31 research outputs found
Rh-Catalyzed Intermolecular Reactions of Cyclic α-Diazocarbonyl Compounds with Selectivity over Tertiary C–H Bond Migration
Intermolecular Rh-catalyzed reactions of cyclic α-diazocarbonyl
compounds with chemoselectivity over β-hydride elimination are
described. These methods represent the first general intermolecular
reactions of Rh-carbenoids that are selective over tertiary β-C–H
bond migration. Successful transformations include cyclopropanation,
cyclopropenation, and various X–H insertion reactions with
a broad scope of substrates. We propose that the intermolecular approach
of substrates to carbenes from acyclic diazo precursors may be relatively
slow due to a steric interaction with the ester function, which is
perpendicular to the π-system of the carbene. For carbenes derived
from five- and six-membered cyclic α-diazocarbonyls, it is proposed
that the carbene is constrained to be more conjugated with the carbonyl,
thereby relieving the steric interaction for intermolecular reactions,
and accelerating the rate of intermolecular reactivity relative to
intramolecular β-hydride migration. However, attempts to use
α-diazo-β-ethylcaprolactone in intermolecular cyclopropanation
with styrene were unsuccessful. It is proposed that the conformational
flexibility of the seven-membered ring allows the carbonyl to be oriented
perpendicular to Rh-carbene. The significant intermolecular interaction
between the carbonyl and approaching substrate is in agreement with
the poor ability of α-diazo-β-ethylcaprolactone to participate
in intermolecular cyclopropanation reactions. DFT calculations provide
support for the mechanistic proposals that are described
Rh-Catalyzed Intermolecular Reactions of Cyclic α-Diazocarbonyl Compounds with Selectivity over Tertiary C–H Bond Migration
Intermolecular Rh-catalyzed reactions of cyclic α-diazocarbonyl
compounds with chemoselectivity over β-hydride elimination are
described. These methods represent the first general intermolecular
reactions of Rh-carbenoids that are selective over tertiary β-C–H
bond migration. Successful transformations include cyclopropanation,
cyclopropenation, and various X–H insertion reactions with
a broad scope of substrates. We propose that the intermolecular approach
of substrates to carbenes from acyclic diazo precursors may be relatively
slow due to a steric interaction with the ester function, which is
perpendicular to the π-system of the carbene. For carbenes derived
from five- and six-membered cyclic α-diazocarbonyls, it is proposed
that the carbene is constrained to be more conjugated with the carbonyl,
thereby relieving the steric interaction for intermolecular reactions,
and accelerating the rate of intermolecular reactivity relative to
intramolecular β-hydride migration. However, attempts to use
α-diazo-β-ethylcaprolactone in intermolecular cyclopropanation
with styrene were unsuccessful. It is proposed that the conformational
flexibility of the seven-membered ring allows the carbonyl to be oriented
perpendicular to Rh-carbene. The significant intermolecular interaction
between the carbonyl and approaching substrate is in agreement with
the poor ability of α-diazo-β-ethylcaprolactone to participate
in intermolecular cyclopropanation reactions. DFT calculations provide
support for the mechanistic proposals that are described
Rh(II)-Catalyzed Reactions of Diazoesters with Organozinc Reagents
RhÂ(II)-catalyzed reactions of diazoesters
with organozinc reagents
are described. Diorganozinc reagents participate in reactions with
diazo compounds by two distinct, catalyst-dependent mechanisms. With
bulky diisopropylethyl acetate ligands, the reaction mechanism is
proposed to involve initial formation of a Rh-carbene and subsequent
carbozincation to give a zinc enolate. With Rh<sub>2</sub>(OAc)<sub>4</sub>, it is proposed that initial formation of an azine precedes
1,2-addition by an organozinc reagent. This straightforward route
to the hydrazone products provides a useful method for preparing chiral
quaternary α-aminoesters or pyrazoles via the Paul–Knorr
condensation with 1,3-diketones. Crossover and deuterium labeling
experiments provide evidence for the mechanisms proposed
Selective Syntheses of Δ<sup>α,β</sup> and Δ<sup>β,γ</sup> Butenolides from Allylic Cyclopropenecarboxylates via Tandem Ring Expansion/[3,3]-Sigmatropic Rearrangements
Allylic cyclopropenecarboxylates undergo ring expansion reactions to give 2-allyloxyfuran intermediates, which subsequently rearrange to Δ<sup>β,γ</sup> butenolides via a Claisen rearrangement or to the corresponding Δ<sup>α,β</sup> butenolides via further Cope rearrangement. Also described are methods for chirality transfer in the rearrangement of nonracemic allylic esters
Selective Syntheses of Δ<sup>α,β</sup> and Δ<sup>β,γ</sup> Butenolides from Allylic Cyclopropenecarboxylates via Tandem Ring Expansion/[3,3]-Sigmatropic Rearrangements
Allylic cyclopropenecarboxylates undergo ring expansion reactions to give 2-allyloxyfuran intermediates, which subsequently rearrange to Δ<sup>β,γ</sup> butenolides via a Claisen rearrangement or to the corresponding Δ<sup>α,β</sup> butenolides via further Cope rearrangement. Also described are methods for chirality transfer in the rearrangement of nonracemic allylic esters
Rh-Catalyzed Intermolecular Reactions of Cyclic α-Diazocarbonyl Compounds with Selectivity over Tertiary C–H Bond Migration
Intermolecular Rh-catalyzed reactions of cyclic α-diazocarbonyl
compounds with chemoselectivity over β-hydride elimination are
described. These methods represent the first general intermolecular
reactions of Rh-carbenoids that are selective over tertiary β-C–H
bond migration. Successful transformations include cyclopropanation,
cyclopropenation, and various X–H insertion reactions with
a broad scope of substrates. We propose that the intermolecular approach
