Zirconocene and Si-Tethered Diynes: A Happy Match Directed toward Organometallic Chemistry and Organic Synthesis

Abstract

Characterizing reactive organometallic intermediates is critical for understanding the mechanistic aspects of metal-mediated organic reactions. Moreover, the isolation of reactive organometallic intermediates can often result in the ability to design new synthetic methods. In this Account, we outline synthetic methods that we developed for a variety of diverse Zr/Si organo-bimetallic compounds and Si/N heteroatom-organic compounds through the detailed study of zirconacyclobutene-silacyclobutene fused compounds. Two basic components are involved in this chemistry. The first is the Si-tethered diyne, which owes its rich reactive palette to the combination of the Si-C bond and the C≡C triple bond. The second is the low-valent zirconocene species Cp(2)Zr(II), which has proven very useful in organic synthesis. The reaction of these two components affords the zirconacyclobutene-silacyclobutene fused compound, which is the key reactive Zr/Si organo-bimetallic intermediate discussed here. We discuss the three types of reactions that have been developed for the zirconacyclobutene-silacyclobutene fused intermediate. The reaction with nitriles (the C≡N triple bond) is introduced in the first section. In this one-pot reaction, up to four different components can be combined: the Si-tethered diyne can be reacted with three identical nitriles, with differing nitriles, or with a nitrile and other unsaturated organic substrates such as formamides, isocyanides, acid chlorides, aldehydes, carbodiimides, and azides. Several unexpected multiring, fused Zr/Si organo-bimetallic intermediates were isolated and characterized. A wide variety of N-heterocycles, such as 5-azaindole, pyrrole, and pyrroloazepine derivatives, were obtained. We then discuss the reaction with alkynes (the C≡C triple bond). A consecutive skeletal rearrangement, differing from that observed in the reactions with nitriles, takes place in this reaction. Finally, we discuss the reaction with the C═X substrates (where X is O or N), including ketones, aldehydes, and isocyanides. Oxa- and azazirconacycles are formed via a new skeletal rearrangement. Our results show that the zirconocene and the Si-tethered diyne cooperate as a "chemical transformer" after treatment with various substrates, leading to a diverse range of cyclic Zr/Si organo-bimetallic compounds. This mechanism-derived synthesis of organometallic and organic compounds demonstrates that the investigation of metal-mediated reactions and the isolation of reactive organometallic intermediates not only contribute to the understanding of complex reactions but can also lead to the discovery of synthetically useful methods.

Recent advances in the chemistry of heterometallacycles of group 4 metallocenes

“…62 However, it should be noted in this context that it is difficult to predict general reactivity patterns, most likely due to the structure of the zirconocene-Si-tethered diyne adduct, which can be described in at least four different resonance forms, thus leading to a multifaceted coupling chemistry. 63 Another example for the formation of seven-membered heterometallacycles was reported by Erker and co-workers. 64 Reaction of the five-membered zirconacycloallenoid Cp 2 Zr-[CH 2 -C(Me)QCQC( t Bu)] with acetonitrile gave a zirconacyclic allene complex, which was most likely formed by insertion of the nitrile into the metal-carbon bond followed by tautomerisation to yield the metallocene enamido product complex (Scheme 20).…”

Formation of silacycles via metal-mediated or catalyzed Si-C bond cleavage

“…Nagao and co-workers [52] developed a highly efficient method to stereoselectively prepare unsymmetrical conjugated (1E,3E)-dienes and -trienes from the zirconium-mediated silacyclobutenes formation (Scheme 8). Xi and co-workers [53] developed the zirconacyclo-butene-silacyclobutene fused intermediate into a powerful "chemical transformer" in organic synthesis. A consecutive skeletal rearrangement was observed from the reaction of alkynes with these Zr/Si organo-bimetallic intermediates to produce various silacyclobututenes.…”

Titanium, zirconium, hafnium

“…9 In the chemistry of Zr, reviews include zirconocene-mediated reductive couplings in the synthesis of macrocycles 10 and zirconacyclobutene-silacyclobutene fused compounds. 11 A review of multinuclear olefin polymerization catalysts features many examples drawn from these metals. 12…”