A general catalytic methodology for the synthesis of pyrazolines from α‐diazo compounds and conjugated alkenes is reported. The direct hydrogen atom transfer (HAT) process of α‐diazo compounds promoted by the
This content will become publicly available on October 4, 2024
Carbon‐centered radicals stabilized by adjacent boron atoms are underexplored reaction intermediates in organic synthesis. This study reports the development of vinyl cyclopropyl diborons (VCPDBs) as a versatile source of previously unknown homoallylic α,α‐diboryl radicals via thiyl radical catalyzed diboron‐directed ring opening. These diboryl stabilized radicals underwent smooth [3+2] cycloaddition with a variety of olefins to provide diboryl cyclopentanes in good to excellent diastereoselectivity. In contrast to the
- Award ID(s):
- 2054897
- NSF-PAR ID:
- 10474445
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Chemistry – A European Journal
- Volume:
- 30
- Issue:
- 2
- ISSN:
- 0947-6539
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
Generation of Diazomethyl Radicals by Hydrogen Atom Abstraction and Their Cycloaddition with Alkenesmore » « less
Abstract tert ‐butylperoxy radical generates electrophilic diazomethyl radicals, thereby reversing the reactivity of the carbon atom attached with the diazo group. The regiocontrolled addition of diazomethyl radicals to carbon‐carbon double bonds followed by intramolecular ring closure on the terminal diazo nitrogen and tautomerization affords a diverse set of pyrazolines in good yields with excellent regioselectivity. This strategy overcomes the limitations of electron‐deficient alkenes in traditional dipolar [3+2]‐cycloaddition of α‐diazo compounds with alkenes. Furthermore, the straightforward formation of the diazomethyl radicals provides umpolung reactivity, thus opening new opportunities for the versatile transformations of diazo compounds. -
Generation of Diazomethyl Radicals by Hydrogen Atom Abstraction and Their Cycloaddition with Alkenesmore » « less
Abstract A general catalytic methodology for the synthesis of pyrazolines from α‐diazo compounds and conjugated alkenes is reported. The direct hydrogen atom transfer (HAT) process of α‐diazo compounds promoted by the
tert ‐butylperoxy radical generates electrophilic diazomethyl radicals, thereby reversing the reactivity of the carbon atom attached with the diazo group. The regiocontrolled addition of diazomethyl radicals to carbon‐carbon double bonds followed by intramolecular ring closure on the terminal diazo nitrogen and tautomerization affords a diverse set of pyrazolines in good yields with excellent regioselectivity. This strategy overcomes the limitations of electron‐deficient alkenes in traditional dipolar [3+2]‐cycloaddition of α‐diazo compounds with alkenes. Furthermore, the straightforward formation of the diazomethyl radicals provides umpolung reactivity, thus opening new opportunities for the versatile transformations of diazo compounds. -
more » « less
Abstract The electrochemical reduction of several α,β ‐epoxyketones was studied using cyclic (linear sweep) voltammetry, convolution voltammetry, and homogeneous redox catalysis. The results were reconciled to pertinent theories of electron transfer. α,β ‐Epoxyketones undergo dissociative electron‐transfer reactions with C−O bond cleavage, via both stepwise and concerted mechanisms, depending on their structure. For aliphatic ketones, the preferred mechanism of reduction is consistent with the “sticky” concerted model for dissociative electron transfer. Bond cleavage occurs simultaneously with electron transfer, and there is a residual, electrostatic interaction in the ring‐opened (distonic) radical anion. In contrast, for aromatic ketones, because the ring‐closed radical anions are resonance‐stabilized and exist at energy minima, a stepwise mechanism operates (electron transfer and bond cleavage occur in discrete steps). The rate constants for ring opening are on the order of 108 s−1, and not significantly affected by substituents on the 3‐membered ring (consistent with C−O bond cleavage). These results and conclusions were fully supported and augmented by molecular orbital calculations.
-
more » « less
Abstract Dinuclear manganese hydride complexes of the form [Mn2(CO)8(
μ ‐H)(μ ‐PR2)] (R=Ph,1 ; R=i Pr,2 ) were used inE ‐selective alkyne semi‐hydrogenation (E ‐SASH) catalysis. Catalyst speciation studies revealed rich coordination chemistry and the complexes thus formed were isolated and in turn tested as catalysts; the results underscore the importance of dinuclearity in engendering the observedE ‐selectivity and provide insights into the nature of the active catalyst. The insertion product obtained from treating2 with (cyclopropylethynyl)benzene contains acis ‐alkenyl bridging ligand with the cyclopropyl ring being intact. Treatment of this complex with H2affords exclusivelytrans ‐(2‐cyclopropylvinyl)benzene. These results, in addition to other control experiments, indicate a non‐radical mechanism forE ‐SASH, which is highly unusual for Mn−H catalysts. The catalytically active species are virtually inactive towardscis totrans alkene isomerization indicating that theE ‐selective process is intrinsic and dinuclear complexes play a critical role. A reaction mechanism is proposed accounting for the observed reactivity which is fully consistent with a kinetic analysis of the rate limiting step and is further supported by DFT computations. -
more » « less
Abstract A new diastereoselective route to 2‐aminotetrahydrofurans has been developed from
N ,O ‐dialkenylhydroxylamines. These intermediates undergo a spontaneous C−C bond‐forming [3,3]‐sigmatropic rearrangement followed by a C−O bond‐forming cyclization. A copper‐catalyzedN ‐alkenylation of anN ‐Boc‐hydroxylamine with alkenyl iodides, and a base‐promoted addition of the resultingN ‐hydroxyenamines to an electron‐deficient allene, provide modular access to these novel rearrangement precursors. The scope of this de novo synthesis of simple nucleoside analogues has been explored to reveal trends in diastereoselectivity and reactivity. In addition, a base‐promoted ring‐opening and Mannich reaction has been discovered to covert 2‐aminotetrahydrofurans to cyclopentyl β‐aminoacid derivatives or cyclopentenones.
