Rhodium-Catalyzed Generation of Anhydrous Hydrogen Iodide: An

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Cite This: Org. Lett. 2018, 20, 6859−6862

Rhodium-Catalyzed Generation of Anhydrous Hydrogen Iodide: An Effective Method for the Preparation of Iodoalkanes Chaoyuan Zeng,† Guoli Shen,† Fan Yang,† Jingchao Chen,*,† Xuexin Zhang,† Cuiping Gu,† Yongyun Zhou,† and Baomin Fan*,†,‡ †

YMU-HKBU Joint Laboratory of Traditional Natural Medicine, Yunnan Minzu University, Kunming 650500, China Key Laboratory of Chemistry in Ethnic Medicinal Resources, Yunnan Minzu University, Kunming 650500, China

‡

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S Supporting Information *

ABSTRACT: The preparation of anhydrous hydrogen iodide directly from molecular hydrogen and iodine using a rhodium catalyst is reported for the first time. The anhydrous hydrogen iodide generated was proven to be highly active in the transformations of alkenes, phenyl aldehydes, alcohols, and cyclic ethers to the corresponding iodoalkanes. Therefore, the present methodology not only has provided convenient access to anhydrous hydrogen iodide but also offers a practical preparation method for various iodoalkanes in excellent atom economy.

I

Table 2. Optimization of Reaction Conditions

odoalkanes, also known as alkyl iodides, are versatile building blocks in organic synthesis due to their outstanding

Table 1. Screening of Ligands and Transition-Metal Precursorsa

entry

metal

ligand

yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Rh(COD)2BF4 Rh(COD)2BF4 Rh(COD)2BF4 Rh(COD)2BF4 Rh(COD)2BF4 Rh(COD)2BF4 Rh(COD)2BF4 [Rh(COD)Cl]2 Rh(CO)2(C5H7O2) Rh(COD)2SbF6 Rh(COD)2OTf RhCl3·3H2O [Rh(C5Me5)Cl2]2 [Ir(COD)Cl]2 Ru(COD)Cl2 Pd(OAc)2

PPh3 (±)-Binap Dppe Dppb Dppf xantphos tricyclohexylphosphine (±)-Binap (±)-Binap (±)-Binap (±)-Binap (±)-Binap (±)-Binap (±)-Binap (±)-Binap (±)-Binap

75 84 80 72 79 70 77 53 61 72 47 trace NR 58 56 trace

solvent

pressure

yield (%)

1 2 3 4 5 6 7 8 9

toluene DCE DCM DMF MTBE DME toluene toluene toluene

40 bar 40 bar 40 bar 40 bar 40 bar 40 bar 10 bar 2 bar atmospheric pressure

92 84 50 NR NR trace 92 92 40

a

Reaction conditions: Rh(COD)BF4 (0.01 mmol) and (±)-Binap (0.012 mol) in solvent (2 mL) were stirred at room temperature for 30 min under Ar, and then 1a (0.2 mmol) and I2 (0.2 mmol) were added. The reaction mixture was stirred in a hydrogen atmosphere.

reaction activities in various reactions such as alkylation,1 cross coupling,2 reduction,3 addition,4 esterification,5 and the organometallic transformations.6 The common present methods for the preparation of iodoalkanes rely on the Finkelstein reaction7 or iodination8 of the corresponding alcohols. Although the direct addition of hydrogen iodide to alkenes has provided an atom-economic strategy for the facile synthesis of iodoalkanes and has been introduced in many textbooks,9 relevant reports are quite few in the literature due to the low

a

Reaction conditions: Metal (0.01 mmol) and ligand (M:P = 1:1.2) in toluene (2 mL) was stirred at room temperature for 30 min under Ar, and then 1a (0.2 mmol) and I2 (0.2 mmol) were added. The reaction mixture was stirred in a hydrogen atmosphere of 40 bar. Note: COD = 1,5-cyclooctadiene; [Rh(C5Me5)Cl2]2 = pentamethylcyclopentadienylrhodium(III) chloride dimer. © 2018 American Chemical Society

entry

Received: September 18, 2018 Published: October 16, 2018 6859

DOI: 10.1021/acs.orglett.8b02980 Org. Lett. 2018, 20, 6859−6862

Letter

Organic Letters Table 3. Scope Exploring of Alkenesa

a Reaction conditions: Rh(COD)BF4 (0.01 mmol) and (±)-Binap (0.012 mol) in toluene (2 mL) were stirred at room temperature for 30 min under Ar, and then 1 (0.2 mmol) and I2 (0.2 mmol) were added. The reaction mixture was stirred in a hydrogen atmosphere of 2 bar. bReacted under a hydrogen atmosphere of 10 bar.

