A Diboracyclopropane with a Planar‐Tetracoordinate Carbon Atom and a Triborabicyclobutane

Abstract

Four‐coordinate in a three‐membered ring and thereby planar: The chemical shift of the skeletal carbon atom of 1, which was generated and spectroscopically characterized at −90 °C, agrees very well with that computed for the model 2, but differs by 70 ppm from that of the similarly substituted, but distorted‐tetrahedral carbon atom in 3. R=SiMe3, Dur=2,3,5,6‐tetramethylphenyl.

Cationic closo‐carboranes 2. Do computed 11B and 13C NMR chemical shifts support their experimental availability?

“…Although the parent compound 5‐vertex trigonal bipyramidal B 5 H 5 2− has not been characterized so far, its mono‐ and dicarbon‐substituted structures are known. [16] The maximum favorability of 1,5‐C 2 B 3 H 5 is explained by the ring‐cap orbital overlap approach. [17] The NMR spectrum of this compound shows an 11 B NMR chemical shift of 3.5 for the ring boron atoms and a 13 C NMR shift of 103.3 for the carbon vertices.…”

Recent advances in planar tetracoordinate carbon chemistry

“…The strategy suggested by Hoffmann et al to stabilize a ptC entity electronically was based on the replacement of the hydrogens by ligands that are (1) -donors (to assist electron transfer to the electron deficient carbon), and (2) -acceptors (to delocalize the p or -type lone pair), or by incorporating the lone pair into a (4n þ 2) delocalized system. 11 Using Hoffmann's strategy (also called the electronic approach), several ptC molecules have been proposed in silico 14,[18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] and/or experimentally detected. [38][39][40][41][42][43][44][45][46][47][48] A few realized compounds of nonclassical structures containing ptCs are shown in Figure 2.…”

“…Using Hoffmann's strategy (also called the electronic approach), several ptC molecules have been proposed in silico 14,[18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] and/or experimentally detected. [38][39][40][41][42][43][44][45][46][47][48] A few realized compounds of nonclassical structures containing ptCs are shown in Figure 2.…”

Towards design of the smallest planar tetracoordinate carbon and boron systems

“…Several elegant and well‐designed attempts lead to realization of a good number of planar tetra coordinated carbon containing molecules in organometallic chemistry. Earlier well‐designed computational strategies involved the incorporation of Li, B, Al and other electron deficient substituents 5, 6, 10, 11. Thus, the quest to understand the basis for the preference for tetrahedral arrangement over the planar one is on 12–19.…”