Selective Propargylation of Diaryl Azo Compounds Using Metallic Barium

Key words

azo compounds - barium - hydrazines - propargylation - propargylic tosylates

Propargylic/allenylic barium compounds, which are generated from Rieke barium[1] and propargylic halides, are useful reagents for the synthesis of organic molecules having a carbon–carbon triple bond and high α-regioselectivity.[2] We have previously reported that a propargylation of azo compounds with propargylic halides occurs via a Barbier-type procedure using reactive barium as the low-valent metal to yield propargylic hydrazines (α-product).[3] In addition, a Grignard-type α-allylation of azo compounds with allylic barium reagents[4] and a Barbier-type benzylation of azo compounds with benzylic chlorides[5] have been achieved. We report herein a metallic-barium-promoted Barbier-type propargylation of azo compounds with propargylic tosylates (Scheme [1]). The results of reductive N–N bond cleavage of the products, propargylic hydrazines, to form the corresponding propargylic amines as well as benzidine rearrangement of the propargylic hydrazines are also disclosed. The propargylic amine structure is often seen as a key framework in pharmaceutical compounds, such as rasagiline mesylate[6] and selegiline hydrochloride.[7] Therefore, the development of useful methods for the synthesis of such propargylic amines has captivated the interest of researchers in the field of organic synthesis.

Scheme 1 Barbier-type propargylation of azobenzenes using metallic barium

We have found that allylic barium reagents can be prepared from metallic barium and allylic chlorides and show high reactivity toward isatin imines with α-selectivity.[8] We envisioned that if a propargylic or an allenylic barium reagent could be generated from metallic barium[9] and the corresponding propargylic halide under mild reaction conditions and displayed α-selectivity in the reaction with an azo compound, the propargylation would provide a practical synthetic procedure for propargylated hydrazines. Thus, we first selected (3-chloroprop-1-ynyl)trimethylsilane (1a) and azobenzene (2a) as the precursor of propargylic or allenylic barium reagent and the electrophile, respectively, and attempted to perform a Barbier-type reaction due to the simplicity of the experimental procedure. When a 3:1 mixture of propargylic chloride 1a (3 equiv) and azobenzene (2a, 1 equiv) was treated with metallic barium (3 equiv) in THF at room temperature for 14 h, the reaction did not proceed and targeted propargylated hydrazine 3aa (α-adduct) was not observed at all (Table [1], entry 1). In contrast, α-methylated trimethylsilyl-substituted propargylic chloride 1b showed remarkable reactivity toward 2a and the desired product was obtained in 49% yield without formation of the corresponding allenylated hydrazine under similar reaction conditions (entry 2). However, the reaction of α-dimethylated propargylic chloride 1c with 2a resulted in a low yield (entry 3). α-Methylated tert-butyl-substituted propargylic chloride 1d showed similar reactivity toward 1b and target adduct 3da was formed in 46% yield (entry 4). Subsequently, we examined solvent effect (entries 4–6) and found that a 4:1 mixture of THF and DMF was the most suitable solvent from the point of view of chemical yield (entry 6). We further focused on electron-withdrawing group X of substrate 1 and when propargylic tosylate 1e was employed, the highest chemical yield (72%) was attained (entry 7). Diphenyl phosphate was also a promising X group (entry 8); in contrast, trifluoroacetate gave unsatisfactory results and desired adduct 3da was not obtained in the reaction (entry 9). The chemical yield of 3da shown in entry 7 was not improved when the amounts of propargylic tosylate 1e and metallic barium were decreased or increased (entries 10 and 11). THF was also a suitable solvent in the reaction of 1e, and 69% yield of product 3da was obtained under the optimized reaction conditions (entry 12).

