Regiospecific and highly stereoselective synthesis of β-amino (Z)-enylphosphonates via β-hydrogen migration reaction of dialkyl α-diazophosphonates catalyzed by AgOTf

Abstract

A series of dialkyl α-diazophosphonates bearing different substituents have been prepared from natural amino acids in order to investigate the steric effect in 1,2-migration reaction of metal carbenes. The diazo decomposition of the diazophosphonates using the AgOTf/NaBAr_F complex resulted in β-hydrogen migration to give β-amino (Z)-enylphosphonates in good yields with high regio- and stereoselectivity. A possible reaction mechanism shows that the steric effect could dramatically influence the geometric isomerism aptitude. This new method for constructing (Z)-β-amino vinylphosphonates should be of general utility in organic synthesis.

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Diazo compounds are commonly used important precursors of metal carbenes, which can subsequently undergo diverse chemical transformation.¹ Numerous efficient transformation such as cyclopropanations,² X–H (X = C, Si, O, N, S, etc.) insertions,³ cycloadditions⁴ and ylide transformations⁵ have been developed, as well as studies on reactivities and mechanisms of the involved transition-metal carbenoids.⁶ In the catalytic reaction of α-diazo carbonyl compounds with both metal carbene and free carbene as reactive intermediates, 1,2-hydrogen migration is a frequently encountered reaction, which referred to as β-hydride shift.⁷ In some cases, 1,2-hydrogen migration can find useful application in organic synthesis. For example, Taber and co-workers have shown that α-diazo esters undergo β-hydride elimination with rhodium(II) trifluoroacetate dimer produce (Z)-α,β-unsaturated esters in good yields.⁸ Wang has found that the diazo decomposition of β-(N-tosyl)amino diazo carbonyl compounds gives either 1,2-aryl migration or 1,2-hydride migration products as the major products, depending on the reaction conditions.⁹

In contrast to that found in α-diazo carbonyl compounds, α-diazophosphonyl compounds have not been studied systematically in metal carbene reactions.¹⁰ Recently, we reported a kind of novel α-diazophosphonyl compounds prepared from natural amino acid, which could afford β-alkoxy substituted β-amino phosphonates derivatives through a combined C–H functionalization/O–H insertion process (Scheme 1, eqn (1)).¹¹ As a natural extension of the diazo compounds 1,2-hydride migration reaction, we developed a stereoselective [Cu(MeCN)₄]PF₆/I₂ catalyzed β,γ-dihydrogen shift reaction for the synthesis of (Z)-β-alkenyl substituted β-aminophosphonates (Scheme 1, eqn (2)).¹²

Scheme 1 Previous and proposed work.

β-Aminophosphonates are the phosphorus analogues of β-amino acids, and therefore have widely used for biological and pharmaceutical applications, such as enzyme inhibitors, agrochemicals, or antivirus activities.¹³ β-Amino vinylphosphonates can be regarded as isosters of β-aminophosphonates. These compounds can be prepared via formation of C–C-, C–N-, and C–P-bonds.¹⁴ For example, Palacios reported the preparation of fluoroalkyl β-enaminophosphonates from alkylphosphonates and perfluoroalkyl nitriles (Scheme 1, eqn (3)).^14a Ionin also reported the addition reaction of secondary amines to alkynylphosphonates catalyzed by Cu(I) salts to form (E)-β-enaminophosphonates (Scheme 1, eqn (4)).^14b In spite of these results up to now there is only one example of the synthesis of β-enaminophosphonates from the corresponding α-diazoethylphosphonates underwent an exclusive 1,2-aryl shift to form the enamine products (Scheme 1, eqn (5)).^14c Continuing with our interest in the chemistry of aminophosphorus derivatives,¹⁵ here we report the first example for converting dialkyl α-diazophosphonates into β-amino (Z)-enylphosphonates in a regiospecific and highly stereoselective manner (Scheme 1, eqn (6)).

Initially, the amino acid derived α-diazophosphonyl compound 1a¹¹ was the first substrate studied to examine the effect of the catalysts on the reaction, and the results were summarized in Table 1. The results revealed that Cu(MeCN)₄PF₆ decomposed 1a mainly afforded (E)-α,β-unsaturated phosphonate 3a in low yield (Table 1, entry 1). CuOTf and Rh₂(OAc)₄ could only decompose 1a with low yields and obtain 2a and 3a with poor stereoselectivities (Table 1, entries 2 and 3). Hg(OTf)₂ could not decompose 1a at all and the starting material recovered (Table 1, entry 4). To our delight AgOTf could promote the reaction smoothly with higher yield and good stereoselectivity (Z/E = 73 : 27) (Table 1, entry 5). When the reaction proceeded in MTBE, the Z/E ratio will increase to 76 : 24 (Table 1, entry 6). To further improve the reactivity and stereoselectivity, the effects of additive and solvent were investigated. In recent years the use of weakly or noncoordinating anions as counter anions is of significant interest in both synthesis and catalysis.¹⁶ The lack of reactivity and non-nucleophilic character of [BAr_F]⁻ have led to the widespread use as noncoordinating.¹⁷ In our investigation, after screening NaBAr_F, it was found that PhCONH₂, CH₃CONH₂, t-BuOH, DMF and PhOH could not give superior results in terms of reactivity and stereoselectivity (Table 1, entries 7–12).

