The Thermal Reaction of Sterically Hindered Nitroxyl Radicals with Allylic and Benzylic Substrates:  Experimental and Computational Evidence for Divergent Mechanisms

Babiarz, Joseph E.; Cunkle, Glen T.; DeBellis, Anthony D.; Eveland, David; Pastor, Stephen D.; Shum, Sai P.

doi:10.1021/jo020426r

Cited by 56 publications

(42 citation statements)

References 18 publications

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“…This hypothesis is based on the well-known fact that TEMPO can react not only trapping radicals, but also abstracting hydrogen atoms from hydrocarbon molecules at 110-120 8C, [24] or undergoing addition to double bonds at lower temperature (70 8C). [25] The experimental synthesis of (4) was then experimentally performed following pathway a in order to confirm the hypothesis, and the product was labeled as PBT.…”

Section: Resultsmentioning

confidence: 99%

Controlled Grafting‐From of Polystyrene on Polybutadiene: Mechanism and Spectroscopic Evidence of the Functionalization of Polybutadiene with 4‐Oxo‐TEMPO

Bonilla‐Cruz

Saldívar‐Guerra

Torres-Lubián

et al. 2008

Macro Chemistry & Physics

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Polybutadienes functionalized with nitroxide (multifunctional macroalkoxyamines) were synthesized by heating PB in the presence of a nitroxide radical (N) and a radical initiator (I). Another functional PB was prepared by using only nitroxide. These functionalized polymers were characterized by FT‐IR, GPC, and NMR and as a result it was possible to elucidate the resulting structure of the macroalkoxyamines, to propose likely functionalization mechanisms, and to estimate functionalization characteristics. One of these polymers was heated in the presence of styrene to obtain PB grafted with polystyrene, which was characterized in detail in order to investigate its structure and level of grafting control.

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Section: Resultsmentioning

confidence: 99%

Controlled Grafting‐From of Polystyrene on Polybutadiene: Mechanism and Spectroscopic Evidence of the Functionalization of Polybutadiene with 4‐Oxo‐TEMPO

Bonilla‐Cruz

Saldívar‐Guerra

Torres-Lubián

et al. 2008

Macro Chemistry & Physics

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“…128 DeBellis et al 129 also studied the reaction of TEMPOL with cyclohexene. From product studies, they inferred a predominant abstraction-addition mechanism.…”

Section: Nitroxide Backgroundmentioning

confidence: 99%

Nitroxide decomposition: Implications toward nitroxide design for applications in living free‐radical polymerization

Nilsen

Braslau

2005

J. Polym. Sci. A Polym. Chem.

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ABSTRACT:Nitroxides bearing an a-hydrogen decompose upon heating in a bimolecular reaction. A new mechanism is proposed for the decomposition of t-butylisopropylphenyl nitroxide (TIPNO) involving the formation of a head-to-tail dimer, single electron transfer to form an oxammonium salt, epimerization to the corresponding nitrone, and elimination to form a conjugated oxime. This mechanism may provide insights into designing new nitroxides for use in controlled polymerization.

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“…1c was prepared by substituting TEMPO for 4-carboxy-TEMPO (1) in a literature method, refluxing overnight in cyclohexene. [57,58] Preparation of 1e was also based on adaptation of an existing method, refluxing 4-carboxy-TEMPO (1) and the radical initiator 1,1'-azobis(cyclohexanecarbonitrile) for 30 hours in methanol. [59] 4-N,N,N,-trimethylamino-TEMPO was prepared as the iodide salt according to the method of Strehmel et al [60] Characterisation of all products by high-resolution mass spectrometry is provided as Supporting Information (Table S1).…”

Section: Synthesismentioning

confidence: 99%

Experimental evidence for competitive N O and O C bond homolysis in gas-phase alkoxyamines

Marshall

Gryn’ova

Coote

et al. 2015

International Journal of Mass Spectrometry

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The extensive use of alkoxyamines in controlled radical polymerisation and polymer stabilisation is based on rapid cycling between the alkoxyamine (R1R2NO-R3) and a stable nitroxyl radical (R1R2NO•) via homolysis of the labile O-C bond. Competing homolysis of the alkoxyamine N-O bond has been predicted to occur for some substituents leading to production of aminyl and alkoxyl radicals. This intrinsic competition between the O-C and N-O bond homolysis processes has to this point been difficult to probe experimentally. Herein we examine the effect of local molecular structure on the competition between N-O and O-C bond cleavage in the gas phase by variable energy tandem mass spectrometry in a triple quadrupole mass spectrometer. A suite of cyclic alkoxyamines with remote carboxylic acid moieties (HOOC-R1R2NO-R3) were synthesised and subjected to negative ion electrospray ionisation to yield [M − H]− anions where the charge is remote from the alkoxyamine moiety. Collision-induced dissociation of these anions yield product ions resulting, almost exclusively, from homolysis of O-C and/or N-O bonds. The relative efficacy of N-O and O-C bond homolysis was examined for alkoxyamines incorporating different R3 substituents by varying the potential difference applied to the collision cell, and comparing dissociation thresholds of each product ion channel. For most R3 substituents, product ions from homolysis of the O-C bond are observed and product ions resulting from cleavage of the N-O bond are minor or absent. A limited number of examples were encountered however, where N-O homolysis is a competitive dissociation pathway because the O-C bond is stabilised by adjacent heteroatom(s) (e.g. R3 = CH2F). The dissociation threshold energies were compared for different alkoxyamine substituents (R3) and the relative ordering of these experimentally determined energies is shown to correlate with the bond dissociation free energies, calculated by ab initio methods. Understanding the structure-dependent relationship between these rival processes will assist in the design and selection of alkoxyamine motifs that selectively promote the desirable O-C homolysis pathway. homolysis is a competitive dissociation pathway because the O-C bond is stabilised by adjacent heteroatom(s) (e.g., R 3 = CH 2 F). The dissociation threshold energies were compared for different alkoxyamine substituents (R 3 ) and the relative ordering of these experimentally determined energies is shown to correlate with the bond dissociation free energies, calculated by ab initio methods.Understanding the structure-dependent relationship between these rival processes will assist in the design and selection of alkoxyamine motifs that selectively promote the desirable O-C homolysis pathway.

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The Thermal Reaction of Sterically Hindered Nitroxyl Radicals with Allylic and Benzylic Substrates: Experimental and Computational Evidence for Divergent Mechanisms

Cited by 56 publications

References 18 publications

Controlled Grafting‐From of Polystyrene on Polybutadiene: Mechanism and Spectroscopic Evidence of the Functionalization of Polybutadiene with 4‐Oxo‐TEMPO

Controlled Grafting‐From of Polystyrene on Polybutadiene: Mechanism and Spectroscopic Evidence of the Functionalization of Polybutadiene with 4‐Oxo‐TEMPO

Nitroxide decomposition: Implications toward nitroxide design for applications in living free‐radical polymerization

Experimental evidence for competitive N O and O C bond homolysis in gas-phase alkoxyamines

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