Table 1 Optimization of Metallic-Barium-Promoted Barbier-Type Propargylation of Azobenzene (2a)^a

Entry	R1	R2	R3	X	x	Solvent	Product	Yield (%)b
1	Me3Si	H	H	Cl 1a	3	THF	3aa	<1
2	Me3Si	Me	H	Cl 1b	3	THF	3ba	49
3	Me3Si	Me	Me	Cl 1c	3	THF	3ca	23
4	t-Bu	Me	H	Cl 1d	3	THF	3da	46
5	t-Bu	Me	H	Cl 1d	3	DMF	3da	38
6	t-Bu	Me	H	Cl 1d	3	THF–DMF (4:1)	3da	60
7	t-Bu	Me	H	OTs 1e	3	THF–DMF (4:1)	3da	72
8	t-Bu	Me	H	OPO(OPh)2 1f	3	THF–DMF (4:1)	3da	46
9	t-Bu	Me	H	OCOCF3 1g	3	THF–DMF (4:1)	3da	<1
10	t-Bu	Me	H	OTs 1e	2	THF–DMF (4:1)	3da	47
11	t-Bu	Me	H	OTs 1e	4	THF–DMF (4:1)	3da	47
12	t-Bu	Me	H	OTs 1e	3	THF	3da	69

^a The Barbier-type reaction was carried out using propargylic compounds 1a–g (x equiv), metallic barium (x equiv), and azobenzene (2a, 1 equiv) in the specified solvent at room temperature for 14 h.

^b The chemical yield was determined by ¹H NMR spectroscopy using 1,4-bis(trimethylsilyl)benzene as the internal standard.

Table 2 Metallic-Barium-Promoted Barbier-Type Propargylation of Azobenzene (2a) with Various Propargylic Tosylates 1e and 1h–m ^a

Entry	R1	R2	Product	Yield (%)b
1	t-Bu	Me (1e)	3da	69
2	t-Bu	Et (1h)	3ea	85
3	t-Bu	i-Pr (1i)	3fa	57
4	Bu	Me (1j)	3ga	50
5	Ph	Me (1k)	3ha	31
6	Me3Si	Me (1l)	3ba	60
7	t-Bu(Me)2Si	Me (1m)	3ia	89

^a The Barbier-type reaction was carried out using propargylic tosylates 1e and 1h–m (3 equiv), metallic barium (3 equiv), and azobenzene (2a, 1 equiv) in THF at room temperature for 14 h.

^b The chemical yield was determined by ¹H NMR spectroscopy using 1,4-bis(trimethylsilyl)benzene as the internal standard.

With the optimum reaction conditions in hand, we examined the propargylation of azobenzene (2a) with propargylic tosylates 1e and 1h–m derived from various propargylic alcohols (Table [2]). Higher reactivity was observed for the reaction of propargylic tosylate 1h, which has an ethyl group as the R² group (entry 2). In contrast, propargylic tosylate 1i, which has an isopropyl group, afforded product 3fa in a lower yield than propargylic tosylate 1e probably due to its steric hindrance (entry 3 vs. entry 1). Employment of phenyl-substituted propargylic tosylate 1k caused a significant decrease in the yield (31%) of its product 3ha (entry 5). Trialkylsilyl-substituted propargylic tosylates 1l and 1m furnished products in satisfactory yields (entries 6 and 7).

We performed the metallic-barium-promoted Barbier-type propargylation of symmetrical azobenzenes 2b–k derived from a diversely substituted aniline (Table [3]). The effect of a substituent on the aromatic ring of azobenzenes 2b–k on the chemical yield was notable: an electron-withdrawing group (Cl or F) at 4-position of the phenyl group enhanced the electrophilicity of 2e and 2f relative to 2d, which has an electron-donating MeO group (entries 4 and 5 vs. entry 3). A methyl group at 2-position reduced the reactivity of 2i (entry 8), whereas 2-F substituted azobenzene showed significant reactivity probably due to its electronic effect rather than its steric hindrance (entry 9).

