1-Phosphaallenes

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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<.

Contents

Introduction

The first example of a stable heteroalkene E=C or E=E' (E, E' = P) containing a heavy group 15 element was reported in 1982 by Yoshifuji. [1] [2] After the realization of heavier heteroalkenes, the field of organometallic chemistry began exploring the idea of heteroallenes E=C=E' and E=C=C=E', in which one or more carbon atom of an allene is substituted by a heavier atom. The first of these heteroallenes containing a heavier group 15 element to be reported was synthesized in 1984 by Yoshifuji. [3] 1-phosphaallene, also referred to as ethenylidenephosphine or λ3-phosphaallene, was first synthesized using the 2,4,6-tri-t-butylphenyl (supermesityl, or Mes*) moiety as a sterically protecting group. [4] [5] The most widely known example of 1-phosphaalenes is the Mes* substituted 3H-phosphallene, where a hydrogen is bonded to the terminal carbon atom, which was synthesized by Märkl and Reitinger in 1988. [6] [7] Some more recent advances made in regards to 1-phosphaallenes include the development of simpler synthetic routes and the discovery of synthetic pathways using phosphaallenes to create molecules that are typically only synthesized through complicated methods.

Synthesis

Multiple synthetic routes to 1-phosphaallenes have been reported. The most common are Phospha-Wittig-Horner reactions, rearrangement of alkylynlphosphines with a base present, dehydrochlorination of a chlorovinylphospine, and the reaction of lithium with a C,C-dichlorophosphirane. [3] [4] More recent synthetic advances include the dehydrobromination of phosphaalkenes the hydroalumination of alkynylphosphines. [8] [9]

Phospha-Wittig-Horner reaction

The Phospha-Wittig-Horner reaction, first reported by Mathey et al. in 1992, was initially developed as a synthetic route to convert aldehydes and ketones into C-mono and C,C-disubstituted phosphaalkenes, respectively. [10] [11] This method was then adapted to rapidly synthesize phosphaallenes by reacting phosphaketenes with an organolithium base, such as lithium diisopropylamide or t-BuOLi. [4] [10] [12]

Rearrangement of alkynylphosphines

Rearrangement of alkynylphosphine.png
n-BuLi is used as a strong base in the synthesis of a phosphaallene.
Rearrangement of alkynylphosphine2.png
A lithium complex can be used to synthesize phosphaallenes.

Like the transition from an alkyne to an allene in organic chemistry, phosphaallenes can be synthesized from alkynylphosphines by adding a strong base such as n-butyllithium (n-BuLi). These complexes are typically useful due to the broad range of stable products that can be obtained from this synthetic route. [3] [4] [13]

Multiple stable phosphaallenes have been synthesized from LiC≡CR and ArP(H)Cl (R = Ph, t-Bu, Me, CH2OSiMe3). The basicity of the lithium complex is essential to the success of these reactions. [4]

Dehydrochlorination of a chlorovinylphosphine

The dehydrochlorination of 1-chlorovinylphosphine by DBU (1,8-diazabicyclo-[5.4.0]undec-7-ene) yields a variety of results, depending on the structure of the initial chlorovinylphosphine. Many of these reactions require temperatures between -90 °C and 0 °C to ensure a stable product is formed. As many of these reactions are warmed, oligomerization of the phosphaallene occurs. Other reactions following this synthetic route occur with gas phase reagents at 250 °C. [4] [14]

Reaction of Lithium with a C,C-dichlorophosphirane

In this synthetic route, a dichlorocarbene is first reacted with a phosphaethylene. The addition of t-BuLi to the resulting 2,2-dichlorophosphirane yields the phosphaallene. This route includes an insertion of carbon into the P=C bond of a phosphaalkene. [4] [15]

Li with dichlorophosphirane.png

Dehydrobromination of phosphaalkenes

(Z)-2,5-dibromo-1-phosphapent-1-ene with the sterically cumbersome Mes* moiety was reacted with t-BuOK to yield the cyclopropylidenephosphaethene product. Treatment of the (Z)-2,5-dibromo-1-phosphapent-1-ene reagent with a weaker base, EtONa yields a non-cyclic isomer that can undergo cyclization in the presence of t-BuOK. [8]

