Cation radical cycloadditions and related sigmatropic reactions

In the past three decades, many studies on the production of biofuels and other chemicals have been conducted using renewable sources such as lignocellulosic biomass. Lignocellulosic biomasses are abundantly available in most countries and furthermore they are carbon neutral. However, the main problem in utilizing lignocellulosic materials lies in the recalcitrance of its bonding. This review provides a comprehensive overview and a brief discussion on producing biofuel and valuable chemicals from lignocellulose biomass. Various aspects of the physical, chemical, thermophysical, thermochemical and biological pretreatment of lignocellulosic materials are discussed in this review. The success in biofuel and chemical production strongly depends on the pretreatment method used. Overall, pretreatment is the major step in the successful production of valuable products from lignocellulosic biomass.

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Pretreatment and conversion of lignocellulose biomass into valuable chemicals

Carr−Purcell−Meiboom−Gill (CPMG) relaxation measurements employing trains of 180° pulses with variable pulse spacing provide valuable information about systems undergoing millisecond-time-scale chemical exchange. Fits of the CPMG relaxation dispersion profiles yield rates of interconversion, relative populations, and absolute values of chemical shift differences between the exchanging states, |Δω|. It is shown that the sign of Δω that is lacking from CPMG dispersion experiments can be obtained from a comparison of chemical shifts in the indirect dimensions in either a pair of HSQC (heteronuclear single quantum coherence) spectra recorded at different magnetic fields or HSQC and HMQC (heteronuclear multiple quantum coherence) spectra obtained at a single field. The methodology is illustrated with an application to a cavity mutant of T4 lysozyme in which a leucine at position 99 has been replaced by an alanine, giving rise to exchange between ground state and excited state conformations with a rate on the o...

Reconstructing NMR spectra of "invisible" excited protein states using HSQC and HMQC experiments.

Carbohydrates, with one oxygen atom per carbon, are actually too oxygen-rich relative to many important oxygenated organics. Removal of carbohydrate hydroxyl groups would therefore be useful, particularly if a different reactive functional group could be introduced. This has now been accomplished via a new catalytic reaction, the stereospecific deoxydehydration of diols and polyols to alkenes and allylic alcohols. In summary, the present catalytic method for net didehydroxylation of diols is potentially competitive in many cases with the standard Corey-Winter approach, treatment of the diol thionocarbonate with phosphite reagents at elevated temperatures. Other metal diolate complexes are known to extrude alkene at room temperature. Hence we believe that it should be possible to develop related catalytic cycles for net diol didehydroxylation that function under milder conditions, comparable to the best current stoichiometric methods. Finally, catalytic cycles utilizing more inexpensive, environmentally friendly reductants, such as hydrogen or carbon monoxide, would facilitate bulk carbohydrate deoxydehydration reactions to produce desirable oxygenated organics. 24 refs., 1 fig.

Toward Nonoxidative Routes to Oxygenated Organics: Stereospecific Deoxydehydration of Diols and Polyols to Alkenes and Allylic Alcohols Catalyzed by the Metal Oxo Complex (C5Me5)ReO3

Redox induced radical and radical ionic carbon-carbon bond forming reactions

The classical example of a neutral carbon-centered radical cyclization reaction is the regioselective 1,5-ring closure (exocyclization) of the 5-hexenyl radical to the cyclopentylcarbinyl radical. Here the authors report the title reaction, a comparable addition process whereby an ..cap alpha.., omega-diene radical cation reacts by endocyclization and hydrogen shift(s) to produce a cycloolefin radical cation.

Radical cation cyclization of 1,5-hexadiene to cyclohexene via the cyclohexane-2,5-diyl radical cation intermediate

ESR evidence for the stereospecific formation of the chair cyclohexane-1,4-diyl radical cation from both bicyclo[2.2.0]hexane and 1,5-hexadiene

ESR results are reported that demonstrate that the rigid 2,3-diazabicyclo(2.2.2)oct-2-ene (1) radical cation has the expected sigma structure with a b{sub 2}(n-)SOMO in C{sub 2} {sigma} symmetry. The effects of gamma radiation on the above mentioned radical in Freon matrices are reported. Photoconversion of 1 to the cyclohexene radical cation was also observed in CF{sub 2}ClCFCl{sub 2} and CFCl{sub 2}CFCl{sub 2}, and reaction schemes are suggested. 17 refs., 2 figs., 1 tab.

Qing Xiang Guo

Papers

Radical cation cyclization of 1,5-hexadiene to cyclohexene via the cyclohexane-2,5-diyl radical cation intermediate

ESR evidence for the stereospecific formation of the chair cyclohexane-1,4-diyl radical cation from both bicyclo[2.2.0]hexane and 1,5-hexadiene

ESR study of the 2,3-diazabicyclo[2.2.2]oct-2-ene radical cation in Freon matrixes

Reinvestigation of the hexamethyl (Dewar benzene) radical cation in Freon matrices

Radical cation Cope rearrangement of 1,5-hexadiyne to 1,2,4,5-hexatetraene (bis(allene)) at 77 K