Isocyanide and Ylidene Complexes of Boron:  Synthesis and Crystal Structures of (2-(Trimethylsiloxy)phenyl isocyanide)−Triphenylborane and (1,2-Dihydrobenzoxazol-2-ylidene)−Triphenylborane

Abstract

2-(Trimethylsiloxy)phenyl isocyanide (1) reacts at −60 °C with triphenylborane to form the isocyanide adduct (2-(trimethylsiloxy)phenyl isocyanide)−triphenylborane (2). Upon desilylation in MeOH with a catalytic amount of KF at −30 °C 2 is converted into the ylidene adduct (1,2-dihydrobenzoxazol-2-ylidene)−triphenylborane (3). The X-ray crystal structures of 2 and 3 are reported. When it is heated, 2 dimerizes to give 1,4-bis(2-(trimethylsiloxy)phenyl)-2,2,3,5,5,6-hexaphenyl-2,5-dibora-2,5-dihydropyrazine (4). Si−O bond cleavage leads to 1,4-bis(2-hydroxyphenyl)-2,2,3,5,5,6-hexaphenyl-2,5-dibora-2,5-dihydropyrazine (5). Attempts to induce formation of a boron-bridged diylidene complex via attack of the hydroxyl oxygens in 5 at the pyrazine carbon atoms failed. Instead, complex 6, containing one bridging N,O-ylidene ligand, was obtained by partial hydrolysis of 5. The molecular structure of 6, which crystallizes with one molecule of methanol and acetone each per formula unit, is reported.

Heterocyclic Carbenes: Synthesis and Coordination Chemistry

“…The N-alkylation of the heterocycle to give 191 proceeds readily (Scheme 103). A large number of complexes with NH,O-and NR,O-stabilized benzoxazolin-2-ylidene ligands containing W, [325,326] Cr, [326] Pt, Pd, [327] B, [328] Fe, [329] and Re [308] have been prepared. Michelin et al describe related palladium and platinum complexes with the six-membered benzo- [1,3]oxazin-2-ylidene heterocycle which was also obtained by the cyclization of a b-functionalized phenyl isocyanide.…”

Carbene Complexes of Nonmetals

“…None of the aminocarbene-borane adducts gave evidence of dissociation at ambient temperature. [179] [MeBF 3 ]K 157.5 [186] 107 163.5 [180] [PhBF 3 ]K 161 [187] 108 166.9 [180] 109 168.8 [181] 110 160.3 [182] [MeBH 3 ]Li 159.7 [188] 111 159.6 [183] [(tBu) 2 BH 2 ]K 164.3 [188] 118 159.6 [191] 120 164.5 [192] [Ph 4 B][NR 4 ] 164-165 [189] 121 163.4 [193] Ph 3 B-NR 3 162-164 [189] 125 162.3 [194] 163.2 [190] 127 163.3 [195] Borane adducts of phosphanylcarbenes proved to be less robust. The adduct 112, derived from a phosphanyl(silyl)-carbene, was observed by NMR spectroscopy at Ϫ80°C, but decayed rapidly due to elimination of Et 2 BNR 2 .…”

“…The same procedure was used to convert 126 into the dihydrobenzoxazin-2-ylidene-borane adduct 127. [195] Compound 127 was also obtained from benzoxazole (128) by a route that is analogous to the conversion of 122 into 121. [196] Dihydrooxazol-2-ylidenes [ϭ amino(oxy)carbenes] would be expected to be weaker donors than dihydroimidazol-2-ylidenes (ϭ diaminocarbenes).…”

“…However, the CϪB bond lengths of their borane adducts are nearly the same, as attested by crystal structures of 125 [194b] and 127 (Table 4). [195] Several dihydroimidazol-2-ylidenes 102 afforded adducts with SiCl 4 , SiMe 2 Cl 2 , SiPh 2 Cl 2 , and SnPh 2 Cl 2 . [197] The crystal structure of 129 shows that Si is in a trigonal-bipyramidal environment, the carbene ligand assuming an equatorial position (Scheme 27).…”

Komplexe von N‐heterocyclischen Carbenen mit Boranen: Synthese und Reaktionen

“…Andere Arbeitsgruppen erzeugten später Triphenylboran-Benzo[d]oxazol-2-yliden-Komplexe wie 10 durch die Reaktion von lithiiertem Oxazol 8 mit Triphenylboran. [20] Als Zwischenstufe wird wahrscheinlich ein Anion wie 9 durchlaufen. Der Komplex 10 lässt sich auch durch Cyclisierung eines b-HydroxyisonitrilBPh 3 -Komplexes herstellen.…”