Ketenyl anion

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A ketenyl anion contains a C=C=O allene-like functional group, similar to ketene, with a negative charge on either terminal carbon or oxygen atom, forming resonance structures by moving a lone pair of electrons on C-C-O bond. Ketenes have been sources for many organic compounds with its reactivity despite a challenge to isolate them as crystal. Precedent method to obtain this product has been at gas phase or at reactive intermediate, and synthesis of ketene is used be done in extreme conditions (i.e., high temperature, low pressure). [1] [2] [3] Recently found stabilized ketenyl anions become easier to prepare compared to precedent synthetic procedure. A major feature about stabilized ketene is that it can be prepared from carbon monoxide (CO) reacting with main-group starting materials such as ylides, silylene, and phosphinidene to synthesize and isolate for further steps. As CO becomes a more common carbon source for various type of synthesis, [4] this recent finding about stabilizing ketene with main-group elements opens a variety of synthetic routes to target desired products.

Contents

Synthesis

Gessner et al. first revealed a synthetic route for stabilized ketenyl anion using metalated ylides in 2022. [5] In their paper, upon introducing CO, metalated ylide with posassium cation exchange CO with phosphine group R, also known for carbonylation of ylide. Their isolated ketenyl anion [K(PPh2(=S)CCO] is stable solid for a week under inert atmosphere, and its crystal structure was characterized. An alternate synthetic pathway for synthesizing ketenyl anion from ylide, shown in Figure 2, includes sulfuration on diphenylphosphine group, deprotonation on carbon center, and CO substitution in exchange of triphenylphosphine leaving. This synthesis resulted in 88% isolation of the product. Later in their studies, the ketenyl anion product upon carbonylation can be selective by changing electron-withdrawing ability on a certain leaving group and Lewis acidity of coordinated alkali metal cation. [6] In their example with ylide containing phosphine group and tosyl group (Ts), Gessner et al. was able to produce the ketenyl anion product more selective by modifying those parameters, shown in Figure 2. As R group is more electron-withdrawing group, it becomes more likely to leave than tosyl group. For example, changing R group from cyclohexyl group (Cy) to phenyl group (Ph) favored the ketenyl anion product with R1 group leaving by 76%. This is because phenyl group is less electron rich and less nucleophilic compared to cyclohexyl group, resulting in more stable by itself. [7] For alkali metal cation trend, when triphenylphosphine group is present, changing from M = Li to M = K favored in phosphine group leaving by 9%. Although it is a small effect compared to leaving group effect, this is due to Lewis acidity [8] on metal cations because a stronger Lewis acidic metal cation (Li > K in Lewis acidity) attracts tosyl group to interact, resulting in increasing leaving group ability.

Figure 1. Synthetic scheme of ketenyl anion from ylide upon sulfuration, deprotonation, and carbonylation in order. 88 % yield was reported in this reaction. 5.05 F23 ketenyl anion synthesis 4.png
Figure 1. Synthetic scheme of ketenyl anion from ylide upon sulfuration, deprotonation, and carbonylation in order. 88 % yield was reported in this reaction.
Figure 2: Synthesis of ketenyl anion from CO carbonylation of ylide, reference from Gessner et al.. M = Li, Na, K. R1 = Cy, Ph, p-(CF3)C6H4. 5.05 F23 ketenyl anion synthesis 1.jpg
Figure 2: Synthesis of ketenyl anion from CO carbonylation of ylide, reference from Gessner et al.. M = Li, Na, K. R1 = Cy, Ph, p-(CF3)C6H4.
Figure 3: Synthesis of ketenyl anion using silylene molecules stabilized by silyl group. R = SiMe3, Si Bu3. Charge of both starting materials and product is balanced by cation [K(18-6-crown)] . 5.05 F23 ketenyl anion reactivity 4.jpg
Figure 3: Synthesis of ketenyl anion using silylene molecules stabilized by silyl group. R = SiMe3, Si Bu3. Charge of both starting materials and product is balanced by cation [K(18-6-crown)] .

Inoue et al. presented synthetic route of stabilizing ketene via silica-carbonyl anion, silicon analogue of ketene. [9] They motivated this goals from recent reactivity study of silylene and disilane activating CO and isolating intermediate, hypothesizing that silica-ketenyl anion is also capable to stabilize ketene. [10] [11] [12] While Gessner et al. uses ylides to accept CO, Inoue et al. uses silylene anion with another silyl group substituted to afford insertion of CO or carbonylation at room temperature in exchange of silyl group.