metal catalyst. The reaction mechanism of hydrogenation reactions was known as the Horiuti−Polanyi mechanism,18 which describes hydrogen molecule dissociation followed by the sequential addition of atomic hydrogen to the alkenes. We envisioned that using iodine as an atomic hydrogen acceptor might result in the generation of anhydrous hydrogen iodide. Herein, we report the unprecedented rhodium-catalyzed preparation of anhydrous hydrogen iodide from hydrogen and iodine. This robust reagent was successfully applied to the reaction of alkenes, aldehydes, alcohols, and ethers, affording various iodoalkanes in one-pot reaction. Our study commenced with the hydroiodination reaction of alkenes, carried out by the reaction of hydrogen gas and iodine with the complex of Rh(COD)BF4 and PPh3 as catalyst and DCE as solvent, performed under a hydrogen atmosphere of 40 bar (Table 1). To our delight, we have observed the fading of reddish brown iodine solution, and finally a light yellow solution resulted after 3 h. After the addition of styrene 1a, the addition product 1-iodo-1-phenylethane 2a was obtained as expected in 75% yield over 4 h (Table 1, entry 1). To improve

reaction activity of aqueous hydrogen iodide in the reactions with alkenes and the uncontrollable iodine liberation of the gaseous hydrogen iodide.10 Thus, methods have been developed to obviate this drawback by allowing for the in situ HI generation via a combination of reagents. The reaction of silanes and iodine has provided a method for the generation of anhydrous hydrogen iodide, and its application in hydroiodination reactions has been reported.11 However, the unavoidable byproduct iodosilane has weakened its utility, especially in the chemical industry. Alternative reagents such as Et3N·HI,12 Al2O3·I2,13 TiI4,14 I2/thiol,15 and Me3SiCl/NaI16 were also developed. Clearly, the development of a practical, effective, atom-economic, and inexpensive iodination feedstock is still urgent and is met with broad interest. To the best of our knowledge, there has been no example of generating anhydrous hydrogen iodide directly from molecular hydrogen and iodine. The hydrogenation reaction of alkenes, which is the most widely used reaction in the organic chemistry industry,17 represents an example of hydrogen activation by a transition 6860

DOI: 10.1021/acs.orglett.8b02980 Org. Lett. 2018, 20, 6859−6862

Letter

Organic Letters

screening results show that the hydroiodination reaction can proceed in toluene, DCE, and DCM (Table 2, entries 1−3). Other solvents, including DMF, MTBE, and DME, were not suitable (Table 2, entries 4−6). The following pressure experiments indicated that a hydroiodination reaction took place smoothly at 10 and 2 bar (Table 2, entries 7 and 8). However, the reaction under atmospheric pressure which was carried out by using a hydrogen balloon reacted slowly, giving the desired product only in 40% yield with the majority of 1a unreacted after 72 h (Table 2, entry 9). With the optimized reaction conditions in hand [5 mol % of Rh(COD)2BF4, 6 mol % of Binap, toluene as solvent and performed at room temperature under 2 bar], the scope and limitations for this hydroiodination reaction of various alkenes were investigated. The experimental results are presented in Table 3. In general, most of the alkene hydroiodination reactions proceed smoothly to afford the corresponding iodoalkanes as Markovnikov addition products. Notably, high reaction yields were achieved by using halogenated styrenes (Table 3, entries 2−6) and diphenylethenes (Table 3, entries 10−11) with halogen groups kept intact in the products. In contrast, electron-rich styrenes were less reactive than electronpoor substrates. For example, the hydroiodination reaction of para-methyl-styrene 1g only afforded the desired product in a trace amount under an enhanced pressure of 10 bar (Table 3, entry 7); trimethyl-2-vinylbenzene 1h and para-methoxylstyrene 1i failed to produce any hydroiodination products (Table 3, entries 8−9). We were delighted to notice the aliphatic alkenes reacted efficiently to give the corresponding iodoalkanes (Table 3, entries 12−16). In addition, internal alkenes also performed well; the reaction of cyclooctene almost gave quantitative yield (Table 3, entry 17). cis-4-Octene gave a moderate yield, probably because of its unfavorable electronic property (Table 3, entry 18). Conjugated alkene, such as methyl cinnamate, also participated readily in this hydroiodination protocol to give the Michael addition product 2s (Table 3, entry 19). To our delight, the current methodology was suitable for the hydroiodination of various phenyl aldehydes by switching the rhodium precursor and phosphine ligand (Scheme 1a). Benzaldehyde was converted to benzyl iodide 4a under the standard conditions, and its derivatives with methyl or methoxyl substitutions were also applicable to give the benzyl iodide derivatives 4b−d. Substrates with halogen substitutions on the phenyl ring were also compatible and afforded benzyl iodide derivatives 4e−g. Nevertheless, no iodoproduct 4h was detected by using 3-phenylpropanal, indicating that the aliphatic aldehyde may not applicable. Furthermore, the current preparation method of anhydrous hydrogen iodide has provided access to benzyl iodides from benzyl alcohols. As it was summarized in Scheme 1b, most of the halogenated benzyl alcohols and the alkyl-substituted benzyl alcohols were transformed to the corresponding benzyl iodides in excellent yield; benzyl alcohol and 4-nitrobenzyl alcohol also reacted well. Notably, the aliphatic alcohols such as 4n and 4o were also applicable. In addition, anhydrous hydrogen iodide successfully participated in the ring-opening reactions of cyclic ethers to afford various iodohydrins (Scheme 1c). Epoxides such as cyclopentene oxide and 1,2-epoxycyclohexane were efficiently transformed to the vicinal iodohydrins 7a and 7b. 4-Membered cyclic ether, together with 5- and 6-membered cyclic ethers, all have reacted smoothly and afforded the corresponding ring-

