Reactions of a Four-Membered Borete with Carbon, Silicon, and Gallium Donor Ligands: Fused and Spiro-Type Boracycles

Abstract

Donor-acceptor cyclopropanes or cyclobutanes are dipolar reagents, which are widely used in the synthesis of complex organic (hetero)cycles in ring expansion reactions. Applying this concept to boron containing heterocycles, the four-membered borete cyclo-iPr₂ N-BC₁₀ H₆ reacted with the carbon donor ligands 2,6-xylylisonitrile and the carbene IMes :C(NMesCH)₂ with ring expansion and ring fusion, respectively. In particular, the tetracyclic structure formed with IMes displays zwitterionic character and absorption in the visible region. In contrast to the carbene IMes, the heavier carbenoids :Si(NDippCH)₂ and :Ga(AmIm) with a two-coordinate donor atom afford spiro-type bicyclic compounds, which display four-coordinate geometry at silicon or gallium. (TD-)DFT calculations provide deeper insight into the mechanism of formation and the absorption properties of these new compounds.

Keywords: N-heterocyclic carbenes; donor-acceptor cyclobutanes; gallium carbenoids; inorganic polycycles; inorganic spirocycles; ring expansion reactions; silicon carbenoids.

Scheme 1

Donor‐acceptor‐substituted cyclopropanes and cyclobutanes. D (donor) or A (acceptor) denote substituents of +M‐ or ‐M‐effect, respectively. The 1,8‐substituted naphthalene platform stabilises four‐membered borete 1 or boretate 2, which reacted exclusively with electrophiles in the past. Counter cations for 2 were omitted for clarity. Mes=2,4,6‐trimethylphenyl.

Scheme 2

Conditions: toluene, rt, 5 min. Molecular structure of compound 6. Thermal ellipsoids are presented on the 50 % level of probability. Hydrogen atoms except for relevant C−H moieties are omitted for clarity. Selected bond lengths (Å) and bond angles (°): N1‐B1 1.394(2), N2‐C1 1.280(2) N2‐C1 1.280(2), C2‐C7 1.422(2), C7‐C8 1.432(2), C8‐B1 1.591(2), C2‐C1‐B1 106.24(13), C7‐C2‐C1 108.71(13), C7‐C8‐B1 106.19(13), C8‐B1‐C1 102.88(13). Xyl=2,6‐dimethylphenyl.

Figure 1

¹H NMR spectra of compound 6 in toluene‐D₈. A) 25° C. B) −60 °C. The arrows indicate diagnostic signals of conformers 6 a or 6 b. The complete assignment can be found in the Supporting Information. Xyl=2,6‐di‐methylphenyl.

Scheme 3

Conditions: toluene, rt, 60 min. Molecular structures of compound 7. Thermal ellipsoids are illustrated at the 50 % level of probability. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (°): N1‐B1 1.5328(15), N2‐B1 1.5905(15), C1‐B1 1.6346(17), C4‐B1 1.6360(16), N3‐C1 1.3253(14), N3‐C12 1.4028(14), N2‐C13 1.3527(14), C2‐C1 1.4664(14), C1‐B1 1.6346(17), N3‐C1‐C2 125.22(10), N3‐C1‐B1 123.98(9), C2‐C1‐B1 109.69(9). Mes=2,4,6‐trimethylphenyl.

Figure 2

A) Experimental UV/VIS spectrum of 8 in CH₂Cl₂. B) Calculated UV/VIS spectrum of 8. C) Relevant frontier orbitals of 8.

Scheme 4

Proposed mechanism for the formation of 8 based on DFT calculations. Values of the standard free enthalpy of intermediates and transition states (▵G^° or ▵G^ǂ) are reported in kJ/mol. Mes=2,4,6‐trimethyl‐phenyl.

Scheme 5

Conditions: i) toluene, 80 °C, overnight. ii) THF, 50 °C, overnight. Molecular structures of compounds 10 and 12. Thermal ellipsoids are illustrated at the 50 % level of probability. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (°): For 10: Si1‐N3 1.7570(7), Si1‐N2 1.7587(7), Si1‐C7 1.8825(8), Si1‐B1 2.0508(9), N1‐B1 1.3972(11), C1‐B1 1.5877(12), N3‐Si1‐N2 91.07(3), N3‐Si1‐C7 114.23(4), N2‐Si1‐C7 109.65(3), N3‐Si1‐B1 129.00(3), N2‐Si1‐B1 123.47(3), C7‐Si1‐B1 90.00(4), C1‐B1 ‐Si1 102.07(5). For 12: Ga1‐N5 1.9436(17), Ga1‐N2 2.0047(19), Ga1‐C7 2.023(2), Ga1‐B1 2.102(3), N1‐B1 1.400(3), N5‐Ga1‐N2 82.98(8), N5‐Ga1 ‐C7 114.87(9), N2‐Ga1‐C7 103.64(8), N5‐Ga1‐B1 139.16(9), N2‐Ga1‐B1 125.51(9), C7‐Ga1‐B1 88.44(9). Dipp=2,6‐diisopropylphenyl.