This review summarizes the recent developments in transition metal-catalyzed 1,n-enyne cyclization reactions that have been reported in the literature from January 1, 2002 to April 30, 2005. The cyclizations described herein include four parts according the reaction mechanism, i. e. cyclization initiated by oxidative cyclometallation; by activation of the triple bond to form a vinyl metal species; by electrophilic activation of the triple bond; and by activation of the double bond. Enyne cyclizations that do not follow the above reaction mechanism are not included. II-1 CYCLIZATION VIA CYCLOMETALLATION PATHWAYCyclometallation pathway is very popular in transition metal-catalyzed enyne cyclization reaction. Palladium [6], Ruthenium [7], Titanium [8], Platinum [9], Nickel [10], and Cobalt [11], could catalyze such a transformation.Zhang's group [12] has been working in developing an asymmetric rhodium-catalyzed 1,6-enyne cyclization reaction, which proceeds via cyclometallation pathway. Since their first publication in 2000 [12a], an excellent method to construct five-membered ring compounds with perfect enantioselectivity was developed [12c-f].Treatment of compound 1 in the absence of a phosphane ligand, [Rh(cod)Cl]2 was an ineffective catalyst for this reaction at either room temperature or temperature up to 65 o C. Using Cn-tunephos as ligand, high catalytic efficiency was observed. When rac-C4-tunephos was used as the ligand in the presence of [Rh(cod)Cl]2 and AgSbF6, high conversion and high yields were obtained at room temperature within 20 min. (Scheme 2). This is quite different from their earlier protocol using [Rh(bisphosphine)Cl]2 as catalytic precursor, in which [Rh(binap)Cl]2 was inactive towards this reaction. Under the modified reaction conditions, a wide spectrum of oxygen-tethered 1,6-enyne substrates with a Z-configured double bond were cyclized to give substituted tetrahydrofuran with high yields (82-98%) and excellent enantioselectivities (99.0-99.9% ee). Scheme 2.

In aerated ponds, oxygen is generally supplied through either diffused or mechanical aeration means. Surface transfer and bubble transfer both contribute significantly to oxygen transfer in a diffused aeration system. In the present study, a liquid-film-forming apparatus (LFFA) is successfully developed on a laboratory scale to improve considerably the surface transfer via the unique liquid film transfer technique. The experimental results show that the volumetric mass transfer coefficient for LFFA alone is found to be as much as 5.3 times higher than that for water surface and that the total volumetric mass transfer coefficient for the liquid film aeration system increases by 37% in comparison with a conventional aeration system. Additionally, by tuning finely the structural parameters of the LFFA, it can also lead to high dissolved oxygen (DO) water with the DO percent saturation greater than 90%. More importantly, this result is accomplished by simply offering a single-pass aeration at a depth as shallow as 26 cm. As a result, the objective of economical energy consumption in aerated ponds can be realized by lowering the aeration depth without sacrificing the aeration efficiency. It is noteworthy that the data presented in this study are acquired either numerically or experimentally.

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