Hydrogenolysis reactions of so-called lignin model dimers using a Ru-xantphos catalyst are presented (xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene). For example, of some nine models studied, the alcohol, 2-(2-methoxyphenoxy)-1-phenylethanol (1), with 5 mol% Ru(H)2(CO)(PPh3)(xantphos) (18) in toluene-d8 at 135 °C for 20 h under N2, gives in ∼95% yield the C–O cleavage hydrogenolysis products, acetophenone (14) and guaiacol (17), and a small amount (<5%) of the ketone, 2-(2-methoxyphenoxy)-1-phenylethanone (4), as observed by 1H NMR spectroscopy. The in situ Ru(H)2(CO)(PPh3)3/xantphos system gives similar findings, confirming a recent report (J. M. Nichols et al., J. Am. Chem. Soc., 2010, 132, 12554). The active catalyst is formulated ‘for convenience’ as ‘Ru(CO)(xantphos)’. The hydrogenolysis mechanism proceeds by initial dehydrogenation to give the ketone 4, which then undergoes hydrogenolysis of the C–O bond to give 14 and 17. Hydrogenolysis of 4 to 14 and 17 also occurs using the Ru catalyst under 1 atm H2; in contrast, use of 3-hydroxy-2-(2-methoxyphenoxy)-1-phenyl-1-propanone (7), for example, where the CH2 of 4 has been changed to CHCH2OH, gives a low yield (≤15%) of hydrogenolysis products. Similarly, the diol substrate, 2-(2-methoxyphenoxy)-1-phenyl-1,3-propanediol (9), gives low yields of hydrogenolysis products. These low yields are due to formation of the catalytically inactive complexes Ru(CO)(xantphos)[C(O)C(OC6H4OMe)C(Ph)O] (20) and/or Ru(CO)(xantphos)[C(O)CHC(Ph)O] (21), where the organic fragments result from dehydrogenation of CH2OH moieties in 7 and 9. Trace amounts of Ru(CO)(xantphos)(OC6H4O), a catecholate complex, are isolated from the reaction of 18 with 1. Improved syntheses of 18 and lignin models are also presented.

Hydrogenolysis of β-O-4 lignin model dimers by a ruthenium-xantphos catalyst.

Flash vacuum pyrolysis (FVP) of azides is an extremely valuable method of generating nitrenes and studying their thermal rearrangements. The nitrenes can in many cases be isolated in low-temperature matrices and observed spectroscopically. NH and methyl, alkyl, aralkyl, vinyl, cyano, aryl and N-heteroaryl, acyl, carbamoyl, alkoxycarbonyl, imidoyl, boryl, silyl, phosphonyl, and sulfonyl nitrenes are included. FVP of triazoloazines generates diazomethylazines and azinylcarbenes, which often rearrange to the energetically more stable arylnitrenes. N2 elimination from monocyclic 1,2,3-triazoles can generate iminocarbenes, 1H-azirines, ketenimines, and cyclization products, and 1,2,4-triazoles are precursors of nitrile ylides. Benzotriazoles are preparatively useful precursors of cyanocyclopentadienes, carbazoles, and aza-analogues. FVP of 5-aryltetrazoles can result in double N2 elimination with formation of arylcarbenes or of heteroarylcarbenes, which again rearrange to arylnitrenes. Many 5-substituted and 2...

http://pubs.acs.org/doi/pdf/10.1021/acs.chemrev.6b00738

Flash Vacuum Pyrolysis of Azides, Triazoles, and Tetrazoles

Chemistry remains an experimental science. The theory of chemical bonding leaves much to be desired. Yet, the past 20 years have been marked by a fruitful symbiosis of organic chemistry and molecular orbital theory. Of necessity this has been a marriage of poor theory with good experiment. Tentative conclusions have been arrived a t on the basis of theories which were such a patchwork on approximations that they appeared to have no right to work; yet, in the hands of clever experimentalists, these ideas were transformed into novel molecules with unusual properties. In the same way, by utilizing the most simple but fundamental concepts of molecular orbital theory we have in the past 3 years been able to rationalize and predict the stereochemical course of virtually every concerted organic reaction.' In our work we have relied on the most basic ideas of molecular orbital theory-the concepts of symmetry, overlap, interaction, bonding, and the nodal structure of wave functions. The lack of numbers in our discussion is not a weakness-it is its greatest strength. Precise numerical values would have to result from some specific sequence of approximations. But an argument from first principles or symmetry, of necessity qualitative, is in fact much stronger than the deceptively authoritative numerical result. For, if the simple argument is true, then any approximate method, as well as the now inaccessible exact solution, must obey it. The simplest description of the electronic structure of a stable molecule is that i t is characterized by a finite band of doubly occupied electronic levels, called bonding orbitals, separated by a gap from a corresponding band of unoccupied, antiboding levels as well as a continuum of higher levels. The magnitude of the gap may range from 40 kcal/mole for highly delocalized, large aromatic systems to 250 kcal/mole for saturated hydrocarbons. It should be noted in context that socalled nonbonding electrons of heteroatoms are in fact bonding. Consider a simple reaction of two molecules to give a third species, proceeding in a nonconcerted manner through a diradical intermediate I. A + B + [I] + C

The Conservation of Orbital Symmetry (福井謙一とフロンティア軌導理論) -- (参考論文)

Thermochemistry of aromatic ketones. Experimental enthalpies of formation and structural effects

The enthalpies of formation of two dibenzocyclooctadienones

The standard enthalpies of formation of some dibenzocycloheptane ketones

Flow-vacuum pyrolysis of three dibenzocycloalkanones

Hydrogenolysis of 1,1a,6,10b-tetrahydro-1,6-methanodibenzo[ a , e ]cyclopropa[ c ] cycloheptene ( I ) over Ru-beta and Ru-Y zeolites is reported. The catalysts were prepared via impregnation of commercial beta and Y zeolites using Ru(NH 3 ) 6 Cl 3 , RuCl 3 or Ru(acac) 3 , as precursors. The catalysts were characterized by chemical analysis, adsorption–desorption isotherms of N 2 at 77 K, hydrogen chemisorption and temperature programmed oxidation (TPO). Hydrogenolysis of ( I ) was carried out in methanol or 2-propanol, as solvent, at temperatures in the range 60–100 °C. The reaction was mainly influenced by the type of zeolite and solvent.

Anca Banciu

Papers

The standard enthalpies of formation of some dibenzocycloheptane ketones

Flow-vacuum pyrolysis of three dibenzocycloalkanones

Hydrogenolysis of 1,1a,6,10b-tetrahydro-1,6-methanodibenzo[a,e]cyclopropa[c] cycloheptene over Ru-zeolites

Flash vacuum pyrolysis (FVP) on zeolites: Pyrolysis of 5-β (hydroxyethyl)-5H-dibenzo[a,d]cycloheptene

Flow-vacuum pyrolysis of 5-methylene-5H-dibenzo[a,d]cycloheptene