Rhodium-Catalyzed Reductive Cyclization of 1,6-Diynes and 1,6-Enynes Mediated by Hydrogen: Catalytic C−C Bond Formation via Capture of Hydrogenation Intermediates

Catalytic hydrogenation of carbon-, nitrogen- and oxygen-tethered 1,6-diynes 1a−9a and 1,6-enynes 10a−18a using cationic Rh(I) precatalysts at ambient temperature and pressure enables reductive
carbocyclization to afford 1,2-dialkylidene cyclopentanes 1b−9b and monoalkylidene cyclopentanes 10b−18b, respectively, in good to excellent yields and as single alkene stereoisomers. Notably, the 1,3-diene
and alkene containing cyclization products 1b−9b and 10b−18b are not subject to over-reduction under
the conditions of catalytic hydrogenation in which they are formed. Reductive cyclization 1,6-diyne 1a and
1,6-enyne 10a performed under an atmosphere of D2 provides the carbocyclization products deuterio-1b
and deuterio-10b, respectively, which incorporate two deuterium atoms. The collective data are consistent
with a catalytic mechanism involving heterolytic activation of elemental hydrogen (H2 + Rh+X- → Rh−H +
HX) followed by Rh(I)-mediated oxidative cyclization of the 1,6-diyne or 1,6-enyne substrates to afford
(hydrido)Rh(III)-based metallocyclopentadiene and metallocyclopentene intermediates, respectively. These
transformations represent the first examples of metal-catalyzed reductive carbocyclization mediated by
hydrogen.

在室温常压条件下，以阳离子型Rh(I)预催化剂催化碳、氮、氧连接的1,6-二炔（1a−9a）与1,6-烯炔（10a−18a）的加氢反应，可分别实现还原环化，以良好至优异的收率得到单一烯烃立体异构体的1,2-双亚烷基环戊烷（1b−9b）与单亚烷基环戊烷（10b−18b）。值得注意的是，所生成的含1,3-二烯与烯烃结构的环化产物1b−9b和10b−18b，在该催化加氢反应条件下不会发生过度还原。以氘气（D₂）为反应氛围开展1,6-二炔1a和1,6-烯炔10a的还原环化反应，可分别得到氘代产物deuterio-1b与deuterio-10b，二者均引入了两个氘原子。整体实验数据与该催化反应机理相符：首先发生单质氢气的异裂活化（H₂ + Rh⁺X⁻ → Rh−H + HX），随后由Rh(I)介导1,6-二炔或1,6-烯炔底物发生氧化环化，分别生成基于氢化Rh(III)的金属环戊二烯与金属环戊烯中间体。此类转化是首例以氢气为介导的金属催化还原环化反应。