Facile activation of alkynes with a boraguanidinato-stabilized germylene: a combined experimental and theoretical studyElectronic supplementary information (ESI) available: Table S1 summarizing the crystallographic data, Tables S2 and S3, Schemes S1 and S2, and Fig. S1-S4 showing further computational results. CCDC 1533462-1533469. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7dt01950e

• Main Authors: Böserle, Ji í, Zhigulin, Grigory, Št pni ka, Petr, Horký, Filip, Erben, Milan, Jambor, Roman, R i ka, Aleš, Ketkov, Sergey, Dostál, Libor
• Format: Article
• Language: English
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/c7dt01950e

A boraguanidinato-stabilized germylene, [(i-Pr) 2 NB( N -2,6-Me 2 C 6 H 3 ) 2 ]Ge, reacts with alkynes RC&z.tbd;CR selectively in a 2 : 1 molar ratio to afford 3,4-R,R′-1,2-digermacyclobut-3-enes 1a-e as the products of formal [2 + 2 + 2] cyclization [R/R′ = Me/Me ( 1a ), Ph/Ph ( 1b ), Ph/H ( 1c ), t -Bu/H ( 1d ) and Cy/H ( 1e )]. Ferrocenyl-substituted alkynes react similarly, yielding the corresponding ferrocenylated 3,4-R,R′-1,2-digermacyclobut-3-enes 2a-d [where R/R′ = Fc/H ( 2a ), Fc/Me ( 2b ), Fc/Ph ( 2c ), and Fc/Fc ( 2d ); Fc = ferrocenyl]. By contrast, only one of the triple bonds available in conjugated diynes RC&z.tbd;CC&z.tbd;CR is activated with the germylene, while the second one remains intact even in the presence of an excess of the germylene. The exclusive formation of 3,4-R,(C&z.tbd;CR)-1,2-digermacyclobut-3-enes 3a-c [R = Ph ( 3a ), t -Bu( 3b ), and Fc ( 3c )] was ascribed to a steric repulsion around the second triple bond. On the other hand, the reaction of the germylene with more flexible dialkyne fc(C&z.tbd;CPh) 2 (fc = ferrocene-1,1′-diyl) proceeded in the expected manner, producing compound 4 , where both triple bonds are transformed into 1,2-digermacyclobut-3-ene rings by reaction with four equivalents of the germylene. All compounds were characterized by multinuclear NMR spectroscopy, Raman and IR spectroscopy, and in the case of 1a-c , 2a , 2c , 3a , 3b and 4 , also by single-crystal X-ray diffraction analysis. The ferrocenyl substituted compounds were studied by cyclic voltammetry (CV). Finally, the plausible reaction pathway was studied for a model reaction of [(i-Pr) 2 NB( N -2,6-Me 2 C 6 H 3 ) 2 ]Ge with MeC&z.tbd;CMe using DFT computations. A comprehensive study of the activation of various alkynes by boraguanidinato-stabilized germylene is presented.