Studies On The Addition Reactions Of Allenes

Although allenes had been considered as highly unstable for a long period of time, recent observation of nice reactivates and selectivities of allenes in organic synthesis has made their chemistry a hot research area, thus much attention has been paid to the transition metal-catalyzed chemistry of allenes. However, the electrophilic addition of allenes has not been well developed mainly due to the fact that the control of regio-and stereoselectivity is not easy. In this dissertation, much attention has been focused on the study of the electrophilic additions of functionalized allenes with electrophiles and Pd(0)/Ag⁺-catalyzed reactions of aryl halides with functionalized allenes. The whole dissertation can be divided into two parts.In the first part of this dissertation, we have developed four reactions:Firstly, we modified the iodohydroxylation of 1,2-allenyl sulfoxides with I₂ in the presence of EtOH in acetonitrile at 0 ℃, which afforded a series of E-3-phenylsulfinyl -2-iodo-2-alkenols stereoselectively in high yields. On the other hand, the iodohydroxylation of 1,2-allenyl sulfoxides with I₂ in the presence of BnSH afforded E-3-hydroxy-2-iodo-2-alkenyl sulfides in good yields and selectivities. The stereochemistry for the products of this transformation is opposite to what was obtained from the iodohydroxylation of 1,2-allenyl sulfides developed in this group. Based on the results of some control experiment, a mechanism was proposed.Secondly, we designed and developed an effective one-step methodology for the synthesis of 4-halo-2(5H)-furanones by the iodolactonization of ethyl 2,3-allenoates in moderate to high yields via the direct participation of the carbonyl oxygen in the regioselective electrophilic addition of the allene moiety with I₂ in aqueous MeCN. Moreover, the current reaction can be expanded from 2,3-allenoates to 3,4-allenoates. Thus, under the standard conditions successfully applied for the 2,3-allenoates, the reaction of 3,4-allenoates with I₂ afforded 3,6-dihydro-2H-pyranones in moderate yields.Based on the above results, we developed a similar reaction of 2,3-allenols and3,4-allenols with I2, which afforded an efficient route to iodo-substituted 2,5-dihydrofurans and 2,6-dihydropyrans in moderate to good yields.We have also developed an effective methodology for the synthesis of the synthetically useful 3-halo-3-alkenals in good yields via the reaction of readily available l-aryl-2,3-allenols with Br2, NBS, or I2. We believed that the reaction was proceeded via a subsequent electrophilic interaction of X+ with the allene moiety forming 7t—allyl cationic intermediates, 1,2-aryI shift, and H+-elimination process. The structures of the products were established by X-ray diffraction study.In the second part of this dissertation, 2(ï¿¡),4-alkadienoates were prepared highly stereoselectively via the Pd(0)/Ag2CO3-cocatalyzed reaction of 3,4-alkadienoates and aryl halides. The reaction is believed to proceed via the oxidative addition-carbopalladation-P-H elimination process. Compared to the reported protocols, in which usually only disubstituted C=C bonds were formed, the current reaction forms the trisubstituted or even tetrasubstituted C=C bond highly stereoselectively. We have also observed the first Heck-type cross-coupling reaction of allenes with aryl halides. The regioselectivity of intermolecular carbopalladation of 1,2-allenyl sulfones affording vinylic Pd intermediates is completely opposite to what was reported in the literature.