Facile activation of alkynes with a boraguanidinato-stabilized germylene: a combined experimental and theoretical study

Abstract

A boraguanidinato-stabilized germylene, [(i-Pr)NB(N-2,6-MeCH)]Ge, reacts with alkynes RC[triple bond, length as m-dash]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[triple bond, length as m-dash]CC[triple bond, length as m-dash]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[triple bond, length as m-dash]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[triple bond, length as m-dash]CPh) (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)NB(N-2,6-MeCH)]Ge with MeC[triple bond, length as m-dash]CMe using DFT computations.

Reactivity of an N,N‐Chelated Germylene Toward Substituted Alkynes, Alkenes, and Allenes

“…The treatment of germylene 1 with above mentioned alkynes resulted in a smooth [2+2+2] cyclization and formation of the respective 3‐Fc‐4‐C(O) R ‐1,2‐digermacyclobut‐3‐enes 2 – 4 [ R = Me ( 2 ), OEt ( 3 ), and NMe 2 ( 4 )] (Scheme ). No products resulting from an attack of the germylene on the carbonyl moiety could be isolated and the reactivity of 1 thus resembles that described earlier for simple alkynes …”

“…Molecular structures determined by single‐crystal X‐ray diffraction analysis revealed that the central 1,2‐digermacyclobut‐3‐ene cores of 3 and 4 (Figure and Figure ) are structurally similar to those found in 3‐Fc‐4‐ R ‐1,2‐digermacyclobut‐3‐enes ( R = H or Ph). These rings comprise C=C double bond [1.334(10) in 3 , and 1.345(12) Å in 4 , cf.…”

“…In cyclic voltammetry, compounds 2 – 4 undergo reversible oxidations that are followed by ill‐defined, irreversible multielectron processes at more positive potentials (Figure ) . The first oxidations (Table ), attributed to the ferrocene/ferrocenium couple, occur more positively than the oxidation of ferrocene itself.…”

“…A high propensity to react with unsaturated substrates such as alkynes,, alkenes, dienes, carbonyl compounds,, isocyanides represents another typical property of germylenes. As a contribution to this field, we have recently reported the synthesis of an N , N ‐chelated germylene, [( i Pr) 2 NB(N‐2,6‐Me 2 C 6 H 3 ) 2 ]Ge ( 1 ), that exerts the typical, germylene‐like, reactivity toward dienes and alkynes . Its reactivity with pseudohalides R N 3 , R NCO, and R NCS has also been studied, leading to the formation of various inorganic rings …”

Spirocyclic germanes via transannular insertion reactions of vinyl germylenes into Si–Si bonds

“…In addition to constituting an interesting class of new transition metal ligands, 10 – 15 several of these compounds are able to participate in small molecule bond activation reactions. 16 – 19 …”