Electron paramagnetic resonance and the art of physical-organic chemistry

Griller, D.; Ingold, K. U.

doi:10.1021/ar50151a001

Cited by 63 publications

(30 citation statements)

References 96 publications

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“…Rate constants of ring-closure reactions can be determined for sterically unhindered radicals by measurements of the concentrations of the ring-open and cyclised radicals under EPR conditions [43–45]. Steady-state concentrations of 12a were determined in the usual way from the spectra [46].…”

Section: Resultsmentioning

confidence: 99%

Interplay of ortho- with spiro-cyclisation during iminyl radical closures onto arenes and heteroarenes

McBurney¹,

Walton²

2013

Beilstein J. Org. Chem.

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SummarySensitised photolyses of ethoxycarbonyl oximes of aromatic and heteroaromatic ketones yielded iminyl radicals, which were characterised by EPR spectroscopy. Iminyls with suitably placed arene or heteroarene acceptors underwent cyclisations yielding phenanthridine-type products from ortho-additions. For benzofuran and benzothiophene acceptors, spiro-cyclisation predominated at low temperatures, but thermodynamic control ensured ortho-products, benzofuro- or benzothieno-isoquinolines, formed at higher temperatures. Estimates by steady-state kinetic EPR established that iminyl radical cyclisations onto aromatics took place about an order of magnitude more slowly than prototypical C-centred radicals. The cyclisation energetics were investigated by DFT computations, which gave insights into factors influencing the two cyclisation modes.

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Section: Resultsmentioning

confidence: 99%

Interplay of ortho- with spiro-cyclisation during iminyl radical closures onto arenes and heteroarenes

McBurney¹,

Walton²

2013

Beilstein J. Org. Chem.

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“…termination rates of small to moderately sized transient radicals are diffusion controlled in solution and depend on the solution viscosity.' [1][2][3][4][5][6][7][8][9][10][11][12][13] In order to evaluate 2kt we have used Fischer's accurate data for the self-termination of t-butyl radicals in heptane.12 At each temperature 2kt was calculated from the Arrhenius equation given by Fischer, equation (2), by correcting for the difference in viscosity of cyclopropane Hydrogen Abstraction from Higher Spiro[2.n]aikanes.-Hydrogen abstraction by t-butoxyl radicals was also studied for spiro [2.4]heptane (Sc), spiro[2S]octane (a), spiroC2.6lnonane (S), and spiroC2.1 lltetradecane (5f). The first three compounds were examined at temperatures down to 120 K, but in all cases the spiro[2.n]alk-2-yl radicals were completely rearranged to cycloalkenylethyl radicals (7), which were observed throughout the whole temperature range.…”

Section: Resultsmentioning

confidence: 99%

Hydrogen abstraction from spiro[2.n]alkanes

Roberts¹,

Walton²

1985

J. Chem. Soc., Perkin Trans. 2

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Hydrogen abstraction from spiro[2.3] hexane by t-butoxyl radicals gave spiro[2.3] hex-2-yl radicals; their rearrangement t o cyclobutenylethyl radicals was followed by kinetic e.s.r. spectroscopy. Hydrogen abstraction at the methylene groups adjacent to the cyclopropyl rings [cyclopropylmethyl (cpm) sites] in higher spiro [ 2.n] alkanes gave spiro [ 2.n] al k -2yl radicals, which rearranged t o cycloal kenylet hyl radicals too rapidly for detection, together with secondary radicals from abstraction at the other methylene groups in the larger ring. From the measured concentrations of the cycloalkenylethyl and secondary radicals the rate of hydrogen abstraction at the cpm sites relative to the rate of hydrogen abstraction at the secondary sites was determined; significant activation of the cprn hydrogens was found. This activation was attributed to a pseudo-ally1 type of effect, i.e., t o delocalisation of the unpaired electron into the Walsh orbitals of the cyclopropane ring of the spiro[2.n]alk-2-yl radicals; semiempirical SCF-MO calculations supported this explanation. Photobromination of spiro[2.3] hexane occurred mainly by SH2 attack of bromine atoms at the cyclopropane methylene carbons with fission of either C-C bond.We showed recently that the mode and rate of homolytic ring fission in cis-bicyclo[n. 1 .O]alk-2-yl radicals (2) is controlled by two factors. For the first two members of the series (1, n = 1 or 2) relief of ring strain outweighs the unfavourable overlap of the SOMO with the orbitals of the inter-ring bond; this bond breaks to give a cycloalkenyl radical (3) (Scheme 1). For the higher members of the series (2, n > 2) stereoelectronic control predominates; i.e., the favourable overlap of the SOMO with the outer cyclopropane bond orbitals ensures that this bond breaks to give cycloalkenylmethyl radicals (4).Hydrogen abstraction from spiroC2.n)alkanes (5) will occur almost exclusively in the larger ring because of the much higher C-H bond strengths in the cyclopropyl rings. Hydrogen abstraction at C(2) [and C(n)] will produce spiroC2.nIalk-2-yl radicals (6) and hydrogen abstraction at other sites will produce secondary cycloalkyl type radicals (8). The radicals (6) are of the cyclopropylmethyl type and will rearrange by p-scission of the P,y-cyclopropane bond to give cycloalkenylethyl radicals (7) (see Scheme 2). The spiro[2.n]alkyl radicals provide, therefore, a second series in which the interplay of the two effects, relief of ring strain and the stereoelectronic factor, on the ring-opening reaction can be studied. The rate of ring fission will depend on the size of the ring and also on the extent of overlap of the SOMO with the P,y-bond. The geometry of the system will hold radicals (6) in conformation (9), but overlap of the SOMO with the cyclopropane bond will depend on the conformation of the larger ring and the s / p character of the SOMO. The first member of the series (6a), i.e., spiropentyl, is a o-radi~al;~ thus this radical centre is not planar as in (9) but bent as in (lo), with conse...

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“…The temperature of the sample is adjusted so that R1 and Rz can be observed simultaneously. Provided there are no other reactions besides the unimolecular process of interest and the radical self-and cross-terminations, the mechanism (1)- (5) (1) source -R , (2) Ri --+ Rz…”

Section: Introductionmentioning

confidence: 99%

Rate constants for the decarboxylation of the t‐butoxicarbonyl radical and concurring radical terminations directly obtained by kinetic ESR

Rüegge

Fischer

1986

Int J of Chemical Kinetics

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Rate constants of various simultaneous reactions of t-butoxicarbonyl and t-butyl radicals generated by photolysis of t-butylpivalate in n-heptane are directly determined by kinetic electron spin resonance. The temperature dependence of the decarboxylation reaction t-BuOCO + t-Bu + CO, obeys log(k/s-') = 13.8-49.0/8 where 8 = 2.303 RT/ kJ . mol-'. The self-and cross-termination of the radicals are diffusion limited.

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Electron paramagnetic resonance and the art of physical-organic chemistry

Cited by 63 publications

References 96 publications

Interplay of ortho- with spiro-cyclisation during iminyl radical closures onto arenes and heteroarenes

Interplay of ortho- with spiro-cyclisation during iminyl radical closures onto arenes and heteroarenes

Hydrogen abstraction from spiro[2.n]alkanes

Rate constants for the decarboxylation of the t‐butoxicarbonyl radical and concurring radical terminations directly obtained by kinetic ESR

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