Alkali Metal Carbenoids: A Case of Higher Stability of the Heavier Congeners

Abstract

As a result of the increased polarity of the metal-carbon bond when going down the group of the periodic table, the heavier alkali metal organyl compounds are generally more reactive and less stable than their lithium congeners. We now report a reverse trend for alkali metal carbenoids. Simple substitution of lithium by the heavier metals (Na, K) results in a significant stabilization of these usually highly reactive compounds. This allows their isolation and handling at room temperature and the first structure elucidation of sodium and potassium carbenoids. The control of stability was used to control reactivity and selectivity. Hence, the Na and K carbenoids act as selective carbene-transfer reagents, whereas the more labile lithium systems give rise to product mixtures. Additional fine tuning of the M-C interaction by means of crown ether addition further allows for control of the stability and reactivity.

Alkali Metal Chlorine and Bromine Carbenoids: Their Thermal Stability and Structural Properties

“…Interestingly, despite the metal–halogen interaction none of the carbenoids exhibits a contact between the metal and the carbon atom. Such an arrangement has also been observed for other alkali metal carbenoids and has been described as a carbene–donor complex, in contrast to bridged and classical structures with direct M−C bonds. Despite the metal–halogen interactions, the carbon–halogen bonds in 1‐M and 2‐M show no obvious elongation compared to the protonated starting materials 1 and 2 , respectively.…”

“…Also, the 1 J PC coupling constant is significantly higher for the lithium compounds, suggesting a higher s‐character in the P−C bonds and hence a higher p‐character in the C−Cl/Br bond relative to 1/2‐Na and 1/2‐K . The latter accounts for a weaker C−Hal interaction in the lithium carbenoids and thus suggests a higher stability of the heavier carbenoids as has previously been observed for the more labile silyl‐substituted systems III …”

“…Only very recently, our group has reported on the impact of the nature of the alkali metal on the carbenoid stability using the silyl system III‐M (with M=Li, Na, K). Interestingly, replacement of lithium by the heavier congeners, sodium and potassium, resulted in a remarkable increase of stability, which allowed for selectivity control in carbene‐transfer reactions …”

“…Although many reports have appeared in the last few years focussing on the influence of the substitution pattern and the M/X combination (M=Li, Mg, Zn; X=Hal, OR, NR, SR) on the carbenoid reactivity, systematic studies particularly on the structural properties of a given system depending on the M/X combination are missing. For example, while several structures of Li/Cl carbenoids have been elucidated, no structure of any Li/Br carbenoid—except for a phosphaalkene system reported by Niecke—is known to date. Likewise, studies on sodium or potassium compounds are limited to the chloro compounds III‐Na and III‐K (Figure ) .…”

“…For example, while several structures of Li/Cl carbenoids have been elucidated, no structure of any Li/Br carbenoid—except for a phosphaalkene system reported by Niecke—is known to date. Likewise, studies on sodium or potassium compounds are limited to the chloro compounds III‐Na and III‐K (Figure ) . To provide further knowledge about the structural diversity of carbenoids, we addressed the systematic synthesis and structure elucidation of chlorine and bromine systems with different alkali metals .…”