Dehydrobromination.png

Hydroalumination of alkynylphosphines

In 2019, a new synthetic route for the synthesis of phosphaallenes was reported by Klöcker and coworkers. The hydroalumination of dialkynylphosphines with different alkylaluminium hydrides resulted in the elimination of dialkylaluminium alkynides and the desired phosphaallenes. Many of the phosphaallenes Klöcker and coworkers synthesized spontaneously decomposed into less-desired products, but the newly reported synthetic method did allow for the isolation of some highly reactive compounds that had not been isolated previously. [9]

Hydroalumination of alkynylphosphine.png

Reactivity

In 2022, the most notable aspect of 1-phosphaallenes is their reactivity. Phosphaallenes contain three reactive centers: the P=C bond, the C=C bond, and the phosphorus atom. The P=C bond and the C=C bond are thought to be electronically independent of one another. This gives rise to a wide variety of reactivity, which is similar to that of phosphaalkenes.

Dimerization

Without a large steric hindrance, most phosphaallenes quickly dimerize in a [2+2]- or [2+3]-cycloaddition to yield four- or five-membered rings. [16] Thus, in order to prevent dimerization, a bulky moiety such as Mes* is often used. Dimerization frequently occurs in a head-to-tail manner, exhibiting coupling between either the two C=C bonds or the two P=C bonds. The regioselectivity of these dimerization reactions depends on the substituents on the C in the 3 position. [3]

Phosphaallene dimerization.png

Reaction with alkoxide or lithium compounds

Nucleophilic alkoxide as well as n-BuLi will add to the P=C bond of a phosphaallene. As seen in the reaction scheme, the central C atom is the preferred location for proton attack due to the partial negative charge it possesses. [4] [9] [13] [17] [18] [19]

Phosphaallene via nucliophilic alkoxide.png

Reactions involving carbenes

The reaction of phosphaallenes with dichlorocarbenes results in a [2+1]-cycloaddition to the P=C double bond, which results in stable heterocyclic compounds. [4] [20]

Phosphaallene 2+1 carbene.png

Phosphaallenes have also been investigated as reagents to form carbenes. Reacting the phosphaallene H2P=C=PH with acetylene yields a carbene via [3 + 2]-cycloaddition. [21]

Phosphaallene 3+2 carbene.png

Reaction with transition metal complexes

Crystal structure of Phosphaallene-W bond.png
Phosphaallene-tungsten complex displaying ɳ1 coordination.
Crystal structure of Phosphaallene-Pt bond.png
Phosphaallene-platinum complex displaying ɳ2 coordination.
Crystal structure showing phosphaallene-transition metal coordination. [9]

Phosphaallenes can coordinate in an ɳ1 or ɳ2 fashion depending on the metal and experimental conditions. Recent studies into the coordination behavior of the 3H-phosphaallene with tungsten (W) show that both stable and unstable phosphaallenes can bond to W through and ɳ1-coordination mode via a P-W bond. [9] [13] Due to the stabilizing effect of coordination, reacting transient phosphaallenes with W has become a common method to create stable phosphaallenes that can then be characterized. These studies have also demonstrated that 3H-phosphaallenes exhibit ɳ2-coordination to Pt via side-on overlap between Pt and the P=C double bond resulting in a three-membered PtPC heterocycle. [9] [13] [22]

Decomposition

3-Phosphaallene.png
Head-to-tail dimerization of 3H-phosphaallene.
Phosphaallene formed tricyclic phosphine.png
Tricyclic phosphine resulting from decomposition via irradiation.

Decomposition of the 3H-phosphaallene occurs at room-temperature to produce several products via P=C (head-to-tail) or C=C dimerization. [15] One decomposition pathway results in an unusual tricyclic phosphine which can be selectively formed by irradiating a solution of the phosphaallene, iBu2AlH, and TripP(C≡CtBu)2 in n-hexanes for 60h with a mercury vapor lamp. Under these conditions, the tricyclic phosphine was isolated in high yield and a purity of >95%. [16]

Formation of bicyclic hydrophosphine

Recently, two novel 1-benzodihydrophosphetes were synthesized by heating 3H-phosphaallene Mes*P=C=C(H)R (R = tBu, Ad) in toluene at 110 °C for 16-20h. The resulting 1- benzodihydrophosphetes were not stable unless coordinated to tungsten. [16] The phosphorus-bound H of the product can be used in catalyst-free hydrophosphination reactions and in the synthesis of bulky phosphine ligands. [19]

