yielded 3 quantitatively according to the NMR measurements, but the isolation as a solid was failed. In contrast, a reaction of 4 with Me2NCN yielded 5, which could be isolated as dark-red powders in 85% yield. The unprecedented ^2-amidino complex was confirmed using X-ray analysis (Figure 3). The iron takes a distorted three-legged piano-stool structure with an ^-amidino fragment. The bond distance of N1-C2 (1.327 A) is shorter than that of a typical N-C single bond (e.g., C3-N1 = 1.455 A, C4-N1 = 1.458 A), and is rather similar to that of an N=C double bond (e.g., N2-C2 = 1.303 A). The sum of angles around N1 is 359.9°. These structural characters are consistent with sp2 hybridization of N1. The C3-N1-C2-N2 fragment is nearly planar with a torsion angle of 2.6(4)°. Both :H and 13C NMR spectra show that the structure in a solid state is maintained in solution. Two NCH3 resonances were observed at room temperature in :H and 13C NMR, reflecting that the C2-N1 bond does not rotate freely at room temperature.

Mf^NCN

R = H; R',R" = Et 2 R = Me; R' = Ph; R" = Me 4

R = H; R',R" = Et 2 R = Me; R' = Ph; R" = Me 4

Figure 3: ORTEP drawing of 5. Selected bond distances (A) and angles (deg): N1-C2 = 1.327 (3); N2-C2 = 1.303(2); C2-N1-C3 = 123.4(2); C2-N1-C4 = 119.5(2); C3-N1-C4 = 117.0(2).

Complexes 3 and 5 were subjected to a thermal reaction. Although 5 produced a small amount of PhMe2SiCN on heating in toluene at 110°C for 24 h, 3 gave Et3SiCN in 62% yield on heating in benzene at 70°C for 24 h. The results show that an N-silylated ^-amidino complex is an intermediate in the N-CN bond cleavage of cyanamide.

Table 4: Catalytic cleavage under photolysis or heating.

toluene

Entry Cat. [M]:[N]:[Sif Condition Temp/°C Time/h TONc

1

6

1 : 10 : 10

hv

25

24

0.4

2

7

1 : 10 : 10

hv

25

24

1.4

3

6

1 : 1 : 1

A

80

12

0

4

7

1 : 1 : 1

A

100

12

0.52

5d

7

1:10:1000

A

100

48

7.9

6d

7

1:1000:5000

A

100

120

32.3

a No Me2NCN bond cleavage took place in the absence of 6 or 7. b Molar ratio of a transition metal complex, Me2NCN, and EtjSiH. 0 Calculated from the isolated a No Me2NCN bond cleavage took place in the absence of 6 or 7. b Molar ratio of a transition metal complex, Me2NCN, and EtjSiH. 0 Calculated from the isolated

Catalytic N-CN bond cleavage of cyanamides has been attempted. Table 4 shows the results of reactions of Me2NCN and Et3SiH in the presence of a catalytic amount of CpFe(CO)2Me or CpMo(CO)3Me. Entry 1 shows that the Fe complex does not work as a catalyst, whereas the Mo complex does under photoirradiation conditions (Table 4, entry 2). The Mo complex shows catalytic activity even under thermal conditions (Table 4, entries 4-6). These results show the first example of catalytic N-CN bond cleavage of cyanamides by a transition metal complex.

8 Conclusions

It was found that a 16e species bearing a transition metal-silyl bond, such as

CpFe(CO)(SiR3) and CpMo(CO)2(SiR3), serves as a real catalyst for C-CN and

N-CN bond cleavage in organonitriles and cyanamides, respectively. Silyl migration from a transition metal to the nitrogen of the nitrile group in RCN and

R2NCN coordinating to the transition metal in -q2-fashion leads to the strong C-

CN and N-CN bond cleavage.

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