Rhodium-catalyzed cyclization reactions of 1,5-bisallenes involving alkenes and alkynes: experimental and theoretical studies

The development of novel catalytic methodologies involving the formation of carbon – carbon/heteroatom bonds to produce cyclic systems constitutes a field of great interest in the modern synthetic organic chemistry. Over the last 30 years, the combined use of transition metals with allenes allowed considerable progress in this field. Allenes are cumulated dienes, constituted by two perpendicular double bonds. The unsaturation spread over three contiguous carbons gives allenes additional versatility in cyclization reactions compared to other unsaturations, such as alkenes or alkynes, although the control of the selectivity becomes more challenging when allenes are used. One paradigmatic process to produce cyclic systems is the transition metal-catalyzed [2+2+2] cycloaddition reaction, which enable the simultaneous formation of several bonds and/or stereogenic centers in a single step to produce six-membered rings in perfect atom economy. Our group has developed several rhodium-catalyzed [2+2+2] cycloaddition reactions and has made great mechanistic contributions employing DFT calculations and experimental techniques. Regarding the use of allenes, our group studied the rhodium(I)-catalyzed intramolecular [2+2+2] cycloaddition of linear substrates accommodating allenes and alkynes and/or alkenes. However, [2+2+2] cycloadditions involving two allenes with another unsaturated carbon – carbon bond remain scarce. Considering this, and as part of our continuous interest in the use of allenes, we envisaged the development of partially intramolecular [2+2+2] cycloaddition reactions of 1,5-bisallenes with alkenes and alkynes. Although 1,5-bisallenes exhibited rich chemistry with transition metals, very few examples were reported involving the incorporation of a third unsaturated partner in a cycloaddition reaction. The main goal set within the context of this thesis was to develop new rhodium(I)-catalyzed cycloaddition reactions of 1,5-bisallenes with alkenes and alkynes. As reported in chapter 3, we have succeeded in reacting 1,5-bisallenes with a series of alkenes employing rhodium(I) catalysis. The reaction provided a series of polycyclic dihydroazepine and dihydrooxepine derivatives in a single step and perfect atom economy. The results obtained in chapter 3 allowed us to develop an homodimerization process of 1,5-bisallenes to obtain spirocyclic derivatives featuring six- and seven- membered rings in the spiro carbon (chapter 4). The process was found to be highly chemo- and regioselective, affording a single isomer out of six possible adducts. Moving forward to the use of alkynes, in chapter 5 and 6 we accomplished the proposed objectives by reacting 1,5-bisallenes and alkynes to form cis-3,4-arylvinyl pyrrolidines and cyclopentanes (chapter 5), and bicyclic trans-fused 3,6-dimethylenecyclohex-1-ene derivatives (chapter 6) from 1,5-bisallenes and alkynes using rhodium(I) as a catalyst. Additionally, the mechanism of the reaction was studied in all the developed protocols by means of DFT calculations and experimental techniques that allowed justify the selectivities observed ​
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