[19093-51-7] · C5H7Cu · 1-Pentynylcopper(I) · (MW 130.67)
Solubility: sol pyridine (10-5 M, 25 °C); slightly sol ether, THF; insol DMF, water.
Purification: can be dried under vacuum (20 mmHg) at 50 °C; traces of unremoved ammonia impart a greenish-yellow color.5
Handling, Storage, and Precautions: all copper(I) acetylides are potentially explosive when dry and should be treated with due caution. Use in a fume hood.
Mild substitutions of acyl and sulfenyl halides can be accomplished with high efficiency (eqs 1 and 2).7 Thermolysis is either carried out neat or in acetonitrile solution.
Halides that are prone to homolytic scission, however, instead of yielding substitution products give rise to acetylenic oxidative coupling products.7 Addition of pentynylcopper to aryl iodides results in formation of coupled tolanes; ortho-amino substituted tolanes can be thermolyzed to give a range of 2-propyl indoles in good yield (eq 3).2,5,7 The cyclizations, which can be carried out either neat or in refluxing pyridine, cyclohexane or DMF, can be catalyzed by copper(I) iodide or a range of salts including HgI2, AgNO3, Hg2Cl2, PdCl2, and PtCl2.7
A similar protocol has been used to convert substituted o-bromophenols into the corresponding 2-propylbenzofurans (eq 4).2 In this series it is not possible to isolate the intermediate tolanes.
Intramolecular additions to tolanes can be effected by other groups, including carboxylic acids, yielding isocoumarins and benzylidenephthalides (eq 5). The yields of such bicyclic adducts are very dependent on both the conditions used and ring size. Intramolecular cyclization of the tolane derived from addition to o-iodobenzyl alcohol gives a 50% yield of the corresponding dihydrobenzopyran bearing an n-propyl substituent. However, reaction of pentynylcopper with o-iodophenethyl alcohol results in formation of dihydrobenzofuran and 1-pentyne in 79% yield.7
Pentynylcopper has been employed to effect coupling to acetylenic halides. Treatment with bromoethynyltrimethylsilane in pyridine at room temperature results in a near-quantitative yield of 1-trimethylsilyl-1,3-heptadiyne (eq 6).8
Mixed organocuprates derived from addition of the copper acetylide to vinyllithium reagents can be used to selectively transfer the vinylic group to a suitable electrophile (eq 7).9 In this manner the pentynyl group behaves as a nontransferable dummy ligand.3 The solubility of pentynylcopper in ether and THF is increased on addition of HMPA; thus the complex is more versatile for use in the preparation of mixed lithioorganocuprates for selective ligand transfer (see 1-Pentynylcopper(I)-Hexamethylphosphorous Triamide).3
Dilithium tris(1-pentynyl)cuprate, prepared by addition of pentynyllithium to copper iodide, adds to a,b-unsaturated systems with 1,2-chemoselectivity (eq 8).10 The additions, which are best effected in the presence of HMPA, are effective in cyclohexenone and cyclopentenone systems, and tolerate b-substitution on the enone.10 As an example of its utility, oxidative rearrangement of the product of addition to 2-allylcyclopent-2-enone gives access to a possible prostaglandin precursor (eq 8).
Graham B. Jones & Brant J. Chapman
Clemson University, SC, USA