Polar and free-radical halogenation of amine-boranes with halocarbons

Abstract

Amine-boranes will react cleanly with halocarbons to give amine-haloboranes and an alkane. Polar reactions are observed with organic halides, such as chlorotriphenylmethane, which form carbonium ions readily, with reactivity increasing with the stability of the carbonium ion. Free-radical reactions are observed with CC14 or CCUBr; they can be initiated with benzoyl peroxide. 4-Methylpyridine-monochloroborane and -dichloroborane have been synthesized. In a recent paper1 evidence was presented which suggested that substitution of halogen for hydrogen in amine-boranes may proceed either by a polar or by a free-radical pathway, depending on the nature of the (1) J. W. Wiggins and G. E. Ryschkewitsch, Inorg. Chim. Acta, 4, 33 (1970). chlorinating agent. During a subsequent investigation of the reaction between dimethylchloramine and amineboranes,2 it was observed that the solvent, CC14, could become involved in the reaction and act as a chlorinat-(2) V.

Electrophilic Activation of Lewis Base Complexes of Borane with Trityl Tetrakis(pentafluorophenyl)borate

“…We therefore attempted to observe 3 9 using NMR methods. While the 11 B NMR spectrum of 1 activated by Ph 3 C + − B(C 6 F 5 ) 4 (TrTPFPB, 5) 10 in CD 2 Cl 2 (rt) has a major peak (among several) at δ 44 ppm, well within the range where trisubstituted borenium ions have been reported, 4 the signal is not coupled to protons and cannot be due to 3 nor to the solvent adduct (4, Nuc = CD 2 Cl 2 ). We have assigned this signal as PyBCl 2 + (6) based on 11 B chemical shift comparisons and a pyridine quench to form the known Py 2 BCl 2 + .…”

“…A highly shielded peak appeared at δ 1 H = −2.6 ppm that integrated for 1H relative to Ph 3 CH. By 11 B NMR, signals were observed for − B(C 6 F 5 ) 4 (sharp singlet at −17 ppm) and for a new broad peak at −3 ppm. Warming the sample to rt resolved coupling to two protons for this peak, but did not result in greater conversion of trityl cation prior to quenching with methanol, and did not produce signals in the trivalent boron region.…”

In Situ Generation of Radical Initiators: Air Stable Amine-Borane Complexes Promote Atom Transfer Radical Additions of Alkyl Halides to Alkenes

“…The formation of a boryl radical was also envisioned, as alkyl radicals are known to abstract hydrogen from borane. These radicals were proven by Ryschkewitsch to be involved in halogenation of similar amine borane complexes 58 and by Roberts [59][60][61][62] to act as a polarity reversal catalyst using peroxide initiation.…”

“…Ryschkewitsch in his seminal paper. 58 However, no decane, chloroform or trichloroundecane were detected in the GC/MS analysis of the crude reaction mixture, hence eliminating the hydrogen abstraction on the complex by an alkyl radical (Scheme 2). Additionally, the bromoborane complex is not observed in the 11 B NMR analysis of the crude mixture.…”

In Situ Generation of Radical Initiators: Air Stable Amine-Borane Complexes Promote Atom Transfer Radical Additions of Alkyl Halides to Alkenes

“…59 This intermediate is observed in 11 B NMR, when a stoichiometric amount of BrCCl 3 is added to a DIPAB solution as shown by Ryschkewitsch in his seminal paper. 58 However, no decane, chloroform or trichloroundecane were detected in the GC/MS analysis of the crude reaction mixture, hence eliminating the hydrogen abstraction on the complex by an alkyl radical (Scheme 2). Additionally, the bromoborane complex is not observed in the 11 B NMR analysis of the crude mixture.…”