Table 3 Metallic-Barium-Promoted Barbier-Type Propargylation of Symmetrical Azobenzenes 2b–k with Propargylic Tosylate 1h ^a

Entry	Ar	Product	Yield (%)b
1c	4-MeC6H4 2b	3eb	80
2	4-i-PrC6H4 2c	3ec	73
3	4-MeOC6H4 2d	3ed	27
4	4-ClC6H4 2e	3ee	56
5	4-FC6H4 2f	3ef	60
6d	3-MeC6H4 2g	3eg	77
7	3-ClC6H4 2h	3eh	51
8	2-MeC6H4 2i	3ei	10
9d	2-FC6H4 2j	3ej	>99
10	2,4-F2C6H3 2k	3ek	51

^a The Barbier-type reaction was carried out using propargylic tosylate 1h (3 equiv), metallic barium (3 equiv), and azobenzenes 2b–k (1 equiv) in THF at room temperature for 14 h.

^b The chemical yield was determined by ¹H NMR spectroscopy using 1,4-bis(trimethylsilyl)benzene as the internal standard.

^c The reaction was performed for 15 h.

^d The reaction was performed for 8 h.

To investigate the electronic effect on the site selectivity of the nitrogen atoms, we carried out the propargylation of unsymmetrical azobenzene derivatives having an electron-deficient group on one aromatic ring and/or an electron-rich group on the other aromatic ring. As a result, a 44:56 mixture of two regioisomers A and B was obtained as product 3el + 3el′ in the reaction of 1-(4-tolyl)-2-phenyldiazene (2l) with 6,6-dimethylhept-4-yn-3-yl 4-methylbenzenesulfonate (1h) almost quantitatively (Table [4], entry 1). In contrast, the reaction of unsymmetrical diaryl azo compounds 2m and 2n, which have a 4-halophenyl group as the Ar² group, resulted in the formation of regioisomer A selectively (entries 2 and 3). Similar site selectivities were observed in the cases of 1-(4-fluorophenyl)-2-(4-tolyl)diazene (2o) and 1-(4-fluorophenyl)-2-(4-isopropylphenyl)diazene (2p), but with unsatisfactory isolated yields (entries 4 and 5). The highest site selectivity was realized with a 2-tolyl group as the Ar¹ group and a 2-fluorophenyl group as the Ar² group (entry 6). A similar site selectivity was achieved by using 1-(2,4-difluorophenyl)-2-(3-tolyl)diazene (2r) as the substrate: a 64:36 mixture of propargylic hydrazines A and B was obtained in 40% combined yield (entry 7).

Table 4 Metallic-Barium-Promoted Barbier-Type Propargylation of Unsymmetrical Azobenzenes 2l–r with Propargylic Tosylate 1h ^a

Entry	Ar1	Ar2	Product	Yield (%)b	A/B c
1	Ph	4-MeC6H4 2l	3el + 3el′	>99	44:56
2	Ph	4-FC6H4 2m	3em + 3em′	78	57:43
3	Ph	4-ClC6H4 2n	3en + 3en′	92	55:45
4	4-MeC6H4	4-FC6H4 2o	3eo + 3eo′	57	58:42
5	4-i-PrC6H4	4-FC6H4 2p	3ep + 3ep′	39	56:44
6	2-MeC6H4	2-FC6H4 2q	3eq + 3eq′	>99	66:34
7	3-MeC6H4	2,4-F2C6H3 2r	3er + 3er′	40	64:36

^a The Barbier-type reaction was carried out using propargylic tosylate 1h (3 equiv), metallic barium (3 equiv), and azobenzenes 2l–r (1 equiv) in THF at room temperature for 15 h.

^b The chemical yield was determined by ¹H NMR spectroscopy using 1,4-bis(trimethylsilyl)benzene as the internal standard.

^c The ratio was determined by ¹H NMR spectroscopy or ¹⁹F NMR spectroscopy. The structure of the major isomer was determined by N–N bond cleavage.

