62nd Annual Report on Research 2017

1. Regio- and Stereoselective Dehydrogenative Silylation and Hydrosilylation of Vinylarenes Catalyzed by Ruthenium Alkylidenes

Vinylsilanes and alkylsilanes are important building blocks in the synthesis of small molecules and polymers, based, in part, on their relatively high stability and virtually non-toxic nature. These organosilanes have been extensively exploited as useful synthetic intermediates whose silicon functional groups can be directly converted to many other useful moieties through further reactions. Regio- and stereoselective dehydrogenative silylation to provide vinylsilanes are challenging, owing to either competitive hydrosilylation to afford alkylsilanes or alternative beta-hydride elimination to furnish allylsilanes. Because alkenes are more readily accessible than alkynes and serve as one of the most important starting materials, more direct silylation methods to afford vinylsilanes are highly attractive. For example, Falck and Hartwig recently reported Ir-catalyzed regio- and stereoselective dehydrogenative silylation of terminal alkenes with norbornene as a stoichiometric sacrificial hydrogen acceptor (SHA). Watson demonstrated Pd-catalyzed silyl Heck reaction utilizing terminal alkenes and silyl triflates. Although there are a number of developments in the dehydrogenative silylation to afford vinylsilanes utilizing metal catalysts, such methods generally require either excess alkene substrates or silanes albeit employing excess SHA, air- and moisture sensitive catalysts, or more reactive alkylsilanes in lieu of more useful alkoxysilanes, for further manipulations. Chirik and coworkers recently demonstrated highly selective Co-catalyzed dehydrogenative silylation of alkenes for preparation of allylsilanes, where for catalytic turnover, half of the alkenes served as sacrificial hydrogen acceptor to furnish simple alkanes as byproduct.

In a previous study, we first demonstrated that the preferential Si–H activation over alkene activation utilizing Ru-alkylidene complexes was feasible to achieve intramolecular alkene hydrosilylation. In contrast to a generally accepted Chauvin type silylation mechanism of addition of Si–H across the pi-bond of a Ru-benzylidene, a mechanism involving direct Si–H activation by Ru–Cl was proposed, based on a series of spectroscopic and isotope-labeling experiments. However, there are no examples of this type of Si–H activation by metal alkylidenes (i.e., catalytic deprotonative silyl metalation) for dehydrogenative silylation to afford vinylsilanes. We have developed regio- and stereoselective dehydrogenative silylation to afford only (E)-vinylsilanes and hydrosilylation of vinylarenes by altering the ruthenium alkylidene catalysts. Notably, preparation of both alkylsilanes and vinylsilanes was achieved using a nearly equimolar ratio of alkenes and silanes with a new sacrificial hydrogen acceptor.

Achievements:

a) Selective dehydrogenative silylation and hydrosilylation of a virtually equimolar ratio of terminal aryl-substituted alkenes and alkoxysilanes exploiting ruthenium alkylidene catalysts to access only (E)-vinylsilanes and alkylsilanes.

b) Divergent access to either regio- and stereoselective dehydrogenative silylation or hydrosilylation by varying ligands within ruthenium catalysts.

c) Mechanistic studies indicates that the Si–H bond cleavage to generate the putative ruthenium silyl complex and HCl, is the turnover-limiting step which is consistent with observation of ruthenium alkylidene as the resting state.

2. Synthetic Applications of Silyl Acetals

Another area of our study was synthetic applications of benzodioxasilines, prepared through exploiting catalytic reductive C–H ortho-silylation of phenols with traceless acetal directing groups. We developed a new synthetic strategy to access diversely substituted silylaryl triflates, as aryne precursors for aryne cycloaddition reactions, from benzodioxasilines. Specifically, sequential addition of MeLi and then trifluoromethanesulfonic anhydride to benzodioxasilines provided arylsilyl triflates in a single pot. Notably, this approach was successfully utilized to prepare sterically hindered 1,2,3-trisubstituted arylsilyl triflates. We demonstrated that fluoride-mediated [4+2] aryne cycloaddition reaction of the resulting diethylmethylsilylaryl triflates with furan to afford distinctly substituted cycloadducts, some of which were not accessed previously.

In summary, the ACS PRF supports to our research program has provided research training for students and allows the PI’s team to develop several new research programs. Through this research opportunity students learned and equipped necessary synthetic skills.