Transmission of electronic effects by icosahedral carboranes; skeletal carbon-13 chemical shifts and ultraviolet–visible spectra of substituted aryl-p-carboranes (1,12-dicarba-closo-dodecaboranes)

Abstract

A series of para-carboranes (1,12-dicarba-closo-dodecaboranes) substituted at one or both carbon atoms by phenyl groups bearing a range of electron-releasing or -withdrawing substituents (X = NMe 2 , NH 2 , OH, OMe, Me, CF 3 or NO 2 ), principally at the para positions, has been prepared. The carbon-13 NMR shifts of the monoaryl compounds (4-XC 6 H 4 )CB 10 H 10 CH showed a linear increase in shielding of the unsubstituted (antipodal) cage carbon, C12, and a linear decrease at the substituted position, C1, with increasing electrondonating power of the substituent represented by its Hammett σ P value. Purely inductive, σ I , or resonance σ R , parameters gave less satisfactory plots. The diaryl series of 4-nitrophenyl derivatives showed the same relationships. Calculations of electron densities showed that the 13 C NMR shifts correspond with charge density in the tangential p orbitals. The UV absorptions of the monoaryl p-carboranes have substantial red-shifts compared with the corresponding substituted benzenes, while those of the 4-nitrophenyl compounds having 4-dimethylamino or 4-oxide anion substituents in the second phenyl group extend into the visible. The colour is attributed to changes in the nitro n → π* absorption rather than conjugation or intermolecular charge transfer. Two anhydrous polymorphs and a hydrate of the (18-crown-6)potassium salt of this nitrophenoxide have been identified (18-crown-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane). The results establish that p-carborane transmits electronic effects, perhaps as a formally unsaturated system or possibly in a similar manner to the staffanes (bicyclo[1.1.1]pentanes) and other formally saturated hydrocarbon linkages.* Michl and co-workers 17 have effected coupling between p-carboranylcopper() and aryl iodides using chloride bis(triphenylphosphine)palladium() as catalyst. We have observed efficient phenylation of this copper intermediate with idobenzene using triphenylphosphine instead of pyridine, and suggest that catalysis by the palladium complex may be due to exchange with this ligand rather than to the palladium. † 1,7-Bis(3-nitrophenyl)-m-carborane is an exception which gives several products under these conditions; all are destroyed by hydrogenation at higher temperatures.

Camouflaged Carborarods Derived from B‐Permethyl‐1,12‐diethynyl‐para‐ and B‐Octamethyl‐1,7‐diethylnyl‐meta‐carborane Modules

“…Precipitated material was filtered off and the filtrate was purified by means of chromatography (silica gel, ethyl acetate/hexanes 1:3) yielding 20 as a colorless solid (1.74 g, 60 %, R f = 0.55). 1- 3,4,5,6,7,8,9,10,]-1'-[closo -2,3,4,5,6,7,8,9,10,11-decamethyl-1,12-dicarbadodecaboranyl(12) 1- 3,4,5,6,7,8,9,10,]-1'-[closo -2,3,4,5,6,7,8,9,10,11-decamethyl-1,12-dicarbadodecaboranyl(12)]-1,4-butadiyne (23) 1- 3,4,5,6,7,8,9,10,]-1'-[closo -2,3,4,5,6,7,8,9,10,11-decamethyl-1,12-dicarbadodecaboranyl(12)]-1,4-butadiyne (24) and 1-[12-(methyl propiolyl)-closo-2,3,4,5,6,7,8,9,10,11-decamethyl-1,12-dicarbadodecaboranylene(12)]-1'-[closo -2,3,4,5,6,7,8,9,10,11-decamethyl-1,12-dicarbadodecaboranyl(12) 12,12 3,4,5,6,7,8,9,10,]-1,4-butadiynediyl}}-{1,1'-{bis [closo-2,3,4,5,6,7,8,9,10,11-decamethyl-1,12-dicarbadodecaboranylene(12)]}-1,4-butadiyne} (26): A mixture of 24 (0.900 g, 1.4 mmol), CuCl (1.00 mg, 0.10 mmol), and DBU (0.100 mL, 0.600 mmol) in anhydrous pyridine (25 mL) was reacted according to the procedure described for the synthesis of 19. The product was extracted with hot toluene.…”

“…UV-visible spectroscopic studies with 1,12-bis(aryl) derivatives of the latter were conducted by Mingos et al [6] and Wade and co-workers. [7] Derivatives of 1,12-carborane bearing sigma-bonded metal centers at their carbon vertices have been examined by Hawthorne and collaborators [8] and Low et al [9] by using electrochemical methods. All of these investigations have shown that electronic effects can be transmitted transversely through the icosahedral 1,12-carborane cage.…”

Four Decades of Organic Chemistry of closo-Boranes: A Synthetic Toolbox for Constructing Liquid Crystal Materials. A Review

“…91 ), and C-acetylene 27 (refs 23,82,91 ) derivatives (Scheme 5). Unlike the carboranyllithiums, the carboranylcopper reagents do not equilibrate with the C-H acid form and high degrees up to 80% of monoarylation of 6 have been obtained 13,82,92 . The original Wade procedure 76 for arylation of the twelve-vertex carboranes 13,82,92,93 has been extended to arylation 58,94 and ethynylation 23 of the ten-vertex para-carborane 10.…”

“…Unlike the carboranyllithiums, the carboranylcopper reagents do not equilibrate with the C-H acid form and high degrees up to 80% of monoarylation of 6 have been obtained 13,82,92 . The original Wade procedure 76 for arylation of the twelve-vertex carboranes 13,82,92,93 has been extended to arylation 58,94 and ethynylation 23 of the ten-vertex para-carborane 10. Attempts to use the procedure for the introduction of perfluoroalkyl de- rivatives proved unsuccessful, and only carboranyl halides 28 (bromides and iodides) were obtained in the metal-halogen exchange process (Scheme 5) 82 .…”

“…Phenol-substituted twelve-vertex carboranes have been obtained in good yields by pyridinium salt or HI cleavage of methyl aryl ethers 92 . Electrophilic substitution of the benzene ring preferentially occurs at the 4 position but typically mixtures of the para and meta isomers are obtained 25 .…”