Conspectus The ynamide functional group activates carbon-carbon triple bonds through an attached nitrogen atom that bears an electron-withdrawing group. As a result, the alkyne has both electrophilic and nucleophilic properties. Through the selection of the electron-withdrawing group attached to nitrogen chemists can modulate the electronic properties and reactivity of ynamides, making these groups versatile synthetic building blocks. The reactions of ynamides also lead directly to nitrogen-containing products, which provides access to important structural motifs found in natural products and molecules of medicinal interest. Therefore, researchers have invested increasing time and research in the chemistry of ynamides in recent years. This Account surveys and assesses new organic transformations involving ynamides developed in our laboratory and in others around the world. We showcase the synthetic power of ynamides for rapid assembly of complex molecular structures. Among the recent reports of ynamide transformations, ring-forming reactions provide a powerful tool for generating molecular complexity quickly. In addition to their synthetic utility, such reactions are mechanistically interesting. Therefore, we focus primarily on the cyclization chemistry of ynamides. This Account highlights ynamide reactions that are useful in the rapid synthesis of cyclic and polycyclic structural manifolds. We discuss the mechanisms active in the ring formations and describe representative examples that demonstrate the scope of these reactions and provide mechanistic insights. In this discussion we feature examples of ynamide reactions involving radical cyclizations, ring-closing metathesis, transition metal and non-transition metal mediated cyclizations, cycloaddition reactions, and rearrangements. The transformations presented rapidly introduce structural complexity and include nitrogen within, or in close proximity to, a newly formed ring (or rings). Thus, ynamides have emerged as powerful synthons for nitrogen-containing heterocycles and nitrogen-substituted rings, and we hope this Account will promote continued interest in the chemistry of ynamides.

[1] For the 2008 Beijing Olympic Games full-scale control (FSC) of atmospheric pollution was implemented to improve the air quality from 20 July to 20 September 2008, resulting in a significant decrease in the emission of pollutants in urban Beijing, especially vehicular emissions. The combination of reduced emissions and weather condition changes provided us with a unique opportunity to investigate urban atmospheric chemistry. Hydrogen peroxide (H 2 O 2 ) and organic peroxides play significant roles in atmospheric processes, such as the cycling of HO x radicals and the formation of secondary sulfate aerosols and secondary organic aerosols. We measured atmospheric H 2 O 2 and organic peroxides in urban Beijing, at the Peking University campus, from 12 July to 30 September, before and during the FSC. The major peroxides observed were H 2 O 2 , methyl hydroperoxide (MHP), and peroxyacetic acid (PAA), having maximal mixing ratios of 2.34, 0.95, and 0.17 ppbv (parts per billion by volume), respectively. Other organic peroxides were detected occasionally, such as bis-hydroxymethyl hydroperoxide, hydroxymethyl hydroperoxide, ethyl hydroperoxide, and 1-hydroxyethyl hydroperoxide. On sunny days the concentrations of H 2 O 2 , MHP, and PAA exhibited pronounced diurnal variations, with a peak in the afternoon (1500-1900) and, occasionally, a second peak in the evening (2000-0200). The night peaks can be attributed to local night production from the ozonolysis of alkenes, coupled with the reaction between NO 3 radicals and organic compounds. Sunny-day weather dominated during 16-26 July, and we found that the concentrations of H 2 O 2 , MHP, and PAA increased strikingly on 22-26 July, compared with the concentrations during 16-19 July. This effect was mainly attributed to the NO x (NO and NO 2 ) decline because of the FSC, due to (i) the suppressing effect of NO and NO 2 on the production of peroxides and (ii) the indirect effect of reduced NO x on the concentration of peroxides via O 3 production in the volatile organic compound-sensitive area. Although the time period from 29 July to 15 August fell within the FSC, the concentrations of H 2 O 2 , MHP, and PAA decreased significantly. This can be explained by a combination of chemical and physical factors during this period, when rainy-and cloudy-day weather dominated. Weaker irradiation and lower temperatures resulted in a lower photochemical production of peroxides; the higher humidity resulted in their greater loss through their aqueous-phase oxidation of S(IV) and through heterogeneous removal, and lower temperatures and higher nighttime humidity resulted in a quicker surface deposition of peroxides. Furthermore, our observations seem to imply that the heterogeneous removal of H 2 O 2 is faster than that of MHP, as indicated by the strong negative correlation between the H 2 O 2 -to-MHP ratio and the aerosol surface area.

Abstract. Peroxyacetic acid (PAA) is one of the most important atmospheric organic peroxides, which have received increasing attention for their potential contribution to the oxidation capacity of the troposphere and the formation of secondary aerosols. We report here, for the first time, a series of data for atmospheric PAA concentrations at urban and rural sites, from five field campaigns carried out in China in summer 2006, 2007 and 2008. For these five measurements, daytime mean (08:00-20:00 LT) PAA concentrations on sunlit days were 21.4-148.0 pptv with a maximum level of ∼1 ppbv. The various meteorological and chemical parameters influencing PAA concentrations were examined using Principal Factor Analysis. This statistical analysis shows that the local photochemical production was the major source of PAA, and its concentration increased with increasing temperature, solar radiation and ozone but decreased with increasing NO x (NO and NO 2 ), CO, SO 2 , and relative humidity. Based on the dataset, several issues are highlighted in this study: (i) Because PAA is a product from the photochemical oxidation of some specific volatile organic compounds (VOCs) that lead to acetyl peroxy radicals, the importance of various VOCs with respect to the PAA formation is therefore ranked using the incremental reactivity method. (ii) The contribution of PAN thermal degradation to PAA formation under conditions of different NO x concentrations is estimated based on the chemical kinetics analysis. The result shows that PAN seems to play an important role in the formation of PAA when the NO/NO 2 concentration ratio was less than 0.2 and PAA would correspondingly have feedback on the PAN-NO x cycle. (iii) PAA and other peroxides, such as methyl Correspondence to: Z. M. Chen (zmchen@pku.edu.cn) hydroperoxide (MHP) and H 2 O 2 , usually exhibited a similar asymmetric shape typically shifted to the afternoon. However, under some conditions, H 2 O 2 diurnal cycle was out of phase with MHP and PAA. The combination of linear regression and kinetics analysis indicate that the formation and removal processes of H 2 O 2 may be different from those of MHP and PAA. (iv) Considering that PAA is the reservior of free radicals, its fate is expected to have an effect on the free radical budget in the atmosphere. A box model based on the CBM-IV mechanism has been performed to access its influence on the radical budget. We suggest that the detailed information on PAA in the atmosphere is of importance to better understand the free radical chemistry.

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