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Efficient Construction of Energetic Materials via Non-metallic Catalytic Carbon-Carbon Cleavage/Oxime-Release-Coupling Reactions Gang Zhao, Chunlin He, Ping Yin, Gregory H. Imler, Damon A. Parrish, and Jean'ne M. Shreeve J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b01260 • Publication Date (Web): 25 Feb 2018 Downloaded from http://pubs.acs.org on February 25, 2018

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Journal of the American Chemical Society

Efficient Construction of Energetic Materials via Non-metallic Catalytic Carbon−Carbon Cleavage/Oxime-Release-Coupling Reactions Gang Zhao,‡ Chunlin He,‡ Ping Yin,‡ Gregory H. Imler,§ Damon A. Parrish,§ and Jean’ne M. Shreeve*‡ ‡ Department of Chemistry, University of Idaho, Moscow, ID 83844-2343 USA, E-mail: [email protected] § Naval Research Laboratory, 4555 Overlook Avenue, Washington, D.C. 20375, USA Supporting Information Placeholder ABSTRACT: The exploitation of C-C activation to facilitate chemical reactions is well-known in organic chemistry. Traditional strategies in homogeneous media rely upon catalystactivated or metal-mediated C-C bonds leading to the design of new processes for applications in organic chemistry. However, activation of a C-C bond, compared with C-H bond activation, is a more challenging process and an underdeveloped area since thermodynamics does not favor insertion into a C-C bond in solution. Carbon-carbon bond cleavage through loss of an oxime moiety has not been reported. In this paper, a new observation of self-coupling via C-C bond cleavage with concomitant loss of oxime in the absence of metals (either metalcomplex mediation or catalysis) results in dihydroxylammonium 5,5-bistetrazole-1,10-diolate (TKX-50) as well as N,N'([3,3'-bi(1,2,4-oxadiazole)]-5,5'-diyl)dinitramine, a potential candidate for a new generation of energetic materials.

Non-metallic catalytic carbon−carbon cleavage seems impossible because of the high energy barrier. Now reported is the first example where oxime-bridged heterocyclic compounds undergo CC bond cleavage with the concomitant loss of the oxime group followed by self-coupling without transition-metal catalysis. The utility of this method is seen in the synthesis of N,N'-([3,3'bi(1,2,4-oxadiazole)]-5,5'-diyl)dinitramine as well as of dihydroxylammonium 5,5'-bistetrazole-1,10-diolate (TKX-50).1 This method to obtain TKX-50 avoids using DMF or NMP which is expensive and difficult to remove.1 The breaking of carbon−carbon bonds is a key industrial process for crude oil refining. Homogeneous transition-metal complex catalyzed C-C bond cleavage reactions are often studied and are reported sources of spectacular chemistry.2 However, efficient transformations involving this step are rarely reported other than in petrochemistry.3 In the de novo synthetic realm, modification of C-H (or C- X) bonds often give rise to the target molecules.4 In this regard, C-C bonds which are activated followed by transformation offers an attractive alternative approach in terms of disconnecting atoms, forming more active linkages and producing more versatile scaffolds.5 However, it is always a challenge to break strong C-C bonds to form relatively weak carbon–metal bonds.6-10 Until now, successful examples are limited to the highly strained three or four-membered carbon-ring substrates with released strain as the driving force to facilitate the thermodynamically unfavorable cleavage of C-C bonds (Fig 1A).7-8 Recently, a

general approach to the catalytic activation of C-C bonds in simple cyclopentanones and some cyclohexanones was reported. The key to this success is the combination of a rhodium precatalyst, a N-heterocyclic carbene ligand, and an amino-pyridine co-catalyst (Fig 1B). 9 Catalytic C–C activation is rarely reported in less strained or non-strained rings. The most usual strategy for cleaving a C-C bond involves Rh-catalyzed reactions, such as rhodium-mediated decarbonylation of cycloalkanones; however, lower efficiency with C-C cleavage occurs with less strained cyclopentanones (Fig 1C).10 Despite this progress, cleavage of C-C bonds as found in typical oximes is still highly unusual. With the exception of a flash vacuum pyrolysis study of aromatic oximes at extreme temperatures (800 oC) and reduced pressures, the literature is silent on such chemistry.11 Now we show a process in developing the chemistry of novel carbon−carbon cleavage, oxime release, and selfcoupling reactions without transition-metal catalysis.

Fig. 1 A) Activation of carbon–carbon bonds. B) Activation of carbon-carbon bonds via Rh-catalysis in cyclopentanones. C) Rhodium-mediated decarbonylation of cycloalkanones. D) This work. Based on our energetic materials research, in order to synthesize 1, 2, 4-oxadiazole derivatives as intermediates, we focused on the study of 2. An ethanol solution of 2 was treated with BrCN in aqueous potassium bicarbonate solution (Scheme 1; Table 1, entry 7) to synthesize 3’ following the literature.11 Unexpectedly, 3,3'bi(1,2,4-oxadiazole)]-5,5'-diamine, 3, formed because of the loss of the oxime moiety (Scheme 1). This structure is confirmed by using single crystal X-ray diffraction. Compound 4 was separated and characterized.