Phosphaallene - bicyclic hydrophosphine.png

Hydroboration of the C=C Bond

Hydroboration is used as a method to reduce unsaturated compounds, and has been used to reduce P-C double or triple bonds in phosphaalkenes and phosphaalkynes. Hydroboration of the 3H-phosphaallene Mes*-P=C=C(H)-tBu with 1:1 molar ratio of phosphaallene to H3B(SMe2) yielded a di(phosphaalkenyl)borane, and the reaction in 2:1 molar ratio of phosphaallene to H3B(SMe2) yielded the desired product with a B atom bound to two P=C double bonds. The hydroboration of the C=C bond was preferred over the hydroboration of the P=C bond. [19]

Hydroboration of CC bond.png

Properties

P=C=C bonding

Phosphaallenes are typically colorless or pale yellow in color, and are often crystalline in structure; however, few crystal structures have been reported for this class of compounds. [4] One compound, ArP=C=CPh2 (Ar =Mes*) was synthesized and characterized by single crystal X-ray diffraction. This compound showed a relatively short P=C double bond length of 1.625(4)Å and a short C=C bond length of 1.1327(5)Å, which closely resembles the bond length in an allene. [23]

The crystal structure also showed a bent geometry with the P=C=C bond angle at 168.0(3)°. [23] This bent geometry is thought to be a result of both steric and electronic effects. One possible explanation for the observed bent geometry is the bonding model postulated by Lappert et al. which described the bonding between P and the C as singlet states that act in a double π-donor-acceptor fashion. [24] [25] [26] [27]

MO overlap of bent phosphaallenes.png

The crystal structure of another 1-phosphaallene RP=C=CHR' (R= Mes*, R' = tBu) displays the same bent geometry with the P=C=C bond angle being reported as 170.7°. Further, this compound also contains a short P=C bond length (1.634Å) and a short C=C bond length (1.136Å). [9]

Crystal structure of bent phosphaallene.png

Electronic structure

The electronic structure of 1-phosphaallene is nearly identical to that of phosphaalkene. The HOMO in both molecules is the π-orbital of the C=P bond, which is primarily P in character. The LUMO in both structures, is also heavily localized over the C=P bond. However, the dipole moment in phosphaallenes is larger than that of phosphaalkenes. [4]

HOMO-LUMO orbitals in phoshpaallenes.png

Vibrational frequencies

In compounds with the antisymmetric form X=C=Y, IR stretching bands are observed in the 1600–2300 cm−1 region. However, the IR spectra of 1-phosphaallenes do not contain bands in this region, which suggests that the phosphaallene P=C=C is relatively symmetric. In the phosphaallene H2C=C=PH, stretching frequencies are observed at 1788 and 789 cm−1, which are attributed to the C=C bond and the C=P bond, respectively. In similar compounds containing a C=C=N skeleton, there is a very strong band present within the 1600 – 2300 cm−1 region. The presence of two stretching frequencies rather than one strong frequency in the phosphaallene supports that the C=C and C=P bonds are nearly independent of one another electronically. [4]

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<span class="mw-page-title-main">Allenes</span> Any organic compound containing a C=C=C group

In organic chemistry, allenes are organic compounds in which one carbon atom has double bonds with each of its two adjacent carbon atoms. Allenes are classified as cumulated dienes. The parent compound of this class is propadiene, which is itself also called allene. An group of the structure R2C=C=CR− is called allenyl, where R is H or some alkyl group. Compounds with an allene-type structure but with more than three carbon atoms are members of a larger class of compounds called cumulenes with X=C=Y bonding.

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

Phosphorine is a heavier element analog of pyridine, containing a phosphorus atom instead of an aza- moiety. It is also called phosphabenzene and belongs to the phosphaalkene class. It is a colorless liquid that is mainly of interest in research.

<span class="mw-page-title-main">Organoboron chemistry</span> Study of compounds containing a boron-carbon bond

Organoboron chemistry or organoborane chemistry studies organoboron compounds, also called organoboranes. These chemical compounds combine boron and carbon; typically, they are organic derivatives of borane (BH3), as in the trialkyl boranes.

