A Theoretical Study Of The Diels-Alder Reaction Of High Tension Dienophiles

The synthesis and application of highly strained organic molecules have always been of interest to scientists.In the early stage,scientists only studied cycloalkenes and six-membered rings containing triple bonds or allene bonds out of curiosity,but now lots of such molecules have been obtained in experiments,such as cyclopropene,benzyne,cyclohexyne,1,2-cyclohexadiene,oxacyclic and azacyclic allenes,and applied in the field of organic chemistry.In this paper,three kinds of highly strained molecules were selected:derivatives of 2-Borabicyclo[1.1.0]but-1(3)-ene(2BB),perfluorobicyclo[2.2.0]hex-1(4)-ene and bicyclo[2.2.0]hex-1(4)-ene.Then their structures,strain energies,frontier orbital were studied systematically.The similarity of LUMO between three kinds of strained molecules and ethylene and cyclopropene indicates that they are dienophile.Therefore,we further investigated the Diels-Alder reactions of the three types dienophile.The main contents are as follows:1.By using M06-2X/6-311++G**method,the stable structures,the strain energies and frontier orbital of the 2BB derivatives were obtained.Calculted strain energies of 2BB derivatives range from 75.2 to 96.2 kcal/mol,indicating that they are highly strained dienophiles.The Diels-Alder reactions of 2BB derivatives with butadiene,cyclopentadiene and 3,6-bis(trifluoromethyl)-tetrazine were studied.The results show that the activation energies in the endo Diels-Alder reactions of butadiene and cyclopentadiene with 2BB derivatives are lower than those of the exo reactions.And the activation energies are lower in the reaction of 2BB derivatives with 3,6-bis(triflfluoromethyl)-tetrazine due to the electron-with-drawing group CF3.Moreover,the activation energies well correlate with the distortion energies of diene,and the difference of the energy barrier of endo/exo transition state also arise from the distortion energies of diene.Therefore,it is theoretically predicted that the derivatives of 2BB are highly strained dienophiles,in addition,the Diels-Alder reactions of the 2BB derivatives can give novel propellanes.2.The didehydrogenative Diels-Alder reactions of 2BB derivatives with 1,3-butyne were studied by using ?B97XD/6-311++G**method.The results show that these reactions are feasible thermodynamically and kinetically,and most stable products are anti products 8-substituted-7-fluoro-7-borotricycline[4.1.1.0]2(3)octadiene(C6H4BFCHX).Then the Diels-Alder reactions of C6H4BFCHX and butadiene were further studied,including the regioselectivity,stereoselectivity,reactivity and influencing factors.The results show that the Diets-Alder reactions of C6H4BFCHX with butadiene are feasible thermodynamically and kinetically and give Z-type products.The reaction energy barriers of the Diets-Alder reactions of C6H4BFCHX with butadiene well corelate with the distortion energies of butadiene.Additionally,the distortion energies of butadiene have effect on the regioselectivity of the double bond and the syn/anti selectivity.3.Using density functional theory with ?B97XD/6-311++G**method,the Diels-Alder reactions of perfluorobicyclo[2.2.0]hex-1(4)-ene(C6F8)and bicyclo[2.2.0]hex-1(4)-ene(C6H8)with five kinds of dienes were studied.These reactions are exothermic,which are consistent with the experimental reactions.The order of the reaction barriers of the Diels-Alder reactions of C6F8 and C6H8 are butadiene>cyclopentadiene>furan>1,3-cyclohexadiene>benzene.In addition,the stronger electrophilicity of C6F8 result in the lower reaction energy barriers compare with C6H8.The changes of the distortion energy of dienes and the interaction energy are well consistent with the reaction energy barrier,indicating that the Diels-Alder reactions of C6F8 and C6H8 are controlled by distortion energy of the diene and interaction energy.We also studied the Diels-Alder reaction of C6F8 and C6H8 with 5-substituted cyclopentadiene and found that the distortion energy of dienes effect on syn/anti selectivity.