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A preliminary examination of the impact of solvents on reactions shows that 4 is obtained as the major product with most of the solvents used. Perhaps surprisingly, with an excess of BrCN in water (Table 1, entry 4) or in DMSO/H2O (1:1) (Table 1, entry 7), 3 was obtained as the major product. With an increase in reaction temperature to 100 oC, only 4 was isolated. In the case of oxime chemistry,13 an oxime loss reaction has not been reported previously by C ̶ C bond cleavage, oxime group release, or selfcoupling.

Scheme 2. Relative stabilities of oxime salts with different cations.

Scheme 1. Oxime loss to form 3; crystal structure of 3. Table 1. Impact of solvents on yields of 3 and 4. entry

Time solvent (h) EtOH/H2O(1:1) 24

attempt to understand this loss of the oxime, 12 was synthesized from 11. But 12 is stable with respect to loss of oxime (Scheme 3).

1

BrCN eq 2

T (oC) 25

Yield 3 (%) 20

Yield 3’ (%) 0

Yield 4 (%) 85

2

2

24

EtOH/H2O(1:1)

0

35

0

65

3

2

24

H2 O

25

40

0

60

4

4

24

H2 O

25

55

0

45

5

2

24

EtOH

25

a

0

a

6

2

24

DMSO

25

b

0

b

7

2

24

DMSO/H2Oc

25

50

0

45

8

2

24

EtOH/H2Oc

100

0

0

>90

9

2

24

DMSO/H2Oc

100

0

0

>90

Therefore, when different rings are bonded to the carbon of the oxime group, e.g., 1-oxytetrazolium (9) compared to tetrazolium (12), the stabilities of the resulting oxime salts are very different which gives rise to very different products.

a

decomposition BrCN + compounds 3 and 4 c (1:1) Another example of oxime loss was observed when aqueous hydroxylamine was reacted with 7. Compound 7 was supported by 1H and 13C NMR, and its ammonium salt, 8, was obtained and confirmed by 1H and 13C NMR, differential scanning calorimetry (DSC), elemental analysis (EA), and by X-ray structuring of a single crystal, which shows that the oxime skeleton was retained for both (Scheme 2). However, preparation of the analogous hydroxylammonium salt, 9, was unsuccessful as shown by properties of the product that did not agree with the expected oxime structure using 13C NMR spectra, and by the lack of agreement with elemental analysis. After recrystallization from water, only a single 13C peak was found; subsequently, the product was identified by X-ray structuring as the recently reported explosive, TKX-50 (10).1 Although unexpected, this result is satisfying since it demonstrates that it is now possible to synthesize this important material using a more efficient and shorter route than previously reported. The 13C-NMR spectrum determined on the reaction mixture of 5 with aqueous hydroxylamine displays only the single peak of 10 (TKX-50) at 134 ppm. The remaining three small resonance peaks (140, 138, 133 ppm) are assigned to 9 (Fig. 2). Initially it was assumed that the side products, CO and hydroxylamine, were formed in the reaction when the oxime disappeared. In search for a mechanism to rationalize the destruction of the oxime moiety, it was thought that because of the effect of electron-withdrawing by the N-O bond of the tetrazole, 9 would lose the oxime carbon by elimination of CO, followed by self-coupling (Scheme 2). In an b

Fig. 2 The 13C-NMR of TKX-50 formed by loss of oxime.

Scheme 3. Comparative stabilities of oxime salts with different anions. As an important class of promising candidates for HEDMs, bisheterocyclic nitramines, have attracted considerable attention due to their excellent energetic performance, good thermal stability, and low impact sensitivity. Recently, bis-heterocyclic energetic derivatives based on triazole and oxadiazole rings have been reported and shown to demonstrate excellent performance.14 N,N'-([3,3'-bi(1,2,4-oxadiazole)]-5,5'-diyl)dinitramine (13) and its analogs bearing a bis-1,3,4-oxadiazole skeleton, which may potentially be a new generation of energetic materials, have been tapped only partially due to the lack of a concise, general synthetic method. Compound 13 was obtained by treating 3 with fuming nitric acid in excess, and in methanol as solvent, it was stable. The methanol solution of 13 was treated directly with ammonia in methanol, hydroxylamine•hydrate or hydrazine• hydrate to give salts, 14, 15 or 16 (Scheme 4).


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Journal of the American Chemical Society Saturated solutions were slowly evaporated to give suitable crystals in methanol or methanol/water mixtures to provide crystals of 3, 8, 14 and 15 making X-ray structuring possible. Crystallographic data, data collection parameters, bond angles, and bond lengths are given in the Supporting Information. Compound 8 falls in the triclinic space group P-1. Compounds 3 and 15 appear in the monoclinic space group P21/c, and 14 crystallizes in the orthorhombic space group Pccn (Figures 3 and 4).

