Lithium Aminoborohydrides 16. Synthesis and Reactions of Monomeric and Dimeric Aminoboranes

Abstract

Aminoboranes are synthesized in situ from the reaction of the corresponding lithium aminoborohydrides (LABs) with methyl iodide, trimethylsilylchloride (TMS-Cl), or benzyl chloride under ambient conditions. In hexanes, the reaction using methyl iodide produces aminoborane and methane, whereas in tetrahydro-furan (THF) this reaction produces amine-boranes (R1R2HN:BH3) as the major product. The reaction of iPr-LAB with TMS-Cl or benzyl chloride yields exclusively diisopropylaminoborane [BH2-N(iPr)2] in THF as well as in hexanes at 25 degrees C. Diisopropylaminoborane and dicyclohexylaminoborane exist as monomers due to the steric requirement of the alkyl group. All other aminoboranes studied are not sterically hindered enough to be monomers in solution, but instead exist as a mixture of monomers and dimers. The dimers are four-membered rings formed through boron-nitrogen coordination. In general aminoboranes are not hydroborating reagents. However, monomeric aminoboranes, such as BH2-N(iPr)2, can reduce nitriles in the presence of catalytic amounts of LiBH4. This BH2-N(iPr)2/LiBH4 reducing system also re-duces ketones, aldehydes, and esters. Diisopropylaminoborane, synthesized from iPr-LAB, can be converted into boronic acids by a palladium-catalyzed reaction with aryl bromides. Aminoboranes derived from heterocyclic amines, such as pyrrole, pyrazole, and imidazole, can be prepared by the direct reaction of borane/tetrahydrofuran (BH3:THF) with these heterocyclic amines. It has been reported that pyrazole-derived aminoborane forms a six-membered dimer through boron-nitrogen coordination, where as, pyrrolylborane forms a dimer through boron-hydrogen coordination. Pyrrolylborane monohydroborates both alkenes and alkynes at ambient temperatures. Hydroboration of styrene with pyrrolylborane followed by hydrolysis gives the corresponding boronic acid, 2-phenylethylboronic acid, in 40% yield. Similarly phenylacetylene is mono-hydroborated by pyrrolylborane, to give E-2-phenylethenylboronic acid in 50% yield.

Ruthenium Bidentate Phosphine Complexes for the Coordination and Catalytic Dehydrogenation of Amine– and Phosphine–Boranes

“…Morpholine borane was fully converted to the more soluble product H 2 B-NA C H T U N G T R E N N U N G {CH 2 CH 2 } 2 O), which, after 2 h, was the only species apparent by 11 B NMR spectroscopy. [21] In an effort to rationalise the contrasting reactivities of 12 and 13, the hydride regions of their 1 H NMR spectra were monitored periodically over the course of H 3 B·NHMe 2 dehydrogenation. In the case of 13, no new Ru À H species were apparent even after 72 h. In contrast, only trace amounts of 12 remained after just 15 min of reaction, with at least three new hydride-containing species present.…”

Alkoxide‐Promoted Selective Hydroboration of N‐Heteroarenes: Pivotal Roles of in situ Generated BH3 in the Dearomatization Process

“…Among them, three peaks in the lowest fields were assigned to pinBO t ‐Bu, KH 2 Bpin, and KO t ‐Bu⋅HBpin, respectively (vide supra) . A small signal at δ =0.2 ppm splits into a triplet ( 1 J BH =102 Hz), attributable to a 1,4‐dihydropyridylborane dimer ( A ) . The 11 B signal at δ= −11.9 ppm (quartet, 1 J BH =97.2 Hz) was unambiguously assigned to an adduct of 1 a with BH 3 ( 1 a ‐BH 3 ) that was confirmed by independently synthesizing 1 a ‐BH 3 .…”

“…The in situ generated pyridine‐BH 3 adduct undergoes a nucleophilic hydride attack by borohydride species to afford 1,4‐dihydropyridyl borohydride ( C ) that is a major resting species. A hydride transfer from C to HBpin is assumed to be slow, regenerating borohydrides with the liberation of 1,4‐dihydropyridylborane presumably in a dimeric form ( D 2 ) . Finally, a monomeric D is presumed to react with HBpin to give N ‐Bpin‐1,4‐dihydropyridine product, releasing BH 3 presumably via σ‐bond metathesis…”

Rehydrogenation of Aminoboranes to Amine–Boranes Using H2O: Reaction Scope and Mechanism

“…B(OH) 3 (99 %) was purchased from Alfa Aesar and used without further purification. i Pr 2 NH · BH 3 ,18a 2,2,6,6‐Me 4 C 5 H 6 NH · BH 3 ,18b,28 Me 2 NH‐BH 2 ‐NMe 2 –BH 3 ,29 [MeNH‐BH 2 ] 3 ,30 [Me 2 N‐BH 2 ] 2 ,18a [NH‐BH] 3 ,18a i Pr 2 N=BH 2 ,31 and 2,2,6,6‐Me 4 C 5 H 6 N=BH 2 32 were synthesized by means of literature methods and purified by sublimation or distillation prior to use. [MeNH–BH 2 ] n ( M n = 180000, PDI = 1.2) and [NH 2 –BH 2 ] n were synthesized by literature methods 33.…”