The thus-obtained propargylic hydrazines can be further converted into propargylic amines through reductive N–N bond cleavage.[10] For example, treatment of propargylic hydrazine derivative 3da with an excess of Zn in acetic acid[11] at room temperature for 15 h afforded corresponding propargylic amine 4da in 32% yield (Table [5], entry 1). Elevating the reaction temperature was effective in acquiring a higher yield and when the reaction was performed at 80 °C, a satisfactory isolated yield of 4da was obtained (entry 3). Decreasing the amount of zinc (entry 4), shortening the reaction time (entry 5), diluting the reaction solution (entries 6 and 7), and employing trifluoroacetic acid instead of acetic acid (entry 8) did not improve the yield.

Table 5 Optimization of Reductive N–N Bond Cleavage of Propargylic Hydrazine (3da)^a

Entry	x	Temp ( °C)	Time (h)	Yield (%)b
1	100	r.t.	15	32
2	100	40	15	57
3	100	80	15	80
4	50	80	15	70
5	100	80	4	78
6c	100	80	4	46
7c	100	120	4	68
8d	100	80	4	33

^a The reaction was carried out using propargylic hydrazine 3da (1 equiv) and zinc dust (x equiv) in acetic acid (1.5 mL) at the specified temperature for 4 h or 15 h.

^b Isolated yield.

^c Acetic acid (3 mL) was used.

^d Trifluoroacetic acid was used in place of acetic acid.

With the optimized reaction conditions in hand, we then executed the N–N bond cleavage of diverse propargylic hydrazines employing Zn in AcOH. α-Ethylated propargylic hydrazine 3ea showed higher reactivity than 3da (Table [6], entry 1 vs. Table [5], entry 3). Not only electron-rich hydrazines but also electron-deficient hydrazines provided the anticipated propargylic amines in satisfactory yields (Table [6], entries 2–5). Regioisomeric mixtures of 3eq and 3eq′ effectively underwent the N–N bond cleavage (Table [6], entry 6) and as a result, regioisomer 3eq was unambiguously determined to be the major product of the reaction shown in Table [4], entry 6. Similarly, regioisomer 3er was found to be the major product in the reaction shown in Table [4], entry 7 from the result of the reduction of a mixture of 3er and 3er′ (Table [6], entry 7).

Table 6 Reductive N–N Bond Cleavage of Various Propargylic Hydrazines 3ea, 3eb, 3ef, 3ej, 3eq+3eq′, and 3er + 3er′ ^a

Entry	Ar1	Ar2	Product	Yield (%)b
1	Ph	Ph 3ea	4ea	82
2c	4-MeC6H4	4-MeC6H4 3eb	4eb	66
3	4-MeC6H4	4-MeC6H4 3eb	4eb	96
4c	4-FC6H4	4-FC6H4 3ef	4ef	55
5	2-FC6H4	2-FC6H4 3ej	4ej	75
6d	2-MeC6H4/2-FC6H4	2-FC6H4 3eq/2-MeC6H4 3eq′, 3eq + 3eq′ (66:34)	4eq + 4eq′	82 (74:26)
7d	3-MeC6H4/2,4-F2C6H3	2,4-F2C6H3 3er/3-MeC6H4 3er′, 3er + 3er′ (64:36)	4er + 4er′	>99 (69:31)

^a The reaction was carried out using propargylic hydrazines 3ea, 3eb, 3ef, 3ej, 3eq + 3eq′, and 3er + 3er′ (1 equiv) and zinc dust (100 equiv) in acetic acid (1.5 mL) at 80 °C for 15 h.

^b Isolated yield.

^c The reaction was performed for 4 h.

^d The reaction was performed using zinc dust (100 equiv) and MeOH (4 equiv) in acetic acid (1.5 mL) at 80 °C for 15 h.

Table 7 Optimization of Benzidine Rearrangement of Propargylic Hydrazine 3ea ^a

Entry	Temp ( °C)	Time (h)	Yield (%)b
1	50	20	47
2	70	20	56
3	70	7	51
4	120	7	28

^a The reaction was carried out using propargylic hydrazine 3ea in a mixture of THF (1 mL) and 2 M HCl aq (1.5 mL) at the specified temperature for 7 h or 20 h.

