Radical cations of strained ring hydrocarbons. Cyclopropane derivatives

Abstract

Bell L[zbortrtorie.s, M~r r r r r~ H i l l . NJ 07974. U.S.A . Receivcd July 27, 1982This l~trper is tletlicnted to Professor Htrrry E . G L~I I I I~I I~ 011 the oc.ctr~ioll Of his 65th birth(1tly HEINZ D. ROTH and MARCIA L. MANION SCHILLING. Can. J. Chern. 61, 1027(1983.A series of radical cations has been generated from various cyclopropane derivatives by photoinitiated electron transfcr to chloranil. Nuclcar spin polarization effects observed during these reactions have been interpreted as evidence for the structure of these intermediates. Tri-and tetraalkylsubstituted cyclopropanes (1, 3. 4) form "trirnethylene" radical cations in which the most highly substituted carbon-carbon bond is broken (or weakened). Thc benzocyclopropanorbornene (12) forms a radical cation in which spin and charge are located in the benzenoid ring; the tertiary cyclopropane protons are coupled by a homohyperconjugative interaction; otherwise, the cyclopropane ring is unaffected. To the radical cation derived from bisrnethanoparacyclophane (17) a trirnethylene structure is assigned in which the benzylic carbons are pyramidal. Finally, benzonorcaradiene (20) forms a radical cation in which the two secondary-tertiary bonds are lengthened and in which the rnethylene group bears spin density. Chcrn. 61, 1027Chcrn. 61, (1983.On a produit une sCrie de cations radicalaires a partir de divers derives du cyclopropane en utilisant un transfert d'Clcctron photoinduit sur le chloranil. On a interprCtC les effets de polarisation de spin nucleaire observCs dans ces reactions cornrne une preuve de structure de ces interrnidiaires. Les cyclopropanes portant dcs substituants tri et titraalkylcs (1, 3, 4) forrnent des cations radicalaires "trirnCthylkne" dans lesquels la liaison carbone-carbone la plus substituCe est rornpue (ou affaiblie). Le benzocyclopropanorbornknc (12) forrne un cation radicalaire dans lequel le spin et la chargc sont IocalisCs dans le cycle benzenique; les protons tertiaires du cyclopropane sont couplts par une interaction hornohyperconjuguCe sans quoi le cycle cyclopropane n'est pas affect& Au cation radicalaire provenant du birnCthanoparacyclophane (17) on a attribuk une structure trirnkthylkne dans laquelle les carbones benzyliques sont pyrarnidaux. Finalcrnent le benzonorcaradi2ne (20) forrne un cation radicalaire dans lequel deux liaisons secondaire-tertiaires sont allongees et dans lequel le groupe rnkthylkne porte la densite de spin.[Traduit par le journal]For several years we have been interested in photoinitiated electron transfer reactions and have utilized chemically induced nuclear polarization (CIDNP) effects to gain an insight into the structure and reactivity of short-lived radical cations which can be generated in this type of reaction (1, 2). The CIDNP technique is based on the interpretation of greatly enhanced nmr signals in either absorption or emission, which may be observed during reactions proceeding via pairs of radicals or radical ions (3). According to the generally accepted radical pair theory the dir...

Radical cations of disubstituted cyclopropanes: stereoelectronic effects on hyperfine coupling

“…90 MHz 1 H CIDNP setup and sample preparation have been described previously . 200 MHz 1 H NMR and CIDNP experiments were carried out on a Bruker AVANCE 200 MHz spectrometer equipped with a CIDNP probe head.…”

“…Structure type B is less common and, therefore, of special interest . The type B structure can be stabilized by conjugation (as in vinylcyclopropane or phenylcyclopropane, viz., 1 •+ ) or homoconjugation (as in norcaradiene derivatives, viz., 2 •+ ) . Hyperconjugation (e.g.…”

“…[7][8][9][10] The 2 A 1 state (orbital s singly occupied) relaxes to a trimethylene structure with one lengthened one-electron C-C bond (type A) whereas the 2 B 2 state forms a structure type resembling a π complex (type B; two lengthened C-C bonds). Structure type A is lowest in energy for many cyclopropane radical cations, [1,3,4,[11][12][13][14][15][16][17][18][19] including the parent. [20] An ESR spectrum at 4.2 K shows two strongly coupled 1 H nuclei (isotropic hyperfine coupling constant, hfcc, a H = 20.4 G, β-protons) and four 1 H nuclei with a smaller splitting (a H = À11.7 G, α-protons), [20] clearly supporting the 2 A 1 structure.…”

“…[15] The type B structure can be stabilized by conjugation (as in vinylcyclopropane [23] or phenylcyclopropane, viz., 1 •+ ) [24,25] or homoconjugation (as in norcaradiene derivatives, viz., 2 •+ ). [15,26] Hyperconjugation [27,28] (e.g. in 1,1-dimethylcyclopropane radical cation, 3 •+ ) also stabilizes the type B structure: it is lower in energy than the corresponding type A structure; however, the putative type B structure is not a stationary point; it undergoes a secondorder Jahn-Teller type distortion to a lower-energy scalene triangular structure.…”

Regioselective Oxidative Ring Opening of Cyclopropyl Silyl Ethers: A Quantum Chemical Study

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Interestingly aliphatic or aromatic substituted cyclopropyl radical cations are-in absence of nucleophiles-structurally and configurationally stable. This has been demonstrated by CIDNP (chemically induced dynamic nuclear polarization) and by other transient spectroscopic experiments based on photoinduced electron transfer (PET).…”