A Highly Effective Ruthenium System for the Catalyzed Dehydrogenative Cyclization of Amine–Boranes to Cyclic Boranes under Mild Conditions

Abstract

We recently disclosed a new ruthenium-catalyzed dehydrogenative cyclization process (CDC) of diamine-monoboranes leading to cyclic diaminoboranes. In the present study, the CDC reaction has been successfully extended to a larger number of diamine-monoboranes (4-7) and to one amine-borane alcohol precursor (8). The corresponding NB(H)N- and NB(H)O-containing cyclic diaminoboranes (12-15) and oxazaborolidine (16) were obtained in good to high yields. Multiple substitution patterns on the starting amine-borane substrates were evaluated and the reaction was also performed with chiral substrates. Efforts have been spent to understand the mechanism of the ruthenium CDC process. In addition to a computational approach, a strategy enabling the kinetic discrimination on successive events of the catalytic process leading to the formation of the NB(H)N linkage was performed on the six-carbon chain diamine-monoborane 21 and completed with a (15) N NMR study. The long-life bis-σ-borane ruthenium intermediate 23 possessing a reactive NHMe ending was characterized in situ and proved to catalyze the dehydrogenative cyclization of 1, ascertaining that bis σ-borane ruthenium complexes are key intermediates in the CDC process.

The Synthesis, Characterization and Dehydrogenation of Sigma‐Complexes of BN‐Cyclohexanes

“…The dehydrogenative cyclization of diaminemonoboranes has been reported to form cyclic diaminoboranes using a Ru(PCy 3 ) 2 (H 2 ) 2 (H) 2 catalyst; [23, 24] the metal-catalyzed dehydrocoupling of base-stabilised diborane(6) [H 2 B(hpp)] 2 to give [HB(hpp)] 2 (hpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinate) has been reported, alongside subsequent coordination chemistry; [25, 26] and there is an early report of sigma complexes formed from cyclic diboranes. [27] Shore and co-workers have developed the coordination chemistry and reactivity of related cyclic anionic organohydroborates of the early transition metals.…”

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

“…Previous work revealed the effective catalytic cyclization of Ia at room temperature in THF to afford N , N′ ‐di‐ tert ‐butyl‐1,3,2‐diazaborolidine ( IVa ) in 88 % yield after 16 h 14b. Replication of the experimental conditions (THF at room temperature) in a J.…”

A General, Rhodium‐Catalyzed, Synthesis of Deuterated Boranes and N‐Methyl Polyaminoboranes

“…[31] An umber of cases where simultaneous chain-and stepgrowth polymerization occur are known. [32] Possible mechanisms for this step-growth process are (Scheme 6B): (i)dehydrogenation of ap olymer-(BH 2 NMeH 2 ) end group, catalyzed by 2,t of orm an amino-borane end group that undergoes dimerization to form ac yclic diborazane, [33] (ii)dehydrogenative BÀNc oupling between aminoborane and amine-polymer end groups to give ad ialkylaminoborane-polymer-NMe-BH-NMe-polymer-linking unit, [34] or (iii)head to tail [9l, 35] coupling of appropriate aminoboranes. We saw no evidencef or ad ialkylaminoborane [ 11 Bc a. d =+30 ppm]; while ab road signal at 11 Bc a. d =+2ppm may be indicative of (i), that also appears broader in the deuterated samples suggesting aB ÀH( or BÀD) group, butw ecannotd iscount this is due to chain-branching.…”