Cyclisation of biscarbenoids – a novel mode of cyclobutadiene stabilisation

Abstract

The reaction of the vicinal biscarbenoid Pip-C[triple bond, length as m-dash]C-Pip with dimethyltin dichloride yields a unique tetraamino-substituted cyclobutadienyl system featuring a dative C-Sn interaction. DFT investigation of the reaction mechanism revealed that coordination of the stannyl fragment to the nucleophilic carbon leads to a metal-stabilised zwitterion, allowing for [2+2] cycloaddition under thermal conditions. The compound features a homoaromatic π-system comprising the three sp-hybridised carbon atoms of the four-membered ring as a consequence of charge separation.

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

“…The reactions of dipiperidinoacetylene ( 1 ) with 0.5 equiv of SnCl 2 or GeCl 2 ⋅dioxane in acetonitrile at room temperature afforded the tin(II) and germanium(II) cyclobutadienyl complexes 2 a and 2 b , respectively, as orange, crystalline solids in moderate (56 % for 2 a ) to good yields (88 % for 2 b ) (Scheme ). The 13 C{ 1 H} NMR spectra (in CDCl 3 ) of 2 a / 2 b exhibit three signals each for the quaternary carbon atoms at 89.8/82.2 (C1), 112.6/114.0 (C2) and 173.7/172.9 ppm (C2+C4), which is in good agreement with the chemical shifts reported for the tin(IV) adduct VI , 78, 113, and 172 ppm (in C 6 D 6 ), and confirms the formation of four‐membered rings with C s ‐symmetry. The observation of the signal for the tin‐coordinated carbon atom in 2 a at lower field (Δ δ =12 ppm) compared with VI is in agreement with the trend observed for other systems, for example N‐heterocyclic carbene (NHC) tin(II) and tin(IV) complexes .…”

“…Nevertheless, the 1,2‐cyclobutadiene complex 4 a does not represent the global minimum on the energy hypersurface, and the corresponding 1,3‐cyclobutadiene adduct 2 a is stabilised by Δ G °=−5.9 kcal mol −1 (Figure S36, Supporting Information). Interconversion between 2 a and 4 a cannot be observed experimentally, and the formation of isomer 2 a from two equivalents of 1 and SnCl 2 can be rationalised by a similar mechanism as proposed for the tin(IV) adduct VI …”

“…The complexes (Ph 3 P=CMe 2 )ECl 2 feature similar bond lengths of 2.3518(14) (Sn−C) and 2.1535(19) Å (Ge−C), whereas shorter bonds were found the ECl 2 adducts of the four‐membered cyclic diphosphete [HCP(NMe 2 ) 2 ] 2 , namely Sn−C=2.267(3), Ge−C=2.070(3) Å . The structural features within the C 4 N 4 unit are virtually identical to those reported for the corresponding tin(IV) adduct VI , with short C−C and C−N bonds along the N2‐C2‐C3‐C4‐N4 chain indicating a high degree of π‐conjugation and electron delocalisation. Accordingly, the piperidine groups at N2 and N4 are in a coplanar arrangement with the four‐membered ring (e.g.…”

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.…”