New reactions of allenes, alkynes, ynamides, enynones and isothiocyanates

This perspective article is related to transformations (both catalytic and non-catalytic) involving allenes, alkynes/enynones/ynamides and isothiocyanates. Part of the work from the author’s group has been reviewed along with some new reactions of (i) allenylphosphonate/allenylphosphine oxide and (ii) a P(III) isothiocyanate. Thus, the allenylphosphine oxide Ph 2 P(O)C(Ph)=C=CH 2 undergoes base (DBU) catalyzed addition to the β -ketophosphonate (OCH 2 CMe 2 CH 2 O)P(O)CH 2 C(O)CH 3 at 90 ∘ C to afford the addition product Ph 2 P(O)C(Ph)=C(Me)CH[C(O)Me][P(O)(OCH 2 CMe 2 CH 2 O)] . In an analogous reaction, with DBU as the base at 140 ∘ C , isomeric vinylphosphine oxides ( Z )- Ph 2 P(O)C(Ph)=C(Me)CH 2 [C(O)Me] and ( E )- Ph 2 P(O)C(Ph)=C(Me)CH 2 [C(O)Me] were isolated. The E -isomer has been characterized by single crystal X-ray structure determination. In another set of studies, the reaction of P(III) isothiocyanate (OCH 2 CMe 2 CH 2 O)P(NCS) with N -(2-bromomethyl)sulfonamide afforded an unusual product with the formula (OCH 2 CMe 2 CH 2 O)P(O)SCH 2 CH 2 NHS(O) 2 - ( C 6 H 4 -4- Me) as shown by X-ray structure determination. This result is different from that obtained in the recently reported analogous reaction using phenyl isothiocyanate. Graphical Abstract Synopsis. Base-catalyzed the addition of β -ketophosphonate or ethyl acetoacetate to allenylphosphine oxide affords vinylphosphine oxides; in the latter case, elimination of the ester group also takes place. It is also shown that reactivity of a P(III) isothiocyanate is different from that of organic isothiocyanates.