of substrates to carbenes from acyclic diazo precursors may be relatively
slow due to a steric interaction with the ester function, which is
perpendicular to the π-system of the carbene. For carbenes derived
from five- and six-membered cyclic α-diazocarbonyls, it is proposed
that the carbene is constrained to be more conjugated with the carbonyl,
thereby relieving the steric interaction for intermolecular reactions,
and accelerating the rate of intermolecular reactivity relative to
intramolecular β-hydride migration. However, attempts to use
α-diazo-β-ethylcaprolactone in intermolecular cyclopropanation
with styrene were unsuccessful. It is proposed that the conformational
flexibility of the seven-membered ring allows the carbonyl to be oriented
perpendicular to Rh-carbene. The significant intermolecular interaction
between the carbonyl and approaching substrate is in agreement with
the poor ability of α-diazo-β-ethylcaprolactone to participate
in intermolecular cyclopropanation reactions. DFT calculations provide
support for the mechanistic proposals that are described
Rh-Catalyzed Intermolecular Reactions of Cyclic α-Diazocarbonyl Compounds with Selectivity over Tertiary C–H Bond Migration
Intermolecular Rh-catalyzed reactions of cyclic α-diazocarbonyl
compounds with chemoselectivity over β-hydride elimination are
described. These methods represent the first general intermolecular
reactions of Rh-carbenoids that are selective over tertiary β-C–H
bond migration. Successful transformations include cyclopropanation,
cyclopropenation, and various X–H insertion reactions with
a broad scope of substrates. We propose that the intermolecular approach
of substrates to carbenes from acyclic diazo precursors may be relatively
slow due to a steric interaction with the ester function, which is
perpendicular to the π-system of the carbene. For carbenes derived
from five- and six-membered cyclic α-diazocarbonyls, it is proposed
that the carbene is constrained to be more conjugated with the carbonyl,
thereby relieving the steric interaction for intermolecular reactions,
and accelerating the rate of intermolecular reactivity relative to
intramolecular β-hydride migration. However, attempts to use
α-diazo-β-ethylcaprolactone in intermolecular cyclopropanation
with styrene were unsuccessful. It is proposed that the conformational
flexibility of the seven-membered ring allows the carbonyl to be oriented
perpendicular to Rh-carbene. The significant intermolecular interaction
between the carbonyl and approaching substrate is in agreement with
the poor ability of α-diazo-β-ethylcaprolactone to participate
in intermolecular cyclopropanation reactions. DFT calculations provide
support for the mechanistic proposals that are described
Rh-Catalyzed Intermolecular Reactions of Cyclic α-Diazocarbonyl Compounds with Selectivity over Tertiary C–H Bond Migration
Intermolecular Rh-catalyzed reactions of cyclic α-diazocarbonyl
compounds with chemoselectivity over β-hydride elimination are
described. These methods represent the first general intermolecular
reactions of Rh-carbenoids that are selective over tertiary β-C–H
bond migration. Successful transformations include cyclopropanation,
cyclopropenation, and various X–H insertion reactions with
a broad scope of substrates. We propose that the intermolecular approach
of substrates to carbenes from acyclic diazo precursors may be relatively
slow due to a steric interaction with the ester function, which is
perpendicular to the π-system of the carbene. For carbenes derived
from five- and six-membered cyclic α-diazocarbonyls, it is proposed
that the carbene is constrained to be more conjugated with the carbonyl,
thereby relieving the steric interaction for intermolecular reactions,
and accelerating the rate of intermolecular reactivity relative to
intramolecular β-hydride migration. However, attempts to use
α-diazo-β-ethylcaprolactone in intermolecular cyclopropanation
with styrene were unsuccessful. It is proposed that the conformational
flexibility of the seven-membered ring allows the carbonyl to be oriented
perpendicular to Rh-carbene. The significant intermolecular interaction
between the carbonyl and approaching substrate is in agreement with
the poor ability of α-diazo-β-ethylcaprolactone to participate
in intermolecular cyclopropanation reactions. DFT calculations provide
support for the mechanistic proposals that are described
Rh(II)-Catalyzed Reactions of Diazoesters with Organozinc Reagents
RhÂ(II)-catalyzed reactions of diazoesters
with organozinc reagents
are described. Diorganozinc reagents participate in reactions with
diazo compounds by two distinct, catalyst-dependent mechanisms. With
bulky diisopropylethyl acetate ligands, the reaction mechanism is
proposed to involve initial formation of a Rh-carbene and subsequent
carbozincation to give a zinc enolate. With Rh<sub>2</sub>(OAc)<sub>4</sub>, it is proposed that initial formation of an azine precedes
1,2-addition by an organozinc reagent. This straightforward route
to the hydrazone products provides a useful method for preparing chiral
quaternary α-aminoesters or pyrazoles via the Paul–Knorr
condensation with 1,3-diketones. Crossover and deuterium labeling
experiments provide evidence for the mechanisms proposed
Facially Selective Cu-Catalyzed Carbozincation of Cyclopropenes Using Arylzinc Reagents Formed by Sequential I/Mg/Zn Exchange
Described is a Cu-catalyzed directed carbozincation of
cyclopropenes
with organozinc reagents prepared by I/Mg/Zn exchange. This protocol
broadens the scope with respect to functional group tolerance and
enables use of aryl iodide precursors, rather than purified diorganozinc
precursors. Critical to diastereoselectivity of the carbozincation
step is the removal of magnesium halide salts after transmetalation
with ZnCl<sub>2</sub>