Figure 4: Synthesis of ketenyl anion from CO carbonylation of phosphinidene-carbone. 5.05 F23 ketenyl anion synthesis 2.jpg
Figure 4: Synthesis of ketenyl anion from CO carbonylation of phosphinidene-carbone.

Liu et al. had another approach to stabilize and isolate ketene by using carbene coordinated by phosphinidene. [13] Carbene coordinated by 2,6-diisopropylphenyl(Dipp)-substituted phosphinidene and dinitrogen (N2) perform N2/CO ligand exchange. The starting material is similar to N-heterocyclic carbene with bulky substituents, invented by Bertrand. [14] In their studies, this reaction is concerted and thermodynamically favorable (-47.4 kcal/mol relative to N2-coordinated carbene) on coordinating CO ligand to NHC. This product is stable at room temperature inert atmosphere for a month, and no decomposition while heating in THF at 80 °C for 12 hours was observed.

Structure

Figure 5: General resonance structure of ketenyl anion. Left structure is ketene form while right structure is ynolate form. 5.05 F23 Wiki ketenyl resonance.jpg
Figure 5: General resonance structure of ketenyl anion. Left structure is ketene form while right structure is ynolate form.

As shown in Figure 5, ketenyl anion has two major resonance structures: ketenyl form and ynolate form. Due to the resonance structures, alkali metal cations can be coordinated to either at central carbon atom or terminal oxygen atom depending on its electronic structure. [5] [6] A series of structural analysis revealed both ketene and ynolate structures evenly contribute to the overall electronic structure of ketenyl anion.

From an example in Gessner's paper, the crystal structure of the ketenyl anion K[PPh2(=S)CCO] had the bond length of C-C bond (1.245 Å) and C-O bond (1.215 Å). [5] By comparing these bond length with Pyykkő's analysis on bond, [15] C-C bond is in between double bond and triple bond whereas C-O bond is in between single bond and double bond. In natural bond orbital (NBO) analysis, [16] [17] Wiberg bond index is found to be 2.06 and 1.72 for C-C bond and C-O bond, respectively. These values also suggests that both double and triple bond character for C-C bond (range of 1.20 - 1.34 Å) and both single bond and double bond character for C-O bond (range of 1.24 - 1.38 Å). The characteristic of allene-like (C=C=C) structure is also applied other ketenyl anion compounds so far. Inoue's silica-ketenyl anion product, shown in Figure 3, had Wiberg bond index of 1.68 and 1.76 for Si-C bond and C-O bond, respectively. [9] Their bond indices demonstrate that both Si-C and C-O bonds have part of double bond character that contributes of Si=C=O structure.

This ketenyl anion can dimerize in solid state as oxygen atoms interacts with alkali metal cation. This dimer can be broken up by adding M(18-crown-6) (where M = alkali metal cation), resulting in isolation of single ketenyl anion structure. [5] [9] Intrinsic bond orbitals (IBO) of the molecule [K(PPh2(=S)CCO] reveal molecular orbital describing π-orbital of C-C and C-O and delocalized orbital on oxygen atom.

Figure 6: Intrinsic bond orbitals of K[PPh2(=S)CCO] (CCDC-2201261), calculating by ORCA def2-TZVP basis set. The geometry of the structure was optimized by r SCAN3c method. 5.05 F23 ketenyl anion iboview.png
Figure 6: Intrinsic bond orbitals of K[PPh2(=S)CCO] (CCDC-2201261), calculating by ORCA def2-TZVP basis set. The geometry of the structure was optimized by r SCAN3c method.

The stability of ketenyl anion is come from the decrease of charge on ketene carbon from parent ketene to ketenyl anion. In Gessner's study, parent ketenyl anion [H-C=C=O]− has smaller positive charge (+4.0 e) on C compared to parent ketene [H2C=C=O] (+7.0 e on C). [5] This drops of charge makes the ketene less amphiphilic, leading to a more stable compound.

Reactivity

The advantage of using ketenyl anion molecule is to synthesize desired compound selectively without concerning dimerization before synthesizing a target product. [21] In ylide-ketenyl anion, electrophile can be substituted in exchange of metal to functionalize the ketene moiety at high yield. [5] Since the central carbon is negatively charged, this nucleophilicity enable substitution with a series of electrophilic compounds such as triphenylmethyl group. Some ketenyl anion can further react with other compounds to form a new functional group. For example, after electrophilic substitution of ketenyl anion with triphenylmethyl group, the treatment with water results in formation of carboxylic acid at C=O moiety. Reported compounds from Gessner et al. had more than 90% yield isolated as solid.

