“…The ODPM rearrangement is found to occur with a loss in stereochemistry at the α and γ carbon atoms, and on the basis of this stereochemical behavior the existence of the two biradical intermediates A and B was proposed by both Van de Weerdt et al and Schaffner et al These floppy intermediates will give rise to complete scrambling of the substituent groups at the α and γ carbon atoms (Chart ), provided they have a discrete lifetime. In the few cases where the ODPM rearrangement has been found to be completely stereospecific, − suggesting a concerted mechanism, the stereospecificity has been put down to steric hindrance. 1 …”
The oxadi-π-methane ([1,2]-acyl shift) and [1,3]-acyl shift
rearrangements of a simple β,γ-enone (but-3-enal) have been investigated using MC-SCF computations in a 6-31G*
basis set. The excited state reaction pathways
and decay funnels for this model compound can be used to explain the
direct and triplet-sensitized photochemistry
of β,γ-enones in general. Our calculations show that the
“classical” biradical intermediates proposed for both
reactions
correspond to decay funnels at which four states
(S1(nπ*), T1(ππ*), T2(nπ*),
and S0) are degenerate. Both
efficient
internal conversion (IC) and efficient intersystem crossing (ISC) can
occur at these points, and the ground state
reaction path is therefore the same regardless of the state initially
populated. The ratio of products formed on photolysis
is governed by the relative heights of the barriers leading to these
decay funnels, and these will be sensitive to
substituent effects on the reactant molecule. The
oxadi-π-methane rearrangement is found to occur via a
three-step
process, where the four-level decay funnel corresponds to the first of
two floppy intermediates on S0. There are
two
possible mechanisms leading to the [1,3]-acyl shift product: one
involving the four-level decay funnel which
corresponds to a “tight” intermediate in a quasi-concerted pathway,
and a second which involves dissociation and
recombination.
“…The ODPM rearrangement is found to occur with a loss in stereochemistry at the α and γ carbon atoms, and on the basis of this stereochemical behavior the existence of the two biradical intermediates A and B was proposed by both Van de Weerdt et al and Schaffner et al These floppy intermediates will give rise to complete scrambling of the substituent groups at the α and γ carbon atoms (Chart ), provided they have a discrete lifetime. In the few cases where the ODPM rearrangement has been found to be completely stereospecific, − suggesting a concerted mechanism, the stereospecificity has been put down to steric hindrance. 1 …”
The oxadi-π-methane ([1,2]-acyl shift) and [1,3]-acyl shift
rearrangements of a simple β,γ-enone (but-3-enal) have been investigated using MC-SCF computations in a 6-31G*
basis set. The excited state reaction pathways
and decay funnels for this model compound can be used to explain the
direct and triplet-sensitized photochemistry
of β,γ-enones in general. Our calculations show that the
“classical” biradical intermediates proposed for both
reactions
correspond to decay funnels at which four states
(S1(nπ*), T1(ππ*), T2(nπ*),
and S0) are degenerate. Both
efficient
internal conversion (IC) and efficient intersystem crossing (ISC) can
occur at these points, and the ground state
reaction path is therefore the same regardless of the state initially
populated. The ratio of products formed on photolysis
is governed by the relative heights of the barriers leading to these
decay funnels, and these will be sensitive to
substituent effects on the reactant molecule. The
oxadi-π-methane rearrangement is found to occur via a
three-step
process, where the four-level decay funnel corresponds to the first of
two floppy intermediates on S0. There are
two
possible mechanisms leading to the [1,3]-acyl shift product: one
involving the four-level decay funnel which
corresponds to a “tight” intermediate in a quasi-concerted pathway,
and a second which involves dissociation and
recombination.
“…Finally, Scheme shows examples of ODPM rearrangements that proceed with preferential inversion of configuration at the methane carbon, a situation frequently observed. The acetone-sensitized irradiation of the enantiomerically pure cyclic ketone 31 gives 32 , in high chemical yield, and with 95% optical purity. , The direct irradiation of 33 brings about the formation of the endo isomer 34 in quantitative yield . A series of studies on the ODPM rearrangement of cyclopentenyl ketones 35 show that all of them yield preferentially the endo product 36 . − The stereochemical results observed for the compounds collected in Scheme suggest concertedness or slow conformational equilibration of intermediates. 22 …”
Section: A the Mechanismmentioning
confidence: 99%
“…In some of the cases reported, the reaction takes place with a high degree of stereochemical control as mentioned before for ketone 29 although in other cases, such as 25 , loss of stereochemistry is observed. Examples of ODPM rearrangement of cyclic ketones, with yields and references, are collected in Table . ,,,,,,,− …”
Section: E the Odpm Rearrangements Of Monocyclic
And
Condensed Polycy...mentioning
confidence: 99%
“…Most of the studies on this subject have concentrated on establishing the stereochemistry of the process. The results obtained show that the three possible alternativessloss of stereochemistry, [56][57][58][59][60] retention of configuration, 61 and inversion of configuration [62][63][64][65][66][67][68][69][70][71] scan occur. Among the three possibilities the latter process is the one that has been observed more frequently.…”
Section: A the Mechanismmentioning
confidence: 99%
“…The acetone-sensitized irradiation of the enantiomerically pure cyclic ketone 31 gives 32, in high chemical yield, and with 95% optical purity. 69,70 The direct irradiation of 33 brings about the formation of the endo isomer 34 in quantitative yield. 71 A series of studies on the ODPM rearrangement of cyclopentenyl ketones 35 show that all of them yield preferentially the endo product 36.…”
This reaction, known as the Zimmerman rearrangement, di‐π‐methane rearrangement, di‐π‐methane photorearrangement, or di‐π‐methane reaction, is a photochemical reaction of a molecular entity comprising two π‐systems separated by a saturated carbon atom (a
sp
3
carbon, e.g. a methylene group) involving the migration of one π‐moiety bonded to the saturated carbon to other π‐moiety accompanied by the concomitant formation of a cyclopropane ring from the remaining π‐moiety and methylene group. This rearrangement has been reported to consist at least six versions and all these variants differ from each other. This rearrangement has high regiospecificity with a strong preference for migration of the less conjugated π‐moiety to the more conjugated one. This rearrangement has proven valuable in preparation of substituted cyclopropanes.
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