Reductive Al-B σ-Bond Formation in Alumaboranes: Facile Scission of Polar Multiple Bonds

Abstract

We present facile access to an alumaborane species with electron precise Al-B σ-bond. The reductive rearrangement of 1-(AlI₂ ), 8-(BMes₂ ) naphthalene (Mes=2,4,6-Me₃ C₆ H₂ ) affords the alumaborane species cyclo-(1,8-C₁₀ H₆ )-[1-Al(Mes)(OEt₂ )-8-B(Mes)] with a covalent Al-B σ-bond. The Al-B σ-bond performs the reductive scission of multiple bonds: S=C(NiPrCMe)₂ affords the naphthalene bridged motif B-S-Al(NHC), NHC=N-heterocyclic carbene, while O=CPh₂ is deoxygenated to afford an B-O-Al bridged species with incorporation of the remaining ≡CPh₂ fragment into the naphthalene scaffold. The reaction with isonitrile Xyl-N≡C (Xyl=2,6-Me₂ C₆ H₄ ) proceeds via a proposed (amino boryl) carbene species; which adds a second equivalent of isonitrile to ultimately form the Al-N-B bridged species cyclo-(1,8-C₁₀ H₆ )-[1-Al(Mes)-N(Xyl)-8-B{C(Mes)=C-N-Xyl}] with complete scission of the C≡N triple bond. The latter reaction is supported with isolated intermediates and by DFT calculations.

Keywords: Alumaborane; Bond Scission; Migration Reactions; Polarized Bonds; σ-Bonds.

Scheme 1

Summary of reported species with covalent Al−B bonds. Cations omitted for clarity. Dipp=2,6‐iPr₂C₆H₃. Mes=2,4,6‐Me₃C₆H₂.

Scheme 2

Synthesis of precursor 4. Cations for 1 omitted. Reagents and conditions. i) 2.0 eq. tBuLi, Et₂O, −78 °C, 30 min, then 1.0 eq. AlMe₂Cl, rt, overnight, 60 %. ii) 1.2 eq. AlI₃, PhMe, rt, 3 h, 90 %. Molecular structures of compounds 3 and 4 with omitted hydrogen atoms and thermal ellipsoids presented at 50 % probability. Selected bond lengths (−) and contacts (⋅⋅⋅) in Å and bond angles in deg. For 3: Al1−C9 1.9755(16), Al1⋅⋅⋅C11 2.4092(14), B1−C1 1.560(2), B1−C11 1.603(2), C30−Al1−C9 114.39(7), C9−Al1⋅⋅⋅C11 94.84(5), C1−B1−C11 126.00(12). For 4: I1−Al1 2.5311(11), I2−Al1 2.5172(11), Al1−C3 1.951(3), Al1⋅⋅⋅C11 2.232(3), C1−B1 1.565(4), C1−B1−C11 125.6(3), C3−Al1⋅⋅⋅C11 100.55(13), I2−Al1−I1 107.10(4). Mes=2,4,6‐Me₃C₆H₂.

Scheme 3

Synthesis of compound 5. Reagents and conditions. i) 2.1 eq. KC₈, Et₂O, rt, 60 min, 65 %. Molecular structure of compound 5 with omitted hydrogen atoms and thermal ellipsoids presented at 50 % probability. Selected bond lengths in Å and bond angles in deg. Al1−O1 1.9183(15), Al1−C9 2.016(2), Al1−C20 2.0045(18), Al1−B1 2.1481(19), C1−B1 1.564(2), C11−B1 1.569(2), C9−Al1−B1 88.72(8), C10−C9−Al1 109.44(13), C10−C1−B1 118.34(15), C1−B1−Al1 102.23(11).

Scheme 4

A) Calculated UV/Vis spectrum. B) Experimental UV/Vis spectrum (in Et₂O) and solution of 5 (in Et₂O). C) Frontier orbitals of compound 5.

Scheme 5

Synthesis of compound 6. Reagents and conditions. i) 1.0 eq. 2,2′‐bipy, benzene, rt, overnight, isolated crystals. Molecular structure of compound 6 with omitted hydrogen atoms except for the bridging hydrogen atom H1. Thermal ellipsoids are presented at 50 % probability. Co‐crystallized benzene molecules are omitted. Selected bond lengths in Å and bond angles in deg. Al1−N1 1.940(3), Al1−N2 2.068(3), Al1−C9 1.985(4), C1−B1 1.625(6), C20−B1 1.626(5), Al1−H1 1.91(3), B1−H1 1.23(3), N1−Al1−N2 78.75(12), C1− B1−C20 101.0(3), C1−B1−H1 106.0(16), C20−B1−H1 102.9(2), C11−B1−H1 106.5(2).

Scheme 6

Synthesis of compound 7. Reagents and conditions. i) 1.05 eq. S=C(NMeCMe)₂, toluene, rt, overnight, 74 %. Molecular structure of compound 7 with omitted hydrogen atoms and thermal ellipsoids presented at 50 % probability. Selected bond lengths in Å and bond angles in deg. S1−B1 1.797(2), S1−Al1 2.2664(6), Al1−N1 1.940(3), Al1−C1 1.966(2), Al1−C(11) 2.0622(19), C9−B1 1.580(3), B1−S1−Al(1) 96.58(7), C1−Al1−C11 109.16(8), C1−Al1−S1 103.56(6), C11−Al1−S1 98.19(5), C9−B1−S1 128.83(14). Mes=2,4,6‐Me₃C₆H₂.

Scheme 7

Synthesis of compound 8. Reagents and conditions. i) 2.05 eq. benzophenone, THF, rt, overnight, 90 %. Molecular structure of compound 8 with omitted hydrogen atoms and thermal ellipsoids presented at 50 % probability. Selected bond lengths in Å and bond angles in deg. Al1−O2 1.6963(13), Al1−O1 1.7229(14), Al1−O3 1.9163(15), O1−B1 1.336(2) C1−C10 1.412(2), C1−B1 1.583(3), C8−C9 1.522(2), C9−C10 1.489(2), C9−C11 1.352(2), C8−C46 1.581(2), O2−Al1−O1 118.01(7), B1−O1−Al1 158.6(2), O1−B1−C1 122.57(16), O1−B1−C1 122.57(16), O1−B1−C24 118.41(17), C1−B1−C24 118.56(16). Mes=2,4,6‐Me₃C₆H₂.

Scheme 8

Synthesis of compound 10 and 11. Reagents and conditions. i) 2.05 eq. 2,6‐xylyl isonitrile, THF, rt, 5 min, then crystallization, ca. 30 min, 71 %. ii) THF, rt, 24 h, 83 %. iii) 2.05 eq. 2,6‐xylyl isonitrile, THF, rt, 24 h, 83 %. THF=tetrahydrofuran. A transient intermediate 9 is proposed for the synthesis of 10. Molecular structures of compounds 10 and 11 with omitted hydrogen atoms and thermal ellipsoids presented at 50 % probability. Selected bond lengths in Å. For 10: Al1−N1 1.8144(15), Al1−O1 1.9229(14), Al1−C1 1.9780(18), N1−C29 1.449(2), N2−C30 1.198(2), N2−C31 1.422(2), C9−B1 1.593(3), C29−B1 1.524(3). For 11: Al1−N1 1.8516(15), Al1−O1 1.9440(14), N1−B1 1.400(2), N2−C21 1.223(2), C11−C21 1.313(2), C11−B1 1.622(2), C1−B1 1.600(3).