Access to 1‐Phospha‐2‐azanorbornenes by Phospha‐aza‐Diels–Alder Reactions

Abstract

The unprecedented phospha‐aza‐Diels–Alder reaction between an activated electron‐poor imine and 2H‐phospholes yields 1‐phospha‐2‐azanorbornenes in a highly chemoselective and moderately diastereoselective reaction. The intermediate 2H‐phospholes, which act as dienes, are formed in situ from the corresponding 1H‐phospholes. Theoretical calculations confirm that the phospha‐aza‐Diels–Alder reaction is of normal electron demand. The reactive P−N bond in 1‐phospha‐2‐azanorbornenes can be cleaved by nucleophiles leading to the formation of 2,3‐dihydrophospholes.

Reductive Rearrangement of a 1‐Phospha‐2‐azanorbornene

“…Reductive P−N bond cleavage reactions are known, [11] and we have already reported that PANs can be converted to 2,3‐dihydrophospholes by reaction with nucleophiles [6] . A proposed mechanism for the formation of 2 is shown in Scheme 3.…”

“…Reductive PÀ N bond cleavage reactions are known, [11] and we have already reported that PANs can be converted to 2,3dihydrophospholes by reaction with nucleophiles. [6] A proposed mechanism for the formation of 2 is shown in Scheme 3. Apparently, the ester group has been reduced besides the expected cleavage of the PÀ N bond via nucleophilic attack of a hydride anion in the first two steps.…”

“…[6] The phosphorus-nitrogen bond in PANs is highly reactive, and we could already show that 2,3-dihydrophospholes can readily be obtained with nucleophiles by PÀ N bond cleavage. [6] In the present work, we have extended the synthetic potential of PANs further. With reducing agents, cleavage of the PÀ N bond also occurs, but the presence and reduction of additional functional groups results in unexpected rearrangements, depending on the reducing agent used.…”

“…Thus, poorly aromatic 1 H ‐phospholes [3,4] form 2 H ‐phospholes in situ by a [1,5]‐sigmatropic shift; [2,5] the latter are powerful dienes and undergo Diels‐Alder reactions with a variety of dienophiles resulting in 1‐phosphanorbornenes or 1‐phosphanorbornadienes [2] . Recently, we reported the first phospha‐aza‐Diels‐Alder reaction between 2 H ‐phospholes and an electron‐poor N ‐sulfonyl‐α‐iminoester, resulting in 1‐phospha‐2‐azanorbornenes (PANs) [6] . The phosphorus‐nitrogen bond in PANs is highly reactive, and we could already show that 2,3‐dihydrophospholes can readily be obtained with nucleophiles by P−N bond cleavage [6] .…”

“…Recently, we reported the first phospha‐aza‐Diels‐Alder reaction between 2 H ‐phospholes and an electron‐poor N ‐sulfonyl‐α‐iminoester, resulting in 1‐phospha‐2‐azanorbornenes (PANs) [6] . The phosphorus‐nitrogen bond in PANs is highly reactive, and we could already show that 2,3‐dihydrophospholes can readily be obtained with nucleophiles by P−N bond cleavage [6] . In the present work, we have extended the synthetic potential of PANs further.…”