UC San Diego

Chapter 1. Historical Development of Stable Metallacyclobutenes

Fred Tebbe and co-workers synthesized the first stable metallacyclobutene complexes in the late 1970’s by treatment of an intermediate titanium methylene species – later popularized as the “Tebbe reagent” – with acetylenes. Robert Grubbs at Caltech further studied this system, using it to detail a degenerate metathesis reaction and to isolate a metallacyclobutane complex – which was implicated in the emerging field of alkene metathesis. Further development of stable metallacyclobutene complexes is explored chronologically, particularly with regard to systems developed by Thorn, Bianchini, Hughes, O’Connor, Casey, Wojcicki, and Chen.

Chapter 2. Reactivity of Stable Metallacyclobutenes and Vinylcarbenes with Alkenes

Cobalt-metallacyclobutene complex (η⁵-C₅H₅)(PPh₃)Co[κ²-(C,C)-C(SO₂Ph)=C(TMS)CH(CO₂Et)] is allowed to react with a variety of acyclic and cyclic alkenes to generate stable η⁴-1,4-pentadienes (9-12) and κ²-metallacyclohexenes (13-14), respectively. The reaction is thought to occur through a vinylcarbene intermediate such as 15-17, and a model complex of an oxametallabenzene containing an internal vinylcarbene moiety is calculated (18). Complex 13-exo reacts with trimethylphosphine to decomplex the ester carbonyl, generating a PMe₃ complex, 20-exo. Oxidative demetallation of complexes 9-ZZ and 9-ZE is effected with iodine to generate organic pentadienes 21-22. Iron vinylcarbene complex (CO)₃Fe(η³-C(OMe)C(CO₂Me)=CH(CO₂Me) (25a) is synthesized, and its reactivity with maleic anhydride and maleimide is found to generate dinuclear iron complex 30, and new organic species 31 and 32. Compounds 31-32 are formulated as the [4 + 2]-cycloaddition product of an intermediate furan or α-pyrone.

Chapter 3. Reactivity of Stable Metallacyclobutenes with C-Nitroso Reagents

Cobaltacyclobutene complex (η⁵-C₅H₅)(PPh₃)Co[κ²-(C,C)-C(SO₂Ph)=C(TMS)CH(CO₂Et)] is found to react with C-nitroso reagents to afford bicyclic [κ²-(C,O)-η¹-(N)-hydroxylamido complexes 5-9. Complexes 5-9 contain a [3.1.0] fused-ring framework, with the 3-membered oxametallaziridine moiety formed in either a cis or trans relationship to the ethyl ester. Formation of products 5-9 is proposed to occur through a [4 + 2]-cycloaddition mechanism from an oxametallabenzene, with stereo-selection via rapid epimerization of nitrogen from the post-cyclization intermediate.

Chapter 4. The Isolation of a Large Cyclopentadienylcobaltsulfide Cluster – Synthesis and

Crystal Structure of Octahedral closo-[(η⁵-C₅H₅)Co]₅S

The metallacyclobutene complex (η⁵-C₅H₅)(PPh₃)Co[κ²-(C,C)-C(SO₂Ph)=C(TMS)CH(CO₂Et)] reacts with added (η⁵-C₅H₅)Co(PPh₃)₂ to give the cluster compound closo-[(η⁵-C₅H₅)Co]₅S (1), the largest (η⁵-C₅H₅)Co cluster known.