Oxophosphonium-alkyne cycloaddition reactions: reversible formation of 1,2-oxaphosphetes and six-membered phosphorus heterocycles

Abstract

While the metathesis reaction between alkynes and carbonyl compounds is an important tool in organic synthesis, the reactivity of alkynes with isoelectronic main-group R 2 EO compounds is unexplored. Herein, we show that oxophosphonium ions, which are the isoelectronic phosphorus congeners to carbonyl compounds, undergo [2 + 2] cycloaddition reactions with different alkynes to generate 1,2-oxaphosphete ions, which were isolated and structurally characterized. The strained phosphorus−oxygen heterocycles open to the corresponding heterodiene structure at elevated temperature, which was used to generate six-membered phosphorus heterocycles via hetero Diels−Alder reactions. Insights into the influence of the substituents at the phosphorus center on the energy profile of the oxygen atom transfer reaction were obtained by quantum-chemical calculations.

Thiophosphonium–Alkyne Cycloaddition Reactions: A Heavy Congener of the Carbonyl–Alkyne Metathesis

“…The different reaction conditions for the ring expansion reactions indicate that ring-opening of the thiaphosphetes is required prior to the hetero-Diels–Alder reactions, which, in agreement with the computational results, is more easily accessible for [ 2d ] + than for [ 2a ] + . The analogous ring expansion reaction with oxaphosphetes proceeds at lower temperature than that with thiaphosphetes, 40 which again is consistent with the energy barrier of the electrocyclic ring-opening reaction.…”

“…In fact, the first barrier ( TS1 ) and the second barrier ( TS2 ) are both only ∼1 kcal/mol lower in energy for the thiophosphonium case (cf. Figure 2 and ref ( 40 )). The most notable deviation between the oxo- and thio systems is the energy difference between CF and OF .…”

“…The 31 P NMR resonance of the thiaphosphete salt [ 2a ][BArF 24 ] appears at lower frequency than that of the analogous oxaphosphete salt (−14.6 ppm). 40 P V thiaphosphete I contains a pentavalent phosphorus atom and exhibits a similar 31 P NMR chemical shift (−40.7 ppm) to [ 2a ] + , 42 whereas the resonance of the P III thiaphosphete II appears at 37.5 ppm. 43 …”

Acid–Base Free Main Group Carbonyl Analogues

“…most recently reported on a heavier carbonyl–yne reaction between a base‐free N,N‐phosphacarbonyl 60(N) and a variety of alkynes to afford isolable oxaphosphete cations (Scheme 28). [11b] Notably, cyclo‐reversion is possible, evidenced by the complete liberation of phenylacetylene from oxaphosphete 60(N)‐HCCPh at 300 °C to regenerate phosphacarbonyl 60(N) , which could alternatively be trapped by DMAP or by exchanging phenylacetylene with 4‐ethynyltoluene. Such reversible P=O double bond formation is also reminiscent of the classical addition–elimination mechanism of carbonyl compounds.…”

The synthesis of 1,2-azaphospholes, 1,2-azaphosphorines and 1,2-azaphosphepines

“…23,24 Much attention has been directed to the synthesis of phosphorus heterocycles due to their wideranging utilities as synthetic intermediates in organic syntheses. [25][26][27][28][29][30][31] Among these phosphorus heterocycles, 1,2-azaphospholes, 1,2-azaphosphorines and 1,2-azaphosphepines are of interest in several laboratories due to their potent pharmacological activities such as antitumor, 32 complexing agents, 33,34 and inhibitor of mammalian dihydroorotase. 35 The present review is focused on the most methodologies for the construction of 1,2-azaphospholes, 1,2-azaphosphorines and 1,2-azaphosphepines up to the end of 2019 and to supplement the information available in literature.…”