Synthesis of Polyynes Using Dicobalt Masking Groups

Abstract

Extended triisopropylsilyl end-capped polyynes have been prepared from the corresponding tetracobalt complexes by removing the complexed dicobalt tetracarbonyldiphenylphosphinomethane (Co(CO)dppm) moieties. Unmasking of this "masked alkyne equivalent" was achieved under mild conditions with elemental iodine at room temperature, making it possible to obtain fragile polyynes with up to 20 contiguous sp-hybridized carbon atoms. The Co(CO)dppm moiety has a strong geometric and steric effect on the polyyne but does not have a marked electronic effect on the terminal alkyne, as indicated by NMR and IR spectroscopy, density functional theory calculations, and X-ray crystallography. An unusual "alkyne hopping" migration of the dicobalt group was noticed as a minor side reaction during copper-catalyzed Eglinton coupling.

Masked Alkyne Equivalents for the Synthesis of Mechanically Interlocked Polyynes**

“…An alternative to photochemically activated systems might be the use of metal complexes as stoppering groups as they are inherently sterically demanding. [21]…”

“…When modified appropriately to increase the bulk, such a system might unmask more efficiently than the MAE presented here. An alternative to photochemically activated systems might be the use of metal complexes as stoppering groups as they are inherently sterically demanding [21] …”

π‐Conjugated Macrocycles Bearing Angle‐Strained Alkynes

“…[15] Other precursors of cyclocarbons are summarized in Figure 10 c. The generation of cyclocarbonsf rom the precursors 2, [17,79] 5, [18,19] 6, [19,65] and 7 [66] was confirmed by mass spectrometry and ultraviolet photoelectron spectroscopy. [80] Although the demetalation of cobaltc omplexes successfully proceeded to form linear oligoynes, [81] the synthesis of strainedc yclo-[n]carbonsf rom precursors 3 has not succeeded so far. [16,82] The acid-mediated hydrolysis of the benzylideneg roups in 4 was unsuccessful.…”

“…Cyclocarbons have not been isolated so far owing to their high reactivity. Because linear polyynes are highly reactive, thermally cross‐linkable, and explosive, the protection by cyclic molecules, such as cyclodextrins, and the capping by transition‐metal complexes is basically essential. In contrast, both cyclic and linear oligo‐ and polyphenylenes are stable under ambient conditions.…”

Masked Alkyne Equivalents for the Synthesis of Mechanically Interlocked Polyynes**

“…Masked alkyne equivalents (MAEs) have been used since the 1980s to release polyyne chains from suitable precursors. Unmasking is achieved using light, [14][15][16] heat, [17] chemical reagents [18][19][20][21] and on-surface using voltage pulses. [22,23] MAEs have opened synthetic avenues to cyclocarbons,r ings consisting entirely of sp-hybridized carbon atoms.…”