Disilane-Catalyzed and Thermally Induced Oligomerizations of Alkynes:  A Comparison

Abstract

The alkynes RCtCR (R ) H, Et, Ph), RCtCH (R ) Me(CH 2 ) 5 , Me(CH 2 ) 7 , Ph, Me 3 Si, EtO 2 C), and RCtCR′ (R ) Ph, R′ ) C 6 F 5 ; R ) Me, R′ ) Ph) trimerize to corresponding benzene derivatives in 30-100% yields in the presence of Si 2 Cl 6 as a procatalyst at 170-200 °C over 20-48 h. These reactions represent only the second example of nonmetal-catalyzed alkyne trimerizations. The unsymmetrical alkynes Me 3 SiCtCH, EtO 2 CCtCH, and PhCtCC 6 F 5 gave symmetrical 1,3,5-substituted benzenes, while the others led to isomeric mixtures. A 1:2 molar mixture of PhCtCH and PhCtCPh provided an isomeric mixture (45% yield) consisting mainly of 1,2,4,5-tetraphenylbenzene. While Si 2 (OMe) 6 also catalyzed alkyne trimerizations (though not as efficiently as Si 2 Cl 6 ), Si 2 Me 6 did not, suggesting an electronegativity influence in the formation of the Cl 3 Si • radicals shown to be involved in these reactions. Somewhat unexpectedly, however, neither Si 2 F 6 nor sym-Si 2 Me 2 Cl 4 catalyzed alkyne trimerizations. Experimental support for the radical pathway proposed for the alkyne trimerization observed herein is presented. In the absence of disilane procatalyst, PhCtCH gave an isomeric mixture of dimers, p-MeC 6 H 4 CtCH afforded predominantly a single dimer, and 1-ethynyl-1-cyclohexene provided exclusively a single dimer, whereas RCtCH (R ) alkyl) and PhCtCMe did not react upon heating under the same conditions.

Investigating Direct Alkynylation at the Bridgehead of Bicyclic Cages Using Silver(I) Acetylides

“…desired product afforded the tetrahydronaphthalene 20. The tetrahydronaphthalene 20 is readily obtained from the known oligomerisation of (cyclohexenyl)acetylene [24] suggesting the presence of (cyclohexenyl)acetylene in our system. This can only occur through deargentation as no cyclohexenyl acetylene was present in the starting silver(I) (cyclohexenyl)acetylide (19) (Scheme 3).…”

Metallic Dysprosium Induced Silyl Radical Reactions: Intermolecular Cyclization and Reduction of Alkynes and Imines

“…Recently, Yang and Verkade and we discovered that silyl radical could effectively catalyze the cyclotrimerization of alkynes. However, these methodologies require the use of either expensive radical precursors such as Si 2 Cl 6 1 or a not easily available initiator (DyI 2 ), along with the need for a special workup procedure.…”

“…To the best of our knowledge, examples of the radical-based intermolecular cyclotrimerization of alkynes are very rare. In comparison with the Cl 3 SiSiCl 3 -mediated identical reaction, the present system not only provides a more convenient method for the formation of SiCl 3 radical under mild conditions from the abundant and cheap SiCl 4 that is ineffective as a precursor in terms of the thermolytic or photochemical production of silyl radical due to its exceedingly high first dissociation energy (ca. 111 kcal/mol) but also typically gave superior regioselectivity.…”

Hexaethylbenzene: A Sterically Crowded Arene and Conformationally Versatile Ligand

“…Catalytic trimerisation of 3-hexyne to form HEB has been reported fora na stonishingly wide range of reagents. These include [Ar 2 M],w here Ar = phenyl or mesityl and M=Co or Ni, [68][69][70][71] CoBr 2 (diimine)/ZnI 2 , [72] [{R 2 P(CH 2 ) 3 PR 2 }Fe(C 2 H 4 ) 2 ], [73] (h 6cycloheptatriene)(h 4 -1,5-cyclooctadiene)iron, [74] K 4 [Ni 2 (CN) 6 ], [75] [(tetrahydrothiophene) 3 M 2 Cl 6 ], where M=Nb or Ta , [76] AuSbF 6 , [77] hexachlorodisilane, [78] and iron vapour in am etal atom co-condensation system. [79] The mechanisms of transition metal catalysis might involvea lkyne coupling to form an (h 4 -cyclobutadiene)-metal complex or am etallocyclopentadiene.…”

“…[79] The mechanisms of transition metal catalysis might involvea lkyne coupling to form an (h 4 -cyclobutadiene)-metal complex or am etallocyclopentadiene. In contrast, the disilane-mediated reaction, which requires much more forcing conditions (170-200 8C, 20-48 h) is thought to be as tepwise process initiated by addition of at richlorosilylr adical to the alkyne, [78] as depicted in Scheme 8.…”