^b Isolated yield.

Subsequently, we investigated the benzidine rearrangement of propargylic hydrazine 3ea, which afforded corresponding biphenylamine 5ea through the N–N bond cleavage of 3ea under acidic conditions.[12] Optimization of the reaction temperature and the reaction time was performed, and the results are shown in Table [7]. When 3ea was exposed to a 2:3 mixture of THF and 2 M HCl (aq) at 50 °C for 20 h, anticipated biphenylamine 5ea was obtained in 47% yield (Table [7], entry 1). When the reaction temperature was elevated to 70 °C, the yield was improved to 56% (entry 2). Attempts to carry out the rearrangement for a shorter reaction time and/or at a higher reaction temperature were not effective in gaining better results ( entries 3 and 4).

A substituent on the aromatic ring of propargylic hydrazines affected the isolated yields of the products. Introduction of a methyl group to 3-position decreased the yield of 5dg because of steric repulsion between the two methyl groups of product 5dg (Table [8], entry 2). 2-Methyl-substituted substrate 3di displayed comparable reactivity to 3da (entry 3 vs. entry 1). The existence of an electron-withdrawing group significantly decreased the yield of 5dj probably due to suppression of protonation of the two amino groups (entry 4).

Table 8 Benzidine Rearrangement of Various Propargylic Hydrazines 3da, 3dg, 3di, and 3dj ^a

Entry	R	Product	Yield (%)b
1	H 3da	5da	56
2	3-Me 3dg	5dg	30
3	2-Me 3di	5di	54
4	2-F 3dj	5dj	9

^a The reaction was carried out using propargylic hydrazines 3da, 3dg, 3di, and 3dj in a mixture of THF (1 mL) and 2 M HCl aq (1.5 mL) at 70 °C for 20 h.

^b Isolated yield.

A proposed reaction mechanism is illustrated in Scheme [2]. Two pathways are possible for propargylic hydrazines 3 (α-adducts). A barium reagent generated from propargylic tosylate 1-OTs and metallic barium is supposed to be present in equilibrium between propargylic isomer 6 and allenylic isomer 7.[13] Thus, α-adducts are accessible from both isomers 6 and 7 by treating them with azo compound 2 via transition-state models 8 and 9, respectively, although former structure 8 is more favorable due to minimal steric repulsion. Meanwhile, allenylic hydrazines (γ-adducts) can be formed from 6 by an S_E2′-type reaction of 6 with azo compound 2 through six-membered cyclic transition state 10. However, 10 is unstable due to steric repulsion between the R¹ group of the barium reagent and an aryl group of the azo compound. As a consequence, propargylic barium species 6 is anticipated to react preferentially at the α-carbon with azo compound 2 via four-membered cyclic transition state 8,[14] yielding the α-adduct selectively. Furthermore, in the case of unsymmetrical azobenzene (Ar¹ = electron-rich Ar group; Ar² = electron-deficient Ar group), the propargylation occurs selectively at the nitrogen atom which bonds to Ar¹ group, because the nitrogen atom is considered to be relatively electron-deficient probably due to resonance effect. In contrast, another relatively electron-rich nitrogen atom is allowed to coordinate to barium atom.

Scheme 2 Proposed reaction pathways to α-adducts and γ-adducts

In conclusion, we have achieved a novel Barbier-type propargylation of azo compounds with barium reagents that are prepared from propargylic tosylates and metallic barium. The employment of metallic barium as the source of barium reagents has enabled the synthesis of various propargylic hydrazines in a regioselective manner.[15] In addition, the site-selective propargylation of unsymmetrical azo compounds has been realized, giving isomeric ratios of up to 66:34. The utility of the propargylated product has been further demonstrated by their transformation into propargylated amines and propargylated biphenylamines through two types of N–N bond cleavage. Further studies of related reactions promoted by metallic barium are under way.

