Amino Substituted Bisketenes:  Generation, Structure, and Reactivity

Abstract

Hitherto unknown diamino-substituted bisketenes with both free (14) and tethered (16) amino substituents have been generated by using laser flash photolysis for ring opening of the corresponding cyclobutenediones. The time-resolved kinetics of ring closure of the amino bisketenes back to the cyclobutenediones were measured by IR or UV spectroscopy, and give first-order rate constants which vary by a factor of 7.5x10(4), and the bis(Me2N) bisketene 14 is the most reactive in ring closure that has been reported. Rate constants for ring closure of these and previously observed bisketenes vary by a factor of 10(13). The dialkylamino bisketenes 16 (R=Me, n-Bu) with tethered substituents and restricted geometries are less reactive than the bis(Me2N) bisketene 14 by factors of 1700 and 540, respectively. Computational results obtained with DFT methods suggest angle strain in the tethered cyclobutenediones 15 inhibits facile cyclization of bisketenes 16.

Squaramide-Based Pt(II) Complexes as Potential Oxygen-Regulated Light-Triggered Photocages

“…Upon pulsed irradiation at 266 nm, negative transient absorption signals (ΔAbs) were mainly recorded in the UV region (see the Supporting Information), a situation already reported for squaramides and ascribed to their photoisomerization to the less absorbing bisketene product. 8 As previously discussed, such a light-generated compound can either return back thermally to the initial squaramide state on the sub-second scale or undergo irreversible reactions with the surrounding media (e.g., with water molecules). 9 Because no time decay of the negative transient absorption signals was recorded, the latter appears to be the major evolution pathway under our experimental conditions, that is, quantitative degradation of the light-generated bisketene derivatives.…”

“…First, ligands L1 and L2 were synthesized from dibutyl squarate and the corresponding aminothiols, following standard procedures for the synthesis of squaramides (Figure a) . The structure of these two new compounds was confirmed by full characterization (see the Supporting Information).…”

“…Squaramide motifs are a class of interesting photoresponsive species that can be converted into their corresponding bisketenes (Scheme 1). 8 Although this process is thermally reversible in non-nucleophilic solvents and under inert atmosphere, bisketenes undergo irreversible reactions with H 2 O in usual biological media, thereby preventing ring closure to recover the initial squaramide motif. 9 This photobehavior prompted us to design a new class of squaramido-type ligands for new Pt(II) photocages.…”

Reaction mechanisms : Part (iii) Polar reactions

“…Laser flash photolysis-induced ringopening of cyclobutenediones 63 and 64 has been shown to yield amino-substituted bisketenes 65 and 66. 47 First order rate constants for the ring closure of the bisketenes back to the cyclobutenediones have been determined by time-resolved spectroscopy. The bisketenes with tethered substituents and restricted geometries 66 were found to be less reactive towards ring closure than bisketenes 65.…”

Stereoselective Synthesis of 3-(5-Benzoyl-1-methyl-1H-pyrrol-2-yl)-2-azetidinone Derivatives via an in Situ Generated Ketene

“…Ketenes are important intermediates that occur in many organic reactions and remarkable for the variety of ways in which they can be prepared. [24][25][26][27] The most elegant method to prepare a fourmembered ring is through the [2+2]-cycloaddition reaction of a ketene with an unsaturated compound. Syntheses of six-membered rings have also been reported by using a ketene with unsaturated compounds.…”