ABSTRACT
Reported here is the Rh and Zn cocatalyzed [4 + 2] cycloaddition of newly designed yne-vinylcyclobutanones, which can generate 5/6 or 6/6 bicyclic products with an all-carbon quaternary bridgehead center. The reaction has a broad scope and can realize chirality transfer from enantioenriched substrates to the cycloadducts. The key to the success of this [4 + 2] reaction is the introduction of a vinyl group to cyclobutanones, which helps the C-C cleavage of vinylcyclobutanones via oxidative addition. This C-C cleavage step is synergistically aided by Zn coordination to the carbonyl group of vinylcyclobutanones. Of the same importance, visual kinetic analysis and computational studies have been carried out to support the dual activation in the rate-determining C-C cleavage, to derive the rate law of the [4 + 2] reaction, to understand another role of Zn in helping the in situ generation of the cationic Rh catalyst and preventing catalyst deactivation, and to analyze the key transition states and intermediates involved.
Subject(s)
Cyclobutanes , Cycloaddition Reaction , Molecular Structure , Kinetics , Catalysis , ZincABSTRACT
The reaction mechanism of bimetallic Pd-Zn-catalyzed cycloaddition of alkynyl aryl ethers with internal alkynes has been studied theoretically. Besides cycloaddition reaction, the dimerization of alkynyl aryl ethers was also considered. Both C6H5OC[triple bond, length as m-dash]CSiiPr3 and C6H5OC[triple bond, length as m-dash]CSiMe3 were chosen as the substrates. The reactions involve C-H activation of the substrate, acetic acid rotation, H transformation, MeC[triple bond, length as m-dash]CMe or substrate insertion into the Pd-phenyl bond and reductive elimination steps. It is found that cycloaddition is favored for C6H5OC[triple bond, length as m-dash]CSiiPr3, while dimerization is preferred for C6H5OC[triple bond, length as m-dash]CSiMe3, because the steric repulsion between two bulky SiiPr3 groups is relatively large and the steric repulsion between two small SiMe3 groups is relatively small. In addition, besides C6H5OC[triple bond, length as m-dash]CSiiPr3, four other substrates C6H5CH2C[triple bond, length as m-dash]CSiiPr3, C6H5C(O)C[triple bond, length as m-dash]CSiiPr3, C6H5SC[triple bond, length as m-dash]CSiiPr3 and C6H5N(H)C[triple bond, length as m-dash]CSiiPr3 have been calculated as the substrates for cycloaddition reaction with MeC[triple bond, length as m-dash]CMe. Among the five substrates, C6H5OC[triple bond, length as m-dash]CSiiPr3 has the lowest energy barrier (29.9 kcal mol-1), consistent with the experimental observation that C6H5OC[triple bond, length as m-dash]CSiiPr3 is the appropriate substrate for successful cycloaddition.
ABSTRACT
At present, the objective of cutting and pruning Cistanche deserticola is to harvest in successive years and enhance the harvesting yield and quality of C. deserticola in the process of the artificial cultivating C. deserticola. An experiment was conducted focusing on cutting and pruning C. deserticola in artificial forests of Haloxylon ammodendron drip-irrigated with saline water at the hinter-land of the Taklimakan desert, according to different growth stages and lengths. The results were following: (1) The effect of cutting on C. deserticola was similar to that of pruning, which resulted in three kinds of morphological types, not related to the bloom and size of C. deserticola. (2) The growth forms were diversified after pruning. Among them, there had sprouting new body, died or maintaining life with no sprouting, mildewed on its surface layer, etc. However, some of new bodies were sprouting from the lower part of the old body. The death rate of bloomed C. deserticola was higher than that of the underground, and the death rate of the 40 cm in stubble height for C. deserticola was higher than those with the stubble height of 20 cm and 5 cm. (3) Most of the diameter of living C. deserticola after pruning was increasing, but some of them changed little. (4) The mildew and rot of C. deserticola and the broken of the roots of the H. ammodendron and the fallen of the point of the inoculated when it was dug, which would cause the death of the C. deserticola. On the other, the yield-increasing effect and the economic benefit of the techniques of the pruning of Cistanche would need further research and evaluate. Therefore, the application of this technique needs to be cautious.
Subject(s)
Amaranthaceae/growth & development , Cistanche/growth & development , Forests , Fruit/growth & development , Plant Roots/growth & developmentABSTRACT
The Ni atom in the title compound, [Ni(C(8)H(8)NO(3))(2)], lies on a center of inversion in a square-planar coordination enviroment. The hydroxyl group of one anion forms a short hydrogen bond to the metal-coordinated O atom of the other anion.
