Rate constants for ring openings of 2-phenylcyclobutylcarbinyl radicals

Abstract

Kinetics of ring openings of 2‐phenyl‐substituted cyclobutylcarbinyl radicals were studied by laser flash photolysis methods employing N‐hydroxypyridine‐2‐thione (PTOC) esters as radical precursors. The radicals studied were (trans‐2‐phenylcyclobutyl)methyl (1a), 1‐(trans‐2‐phenylcyclobutyl)ethyl (1b), 1‐methyl‐1‐(trans‐2‐phenylcyclobutyl)ethyl (1c), (cis‐2‐phenylcyclobutyl)methyl (2a), 1‐(cis‐2‐phenylcyclobutyl)ethyl (2b) and (ethoxycarbonyl),(trans‐2‐phenylcyclobutyl)methyl (13). Arrhenius parameters for radicals 1 were determined in THF and acetonitrile and those for radicals 2 were determined in THF. Rate constants for ring openings at 20 °C in units of 107 s−1 are 1.3 (1a), 1.0 (1b), 0.8 (1c), 3.2 (2a) and 3.9 (2b). These ring openings are more than three orders of magnitude faster than those for the parent cyclobutylcarbinyl radicals lacking the phenyl groups. The family can be used in direct studies of radical substituent effects and as internal reporter groups for other radical reactions. A demonstration of the former application is the measurement of the kinetics of ring openings of radical 13 which rearranges with rate constants nearly equal to those of the analogous methyl‐substituted radical 1b and, unlike radicals 1 displays a polar solvent effect in the kinetics of the fragmentation reaction. Copyright © 2000 John Wiley & Sons, Ltd.

Mechanistic Investigation of the Iron-Catalyzed Azidation of Alkyl C(sp3)–H Bonds with Zhdankin’s λ3-Azidoiodane

“…However, the same reactions conducted in the absence of an iron catalyst, in either acetonitrile or DMSO, led to complete decomposition of the terpene reactants. In these reactions, the HAA step generates an α-cyclobutyl radical, which is known to undergo ring opening to give an alkene. − In the iron-catalyzed azidation, trapping of the α-cyclobutyl radical with an iron­(III)-azide, apparently, outcompetes ring opening to form the alkyl azide product.…”

Mechanistic Investigation of the Iron-Catalyzed Azidation of Alkyl C(sp3)–H Bonds with Zhdankin’s λ3-Azidoiodane

“…However, the same reactions conducted in the absence of an iron catalyst, in either acetonitrile or DMSO, led to complete decomposition of the terpene reactants. In these reactions, the HAA step generates an α-cyclobutyl radical, which is known to undergo ring opening to give an alkene. − In the iron-catalyzed azidation, trapping of the α-cyclobutyl radical with an iron­(III)-azide, apparently, outcompetes ring opening to form the alkyl azide product.…”

Rate Constants and Arrhenius Functions for Ring Opening of a Cyclobutylcarbinyl Radical Clock and for Hydrogen Atom Transfer From the Et 3 B−MeOH Complex

“…Among the slower radical clocks, ring openings of cyclobutylcarbinyl radicals to 4-pentenyl radicals are noteworthy. At room temperature, these unimolecular reactions are about 2 orders of magnitude less rapid than cyclizations of 5-hexenyl radicals, but only limited kinetic studies , and computational studies of cyclobutylcarbinyl radical ring openings are available. Recently, we used the ring-opening reaction of the 1-cyclobutyldodecyl radical ( 1 ) to the trans -4-hexadecenyl radical ( 2 ) (Scheme ) in kinetic studies of hydrogen atom transfer trappings by water and methanol complexes of triethylborane .…”

7 Reaction mechanisms. Part (iv) Radical and radical ion reactions

“…4,5 Scheme 2. 1 Roth and Figge determined the energy profile for the equilibrium between 2.8 and 2.9 (Scheme 2.4) by investigating the oxygen trapping of 2.9. 6 The diradical 2.9 was determined to be a triplet from trapping studies and the heat of its formation was estimated to be 25.7 kcal mol 21 .…”

Mechanistic Investigation of the Iron-Catalyzed Azidation of Alkyl C(sp3)–H Bonds with Zhdankin’s λ3-Azidoiodane

“…However, the same reactions conducted in the absence of an iron catalyst, in either acetonitrile or DMSO, led to complete decomposition of the terpene reactants. In these reactions, the HAA step generates an α-cyclobutyl radical, which is known to undergo ring opening to give an alkene. − In the iron-catalyzed azidation, trapping of the α-cyclobutyl radical with an iron­(III)-azide, apparently, outcompetes ring opening to form the alkyl azide product.…”

Rate Constants and Arrhenius Functions for Ring Opening of a Cyclobutylcarbinyl Radical Clock and for Hydrogen Atom Transfer From the Et 3 B−MeOH Complex

“…Among the slower radical clocks, ring openings of cyclobutylcarbinyl radicals to 4-pentenyl radicals are noteworthy. At room temperature, these unimolecular reactions are about 2 orders of magnitude less rapid than cyclizations of 5-hexenyl radicals, but only limited kinetic studies , and computational studies of cyclobutylcarbinyl radical ring openings are available. Recently, we used the ring-opening reaction of the 1-cyclobutyldodecyl radical ( 1 ) to the trans -4-hexadecenyl radical ( 2 ) (Scheme ) in kinetic studies of hydrogen atom transfer trappings by water and methanol complexes of triethylborane .…”

7 Reaction mechanisms. Part (iv) Radical and radical ion reactions

“…4,5 Scheme 2. 1 Roth and Figge determined the energy profile for the equilibrium between 2.8 and 2.9 (Scheme 2.4) by investigating the oxygen trapping of 2.9. 6 The diradical 2.9 was determined to be a triplet from trapping studies and the heat of its formation was estimated to be 25.7 kcal mol 21 .…”

Mechanistic Investigation of the Iron-Catalyzed Azidation of Alkyl C(sp3)–H Bonds with Zhdankin’s λ3-Azidoiodane

“…However, the same reactions conducted in the absence of an iron catalyst, in either acetonitrile or DMSO, led to complete decomposition of the terpene reactants. In these reactions, the HAA step generates an α-cyclobutyl radical, which is known to undergo ring opening to give an alkene. − In the iron-catalyzed azidation, trapping of the α-cyclobutyl radical with an iron­(III)-azide, apparently, outcompetes ring opening to form the alkyl azide product.…”

Rate Constants and Arrhenius Functions for Ring Opening of a Cyclobutylcarbinyl Radical Clock and for Hydrogen Atom Transfer From the Et 3 B−MeOH Complex

“…Among the slower radical clocks, ring openings of cyclobutylcarbinyl radicals to 4-pentenyl radicals are noteworthy. At room temperature, these unimolecular reactions are about 2 orders of magnitude less rapid than cyclizations of 5-hexenyl radicals, but only limited kinetic studies , and computational studies of cyclobutylcarbinyl radical ring openings are available. Recently, we used the ring-opening reaction of the 1-cyclobutyldodecyl radical ( 1 ) to the trans -4-hexadecenyl radical ( 2 ) (Scheme ) in kinetic studies of hydrogen atom transfer trappings by water and methanol complexes of triethylborane .…”

7 Reaction mechanisms. Part (iv) Radical and radical ion reactions

“…4,5 Scheme 2. 1 Roth and Figge determined the energy profile for the equilibrium between 2.8 and 2.9 (Scheme 2.4) by investigating the oxygen trapping of 2.9. 6 The diradical 2.9 was determined to be a triplet from trapping studies and the heat of its formation was estimated to be 25.7 kcal mol 21 .…”