[56964-06-8]  · C5H5Cu  · 3-Methyl-3-buten-1-ynylcopper(I)  · (MW 128.65)

(coupling agent for introduction of the isopropenyl group;2 nucleophilic species which effects decarbonylative ring contraction of bicyclic lactones3)

Alternate Name: copper(I) isoprenylacetylide.

Physical Data: bright yellow solid; mp 120-200 °C (dec);2 IR (KBr) 3095, 3003, 2970, 2921, 1932, 1603, 1440, 1428, 1380, 1363, 1255, 895, 878, 709, 552, 525 cm-1.4

Solubility: sol pyridine, DMF, ethanol; insol water.

Preparative Method: Hydroxylamine hydrochloride reduction of Copper(II) Sulfate in aq ammonium hydroxide followed by addition of isopropenylacetylene in ethanol.2 Drying: solid dried under P2O5 in vacuo.

Handling, Storage, and Precautions: all solid copper(I) acetylides are potentially explosive. Use in a fume hood.

Coupling Reactions.

Organocuprates such as isopropenylcopper alkynes couple to a variety of electrophiles (see also 1-Hexynylcopper(I), 1-Pentynylcopper(I), Phenylethynylcopper(I)). The couplings, typically performed on aryl and vinyl iodides and bromides, give rise to substituted alkynes, which in one report included products of the dienyne class (eq 1).5

Coupling onto aryl iodides is one of the more studied reactions, due to its importance in the synthesis of natural product systems. A representative example is seen in the synthesis of frustulosin, a tetrasubstituted aromatic natural product (eq 2).6

Isolation of the disubstituted alkynic product is often not possible where an o-hydroxy group is present. Instead, concomitant cyclization to yield benzofuran-type structures usually ensues under the reaction conditions.7 Numerous examples have been documented.2,8-10 Often, heavily functionalized molecules do not pose any problem to the sequence, yielding complex heterocycles (eq 3).11

The requirement for an o-hydroxy group is not strict, as phenolic derivatives such as acetates12,13 and even methyl ethers14 have been shown to yield the corresponding cyclized benzofurans under the harsh reaction conditions, which typically involve thermolysis either in refluxing pyridine or quinoline.12 -14 Thus thermolysis of an o-iodo diacetate in pyridine yields a benzofuran (eq 4), an intermediate which was used in the synthesis of the dibenzofuranoid Cannabis constituents cannabifuran and dehydrocannabifuran.15

Nucleophilic Displacements.

Copper isopropenylacetylide can behave as a simple nucleophile under certain circumstances, effecting decarbonylative ring contractions (eq 5).3

The benzofuran product would presumably not be accessible via the traditional tolane cyclization route due to competing reactions at the bromo site.9

1. (a) Normant, J. F. S 1972, 63. (b) Sladkov, A. M.; Golding, I. R. RCR 1979, 48, 868. (c) Lipshutz, B. H.; Sengupta, S. OR 1992, 41, 135. (d) FF 1980, 8, 123.
2. Schreiber, F. G.; Stevenson, R. JCS(P1) 1977, 90.
3. Reisch, J.; Bathe, A. LA 1988, 543.
4. Aleksanyan, V. T.; Garbuzova, I. A.; Gol'ding, I. R.; Sladkov, A. M. Spectrochim. Acta 1975, 31A, 517.
5. Struve, G.; Seltzer, S. JOC 1982, 47, 2109.
6. Ronald, R. C.; Lansinger, J. M. CC 1979, 124.
7. Pinault, M.; Frangin, Y.; Genet, J. P.; Zamarlik, H. S 1990, 935.
8. Batu, G.; Stevenson, R. JOC 1979, 44, 3948.
9. Scannell, R. T.; Stevenson, R. JCS(P1) 1983, 2927.
10. Schreiber, F. G.; Stevenson, R. JCR(M) 1978, 1201.
11. Scannell, R. T.; Stevenson, R. JHC 1986, 23, 857.
12. Schreiber, F. G.; Stevenson, R. OPP 1978, 10, 137.
13. Bohlmann, F.; Bühmann, U. CB 1972, 103, 863.
14. Gaston, J. L.; Greer, R. J.; Grundon, M. F. JCR(S) 1985, 135.
15. Scannell, R. T.; Stevenson, R. JCR(S) 1983, 36.

Graham B. Jones & Brant J. Chapman

Clemson University, SC, USA

Copyright 1995-2000 by John Wiley & Sons, Ltd. All rights reserved.