Scientific research offers many rewards and benefits that are highly sought in any activity – sharing ideas, interests, and experiences with like-minded individuals; the excitement of the quest for an understanding of the unknown and the function of the surrounding world; the satisfaction of achievement; and the generation of new knowledge uponwhich others may build. The study of the compounds with unusual valency has been and continues to be just such a productive and rewarding activity. In chemistry, the study of compounds with unusual valency offers researchers a rapid path ‘out of the box’ of the science’s conventional wisdom. The traditional grasp of how things are, how things function, and what one should anticipate from molecule–molecule interactions and reactions, or matter– energy interactions is best tested in this realm of the unusual. Accepted ideas are most easily pushed to their limits (and beyond) when applied to that which appears unusual by accepted standards. Carbenes fall within this area of compounds with unusual valency. These compounds contain a divalent carbon that, formally, violates the octet rule. Carbene chemistry has its roots in much simpler times when there was no real cause to be alarmed or concerned about such compounds because there was no octet rule to violate. Hence early chemical research seeking to isolate and study stable carbenes was reasonable. The octet rule has its origins at the turn of the 20th century in Abegg’s rule and Lewis’s cubical atom. However, recognizable ‘carbene’ research started some sixty five years earlier with the attempts of Dumas to isolate and characterize methylene (Scheme 1). The progress of carbene research from its origins has been recounted elsewhere and is left to the reader to retrace in detail; here I only intend to briefly summarize those earlier events. The typically high reactivity of carbenes and lack of suitable spectroscopy methods or laboratory techniques to handle these apparently transient species led to a widespread view that carbenes were too reactive to be isolated as stable entities, but were, nonetheless, important intermediates in some chemical transformations. This view of extreme reactivity could be further supported by reliance on the octet rule. By the late 20th century, the view of carbenes was beginning to change again. Experimental techniques had improved substantially and new spectroscopic techniques, beyond the reach of early carbene researchers, were becoming common-place. There was hope that transient carbene species could at least be captured briefly and studied if not placed in a bottle. Beginning in 1960Wanzlick reported the chemistry of imidazolines and (to a lesser extent) imidazoles in the quest for stable carbenes. Unfortunately, Wanzlick’s efforts were contemporaneous with other rather spectacular experimental mishaps that laid false claims to the isolation of simple singlet carbenes. Wanzlick’s experiments were perhaps nearly capable of producing a ‘bottle-able’ carbene, but the scientific environment and culture in his time was still not welcoming to his views. Publications suggesting flaws or oversights in his work kept a truly ‘bottle-able’ carbene just beyond the community’s grasp. Nonetheless, Wanzlick, Ofele, Lappert, Huttner and others published chemistry that showed that in situ generated imidazol(in)-2-ylidenes give rise to interesting new compounds (Scheme 2). Chemistry of themain group elements sulfur and phosphorus opened another opportunity to approach the quest for carbenes. Seppelt and Bertrand employed the elements of sulfur and phosphorus to construct molecules in which the conceptual ‘resonance’ relationship between a-dicarbenes and acetylenes could be perturbed by the introduction of the heavier main group elements. These pioneering studies provided thiaacetylenes and phosphaacetylenes that exhibited ‘carbene-like’
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Open Access
Jan 1, 2014
Ikuya Yamada 
