Practical Synthesis of Ethynyl(phenyl)-λ3-Iodane Using Calcium Carbide as an Ethynyl Group Source

Stannylation of calcium carbide followed by Sn–hypervalent iodine(III) exchange reaction cleanly afforded the electrophilic ethynylating agent ethynyl(phenyl)-λ3-iodane in high yield. This two-step method uses very inexpensive materials and is readily operable without any special precautions.


Synthesis of Ethynyl(Tributyl)Stannanes
These reactions were carried out in a two-necked round bottom flask. In a typical reaction: To a stirred suspension of wellground calcium carbide (7) (2.48 g, 38.7 mmol) in DMSO (20 mL) were added chloro(tributyl)stannane (8) (3.26 g, 10.0 mmol) and water (0.40 mL, 22.2 mmol) at room temperature under argon. The resulting grayish suspension was warmed to 80 • C for 1 h (the disappearance of 8 was monitored by GCMS analysis), then allowed to cool to room temperature. Hexane was added to it, and the organic phase was filtered under reduced pressure through a K 2 CO 3 -silica gel (1:9) mixture and transferred to a separating funnel. The combined organic phase was washed with water several times, then filtered, and the filtrate was concentrated under reduced pressure to give an oil, which was further purified by chromatography (ø5 mm) on a column packed with K 2 CO 3 -silica gel (1:9). Elution with hexane gave a pale yellow oil (2.51 g). 1 H NMR analysis (mesitylene as an internal standard) showed the formation of a mixture of ethynyl(tributyl)stannane (3) (3.4 mmol, 34%) and bis(tributylstannyl)acetylene (9) (2.3 mmol, 45%). Capillary GC analysis (n-dodecane as an internal standard; Bruker BR-5ms column 0.25 mm × 30 m, 100 • C) showed different yields of 3 (3.6 mmol, 36%) and 9 (1.60 mmol, 32%), probably reflecting partial decomposition of 9 during the GC analysis. This product mixture was used directly for the synthesis of 1a. Spectroscopic data of 3 and 9 were compared to the authentic samples synthesized according to the literatures (Supplementary Material). General Procedure for Synthesis of Ethynyl-λ 3 -Iodane 1a From PhI(OAc) 2 6 To a stirred solution of (diacetoxyiodo)benzene (6) (159 mg, 0.49 mmol) in dichloromethane (1 mL) was added BF 3 -Et 2 O (130 µL, 1.04 mmol) at −78 • C, and then a 60:40 mixture of stannanes 3 and 9 (301 mg, 0.70 mmol) was slowly added. The reaction mixture was stirred at the same temperature for 1 h, then allowed to warm to room temperature, and the solvent was removed under reduced pressure. The resulting pale yellow solid was washed several times with hexane and Et 2 O at 0 • C to give 1a (114 mg, 73%).

General Procedure for Synthesis of Ethynyl-λ 3 -Iodane 1a From PhIO 2
To a stirred solution of iodosylbenzene (2) (77.6 mg, 0.35 mmol) in dichloromethane (0.7 mL) was added BF 3 -Et 2 O (100 µL, 0.77 mmol) at −78 • C, and then a 38:62 mixture of stannanes 3 and 9 (218 mg, 0.46 mmol) was slowly added. The reaction mixture was stirred at the same temperature for 1 h, then allowed to warm to room temperature, and the solvent was removed under reduced pressure. The resulting pale brown solid was washed several times with hexane and Et 2 O at 0 • C to give 1a (90.4 mg, 81%); 1 H NMR analysis shows this product contained a small amount of impurities. 1 H NMR yield: 67% (mesitylene as an internal standard).

RESULTS AND DISCUSSION
We commenced our study by trapping CaC 2 7 with 8. Exposure of well-ground 7 (4 equiv) to 8 in DMSO at room temperature did not give any alkynylstannanes. Addition of small amount of water (2 equiv), which has been reported to be effective for the electrophilic trapping of 7, was fruitless (Rodygin et al., 2016). On the other hand, heating at 80 • C resulted in smooth consumption of 8 and after 1 h, a 6:4 mixture of 3 and bis(tributylstannyl)acetylene (9) was obtained in 79% yield ( Table 1, entry 2). The ratio of 3 and 9 has changed in a range of ca. 3:7-6:4 through several runs, partly due to the reaction scale and the surface area of 7 (entries 1 and 2). Use of longer reaction time increased the ratio of 9 (entries 3 and 4). Under the conditions, the addition of water did not significantly change the yield of 9, but it accelerated the bisstannylation (entries 4-6). Interestingly, this stannylation did not occur in other aprotic solvents such as THF and DMF, even at elevated temperatures (≤110 • C) (Cochran et al., 1990). It should be noted that these alkynylstannanes 3 and 9 could be separated from other organostannane impurities on a short column packed with K 2 CO 3 -silica gel (1:9) mixture (Harrowven et al., 2010). Other crystallogen analog, trimethylsilyl chloride did not afford corresponding ethynyl(trimethyl)silanes under optimized conditions, partly because of the more moisture sensitive character of silyl chloride. Next, we focused on the synthesis of ethynyl-λ 3 -iodane 1a using a mixture of alkynylstannanes 3 and 9. After screening various reaction conditions, we found an efficient method. Exposure of a 6:4 mixture of 3 and 9 (obtained from the reaction shown in entry 1 in Table 1) to a combination of PhI(OAc) 2 6 and BF 3 -Et 2 O in dichloromethane at −78 • C resulted in smooth Sn-I(III) exchange, and after 1 h, 1a was selectively obtained in 73% yield (Figure 2A). The standard PhIO 2-BF 3 -Et 2 O system also afforded 1a in high yield. It should be emphasized that these methods do not require timeconsuming work-up. Simple washing of the reaction mixture with hexane and Et 2 O by decantation gave pure 1a and a mixture of Bu 3 SnX-type organostannanes thus formed by I(III)-Sn exchange was recovered quantitatively in the supernatant. As we expected, these optimized conditions could also be applied to authentic 3 and 9 individually to provide 1a in moderate to high yields ( Figure 2B). In these cases, the combination of I(III)-organostannane pairs (6-3 and 2-9) gave better yields of 1a than opposite pairs (6-9 and 2-3), although the reason remains unclear. From a mechanistic point of view, our results using 9 is somewhat surprising since the Sn-I(III) exchange of 9 with cyano(trifluoromethylsulfonyloxy)iodobenzene (10) selectively affords bis[phenyl(triflato)-λ 3 -iodanyl]acetylene (11) ( Figure 2C; Zhdankin, 1990, 1991). The moderately electrophilic nature of iodine center of the intermediates such as PhI(OAc) 2 -BF 3 (Izquierdo et al., 2016) or PhIO-BF 3 (Ochiai, 2007) might be partly responsible for the selective formation of 1a.

CONCLUSION
In summary, we have developed a safe and inexpensive two-step method for the synthesis of 1a using readily available CaC 2 7 as an ethynyl group source. This method not only provides time-/cost-/labor-saving methodology to prepare unstable ethynyl-λ 3iodane 1, but also serves as an effective approach for synthetically useful but costly bis(stannyl)acetylene 9 (Brend'amour et al., 2018).

DATA AVAILABILITY STATEMENT
All datasets generated for this study are included in the article/Supplementary Material.