Table 1 Optimization of the reaction conditions^a

Entry	Catalysts	Solvent	Additive	Z/E ratio^b (2a : 3a)	Overall yield^c (%)
a Unless otherwise noted, all reactions were carried out using α-diazophosphonate 1a (0.28 mmol, 1 equiv.) in 2 mL solvent with 5 mol% of catalyst and 6 mol% additive in 2 mL solvent at 25 °C for 7.5 h (before addition 1.5 h, after addition 6 h).b The product ratio was determined by ^31P NMR of the crude product.c Overall yield of the mixture of 2a and 3a after silica gel chromatograph.d MTBE = methyl tert-butyl ether.e NaBArF = sodium tetrakis[3,5-bis(trifluoromethyl)-phenyl]borate.f 50 mol% PhCONH2 was used.g 50 mol% CH3CONH2 was used.h 5eq. t-BuOH was used.i 5 eq. DMF was used.j 5 eq. PhOH was used.k 4 mL MTBE was used to dissolve α-diazophosphonate 1a.l 2 mol% AgOTf and 3 mol% NaBArF was used.m 10 mol% AgOTf and 11 mol% NaBArF was used.
1	Cu(MeCN)4PF4	CH2Cl2	—	7 : 93	38
2	CuOTf	CH2Cl2	—	63 : 37	11
3	Rh2(OAc)4	CH2Cl2	—	23 : 77	15
4	Hg(OTf)2	CH2Cl2	—	—	N.R.
5	AgOTf	CH2Cl2	—	73 : 27	64
6	AgOTf	MTBE^d	—	76 : 24	65
7	AgOTf	CH2Cl2	NaBArF^e	63 : 37	77
8	AgOTf	CH2Cl2	PhCONH2^f	64 : 36	63
9	AgOTf	CH2Cl2	CH3CONH2^g	61 : 39	71
10	AgOTf	CH2Cl2	t-BuOH^h	60 : 40	75
11	AgOTf	CH2Cl2	DMF^i	69 : 31	65
12	AgOTf	CH2Cl2	PhOH^j	62 : 38	70
13	AgOTf	MTBE	NaBArF	78 : 22	73
14	AgOTf	Et2O	NaBArF	86 : 14	51
15	AgOTf	PhCH3	NaBArF	78 : 22	61
16	AgOTf	i-Pr2O	NaBArF	80 : 20	60
17	AgOTf	PhOMe	NaBArF	80 : 20	51
18	AgOTf	Acetone	NaBArF	73 : 27	45
19^k	AgOTf	MTBE	NaBArF	80 : 20	75
20^l	AgOTf	MTBE	NaBArF	75 : 25	73
21^m	AgOTf	MTBE	NaBArF	80 : 20	77

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With the best AgOTf catalyst combined with NaBAr_F as additive (Table 1, entry 7), we next carried out the reaction in different solvents to determine the best solvent for this reaction. Among the various solvents tested, diethyl ether, toluene, isopropyl ether, anisole and acetone afforded lower yields of the expected products 2a and 3a with moderate stereoselectivities (Table 1, entries 14–18). The most suitable solvent was found to be methyl tert-butyl ether (MTBE), (Z)-diethyl 2-(1,3-dioxoisoindolin-2-yl)prop-1-enylphosphonate 2a was obtained in good yield and stereoselectivity (Table 1, entry 13). With further optimization of the reaction conditions, we found that increasing the solvent amount to 6 mL, the Z/E ratio could increase to 80 : 20 with good yield (Table 1, entry 19). Furthermore, a decrease in the catalyst loading to 2 mol% of AgOTf and 3 mol% of NaBAr_F led to a decrease in yield and stereoselectivity (Table 1, entry 20). Similar result was obtained when the reaction was performed in MTBE with increasing the catalyst loading to 10 mol% of AgOTf and 11 mol% of NaBAr_F (Table 1, entry 21). Thus, the optimal reaction conditions for this transformation were determined to be 0.28 mmol α-diazophosphonate 1a, 5 mol% of AgOTf as catalyst and 6 mol% of NaBAr_F as additive in 6 mL MTBE as solvent at room temperature. The two stereoisomers 2a and 3a could be separated after purification.

Based on the above optimization efforts, the substrate scope of this reaction was investigated (Table 2). The impact of substituent groups on β-position of dialkyl α-diazophosphonates 1 which derived from different natural amino acids was evaluated. The tested α-diazophosphonates 1a–b with different substituents on β-position, such as methyl and isobutyl groups afforded good level yields of β-amino-α,β-unsaturated phosphonates 2a–b in favor of the Z-isomer (Table 2, entries 1 and 2). In cases where the substituent groups on β-position of dialkyl α-diazophosphonates 1 change to benzyl, 2-propylisoindoline-1,3-dione, and p-tolyl acetate groups, a significant amount of Z-isomers 2c–e was formed (Table 2, entries 3–5).