Scheme 1. Reaction of Anhydrous Hydrogen Iodide with Other Substratesa

a

Reaction conditions: Rhodium catalyst (0.01 mmol) and phosphine ligand (M:P = 1:1.2) in DCE (2 mL) were stirred at room temperature for 30 min under Ar, and then 3 or 5 (0.2 mmol) and I2 (0.2 mmol) were added. The reaction mixture was stirred in a hydrogen atmosphere. *Rhodium catalyst (0.01 mmol) and phosphine ligand (0.012 mol) in 6 (2 mL) were stirred at room temperature for 30 min under Ar, and then I2 (0.2 mmol) was added. The reaction mixture was stirred in a hydrogen atmosphere, and the reaction yields were calculated by the amount of I2 used.

the reaction yield, alternative phosphine ligands were screened. It was found that all of the tested phosphine ligands can promote the present hydroiodination reaction (Table 1, entries 2−7), but all less effective than (±)-Binap in terms of the reaction yield (Table 1, entry 2). With (±)-Binap as ligand, various transition metal catalysts including rhodium, iridium, ruthenium, and palladium precursors were next tested. The results indicate that except for RhCl3·3H20 and [Rh(C5Me5)Cl2]2 all of the tested rhodium precursors showed moderate catalytic abilities (Table 1, entries 8−11). Iridium and ruthenium catalysts were also effective (Table 1, entries 14− 15), but Pd(OAc)2 failed to promote the current reaction (Table 1, entry 16). Other reaction parameters including solvent, temperature, and the hydrogen pressure were next investigated. The solvent 6861

DOI: 10.1021/acs.orglett.8b02980 Org. Lett. 2018, 20, 6859−6862

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opening iodohydrins 7c−f. However, ethylene oxide was not suitable as no desired product was detected. In summary, we have realized the first catalytic preparation method for anhydrous hydrogen iodide from iodine and hydrogen. By employing this technology, various substrates including alkenes, phenyl aldehydes, alcohols, and cyclic ethers were transformed into the corresponding iodoalkanes. Notably, the present catalytic method has not only provided a practical preparation method for anhydrous hydrogen iodide and iodoalkanes but also offered a promising future for other hydrogen iodide participating reactions. Further investigations of other organic reactions using anhydrous hydrogen iodide are underway in our laboratory.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b02980.

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Experimental procedures and characterization data (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Chaoyuan Zeng: 0000-0001-7032-6004 Baomin Fan: 0000-0003-1789-3741 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully thank the National Natural Science Foundation of China (21572198), the Applied Basic Research Project of Yunnan Province (2018FB021, 2017FA004), and the Department of Education of Yunnan Province (2017ZDX046) for their financial support.

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REFERENCES

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DOI: 10.1021/acs.orglett.8b02980 Org. Lett. 2018, 20, 6859−6862