Selective Synthesis of Stannoles by 1,1‐Carboboration of Bis(trimethylsilylethynyl)tin Compounds Using Weakly and Strongly Electrophilic Triorganoboranes: Characterization of a Zwitterionic Intermediate

Abstract

The triorganoboranes BEt 3 , BPh 3 , and B(C 6 F 5 ) 3 were allowed to react with bis(trimethylsilylethynyl)diorganotin com-n Oct (c), Ph (d)] to give selectively and quantitatively stannoles. The reactions proceeded by 1,1-carboboration in two consecutive steps (inter-and intramolecular) and intermediates were detected by NMR spectroscopy. In one case, using the strongly electrophilic B(C 6 F 5 ) 3 , a zwitterionic inter-

1,1‐Carboboration through Activation of Silicon–Carbon and Tin–Carbon Bonds

“…The search for intermediates to explain the formation of stannoles or 1‐stanna‐4‐bora‐cyclohexa‐2,5‐dienes started as early as 1977 and took several years, until a zwitterionic Pb derivative proved to be isolable at low temperature and structurally characterisable 65. Then, it proved possible to crystallise various zwitterionic tin derivatives (Figure 5), all of which supported the proposed mechanism 61,64,66–68. In every case the sum of bond angles at tin relating to the three σ‐bonded substituents approaches 360°, indicating the cationic character of the almost trigonal‐planar surrounded tin atom, stabilized by "side‐on" coordination to the corresponding C≡C bond of an alkynylborate unit.…”

“…With the hex‐1‐ynyl derivatives, again the six‐membered ring is formed together with the stannole, whereas with t Bu or Ph at the C≡C bonds only stannoles are formed. As in the case of siloles (Scheme ), reactions between bis(trimethylsilylethynyl)stannanes – R 2 2 Sn(C≡C–SiMe 3 ) 2 – and many different triorganylboranes, (BR 3 , R = alkyl, Ph or C 6 F 5 ) exclusively give stannoles 63,64…”

“… Molecular structures of zwitterionic intermediates isolated from the reactions between ( A ) Me 2 Sn(C≡C– i Pr) 2 and BEt 3 ,66 ( B ) Me 2 Sn(C≡C–CH 2 NMe 2 ) 2 and BEt 3 ,67 ( C ) CH 2 (CH 2 ) 4 Sn(C≡C–SiMe 3 ) 2 and B(C 6 F 5 ) 3 ,64 and ( D ) CH 2 (CH 2 ) 4 Sn(C≡C–H) 2 and B(C 6 F 5 ) 3 61…”

Activation of Silicon- and Tin-Carbon Bonds. 1,1-Carboboration of Alkynylsilanes and -stannanes

“…As for siloles (Scheme 4), reactions of bis(trimethylsilylethynyl)stannanes with many different triorganylboranes, BR 3 (R = alkyl, Ph, and C 6 F 5 ) give exclusively stannoles (Wrackmeyer et al, 2003c;Wrackmeyer et al, 2014a). Early observations (B.…”

1,1-Carboboration to tellurium–boron intramolecular frustrated Lewis pairs

“…The 1,1-carboboration of these "activated" alkynes using trialkyl boranes can sometimes require very specific reaction conditions depending on the substituents on the alkyne. [16][17][18] 1,1-Carboboration has also led to the facile synthesis of a variety of heterocycles and aromatic compounds, 19 including boroles, 20,21 stannoles, 22 1,4-stannaboracyclohexa-2,5-dienes 23 and 1-silacyclopent-2-ene. The use of B(C 6 F 5 ) 3 has helped to extend the scope of this type of reactions to include phos-phorus-substituted acetylenes 14,15 and unactivated acetylenes.…”