Lithium Dihydropyridine Dehydrogenation Catalysis: A Group 1 Approach to the Cyclization of Diamine Boranes

Abstract In reactions restricted previously to a ruthenium catalyst, a 1‐lithium‐2‐alkyl‐1,2‐dihydropyridine complex is shown to be a competitive alternative dehydrogenation catalyst for the transformation of diamine boranes into cyclic 1,3,2‐diazaborolidines, which can in turn be smoothly arylated in good yields. This study established the conditions and solvent dependence of the catalysis through NMR monitoring, with mechanistic insight provided by NMR (including DOSY) experiments and X‐ray crystallographic studies of several model lithio intermediates.


2--t--Bupyridine·•BH 3
Reaction of 2--tert--butylpyridine with Ia in d 6 --benzene results in formation of two products -2--t--Bupyridine·•BH 3 and an unknown BH 3 containing compound resonating at δ--13.5 ppm, in agreement with the unknown catalytic intermediate. Although the identity of this species is unknown it is clear that it only occurs in reactions involving Ia.  The estimated molecular weight of IIa·•THF in d 8 --THF is 297 g mol --1 , 12% higher than the anticipated value for a monomeric unit (264 g mol --1 ). While this difference is reasonably large the value is still much closer to that of a monomeric unit than to a dimeric species. In addition we cannot rule out a solvation--desolvation event between a second molecule of THF and the lithium atom in IIa·•THF. This would also lead to an increased estimated value.

THF--d8
Internal Reference logDref,fix  2 in d 8 --THF is 265 g mol --1 , 4% higher than the anticipated value for the dimeric species observed in the crystal structure (228 g mol --1 , minus the solvating THF ligands from the crystal structure which were removed under application of vacuum during isolation). This small difference supports the view that a dimeric species is present in solution.

Synthesis and NMR characterisation of IVaPh
Ia (372 mg, 2 mmol) and 1tLi (14 mg, 5 mol%) were dissolved in toluene (2 mL) and heated at 80 °C for 7h to ensure in situ conversion to IVa. Phenyllithium (168 mg , 2 mmol) was added and the reaction stirred overnight. Hexane (5 mL) was added and the reaction placed at -70 °C. After 24 hours, colourless crystals suitable for X--ray diffraction studies formed. Yield 361 mg, 70%.

Synthesis and NMR characterisation of IVbPh
Ib (1.640 g, 4 mmol) and 1tLi (29 mg, 5 mol%) were dissolved in toluene (4 mL) and heated at 80 °C for 24 h to ensure in situ conversion to IVb. Phenyllithium (336 mg , 4 mmol) was added and the reaction stirred overnight. Hexane (5 mL) was added and the reaction placed at -70 °C. After 24 hours, colourless crystals formed and were isolated by filtration. Yield 1.052 g, 81%.

Synthesis and NMR characterisation of IVdPh
Id (420 mg, 2.5 mmol) and 1tLi (18 mg, 5 mol%) were dissolved in toluene (4 mL) and heated at 80 °C for 48 h to ensure in situ conversion to IVd. Phenyllithium (156 mg , 2.5 mmol) was added and the reaction stirred overnight. Hexane (5 mL) was added and the reaction placed at -70 °C. After 24 hours a white solid formed. These were isolated at low temperature by decanting the solution from the solid. The reaction product mixture exists as a waxy white solid in a 3:1 ratio of IVcPh:IVc.