Development of Methods for Boron Reagents

Boron reagents are known to be valuable in the field of organic chemistry due to their abilities to undergo a variety of transformations, resulting in useful pharmaceuticals and synthetic intermediates. It has also been shown that diboron reagents can act as reaction mediators due to the unique properties of the boron atom. To that end, this dissertation discloses three novel methods of employing boron reagents.

Chapter 1 describes a method of utilizing a diboron reagent mediator in the palladium-catalyzed hydrogenation of allenes. In the presence of a palladium catalyst, tetrahydroxydiboron and stoichiometric water, allene semireduction proceeds in good yield. This semireduction is regioselective for the terminal alkene and results in the selective formation of Z-alkenes when used with unsymmetrical allenes (>80:20 Z:E). It is also compatible with more sterically hindered 1,1-diarylallenes, resulting in tri-substituted alkenes in good yields (63-88%).

A borylation, defluorination of alpha-trifluoromethyl-alpha,beta-unsaturated esters is described in Chapter 2. The borylation is copper-catalyzed (10 mol %) and proceeds in the presence of stoichiometric bis(pinacolato)diboron and sodium tert-butoxide. The reaction affords compounds that contain two potentially useful functional handles: boronic esters and gem-difluoroalkenes. The products are obtained in moderate to good yield (up to 75%) with a large substrate scope including compounds with electron-donating, electron-withdrawing, heteroatom, and aryl substituents. In addition, the utility of the products in further transformation is demonstrated. A proposed reaction mechanism that provides rationale for the formation of products is described along with experimental evidence.

Finally, Chapter 3 describes a transition-metal-free trans hydroboration of alkynoate esters and amides. The reaction is phosphine-catalyzed and proceeds with pinacolborane to afford (E)-beta-borylacrylates and (E)-beta-borylacrylamides in good to excellent yields. The reaction products are converted into novel oxaboroles through reduction with sodium borohydride. Theoretical calculations provide mechanistic insight for the transformation. The formation of a key phosphonocyclobutene intermediate is responsible for the observed stereoselectivity.