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Metal-Catalyzed Formation and Transformations of Carbon-Boron Bonds

Our research seeks new methods for functionalizing organic small molecules using organoboronic derivatives as a versatile handle for late-stage manipulations. Metal-catalyzed formation of new carbon-boron bonds and their subsequent transformations are highlighted.

Among the myriad of unsaturated substrates for conducting borylation reactions, allenes have received minimal attention. These substrates are uniquely advantageous given that diboration results in the formation of both allylic and vinylic boronates. Orthogonal reactivity of the sp2 and the sp3 C-B bonds can allow for chemoselective transformations. However, oxidation of the carbon-boron bond is one example in which the conditions are unselective. To address this shortcoming, a platinum catalyst was developed for the diboration of 1,1-diaryl allenes with a differentially protected diboron reagent, pinB-Bdan. The reaction proceeds regioselectively in high yields to furnish olefins bearing a vinylic Bpin and an allylic Bdan moiety. The subsequent chemoselective transformation of each boron center was demonstrated.

Methods for preparing 1,8-diaminonaphthalene protected vinylboronates conjugated to carbonyl groups are severely limited. A simple and efficient protocol was developed for carrying out an environmentally friendly copper(II)-catalyzed beta-borylation of alkynoates and alkynamides in water and open-to-air. Following the discriminative activation of the more Lewis acidic pinacol protected boron center in pinB-Bdan, a regio-, stereo- and chemoselective beta-borylation of acetylenic substrates delivers (Z)-beta-boryl enoates and primary, secondary, and tertiary enamides under very mild conditions.

As an inexpensive and earth abundant metal, catalysts based on copper are highly desirable. An international collaborative project to develop a copper-catalyzed cross-coupling reaction of beta-boryl carbonyl compounds was explored. Preliminary results found these substrates to be either unstable towards or unreactive under the reactions conditions screened. / Ph. D. / The very basis of everything in existence is the atom. The idiosyncratic arrangements and interactions of atoms confer distinctive physical properties which give rise to the biological processes of organic lifeforms or the diverse characteristics of inorganic substances, like salts and minerals. In organic chemistry, the carbon-based backbone of the compound is decorated with socalled functional groups, which govern the physical or biological properties of the molecule. Building the unique structural arrangement of functional groups within a pharmaceutical, for example, requires multi-step reaction sequences and purifications to deliver the desired product. Thus, their assembly must be extremely selective and highly efficient to yield the final compound in useable amounts. The overarching goal of our work is to develop such methods for building complex small molecules from very simple starting materials. The carbon—boron bond is a particularly versatile tool in synthetic chemistry because it offers direct access to a myriad of different functional groups. We utilize the unique properties of boron, a tunable semi-metallic element, in the formation and transformation of carbon—boron bonds with divergent reactivity.

Catalysis offers a modern approach to enhance the selectivity and sustainability of preparative organic chemistry. Energy input is needed to make and break chemical bonds. Conducting the reaction at an elevated temperature, for example, is a conventional way to provide the energy necessary for molecules to come together. Conversely, metal catalysts can be cleverly designed to lower the potential energy barrier, which gives rise to new pathways for carrying out chemical transformations. Moreover, incredibly small amounts of the metal is sufficient because a catalyst propagates the process in a cyclic and repetitive fashion. In this work, metal catalysts were optimized to form carbon—boron bonds from diboron reageants containing two different boron centers. Taking advantage of the orthogonal reactivity of each boron allowed for the selective installation of functional groups in subsequent transformation reactions.

Identiferoai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/83400
Date01 December 2016
CreatorsNelson, Amanda Kay
ContributorsChemistry, Santos, Webster L., Merola, Joseph S., Carlier, Paul R., Tanko, James M.
PublisherVirginia Tech
Source SetsVirginia Tech Theses and Dissertation
Detected LanguageEnglish
TypeDissertation
FormatETD, application/pdf, application/pdf, application/pdf
RightsIn Copyright, http://rightsstatements.org/vocab/InC/1.0/

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