Table 2 Scope of the reaction^a

Entry	Substrate	R^1	R^2	R^3	Z/E ratio^b (2 : 3)	Overall yield^c (%)
a Reaction conditions: α-diazophosphonate 1 (0.28 mmol) in 6 mL of MTBE at 25 °C in the presence of 5 mol% of AgOTf and 6 mol% of NaBArF for 7.5 h (before addition 1.5 h, after addition 6 h).b The product ratio was determined by ^31P NMR of the crude product and the configuration of 2 was assigned as Z by the ^1H–^1H NOESY spectrum.c Overall yield of the mixture of 2 and 3 after silica gel chromatograph.d CH2Cl2 was used as the solvent without the addition of NaBArF.
1	1a	H	H	Et	80 : 20	75
2	1b	CH(CH3)2	H	Et	80 : 20	87
3	1c	Ph	H	Et	93 : 7	87
4	1d	(CH2)3NPhth	H	Et	91 : 9	62
5	1e	p-AcOPh	H	Et	98 : 2	63
6^d	1f	CH3	CH3	Et	100 : 0	76
7^d	1g	CH3	Et	Et	100 : 0	73
8	1h	CH2SCH3	H	Et	—	N.R.
9	1i	H	H	Me	90 : 10 (69 : 31)^d	62 (71)^d
10	1j	H	H	i-Pr	78 : 22 (72 : 28)^d	71 (72)^d
11	1k	H	H	n-Bu	72 : 28 (69 : 31)^d	61 (89)^d
12	1l	Ph	H	Me	90 : 10	79
13	1m	Ph	H	i-Pr	96 : 4 (88 : 12)^d	51 (79)^d
14	1n	Ph	H	n-Bu	92 : 8 (89 : 11)^d	28 (94)^d

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It was found that the Z/E isomer selectivity of 2 and 3 impacted by the size of the R₁ and R₂ groups of α-diazophosphonates 1. With the increasing bulk of R₁ group, the Z/E ratio of 2 and 3 ranged from 80 : 20 to 98 : 2 with moderate to good yields (Table 2, entries 1–5). In cases where the substituent groups on β-position of dialkyl α-diazophosphonates change to isopropyl and isobutyl groups, no desired products 2f–g and 3f–g obtained due to the poor solubility of substrate 1f–g in MTBE. When use CH₂Cl₂ as solvent without the add of NaBAr_F, a significant amount of Z-isomers 2f–g were formed with good yields (Table 2, entries 6 and 7). It is worthwhile to note that diethyl α-diazophosphonate 1h which derived from methionine can not proceed this reaction and get the desired product (Table 2, entry 8).

To access the effect of substrates on product selectivity, we set out to study reactions of a series of dialkyl α-diazophosphonates 1i–n under AgOTf/NaBAr_F catalytic condition. The migratory product aptitude was dependent upon the size of the R³ group. When the bulk of R³ group was increased from methyl to butyl, the Z/E isomer products 2 and 3 were obtained with lower yields. Interestingly, when the reaction proceeding in CH₂Cl₂ without the add of NaBAr_F, the low yields could be improved with some changes for the Z/E isomer selectivities (Table 2, entries 9–14). The structure of 2a was confirmed by single crystal X-ray diffraction (see the ESI† for details).¹⁸

The Z/E stereoselectivity of β-amino-α,β-unsaturated phosphonates implies that conformational factors may play a role in the migration process. For the migration to occur, it is necessary that the migrating bond needs to be parallel to the p orbital of the carbene carbon in the transition states.¹⁹ Of the two conformations A and B, B is likely to be disfavored because of steric hindrance between the phosphonate group and the R₁ group. Thus, the β-hydrogen migration is proposed to occur via transition state A which leads to the observed Z-isomer. The effect is most likely due to the increased steric hindrance between R₁ and the phosphonate group, which affects the populations and barriers to their interconversion, of the stereoelectronically required conformations for migration. Besides the steric hindrance influence, Ag⁺ participates chelation effect should not be ruled out. As shown in Fig. 1, Ag⁺ may coordinate with phosphonate group and phthalimide group in transition state A which lead to the formation of Z-isomer. In transition state B, the chelation effect is lacked, therefore conformation A is more favored than B.

Fig. 1 Conformations leading to β-hydrogen migration.

In conclusion, we have developed a new and convenient synthesis of β-amino (Z)-enylphosphonates from amino acid-derived dialkylphosphonates with complete control over regio- and stereochemistry. The influence factors on the Z/E isomer selectivity have been discussed. The investigation demonstrated that, steric factors play the important role in affecting the geometric isomerism aptitude in this carbene reaction. Further researches for extension of this reaction are currently underway in our laboratory.

Acknowledgements

We thank the National Natural Science Foundation of China (21072102), the Committee of Science and Technology of Tianjin (11JCYBJC04200) and State Key Laboratory of Elemento-Organic Chemistry in Nankai University for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental section, characterization of all compounds, and copies of ¹H and ¹³C NMR spectra for selected compounds. CCDC 893994. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra02520b

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