Utilization of 1,1-dimethyl-4,6-di-tert-butylspiro[2,5]octa-3,6-dien-5-one as a ‘hypersensitive’ probe for single electron transfer to carbonyl compounds
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“…Our interest in this system is related to the fact that 1c has proven to be a highly effective “probe” for distinguishing between SET and conventional polar pathways in reactions of nucleophiles with carbonyl compounds based upon the observed regiochemistry of the products (Scheme ). The observed behavior of this substrate in reactions with nucleophiles which do and do not react with carbonyl compounds via SET has confirmed the key elements of this hypothesis. 1 …”
Results pertaining to the direct and indirect electrochemistry of
5,7-di-tert-butylspiro[2.5]octa-4,7-dien-6-one (1a),
1-methyl-5,7-di-tert-butylspiro[2.5]octa-4,7-dien-6-one
(1b), and 1,1,-dimethyl-5,7-di-tert-butylspiro[2.5]octa-4,7-dien-6-one (1c) are
reported. Product analyses reveal that reduction of all these
substrates
leads to cyclopropane ring-opened products; ring opening occurs with
modest selectivity leading to the more
substituted (stable) distonic radical anion. The direct
electrochemistry of these compounds is characterized
by rate limiting electron transfer (with α ≈ 0.5), suggesting that
while ring opening is extremely rapid, the
radical anions do have a discrete lifetime (i.e., electron
transfer and ring opening are not concerted).
Utilizing
homogeneous redox catalysis, rate constants for electron transfer
between 1a, 1b, and 1c and a series of
aromatic
radical anions were measured; reduction potentials and reorganization
energies were derived from these rate
constants by using Marcus theory.
“…Our interest in this system is related to the fact that 1c has proven to be a highly effective “probe” for distinguishing between SET and conventional polar pathways in reactions of nucleophiles with carbonyl compounds based upon the observed regiochemistry of the products (Scheme ). The observed behavior of this substrate in reactions with nucleophiles which do and do not react with carbonyl compounds via SET has confirmed the key elements of this hypothesis. 1 …”
Results pertaining to the direct and indirect electrochemistry of
5,7-di-tert-butylspiro[2.5]octa-4,7-dien-6-one (1a),
1-methyl-5,7-di-tert-butylspiro[2.5]octa-4,7-dien-6-one
(1b), and 1,1,-dimethyl-5,7-di-tert-butylspiro[2.5]octa-4,7-dien-6-one (1c) are
reported. Product analyses reveal that reduction of all these
substrates
leads to cyclopropane ring-opened products; ring opening occurs with
modest selectivity leading to the more
substituted (stable) distonic radical anion. The direct
electrochemistry of these compounds is characterized
by rate limiting electron transfer (with α ≈ 0.5), suggesting that
while ring opening is extremely rapid, the
radical anions do have a discrete lifetime (i.e., electron
transfer and ring opening are not concerted).
Utilizing
homogeneous redox catalysis, rate constants for electron transfer
between 1a, 1b, and 1c and a series of
aromatic
radical anions were measured; reduction potentials and reorganization
energies were derived from these rate
constants by using Marcus theory.
“…By examining reactions of 1 with nucleophiles which have been shown independently to react with carbonyl compounds via SET, we were able to confirm the first half of Scheme , namely, the behavior of this system in bona fide SET processes …”
1-Methyl-5,7-di-tert-butylspiro[2.5]octa-4,7-dien-6-one
(8) and
1,1-dimethyl-5,7-di-tert-butylspiro[2.5]octa-4,7-dien-6-one (1) react with thiophenoxide ion
to produce cyclopropane ring-opened
products. Thermodynamic considerations effectively rule out any
possibility that single electron
transfer is involved in theses reactions; the process
PhS- + substrate → PhS• +
substrate•- is
endothermic by over 50 kcal/mol! Nucleophilic attack occurs both
at the least- and most-hindered
carbons of the cyclopropyl group, and the product ratio
(R(1°/2°) from 8 and R(1°/3°)
from 1, where
1°, 2°, and 3° refer to the regioisomeric phenyl sulfides formed
from these substrates) is found to
vary with solvent. In dipolar, aprotic solvents, nucleophilic
attack occurs preferentially at the least-hindered carbon of the cyclopropyl group (R(1°/2°) and
R(1°/3°) ≈ 4−5), consistent with an
SN2
mechanism. In protic solvents, products arising from nucleophilic
attack at the more-substituted
carbon of the cyclopropyl group become increasingly important,
consistent with the onset of a
carbocationic (SN2(C+)) pathway. The strengths and
weaknesses of 1 and 8 as probes for
single
electron transfer are discussed in the context of these
results.
“…Our approach to discovering whether radical intermediates were present during the carbon–carbon bond-forming step of the addition of an allylmagnesium reagent to a nonaromatic aldehyde involved the use of a radical clock. The 2,2-diphenylcyclopropylcarbinyl system (Scheme ), one of the fastest radical clocks known, was chosen to maximize the chance of observing products derived from single-electron-transfer reactions. , The 2,2-diphenylcyclopropylcarbinyl radical undergoes ring opening at a rate of 5 × 10 11 s –1 , − and oxygen-containing substituents have been shown to have little effect on this rate. , Consequently, if a radical intermediate were formed, it should undergo ring opening at a rate that is competitive with the rate of geminate radical pairs undergoing recombination. , Cyclopropane-derived radical clocks have been used to provide evidence for radical intermediates. , These radical clocks have been used to identify ketyl radicals as intermediates in reactions of reagents such as SmI 2 and tributyltin hydride with aldehydes, ,, ketones, − esters, − amides, and carboxylic acids …”
mentioning
confidence: 99%
“…18,19 The 2,2-diphenylcyclopropylcarbinyl radical undergoes ring opening at a rate of 5 × 10 11 s −1 , 19−21 and oxygen-containing substituents have been shown to have little effect on this rate. 20,22 Consequently, if a radical intermediate were formed, it should undergo ring opening at a rate that is competitive with the rate of geminate radical pairs undergoing recombination. 23,24 Cyclopropane-derived radical clocks have been used to provide evidence for radical intermediates.…”
Addition of allylmagnesium reagents to an aliphatic aldehyde bearing a radical clock gave only addition products and no evidence of ring-opened products that would suggest single-electron-transfer reactions. The analogous Barbier reaction also did not provide evidence for a single-electron-transfer mechanism in the addition step. Other Grignard reagents (methyl-, vinyl-, t-Bu-, and triphenylmethylmagnesium halides) also do not appear to add to an alkyl aldehyde by a single-electron-transfer mechanism.
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