Figure 7: Electrophilic substitution of ketenyl anion with a series of substrate, reported in Gessner et al. 5.05 F23 ketenyl anion reactivity 1.jpg
Figure 7: Electrophilic substitution of ketenyl anion with a series of substrate, reported in Gessner et al.

Not only at the central carbon where a cation can be coordinated, other carbon atom and terminal oxygen atom can also be functionalized upon electrophilic substitution. This reactivity allows activation of chemical bonds such as S-S and C=O bonds and new bonds C-S bond and C=C bond. [5] These products requires CO and substrates of interests, which highlight new synthetic pathways of organic compounds at room temperature instead of extreme conditions such as pyrolysis. [2]

Figure 8: Reported reactivity studies on S-S and C=O bond activation by ylide-ketenyl anion by Gessner et al. 5.05 F23 ketenyl anion reactivity 2.jpg
Figure 8: Reported reactivity studies on S-S and C=O bond activation by ylide-ketenyl anion by Gessner et al.

A stabilized ketenyl anion also undergoes dimerization with disubstituted phosphine compound to form a heterocyclic product. [5] In this reaction, an intermediate is proposed to be electrophilic substitution of a disubstituted phosphine compound followed by dimerization.

Figure 9: Reported dimerization of ylide-ketenyl anion upon electrophilic substitution of disubstituted phosphines by Gessner et al. 5.05 F23 ketenyl anion reactivity 3.jpg
Figure 9: Reported dimerization of ylide-ketenyl anion upon electrophilic substitution of disubstituted phosphines by Gessner et al.

In different ketenyl anion compound, cleavage of Csp-H bond, C=N bond, and I2 bond at room temperature were also reported in phosphinidene-stabilized ketene. [13] For I2 cleaving reaction, the mechanism is proposed to be cleavage of the bond at central carbon and migration of I to phosphorus atom.

Figure 10: Reported bond cleavage reactions with phosphinidene-ketenyl anion. Charge of both starting materials and product is balanced by cation [K(18-6-crown)] . 5.05 F23 ketenyl anion reactivity 5.jpg
Figure 10: Reported bond cleavage reactions with phosphinidene-ketenyl anion. Charge of both starting materials and product is balanced by cation [K(18-6-crown)] .

Related Research Articles

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In organic chemistry, a ketene is an organic compound of the form RR'C=C=O, where R and R' are two arbitrary monovalent chemical groups. The name may also refer to the specific compound ethenone H2C=C=O, the simplest ketene.

<span class="mw-page-title-main">Organometallic chemistry</span> Study of organic compounds containing metal(s)

Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and sometimes broadened to include metalloids like boron, silicon, and selenium, as well. Aside from bonds to organyl fragments or molecules, bonds to 'inorganic' carbon, like carbon monoxide, cyanide, or carbide, are generally considered to be organometallic as well. Some related compounds such as transition metal hydrides and metal phosphine complexes are often included in discussions of organometallic compounds, though strictly speaking, they are not necessarily organometallic. The related but distinct term "metalorganic compound" refers to metal-containing compounds lacking direct metal-carbon bonds but which contain organic ligands. Metal β-diketonates, alkoxides, dialkylamides, and metal phosphine complexes are representative members of this class. The field of organometallic chemistry combines aspects of traditional inorganic and organic chemistry.

<span class="mw-page-title-main">Acyl group</span> Chemical group (R–C=O)

In chemistry, an acyl group is a moiety derived by the removal of one or more hydroxyl groups from an oxoacid, including inorganic acids. It contains a double-bonded oxygen atom and an organyl group or hydrogen in the case of formyl group. In organic chemistry, the acyl group is usually derived from a carboxylic acid, in which case it has the formula R−C(=O)−, where R represents an organyl group or hydrogen. Although the term is almost always applied to organic compounds, acyl groups can in principle be derived from other types of acids such as sulfonic acids and phosphonic acids. In the most common arrangement, acyl groups are attached to a larger molecular fragment, in which case the carbon and oxygen atoms are linked by a double bond.

An ylide or ylid is a neutral dipolar molecule containing a formally negatively charged atom (usually a carbanion) directly attached to a heteroatom with a formal positive charge (usually nitrogen, phosphorus or sulfur), and in which both atoms have full octets of electrons. The result can be viewed as a structure in which two adjacent atoms are connected by both a covalent and an ionic bond; normally written X+–Y−. Ylides are thus 1,2-dipolar compounds, and a subclass of zwitterions. They appear in organic chemistry as reagents or reactive intermediates.

