Formation and reactions of monovalent carbon intermediates. III. Reaction of carbethoxymethyne with olefins

Abstract

Carbethoxymethyne (EtOCOC:) has been generated by the short wavelength photolysis of diethylmercury bisdiazoacetate, (N2CC02Et)2Hg + hv -* 2EtOCOC: + 2N2 + Hg, and its reactions with cyclohexene, c/s-2-butene, and rra«s-2-butene were investigated. The carbyne, formed in its 2 ground state, adds to the olefinic bond in a spin allowed but, according to INDO MO calculations, orbital symmetry forbidden concerted step with retention of the geometrical configuration of the parent olefin to yield a cyclopropyl radical. Parallel to addition, preferential insertion into the allylic C-H bonds also occurs yielding an alkyl radical. Thus, the basic chemical behavior of doublet ground state carbynes appears to bear a close resemblance to that of singlet carbenes. The cyclopropyl and alkyl radicals undergo free radical combination and hydrogen abstraction reactions with solvent molecules. The hydrogen abstraction reaction of cyclopropyl radicals is stereoselective and occurs from the sterically less hindered side of the ring, resulting in the preponderant formation of the less stable endo product. In the longer wavelength photolysis of the source compound N2 is eliminated in a stepwise fashion and a singlet state mercury carbene, which is capable of insertion and stereospecific addition, is formed. Cleavage of the C-Hg linkage in the source compound results in the formation of ethyl diazoacetate which was detected and monitored during the photolysis by infrared spectroscopy. The effect of conversion and wavelength of photolysis on product yields and the relative importance of the various competing primary modes of decomposition were investigated and the overall mechanism is discussed.arbon free radicals constitute the most common and frequently encountered class of organic intermediates. The chemistry of the trivalent, divalent, and to a lesser extent of the zerovalent carbon radicals, the simplest representatives of which are H3C•, H2C:, and the free carbon atom, respectively, comprises one of the fundamental pillars of modern organic chemistry. Little, however, is known about the family of monovalent carbon radicals, the carbynes. The simplest of

Formation and Characterization of a BeOBeC Multiple Radical Featuring a Quartet Carbyne Moiety

“…The observation of a quartet ground state carbyne is quite unusual, as the previously reported carbyne radicals have doublet ground state . The preference of a doublet ground state for carbyne radicals can be understood from the bonding of the methylidyne HC radical, which has a doublet 2 Π ground state with a quartet state lying 17.1 kcal mol −1 higher in energy .…”

“…Previous experimental and theoretical studies indicate that the reactivity patterns of the carbyne radicals in their doublet and quartet spin states are rather different . Such radicals in the doublet state with one unpaired electron and one electron pair on carbon are found to be very reactive toward alkenes and they can even insert into the well‐known inert C−H bonds.…”

“…By contrast, the quartet state radicals with three unpaired electrons prefer to react with radicals and are prone to H‐abstraction reactions. The experimentally known carbynes predominately have doublet electronic ground states . The possibility of tuning the ground state electronic spin from the low‐spin doublet to high spin quartet for carbynes has been theoretically discussed and predicted .…”

Formation and Characterization of a BeOBeC Multiple Radical Featuring a Quartet Carbyne Moiety

“…Theobservation of aquartet ground state carbyne is quite unusual, as the previously reported carbyne radicals have doublet ground state. [2,[9][10][11][12][13][14][15][16] Thep reference of ad oublet ground state for carbyne radicals can be understood from the bonding of the methylidyne HC radical, which has ad oublet 2 P ground state with aq uartet state lying 17.1 kcal mol À1 higher in energy. [31] Theg round state HC radical has an electron configuration of (1s) 2 (2s) 2 (3s) 2 (1p) 1 .…”

“…[10][11][12][13][14][15][16] Such radicals in the doublet state with one unpaired electron and one electron pair on carbon are found to be very reactive toward alkenes and they can even insert into the well-known inert C À Hb onds.Byc ontrast, the quartet state radicals with three unpaired electrons prefer to react with radicals and are prone to H-abstraction reactions.T he experimentally known carbynes predominately have doublet electronic ground states. [2,[9][10][11][12][13][14][15][16] Thep ossibility of tuning the ground state electronic spin from the low-spin doublet to high spin quartet for carbynes has been theoretically discussed and predicted. [17][18][19] Chemical substitutions with greater electropositivity (s-donation) and p-acceptance of the single substituent were predicted to favor the high-spin state.S ome carbynes with quartet ground spin state were predicted.…”