ABSTRACT
In this detailed study we report on the structures of the widely employed diboron(4) compounds bis(pinacolato)diboron (B2pin2) and bis(neopentyl glycolato)diboron (B2neop2), as well as bis(ethylene glycolato)diboron (B2eg2) and tetrakis(dimethylamino)diboron (B2(NMe2)4), and their reactivity, along with that of bis(catecholato)diboron (B2cat2) with backbone saturated and backbone unsaturared N-heterocyclic carbenes (NHCs) of different steric demand. Depending on the nature of the diboron(4) compound and the NHC used, Lewis-acid/Lewis-base adducts or NHC ring-expansion products stemming from B-B and C-N bond activation have been observed. The corresponding NHC adducts and NHC ring-expanded products were isolated and characterised via solid-state and solution NMR spectroscopy and X-ray diffraction. In general, we observed B-B bond and C-N bond activation at low temperature for B2eg2, at room temperature for B2neop2 and at higher temperature for B2cat2. The reactivity strongly depends on steric effects of the NHCs and the diboron(4) compounds, as well as on the corresponding Lewis-basicity and Lewis-acidity. Our results provide profound insight into the chemistry of these diboron(4) reagents with the nowadays ubiquitous NHCs, the stability of the corresponding NHC adducts and on B-B bond activation using Lewis-bases in general. We demonstrate that B-B bond activation may be triggered even at temperatures as low as -40 °C to -30 °C without any metal species involved. For example, the reactions of B2eg2 with sterically less demanding NHCs such as Me2ImMe and iPr2Im in solution led to the corresponding ring-expanded products at low temperatures. Furthermore, boronium [L2B(OR)2]+ and borenium [LB(OR)2]+ cations have been observed from the reaction of the bis-borate B2eg3 with the NHCs iPr2Im and Me2ImMe, which led to the conclusion that the activation of bis-borates with NHCs (or Lewis-bases in general) might be a facile and simple route to access such species.
ABSTRACT
C-F and C-H bond activation reactions of polyfluoroaromatics at the cyclic (alkyl)(amino)carbene (cAAC) cAACmethyl (1) are reported. Studies on the C-F bond activation using the cAAC-stabilized nickel(0) complex [Ni(cAACmethyl )2 ] (2) have shown that 2 does not react with fluorinated arenes. However, these investigations led to the observation of C-F bond cleavage of perfluorinated arenes by the carbene ligand cAACmethyl (1) itself. The reaction of 1 with C6 F6 , C6 F5 -C6 F5 , C6 F5 -CF3 , and C5 F5 N afforded the insertion products of cAAC into one of the C-F bonds of the substrate, that is, the C-F bond activation products (cAACmethyl )F(Arf ) (Arf =C6 F5 4 a, C6 F4 -C6 F5 4 b, C6 F4 -CF3 4 c, C5 F4 N 4 d). These products decompose readily upon heating to 80 °C within a few hours in solution with formation of ionic iminium salts [(cAACmethyl )(Arf )][X] 6 a-d or neutral alkenyl perfluoroaryl imine compounds 7 a-d. The compounds (cAACmethyl )F(Arf ) 4 a-d readily transfer fluoride, which has been exemplified by the fluoride transfer of all compounds using BF3 etherate as fluoride acceptor. Fluoride transfer has also been achieved starting from (cAACmethyl )F(C6 F4 -CF3 ) (4 c) or (cAACmethyl )F(C5 F4 N) (4 d) to other selected substrates such as trimethylchlorosilane, benzoyl chloride and tosyl chloride. Instead of C-F bond activation, insertion of the cAAC into the C-H bond was observed if 1 was treated with the partially fluorinated arenes C6 F5 H, 1,2,4,5-C6 F4 H2 , 1,3,5-C6 F3 H3 , and 1,3-C6 F2 H4 . The compounds (cAACmethyl )H(Arf ) (Arf =C6 F5 12 e, 2,3,5,6-C6 F4 H 12 f, 2,4,6-C6 F3 H2 12 g and 2,6-C6 F2 H3 12 h) have been isolated in good yields and have been characterized including X-ray analysis. Fluorobenzene C6 FH5 (pKa ≈37), the least C-H acidic fluoroarene used in this study, does not react. In order to investigate the scope and limitations of this type of cAAC C-H bond activation reaction, cAACmethyl (1) was treated with several other reagents of different C-H acidity such as imidazolium salts, imidazoles, esters, and trimethylphosphine. These investigations led to the isolation and characterization of the compounds [(cAACmethyl )H(R2 ImMe2 )]X (13 a,b), (cAACmethyl )H(ImR2 ) (14 a-c), (cAACmethyl )H(CH(COOCH3 )2 ) (15 b) and (cAACmethyl )H(CH2 -PMe2 ) (16). Deprotonation of [(cAACmethyl )H(Me2 ImMe2 )][BF4 ] (13 a) at the cAAC carbon atom using KHMDS as a base led to isolation and structural characterization of the cAACmethyl -NHC heterodimer (17).
ABSTRACT
N-heterocyclic carbenes (NHCs) and cyclic (alkyl)(amino)carbenes (CAACs) are of great interest, as their electronic and steric properties provide a unique class of ligands and organocatalysts. Herein, substitution reactions involving novel carbonyl complexes of rhodium and nickel were studied to provide a deeper understanding of the fundamental electronic factors characterizing CAAC(methyl) , which were compared with the large array of data available for NHC and sterically more demanding CAAC ligands.
ABSTRACT
The thermal dehydrogenative coupling of aryl phosphine-borane adducts with iridium complexes bearing a bis(phosphinite) pincer ligand is reported. This catalysis produces high molecular weight poly(phosphinoboranes) [ArPH-BH2]n (Ar = Ph, (p)Tol, Mes). Furthermore, we investigated the reactivity of these pincer complexes towards primary phosphines and their respective borane adducts on a stoichiometric scale.
ABSTRACT
The [Ni(IMes)2]-catalyzed transformation of fluoroarenes into arylboronic acid pinacol esters via C-F bond activation and transmetalation with bis(pinacolato)diboron (B2pin2) is reported. Various partially fluorinated arenes with different degrees of fluorination were converted into their corresponding boronate esters.
ABSTRACT
Novel gold(I) "click" carbene(1,2,3-triazolylidene) complexes have been synthesised, characterised and exploited for the self-assembly of a metallomacrocycle and as precatalysts for gold(I)-catalysed reactions.
