The Inorganic, Bioinorganic and Organometallic Chemistry Program supports the efforts of Professor Joseph M. O'Connor of the University of California San Diego to investigate late-metal metallacyclobutene complexes. Such complexes have been proposed as key intermediates in a large number of important metal-mediated transformations, including alkyne polymerizations, enyne methathesis reactions, and in the synthesis of cyclopropenes, furans, pyrones, and benzannulated compounds. The development of new synthetic routes to these targets maximizes the potential for subsequent reactivity. Isolable metallacyclobutene complexes allow for: a) structural studies, b) well-defined reactivity studies, c) the development of new reactions, d) appropriate modeling of key steps in reactions which are thought to involve metallacyclobutene intermediates, and e) applications of metallacyclobutene-derived products in many areas of the chemical sciences. Undergraduate and graduate students trained in the area of organometallic chemistry are well-positioned to make important contributions to both industrial and academic endeavors which require interdisciplinary research in areas such as materials, inorganic and organic chemistry.
Alkyne-derived metallacycles, such as metallacyclopentadienes, I, and metallacyclobutenes, II, are often invoked as key intermediates or transition states in a number of important metal-catalyzed reactions of alkynes, including alkyne cyclizations, alkyne polymerization, enyne metathesis, cyclopropenation, carbene additions across alkynes, and alkyne cyclization to furans (Figure 1). Although metallacyclobutene chemistry is at a much earlier stage of development than metallacyclopentadiene chemistry, the metallacyclobutenes are expected to exhibit a fascinating reactivity profile due to the presence of an sp3-carbon in the metallacycle ring and the potential for valence isomerization to a bent eta-3-vinylcarbene (III) and/or an eta-1-vinylcarbene (IV). There has been considerable speculation in the literature as to the relative importance of metallacyclobutene and vinylcarbene intermediates /transition states in a variety of organometallic reactions. In an effort to facilitate the observation of a vinylcarbene intermediate, we prepared and characterized metallacyclobutene 1-TIPS (Figure 2). As anticipated, the bulky tri-isopropylsilyl ring substituent (TIPS) in 1-TIPS led to more facile triphenylphosphine (PPh3) dissociation and formation of the new, highly reactive vinylcarbene complex 2-TIPS-E which was characterized spectroscopically and by computation. This work represents the first experimental observation of a metallacyclobutene to eta-3-vinylcarbene conversion and the first experimental observation of a eta-3-vinylcarbene to metallacyclobutene conversion. This unprecedented equilibrium between a metallacyclobutene and an eta-3-vinylcarbene complex (1-TIPS and 2-TIPS-E) provides an experimental foundation for elucidating the factors that control the interconversion chemistry of valence tautomers (2-TIPS-E, 2-TIPS-C,O, and 2-TIPS-Z) that have been implicated in numerous metal-catalyzed reactions of alkynes and carbenes. We have initiated exploratory reactivity studies of 2-TPS-E in order to understand the reactivity patterns exhibited by analogues of 1-TIPS) and have found that a number of reactions (with ethyldiazoacetate, dienophiles, and C5H5Co(PPh3)2) all appear to proceed via the intermediacy of 2-TIPS-E. Ultimately these results will help to explicate the factors that control the selectivity of alkyne – carbene coupling reactions, and lead to the development of new metal-based reaction chemistry.
