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Isolation and Reduction of Sterically Encumbered Arylboron Dihalides: Novel Boranediyl Insertion into C−C σ-Bonds

1996; American Chemical Society; Volume: 118; Issue: 34 Linguagem: Inglês

10.1021/ja960918j

1943-2984

Warren J. Grigsby, Philip P. Power,

Coordination Chemistry and Organometallics

The synthesis and subsequent reduction of the arylboron dihalides 2,6-Mes2C6H3BX2 (X = Cl (1); Br (2)) and 2,6-Trip2C6H3BBr2 (3) (Mes = 2,4,6-Me3C6H2- and Trip = 2,4,6-i-Pr3C6H2-) are described. Treatment of 2 with lithium metal in diethyl ether gave the novel lithium 9-borafluorenyl compounds 4 and 5 (see Scheme 1) in which the boranediyl intermediate has inserted into an o-Me-ring C−C σ-bond to form a borafluorenyl structure incorporating boron in a delocalized five-membered ring. Boranediyl insertion into C−C σ-bonds, as distinct from boranediyl induced rearrangements involving CC cleavage in delocalized aromatic substrates, is unknown. The main difference between the structures of these products is that 5 is dimerized as a consequence of the reduction in the number of solvating ethers. Reduction of 2 with KC8 led to the isolation of the 9-borafluorenyl "ate" compounds 6 and 7. These products also result from C−C bond insertion by boron as seen in 4 and 5. However, the delocalization is not observed owing to the addition of hydrogen (presumably from solvent) to the borons affording borate salts. Reduction of 3 with 3 equiv of KC8 furnishes the new "diborate" species 8. This compound features a unique B−B bonded dianionic structure with a long (1.83(2) Å) B−B bond which arises from the association of two borate radical anion fragments that have a 9-borafluorenyl structure similar to those described above. Compounds 2−8 were characterized by 1H, 13C, 7Li, and 11B NMR spectroscopy and by X-ray crystallography. Crystal data at 130 K with Cu Kα (λ = 1.54178 Å) radiation: 1, C12H12.5B0.5Br, a = 15.562(5) Å, b = 9.156(2) Å, c = 16.368(5) Å, β = 111.23(2)°, Z = 4, space group C2/c, R = 0.060 for 1338 (I > 2σ(I)) data. 3, C18H14.5B0.5Br, a = 11.001(2) Å, b = 12.194(2) Å, c = 25.473(5) Å, Z = 8, space group Pbcm, R = 0.065 for 1876 (I > 2σ(I)) data. 4, C32H45BLi2O2, a = 23.611(6) Å, b = 12.812(5) Å, c = 10.194(3) Å, Z = 4, space group Pbn21, R = 0.085 for 1212 (I > 2σ(I)) data. 5, C48H70B2Li4O2, a = 14.398(3) Å, b = 17.739(4) Å, c = 19.885(4) Å, β = 97.64(3)°, Z = 4, space group P21/c, R = 0.120 for 3125 (I > 2σ(I)) data. 6, C32H44BKO2, a = 9.235(4) Å, b = 22.292(7) Å, c = 14.301(8) Å, β = 100.51(4)°, Z = 4, space group P21/n, R = 0.074 for 2968 (I > 2σ(I)) data. 7·C6H6, C33H35BK, a = 10.867(4) Å, b = 12.197(5) Å, c = 12.358(5) Å, α = 108.28(3)°, β = 111.76(3)°, γ = 101.96(3)°, Z = 2, space group P1̄, R = 0.044 for 3291 (I > 2σ(I)) data. 8·1.5Et2O·hexane, C152H224B4K4O3.5, a = 17.686(13) Å, b = 17.659(5) Å, c = 23.568(10) Å, β = 104.41(4)°, Z = 2, space group Pc, R = 0.097 for 4672 (I > 2σ(I)) data.