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1. Catalytic palladium-mediated carbocyclizations: The [3+2] cycloaddition mode. 2. Chemistry of weak pronucleophiles: Mitsunobu alkylation and palladium-catalyzed dienylation of bis(2,2,2 -trifluoroethyl) malonates
Palladium(0)-mediated carbocyclizations of bisdienes provide a convenient access to functionalized 5- and 6-member-ring carbocycles and heterocycles. The reaction mode selectivity is controlled chiefly by the reaction conditions, but in some instances—by the bisdiene substituents as well. We found that aryl-substituted bisdienes readily undergo carbocyclization in a novel [3+2] cycloaddition reaction mode in preference to the other two carbocyclization modes: cyclization trapping, or cycloisomerization. The products of the cycloaddition are functionalized trans-bicyclo[4.3.0]nonanes. The reaction is diastereoselective, and occurs with the formation of 4 contiguous stereocenters located on the five-member ring of the product. The reaction is tolerant to a range of aryl substituents, and facile. ^ During the work on the [3+2] cycloaddition, two new synthetic methodologies based on alkylation chemistry of bis(2,2,2-trifluoroethyl) malonates emerged in the Takacs group. Bis(2,2,2-trifluoroethyl) malonate has a pKa of <13, and unlike other simple malonates, such as diethyl malonate, readily undergoes Mitsunobo dehydrative alkylattion with a variety of primary alcohols; some secondary alcohols can be employed as well. In contrast to other stabilized carbon-based pronucleophiles that do undergo the Mitsunobo reaction with alcohols, bis(2,2,2-trifluoroethyl) malonate successfully undergoes monoalkylation with one equivalent of the alcohol partner, little competing dialkylation is observed. ^ Due to their acidity, bis(2,2,2-trifluoroethyl) malonates exhibit improved regio- and stereo selectivity in the Pd-catalyzed dienylation reactions. Thus, bis(2,2,2-trifluoroethyl) malonates can be dienylated by 5-substituted 2,4-dienyl-alcohols, or their corresponding divinyl alcohols, to give dienylation products of higher regio- (the external, α position is alkylated only) and stereoisomeric purity (the reaction predominantly gives the E,E-dienes) than the analogous reaction of simple dialkyl malonates with the corresponding acetates and carbonates. The alkylation methodologies described above provide a convenient means for the rapid assembly of unsaturated substrates, including the bisdiene substrates for the Pd-catalyzed bisdiene carbocyclizations. ^
Leonov, Alexei P, "1. Catalytic palladium-mediated carbocyclizations: The [3+2] cycloaddition mode. 2. Chemistry of weak pronucleophiles: Mitsunobu alkylation and palladium-catalyzed dienylation of bis(2,2,2 -trifluoroethyl) malonates" (2004). ETD collection for University of Nebraska - Lincoln. AAI3142091.