(IS: 20 J, FS: 120 N) are somewhat sensitive to friction. Using the calculated values of the heats of formation and experimental densities, EXPLO5 (version 6.01) was used to calculate detonation properties.17 The detonation pressures (P) range from 24.0 to 38.9 GPa, and the detonation velocities (Dv) fall within 8114 and 9698 m/s. With the exception of 8 and 14, all compounds have detonation properties which are superior to RDX, and 16 (P: 34.2 GPa, Dv: 9078 m/s) possesses detonation properties which are close to those of HMX (P: 39.5 GPa, Dv: 9320 m/s).

Table 2. Properties of compounds 6, 10, 14-16. ρa (g·cm-3)

Scheme 4. Routes to N,N'-([3,3'-bi(1,2,4-oxadiazole)]-5,5'diyl)dinitramine (13) and its salts.

Db (m/s)

Pc (GPa)

∆Hfd (kJ/mol)

Tdece (oC)

ISf (J)

Fg (N)

372.3

241

11

240

8

1.62

8114

24.0

4

1.785

8523

29.72

42.3

244

40

360

15

1.84

8889

35.6

149.2

198

30

240

16

1.82

9078

34.2

369.0

211

25

240

1.88

9698

38.9

446.6

221

20

120

1.90

9320

39.5

104.8

280

7

120

1.80

8795

34.9

92.6

204

7

120

10(TKX50)h HMX

h

RDX h a

b

Density measured - gas pycnometer at 25 °C; Calculated detonation velocity; c Calculated detonation pressure; d Heat of formation; e Temperature of decomposition (onset); f Impact sensitivity; g Friction sensitivity. h ref 1.

Figure 3. (a) Labeling scheme and thermal ellipsoid plot (50%) for 14. (b) Packing diagram (ball-and-stick) of 14 as viewed down the b axis. Strong hydrogen bonding indicated by dashed lines.

Figure 4. (a) Labeling scheme and thermal ellipsoid plot (50%) for 15. (b) Packing diagram (ball-and-stick) of 15 as viewed down the b axis. Strong hydrogen bonding indicated by dashed lines. Differential scanning calorimetric (DSC) measurements were used to determine the thermal stabilities of all compounds. As shown in Table 2, compounds 8, 14, 15, and 16 decompose between 198 and 241 oC (exceeding RDX, 204 oC). Using a gas pycnometer, densities were measured at 25 °C and the values obtained range between 1.62 (6) and 1.84 (15) g/cm3. The Gaussian 03 (Revision D.01) suite of programs were used to calculate heats of formation.15 With relatively high positive heats of formation (∆Hf), which fall between 42.3 and 372.3 kJ/mol, 8, 14, 15, and 16 exceed both TNT (-59.4 kJ/mol) and RDX (92.6 kJ/mol) (except 16). Due to the presence of tetrazole groups, 6 (372.3 kJ/mol) has the highest heat of formation. A BAM drophammer apparatus and BAM friction tester, were used to measure values of impact and friction sensitivities, respectively.16 All compounds have lower impact and friction sensitivities relative to RDX; the 1, 2, 4-oxidazole derivatives, 14-16 are insensitive (IS: 25-40 J; FS: 240-360 N) while the tetrazole derivatives, 8 (IS: 11 J, FS: 120 N) and 10 (TKX-50)

A novel, practical and synthetically useful activation of the C-C bond which occurs in the absence of metal-complex mediation or catalysis has been developed. This is particularly unique since, in addition to the loss of the oxime moiety in the cleavage process, a carbon atom is also lost. It has led to the very effective synthesis of a new family of symmetric N,N'-([3,3'-bi(1,2,4-oxadiazole)]5,5'-diyl)dinitramines as well as TKX-50. The option of the oxime moiety for conversion to symmetric 1, 2, 4-oxadiazole suggests a route for further modifying properties of energetic materials. An examination of the physiochemical and energetic properties of compounds 8, 10 (TKX-50), 14, 15, and 16 indicates good density, high heat of formation, and good detonation pressure and velocity. The utilization of the oxime-loss reaction provides a new route to TKX-50, which avoids the use of the expensive and hardto-remove solvents, DMF and NMP. Although there are no experimental and theoretical results to suggest a plausible discussion of the mechanism for the loss of oxime, the results described here present the first example of practical synthetic transformations involving selective cleavage of a C-C bond without being mediated or catalyzed by a transition metal, and demonstrates that oxime loss is a fundamental process in organic chemistry. In summary, the cleavage of an aromatic C-C bond by oxime loss enables oxime-bridged heterocyclic compounds to be converted to selfcoupled heterocyclic compounds, a transformation that may potentially be extended to other systems and thereby provide a new means of functionalizing heterocyclic molecules.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Synthesis, characterization data, calculation detail, and crystallographic data (.docx)

AUTHOR INFORMATION



Corresponding Author [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the Office of Naval Research (N00014-16-1-2089) and the Defense Threat Reduction Agency (HDTRA 1-15-1-0028). We are also grateful to the Murdock Charitable Trust, Vancouver, WA, Reference No.: 2014120:MNL:11/20/2014 for funds supporting the purchase of a 500 MHz nuclear magnetic resonance spectrometer.

REFERENCES

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Efficient Construction of Energetic Materials via ... - ACS Publications

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