Diphosphene is a type of organophosphorus compound that has a phosphorus–phosphorus double bond, denoted by R-P=P-R'. These compounds are not common, but their properties have theoretical importance.

<span class="mw-page-title-main">Metal carbonyl</span> Coordination complexes of transition metals with carbon monoxide ligands

Metal carbonyls are coordination complexes of transition metals with carbon monoxide ligands. Metal carbonyls are useful in organic synthesis and as catalysts or catalyst precursors in homogeneous catalysis, such as hydroformylation and Reppe chemistry. In the Mond process, nickel tetracarbonyl is used to produce pure nickel. In organometallic chemistry, metal carbonyls serve as precursors for the preparation of other organometallic complexes.

<span class="mw-page-title-main">Persistent carbene</span> Type of carbene demonstrating particular stability

A persistent carbene is an organic molecule whose natural resonance structure has a carbon atom with incomplete octet, but does not exhibit the tremendous instability typically associated with such moieties. The best-known examples and by far largest subgroup are the N-heterocyclic carbenes (NHC), in which nitrogen atoms flank the formal carbene.

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Borirenes are a unique class of three-membered heterocyclic compounds characterized by an unsaturated boron atom within their ring structure. First computationally predicted by John Pople and Paul von Rague Schleyer in 1981, the simplest borirene, (CH)2BH, is isoelectronic with the cyclopropenium cation and exhibits Hückel aromaticity. Borirenes undergo ring-opening reactions with polar reagents and form Lewis adducts, due to the significant ring strain in its three-membered structure and the presence of an empty p orbital on the boron atom. The balance of these two properties leads to unique properties as a ligand for transition metals, in addition to observation of photochemical rearrangement and ring expansion. While borirenes were first discovered in the 1980s, new derivatives such as benzoborirenes have led to renewed interest in the field, with their potential applications yet to be fully explored.

<span class="mw-page-title-main">Germylene</span> Class of germanium (II) compounds

Germylenes are a class of germanium(II) compounds with the general formula :GeR2. They are heavier carbene analogs. However, unlike carbenes, whose ground state can be either singlet or triplet depending on the substituents, germylenes have exclusively a singlet ground state. Unprotected carbene analogs, including germylenes, has a dimerization nature. Free germylenes can be isolated under the stabilization of steric hindrance or electron donation. The synthesis of first stable free dialkyl germylene was reported by Jutzi, et al in 1991.

<span class="mw-page-title-main">Phosphinidene</span> Type of compound

Phosphinidenes are low-valent phosphorus compounds analogous to carbenes and nitrenes, having the general structure RP. The parent phosphinidine has the formula PH. More common are the organic analogues where R = alkyl or aryl. In these compounds phosphorus has only 6 electrons in its valence level. Most phosphinidenes are highly reactive and short-lived, thereby complicating empirical studies on their chemical properties.

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<span class="mw-page-title-main">Zirconocene</span> Chemical compound

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<span class="mw-page-title-main">Polyfluoroalkoxyaluminates</span> Class of chemical compounds

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<span class="mw-page-title-main">Carbones</span> Class of molecules

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1-Phosphaallenes is are allenes in which the first carbon atom is replaced by phosphorus, resulting in the structure: -P=C=C<.

<span class="mw-page-title-main">Stable phosphorus radicals</span>

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Pnictogen-substituted tetrahedranes are pnictogen-containing analogues of tetrahedranes with the formula RxCxPn4−x. Computational work has indicated that the incorporation of pnictogens to the tetrahedral core alleviates the ring strain of tetrahedrane. Although theoretical work on pnictogen-substituted tetrahedranes has existed for decades, only the phosphorus-containing species have been synthesized. These species exhibit novel reactivities, most often through ring-opening and polymerization pathways. Phosphatetrahedranes are of interest as new retrons for organophosphorus chemistry. Their strain also make them of interest in the development of energy-dense compounds.

Alkylidene ketenes are a class of organic compounds that are of the form R2C=C=C=O. They are a member of the family of heterocumulenes (R2C=(C)n=O), and are often considered an unsaturated homolog of ketenes (R2C=C=O). Sometimes referred to as methyleneketenes, these compounds are highly reactive and much more difficult to access than ketenes.

References

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