Given the synthetic value of ketenyl anions and their accessibility via a mild and transition metal free protocol using CO as a sustainable C1 source, we became interested in further expanding this methodology by exploring the factors which control the selectivity of the carbonylation reaction, i.e. the ligand exchange at carbon. In addition, we aimed at a deeper understanding of the bonding situation in ketenyl anions and their reactivity depending on their substituents. Therefore, we turned our attention towards the tosyl substituted metallated ylide 1a, previously synthesize in our group. Whereas the thiophosphinoyl substituted metallated ylide I selectively undergoes CO for phosphine exchange, initial attempts to obtain the analogous tosyl compound 2-M under the same reaction conditions by carbonylation of the PCy3-substituted α-metallated ylide 1a-Li failed (Table ). Instead of 2-M formation of the phosphoranylideneketene, Cy3PC=C=O (3a), previously reported by Krempner, with concomitant lithium sulfinate elimination was observed based on 31 P{ 1 H} NMR studies. This observation can be rationalized by the proposed mechanism of the carbonylation reaction, which presumably proceeds via initial nucleophilic attack of the yldiide at the LUMO of CO to form intermediate Int, which via backdonation eliminates either the phosphine or in case of the tosyl system 1a-Li the metal sulfinate to form 3a instead of 2-Li (Scheme 1). We hypothesized that the selectivity of the phosphine displacement can be improved by enhancing the leaving group ability of the phosphine, i.e. by introducing electronwithdrawing substituents at the phosphorus atom. Indeed, treatment of a solution of the PPh3 analogue 1b-Li in THF with 1 bar of CO at room temperature resulted in a switch of selectivity and the preferred formation of the ketenyl anion 2-Li in an approx. 79:21 ratio with Bestmann's ylide 3b. Surprisingly, further decreasing the electron density by using trifluoromethyl groups in para-position of the phenyl groups (1c-Li) led to no further improvement of the selectivity and introduction of even more electronpoor phosphines was hampered by difficulties in the formation of the phosphonium salt or competing metal/fluoride exchange (1d, 1e). Table . Impact of the phosphine, the solvent and alkali metal on the selectivity of the carbonylation of tosylsubstituted metallated ylides. The complete list of screening experiments is given in the Supporting Information (Table In order to further enhance the selectivity, we thus examined the influence of the metal cation. Owing to the different Lewis acidities of the alkali metal ions, the sulfonyl group is expected to more strongly bind to lithium, resulting in an enhanced leaving group ability and the preferred MSO2Tol elimination. Indeed, switching to sodium and potassium, respectively, led to a slightly improved selectivity for 2-M (Entry 6 and 7). Despite the high selectivity for the ketenyl anion 2-M in THF, it was not possible to isolate it in pure form due to its similar solubility compared to the sulfinate. Therefore, we turned our attention towards examining the impact of the solvent and additional additives using 1b-Na as test system. Unfor-tunately, no conversion of the metallated ylide was observed in all solvents (hexane, diethyl ether, toluene, ortho-difluorobenzene (oDFB)) except THF even after prolonged reaction times (7 d), due to the low solubility of 1b-Na in less polar solvents. At elevated temperatures, minor conversion was found in toluene and oDFB (< 10%), but decreased selectivity in THF (see SI for details). Therefore, we attempted to increase the solubility of the sodium salt by addition of crown ethers which were expected to also weaken the coordination of the metal cation by the tosyl group. To our delight, using 15-crown-5 (15-c-5) resulted in high selectivities in all solvents, although transformations in hexane and diethyl ether were incomplete. In THF, full conversion with a 98% selectively for the ketenyl anion was recorded (Entry 8).