Cyclopentannulation on 3-phospholenes: an expedient route to the 2-phosphabicyclo[3.3.0]octene ring system

New Monodentate P,C‐Stereogenic Bicyclic Phosphanes: 1‐Phenyl‐1,3a,4,5,6,6a‐hexahydrocyclopenta[b]phosphole and 1‐Phenyloctahydrocyclopenta[b]phosphole

“…Racemic 1-phenyl-1,3a,4,5,6,6a-hexahydrocyclopenta [-b]phosphole 1-oxide (6) was synthesized by a single-step cyclopentannulation route [5,6] involving deprotonation of 1-phenyl-2,5-dihydro-1H-phosphole 1-oxide (2.1 equiv. of LDA, Ϫ78°C) and treatment of the resulting anionic intermediate with 1,3-diiodopropane.…”

“…The analogous annulation with 1-phenyl-2,5-dihydro-1H-phosphole 1-sulfide proved to be less exothermic and afforded the expected 1-phenyl-1,3a,4,5,6,6a-hexahydrocyclopenta[b]phosphole 1-sulfide (7) in very high yield irrespective of the reaction scale used (91% yield for a 10 mmol scale). [5,6] Reduction of 6 with phenylsilane [7] and radical-based reduction of 7 with tris(trimethylsilyl)silane [8] gave 1-phenyl-1,3a,4,5,6,6a-hexahydrocyclopenta[b]phosphole (4) quantitatively and with complete retention of configuration at P (Scheme 1). [5,7,8] Scheme 1.…”

“…[5,6] Reduction of 6 with phenylsilane [7] and radical-based reduction of 7 with tris(trimethylsilyl)silane [8] gave 1-phenyl-1,3a,4,5,6,6a-hexahydrocyclopenta[b]phosphole (4) quantitatively and with complete retention of configuration at P (Scheme 1). [5,7,8] Scheme 1. Synthesis of racemic 1-phenyl-1,3a,4,5,6,6a-hexahydrocyclopenta [b]phosphole (4) Separation of 4 into its enantiomers was achieved by an oxidative resolution route, [9] as shown in Scheme 2.…”

“…Hydrogenation of (S P )-6 on 10% Pd / C afforded saturated 1-phenyloctahydrocyclopenta[b]phosphole 1-oxide [(S P )-9, 92%], which, after reduction with phenylsilane, cleanly gave (R P )-5 (98%, Scheme 3). As we have found before, [5] the reduction proceeded with complete retention. Scheme 3.…”

“…[1] Recently, monophosphanes with rigid bicyclic structures containing the phosphorus atom embedded in a five-membered ring (e.g., 1؊3) have been identified as both highly efficient chiral ligands and excellent chiral catalysts ( Figure 1). [2Ϫ4] In this paper we wish to demonstrate that the bicyclic P,C-stereogenic monophosphanes 1-phenyl-1,3a,4,5,6,6ahexahydrocyclopenta [b]phosphole [(S P )-4] [5] and 1-phenyloctahydrocyclopenta [b]phosphole [(R P )-5] can be readily obtained in the enantiomerically pure state by an efficient oxidative resolution procedure utilizing menthyl bromoacetates as resolving agents. The use of enantiopure monophosphanes (S P )-4 and (R P )-5 as ligands and catalysts in model asymmetric CϪC bond-formation and hydrogenation processes is also reported.…”