• References and Notes

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For reviews, see:
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• 8 Yanagisawa A, Yamafuji S, Sawae T. Synlett 2016; 27: 2019

Prof. Miyoshi and co-workers have reported various reactions promoted by metallic strontium, see:
• 9a Miyoshi N. In Science of Synthesis, Vol. 7. Yamamoto H. Thieme; Stuttgart: 2004: 685
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• 10 For a review of reductive N–N bond cleavage of hydrazines, see: Gilchrist TL. In Comprehensive Organic Synthesis, Vol. 8. Trost BM, Fleming I. Pergamon Press; Oxford: 1991: 388
• 11 Sapountzis I, Knochel P. Angew. Chem. Int. Ed. 2004; 43: 897
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For reviews, see:
• 13a Epsztein R. In Comprehensive Carbanion Chemistry, Chap. 3. Buncel E, Durst T. Elsevier; New York: 1984: 107
• 13b Yamamoto H. In Comprehensive Organic Synthesis, Vol. 2. Trost BM, Fleming I, Heathcock CH. Pergamon Press; Oxford: 1991: 81
• 14 Exactly what causes propargylic barium reagent 6 to react selectively at the α-carbon with azo compound 2 is not clear; however, the unusually long barium–carbon bond (2.76–2.88 Å) might prevent the formation of a six-membered cyclic transition-state model 10 leading to the γ-adduct, see: Kaupp M, Schleyer P. vR. J. Am. Chem. Soc. 1992; 114: 491
• 15 Typical Experimental Procedure for the Barbier-Type Selective Propargylation of Diaryl Azo Compounds: Synthesis of 1-(6,6-Dimethylhept-4-yn-3-yl)-1,2-diphenylhydrazine (3ea, Table [2], Entry 2) Freshly cut barium (small pieces, 103.0 mg, 0.75 mmol), propargylic tosylate 1h (220.8 mg, 0.75 mmol), and azobenzene (45.6 mg, 0.25 mmol) were placed in a Schlenk tube (25 mL) under an argon atmosphere and covered with dry THF (1 mL). The mixture was stirred for 14 h at room temperature. The mixture was treated with sat. aq NH₄Cl (10 mL), and the aqueous layer was extracted three times with Et₂O (10 mL each). The combined organic extracts were washed with brine, dried over Na₂SO₄, and concentrated in vacuo after filtration. The residual crude product was purified by column chromatography on silica gel (hexane–MeOH, 50:1) to afford propargylic hydrazine 3ea (65.5 mg, 85% yield). ¹H NMR (400 MHz, CDCl₃): δ = 7.25–7.15 (m, 4 H, Ar–H), 7.05–7.03 (d, 2 H, J = 8.2 Hz, Ar–H), 6.91–6.85 (m, 3 H, Ar–H), 6.77–6.74 (t, 1 H, J = 7.3 Hz, Ar–H), 5.67 (br, 1 H, NH), 4.43–4.39 (t, 1 H, J = 7.5 Hz, CH), 1.89–1.66 (m, 2 H, CH₂), 1.16 (s, 9 H, 3 CH₃), 1.04–1.00 (t, 3 H, J = 7.4 Hz, CH₃). ¹³C NMR (99.5 MHz, CDCl₃): δ = 150.8, 149.0, 129.1, 128.9, 120.7, 118.9, 116.2, 112.1, 94.9, 75.1, 58.1, 31.4, 31.1, 27.4, 11.4. IR (neat): 3311, 2965, 2359, 1600, 1496, 1362, 1308, 1239, 1170, 1092, 1025, 992, 857, 819, 745, 691, 628 cm^–1. MS (ESI): m/z calcd for [C₂₁H₂₇N₂]⁺ [M + H]⁺: 307.2169; found: 307.2170; mp 57–60 °C.