The Phosphinoboration of N‐Heterocycles

Abstract

The addition of phosphinoboronate ester Ph PBpin (pin=1,2-O C Me ) (1) to a number of different N-heterocycles has been investigated. Reaction of 1 with pyridine resulted in highly selective formation of the corresponding 1,4-addition product, with addition of the electron-deficient Bpin group to the pyridine nitrogen atom and the phosphido group to the para carbon atom. Conversely, reactions of para-substituted pyridine derivatives occurred predominately to afford 1,2-addition products while quinoline reacted to afford the 1,2-adduct which ultimately isomerized to afford the corresponding 1,4-addition product. Preliminary computational studies have been undertaken to explore possible pathways for these transformations including transfer of the PPh anion from [B(PPh ) pin] to the 4-position of a borenium/boronium activated pyridine and concerted pathways for 1,2-addition via intramolecular nucleophilic attack of PPh at C2 of a Ph PBpin-coordinated pyridine via a four-centered transition state and intramolecular transfer of PPh to the 2-position of a boron-activated pyridine in a phosphido-bridged dimer involving a six-centered transition state.

Double Phosphinoboration of CO2: A Facile Route to Diphospha‐Ureas

“…The first example of ad iphospha-urea (R = Ph) was preparedi nlow yieldb yt he reaction of Ph 2 PSiMe 3 with phosgene at À110 8C. [21] Subsequently,W estcott and co-workersr eported relateda ddition reactions in which Ph 2 PBpin reacts with unsaturatedm ultiple bonds prompting phosphinoboration of ketones, imines, aldehydes, [22] N-heterocycles, [23] and as eries of heteroallenes. [18] Although transition-metalsupported diphospha-ureas have also been reported, [19] we have recentlyd escribed as ynthetic route exploiting nucleophilic attack of the phosphaketene Ph 3 GePCO by phosphides and subsequenta lkylation.…”

Double Hydroboration of Quinolines via Borane Catalysis: Diastereoselective One Pot Synthesis of 3‐Hydroxytetrahydroquinolines

“…The current catalysis proved to have high fidelity towards sp 3 CÀ B bond formation at the β-position to give the C3-borylated tetrahydroquinoline possessing a newly formed tetra-substituted carbon (12). Gratifyingly, a range of isoquinolines bearing substituents at the benzene ring also underwent smoothly the consecutive borylation at 85°C to provide the corresponding 2,4-bis-borylated tetrahydroisoquinolines in high yields (15)(16)(17)(18)(19). It needs to be emphasized that the present metal-free catalytic system operates even in the gram-scale reaction with good efficiency (for 3, 4, 12, and 14: 60-85%).…”

“…[6g, [14][15] Initially, B(C 6 F 5 ) 3 forms a quinoline Scheme 2. [6g, [14][15] Initially, B(C 6 F 5 ) 3 forms a quinoline Scheme 2.…”

“…Gratifyingly, a range of isoquinolines bearing substituents at the benzene ring also underwent smoothly the consecutive borylation at 85°C to provide the corresponding 2,4-bis-borylated tetrahydroisoquinolines in high yields (15)(16)(17)(18)(19). It is interesting to note that when compared to the present borylative conversion, the silylative reduction of isoquinolines was operative only under rather harsher conditions (100°C, 24 h), thus giving moderate yields of β-silylated azacyclic products.…”

“…Based on the previous reports and the present mechanistic insights, we propose an outer-sphere ionic pathway for the present consecutive hydroboration of quinolines involving a boronium ion as a key species (Scheme 3). [6g, [14][15] Initially, B(C 6 F 5 ) 3 forms a quinoline COMMUNICATIONS asc.wiley-vch.de adduct A as an off-cycle resting species [5,16] and the constructive path begins upon the activation of a BÀ H bond of HBcat by B(C 6 F 5 ) 3 to generate a boronium ion pair bearing a borohydride anion B. It is postulated that the subsequent hydride transfer from the [HB (C 6 F 5 ) 3 ] À species occurs mainly at the C4-position of the bound quinoline to afford 1,4-intermediate C. In sequence, this 1,4-adduct C undergoes a turnoverlimiting C3-selective hydroboration to liberate a borylated tertrahydroquinoline G presumably via an iminium intermediate F. As mentioned above, an initially formed allylamine adduct D will be isomerized to its 1,4-isomer C although detailed pathways are not known at the present stage.…”