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

Pawel Löwe,Maike B Röthel,Fabian Dielmann,Lukas F B Wilm,Milica Feldt

doi:10.1021/acs.inorgchem.1c02076

Pawel Löwe, Maike B Röthel + Show 3 more

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https://doi.org/10.1021/acs.inorgchem.1c02076

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Abstract

While the metathesis reaction between alkynes and thiocarbonyl compounds has been thoroughly studied, the reactivity of alkynes with isoelectronic main group R2E=S compounds is rarely reported and unknown for [R2P=S]+ analogues. We show that thiophosphonium ions, which are the isoelectronic phosphorus congeners to thiocarbonyl compounds, undergo [2 + 2]-cycloaddition reactions with different alkynes to generate 1,2-thiaphosphete ions. The four-membered ring species are in an equilibrium state with the corresponding P=C–C=S heterodiene structure and thus undergo hetero-Diels–Alder reactions with acetonitrile. Heteroatom and substituent effects on the energy profile of the 1,2-thiaphosphete formation were elucidated by means of quantum chemical methods.

Highlights

Heavy analogues of carbonyl compounds are generally highly reactive and prone to spontaneous oligomerization owing to the energetic preference of heavy p-block elements in forming σ bonds instead of (p−p)π bonds.[1−4] In this respect, the thiocarbonyl group (C=S) is an exception, but it reacts, due to its rather weak C=S bond and the aptitude of sulfur to stabilize an adjacent charge or radical center, more in nucleophilic reactions and sigmatropic rearrangements than carbonyls.[5]Both carbonyls and thiocarbonyls undergocarbonyl−alkyne metathesis reactions, involving the [2 + 2]-cycloaddition reaction of acarbonyl with an alkyne
The carbonyl−alkyne metathesis proceeds via a fourmembered oxete intermediate, which is usually directly transformed into the α,β-unsaturated ketone,[7−12] unless it is stabilized by strongly electron-withdrawing groups.[13−15] Due to the lower tendency of sulfur to form double bonds, thietes are more stable than oxetes,[16−21] and a dynamic equilibrium between the “closed” thiete and “open” α,β-unsaturated thioketone form was observed with thioether substituents.[22,23]
PV thiaphosphete I contains a pentavalent phosphorus atom and exhibits a similar 31P NMR chemical shift (−40.7 ppm) to [2a]+,42 whereas the resonance of the PIII thiaphosphete II appears at 37.5 ppm.[43]

Summary

■ INTRODUCTION

Heavy analogues of carbonyl compounds are generally highly reactive and prone to spontaneous oligomerization owing to the energetic preference of heavy p-block elements in forming σ bonds instead of (p−p)π bonds.[1−4] In this respect, the thiocarbonyl group (C=S) is an exception, but it reacts, due to its rather weak C=S bond and the aptitude of sulfur to stabilize an adjacent charge or radical center, more in nucleophilic reactions and sigmatropic rearrangements than carbonyls.[5]. The carbonyl−alkyne metathesis proceeds via a fourmembered oxete intermediate, which is usually directly transformed into the α,β-unsaturated ketone,[7−12] unless it is stabilized by strongly electron-withdrawing groups.[13−15] Due to the lower tendency of sulfur to form double bonds, thietes are more stable than oxetes,[16−21] and a dynamic equilibrium between the “closed” thiete and “open” α,β-unsaturated thioketone form was observed with thioether substituents.[22,23] Given these differences between oxetes and thietes, we became curious to explore how the introduction of another heavy main group element would affect the stability of the four-membered ring species. Ragogna and co-workers prepared the neutral PIII 1,2-thiaphosphete II via transfer of a phosphinidene sulfide intermediate to an alkyne.[43]

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ REFERENCES