Elucidating the Reactivity of Vicinal Dicarbenoids: From Lewis Adduct Formation to B−C Bond Activation

Abstract

The reactivity of the diaminoacetylene Pip-C≡C-Pip (Pip=piperidyl=NC H ) towards phenyldichloro- and triphenylborane is presented. In the case of the less Lewis acidic PhBCl , the first example of a double Lewis adduct of a vicinal dicarbenoid is reported. For the more Lewis acidic triphenylborane, coordination to the bifunctional carbene leads to a mild B-C bond activation, resulting in a syn-1,2-carboboration. Ensuing cis/trans isomerization yields a novel ethylene-bridged frustrated Lewis pair (FLP). The compounds were characterized using multinuclear NMR spectroscopy, structural analysis, and mass spectrometry. Reactivity studies of both isomers with the N-heterocyclic carbene 1,3-dimethylimidazol-2-ylidene (IMe) aided in elucidating the proposed isomerization pathway. DFT calculations were carried out to elucidate the reaction mechanism. The rather low free energy of activation is consistent with the observation that the reaction proceeds smoothly at room temperature.

Borabicyclo[3.2.0]heptadiene: A Fused Bicyclic Isomer of Borepin

“…The formation of a borepin ring in these reactions suggests that the diaminoalkyne has undergone a [4+2] cycloaddition reaction with boroles 3 and 5 , thereby paralleling the reactivity of other alkynes . However, its more reactive vicinal dicarbenoid form is displayed in the formal 1,2‐carboboration reaction with the exocyclic B−aryl bond, leading to products 4 and 6 . The facile B−C bond activation by the diaminoalkyne (i.e., alkyne 1,2‐carboboration) has precedence in the literature and was observed for the first time in the reaction with triphenylborane .…”

“…[6][7][8][9][10] However,i ts more reactive vicinal dicarbenoid form is displayed in the formal 1,2-carboborationr eaction with the exocyclic BÀaryl bond, leading to products 4 and 6. [14][15][16] The facile BÀCb ond activation by the diaminoalkyne (i.e.,a lkyne 1,2-carboboration)h as precedence in the literature and was observed for the first time in the reactionw ith triphenylborane. [15] The divergent reactivity of boroles 3 and 5 toward the alkyne can most straightforwardly be rationalized on the basis of steric differences in the BÀaryl substituents, with the mesityl ortho methyl groups in borole 1 hindering the attack of the alkyne at the boron centera nd therefore rendering the carboboration step impossible.…”

“…[14][15][16] The facile BÀCb ond activation by the diaminoalkyne (i.e.,a lkyne 1,2-carboboration)h as precedence in the literature and was observed for the first time in the reactionw ith triphenylborane. [15] The divergent reactivity of boroles 3 and 5 toward the alkyne can most straightforwardly be rationalized on the basis of steric differences in the BÀaryl substituents, with the mesityl ortho methyl groups in borole 1 hindering the attack of the alkyne at the boron centera nd therefore rendering the carboboration step impossible. Given that the borepin is av alence isomer of 7-boranorbornadiene, we suspect that the reactions of boroles 3 and 5 initially follow as imilar pathway to that of 1 and result in the formation of a7 -boranorbornadiene via a[ 4 + +2] Diels-Alder cycloaddition.…”

Dimerisation of Dipiperidinoacetylene: Convenient Access to Tetraamino‐1,3‐Cyclobutadiene and Tetraamino‐1,2‐Cyclobutadiene Metal Complexes

“…[5] Compound 1 also reacted as av icinal dicarbenoid towards main-group Lewis acids such as BPhCl 2 ,w ith formation of ad iboranea dduct; [7] however,B ÀCb ond activation and 1,2-carboboration was found when BPh 3 or cyclic boroles such as 1-mesityl-2,3,4,5-tetraphenylborole wereu sed. [7,8] Similar reactivity was found for diboranea nd dialane species, which proceeded with diborationa nd dialumination of the CCtriplebond in 1. [9] Unprecedented reactivity was also observed upon reaction of 1 with the molybdenuma lkylidyne complex [MesCMo{OC-Me(CF 3 ) 2 } 3 ]( Mes = 2,4,6-trimethylphenyl), which afforded the paramagnetic metallacyclobutadiene (MCBD) complex V with a triplet ground state (S = 1) through cleavage of the carboncarbon triple bond and insertion of the alkylidyne moiety.…”

Stable Boron Dithiolene Radicals

“…readily undergoes thiourea‐mediated boron–carbon bond activation under mild conditions to give diamagnetic zwitterion 4 (Scheme ), which may be regarded as an anionic dithiolene‐modified carbene complex of the sulfenyl cation RS + (R=cyclohexyl). The activation of boron–carbon bonds with high bond dissociation energies (BDE B−C ≈360 kJ mol −1 vs. BDE C−C ≈350 kJ mol −1 ) under mild conditions has been well explored . Transition‐metal species are well known for their utility in boron–carbon bond activation (e.g., Pd 0 species in Suzuki couplings between organoboranes and organic halides) .…”