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 are of theoretical interest. Normally, compounds with the empirical formula RP exist as rings. However, like other multiple bonds between heavy main-group elements, P=P double bonds can be stabilized by a large steric hindrance from the substitutions. The first isolated diphosphene bis(2,4,6-tri-tert-butylphenyl)diphosphene was exemplified by Masaaki Yoshifuji and his coworkers in 1981, in which diphosphene is stabilized by two bulky phenyl group.

<span class="mw-page-title-main">Phosphaalkyne</span>

In chemistry, a phosphaalkyne is an organophosphorus compound containing a triple bond between phosphorus and carbon with the general formula R-C≡P. Phosphaalkynes are the heavier congeners of nitriles, though, due to the similar electronegativities of phosphorus and carbon, possess reactivity patterns reminiscent of alkynes. Due to their high reactivity, phosphaalkynes are not found naturally on earth, but the simplest phosphaalkyne, phosphaethyne (H-C≡P) has been observed in the interstellar medium.

In organic chemistry, hydroboration refers to the addition of a hydrogen-boron bond to certain double and triple bonds involving carbon. This chemical reaction is useful in the organic synthesis of organic compounds.

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<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

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<i>tert</i>-Butylphosphaacetylene Chemical compound

tert-Butylphosphaacetylene is an organophosphorus compound. Abbreviated t-BuCP, it was the first example of an isolable phosphaalkyne. Prior to its synthesis, the double bond rule had suggested that elements of Period 3 and higher were unable to form double or triple bonds with lighter main group elements because of weak orbital overlap. The synthesis of t-BuCP discredited much of the double bond rule and opened new studies into the formation of unsaturated phosphorus compounds.

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<span class="mw-page-title-main">Phosphenium</span> Divalent cations of phosphorus

Phosphenium ions, not to be confused with phosphonium or phosphirenium, are divalent cations of phosphorus of the form [PR2]+. Phosphenium ions have long been proposed as reaction intermediates.

<span class="mw-page-title-main">Phosphasilene</span>

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<span class="mw-page-title-main">Nontrigonal pnictogen compounds</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.

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. 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, this recent finding about stabilizing ketene with main-group elements opens a variety of synthetic routes to target desired products.

References

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  22. ↑ Nixon, John F. (November 1988). "Coordination chemistry of compounds containing phosphorus-carbon multiple bonds". Chemical Reviews. 88 (7): 1327–1362. doi:10.1021/cr00089a015.
  23. 1 2 Yoshifuji, Masaaki; Toyota, Kozo; Inamoto, Naoki; Hirotsu, Ken; Higuchi, Taiichi; Nagase, Shigeru (December 1985). "Structures of Phosphaethylenes and a 1-Phosphaallene Containing Phosphorus in Lower Coordination State". Phosphorus and Sulfur and the Related Elements. 25 (3): 237–243. doi:10.1080/03086648508072740.
  24. ↑ Eichler, Barrett; West, Robert (2000). "Chemistry of group 14 heteroallenes". Advances in Organometallic Chemistry. 46: 1–46. doi:10.1016/S0065-3055(00)46001-6. ISBN   978-0-12-031146-0.
  25. ↑ Davidson, Peter J.; Harris, David H.; Lappert, Michael F. (1976). "Subvalent Group 4B metal alkyls and amides. Part I. The synthesis and physical properties of kinetically stable bis[bis(trimethysilyl)methyl]-germanium(II), -tin(II), and -lead(II)". Journal of the Chemical Society, Dalton Transactions (21): 2268. doi:10.1039/DT9760002268.
  26. ↑ Grützmacher, Hansjörg; Freitag, Stefanie; Herbst-Irmer, Regine; Sheldrick, George S. (April 1992). "Investigations of the Structure and Reactivity of a Stannaketenimine". Angewandte Chemie International Edition in English. 31 (4): 437–438. doi:10.1002/anie.199204371.
  27. ↑ Chen, Xiaodan; Li, Zhongshu; Frenking, Gernot; Fernández, Israel; Zhao, Lili; Grützmacher, Hansjörg (12 June 2019). "Bent Phosphaallenes With "Hidden" Lone Pairs as Ligands". Chemistry – A European Journal. 25 (33): 7912–7920. doi:10.1002/chem.201900645. PMID   30927503. S2CID   88481667.
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