1-Phosphaallenes

Main-group allene analog 1-Phosphaallenes are allenes in which the first carbon atom is replaced by phosphorus, resulting in the structure: -P=C=C<. 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.Yoshifuji M. Shima I. Inamoto N. Hirotsu K. Higuchi T. Synthesis and structure of bis(2,4,6-tri-tert-butylphenyl)diphosphene: isolation of a true phosphobenzene Journal of the American Chemical Society July 1981 103 15 4587–4589 10.1021/ja00405a054Yoshifuji Masaaki Shima Ichiro Inamoto Naoki Hirotsu Ken Higuchi Taiichi Additions and Corrections – Synthesis and Structure of Bis(2,4,6-tri-tert-butylphenyl)diphosphene: Isolation of a True "Phosphobenzene" Journal of the American Chemical Society November 1982 104 22 6167 10.1021/ja00386a605 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.Escudié Jean Ranaivonjatovo Henri Group 14 and 15 Heteroallenes ECC and ECE' Organometallics 1 March 2007 26 7 1542–1559 10.1021/om0610086 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.Escudié Jean Ranaivonjatovo Henri Rigon Leslie Heavy Allenes and Cumulenes ECE' and ECCE' (E = P, As, Si, Ge, Sn; E' = C, N, P, As, O, S) Chemical Reviews 1 October 2000 100 10 3639–3696 10.1021/cr990013z 11749324 Yoshifuji Masaaki Toyota Kozo Inamoto Naoki Isolation and characterisation of the first stable diphospha-allene: P,P-bis(2,4,6-tri-t-butylphenyl)-1,3-diphospha-allene J. Chem. Soc., Chem. Commun. 1984 11 689–690 10.1039/C39840000689 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.Klöcker Hans Layh Marcus Hepp Alexander Uhl Werner Hydroalumination of alkynyl-aminophosphines as a promising tool for the synthesis of unusual phosphines: P–N bond activation, a transient phosphaallene, a zwitterionic AlP 2 C 2 heterocycle and a masked Al/P-based frustrated Lewis pair Dalton Transactions 2016 45 5 2031–2043 10.1039/C5DT02825F 26391528 Märkl G. Reitinger S. 3H-phosphaallen – alkinyl-1H-phosphan – tautomere Tetrahedron Letters January 1988 29 4 463–466 10.1016/S0040-4039(00)80122-7 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. More recent synthetic advances include the dehydrobromination of phosphaalkenes the hydroalumination of alkynylphosphines.Ito Shigekazu Kimura Shigeo Yoshifuji Masaaki Preparation, Structure, and Reaction of a Sterically Encumbered 1-Phosphaallene Containing a Cyclopropylidene Moiety Organic Letters 1 April 2003 5 7 1111–1114 10.1021/ol0341750 12659586 Klöcker Hans Tendyck Jonas C. Keweloh Lukas Hepp Alexander Uhl Werner 3H-Phosphaallenes Revisited: Facile Synthesis by Hydroalumination of Alkynylphosphines and β-Elimination, Stability and Trapping of Transient Species by Coordination to Transition Metal Atoms Chemistry – A European Journal 27 March 2019 25 18 4793–4807 10.1002/chem.201806334 30681211 59252113 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.Arkhypchuk Anna I. Svyaschenko Yurii V. Orthaber Andreas Ott Sascha Mechanism of the Phospha-Wittig-Horner Reaction Angewandte Chemie International Edition 17 June 2013 52 25 6484–6487 10.1002/anie.201301469 23653134 3738942 De Vaumas Rene Marinetti Angela Ricard Louis Mathey Francois Use of prochiral phosphaalkene complexes in the synthesis of optically active phosphines Journal of the American Chemical Society January 1992 114 1 261–266 10.1021/ja00027a034 This method was then adapted to rapidly synthesize phosphaallenes by reacting phosphaketenes with an organolithium base, such as lithium diisopropylamide or t-BuOLi.Appel Rolf Fölling Peter Josten Bernhard Siray Mustafa Winkhaus Volker Knoch Falk Phosphaallenes Angewandte Chemie International Edition in English August 1984 23 8 619–620 10.1002/anie.198406191 Rearrangement of alkynylphosphines center vertical right 450 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.Reactions of a Sterically Protected 2,3-Dichloro-1,4-Diphospha-1,3-Butadiene with Some Alkyl and Hydrido Lithium Reagents 10.1002/(SICI)1098-1071(2000)11:3<171::AID-HC1>3.0.CO;2-6 2000 11 3 171–176 Ito Shigekazu Toyota Kozo Yoshifuji Masaaki Heteroatom Chemistry free 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. 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.Guillemin Jean-Claude Janati Tajdine Denis Jean-Marc Lewis base-induced rearrangement of primary ethyn-1-ylphosphines, a new and efficient route to phosphaalkynes Journal of the Chemical Society, Chemical Communications 1992 5 415 10.1039/C39920000415 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.Yoshifuji Masaaki Yoshimura Hideki Toyota Kozo Reaction of Dichlorocarbene with Phosphaethylenes. Preparation of Phosphaallene from Phosphaethylene via Dichlorophosphirane Chemistry Letters May 1990 19 5 827–830 10.1246/cl.1990.827 500px|center 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. 350px|center 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. 350px|center 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.Tendyck Jonas C. Klöcker Hans Schürmann Lina Würthwein Ernst-Ulrich Hepp Alexander Layh Marcus Uhl Werner Aspects of Phosphaallene Chemistry: Heat-Induced Formation of 1,2-Dihydrophosphetes by Intramolecular Nucleophilic Aromatic Substitution and Photochemical Generation of Tricyclic Phosphiranes The Journal of Organic Chemistry 20 November 2020 85 22 14315–14332 10.1021/acs.joc.9b03056 32022561 211036465 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. 400px|center 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.Hafner Michael Wegemann Thomas Regitz Manfred Organophosphorus Compounds; 72. 1 The Tri- tert -butylcyclopropenyl Group as a Steric Protecting Function for 1-Phosphaallenes Synthesis 1993 1993 12 1247–1252 10.1055/s-1993-26036Nguyen Minh Tho Hegarty Anthony F. Structure and properties of 1-phospha-allene (H2C=C=PH). α-Carbon versus phosphorus protonation? J. Chem. Soc., Perkin Trans. 2 1985 12 1999–2004 10.1039/P29850001999Tendyck Jonas C. Hepp Alexander Würthwein Ernst-Ulrich Uhl Werner New Reactivity Patterns in 3H-Phosphaallene Chemistry [Aryl-P=C=C(H)- t Bu]: Hydroboration of the C=C Bond, Deprotonation and Trimerisation Chemistry – A European Journal 4 December 2020 26 68 15977–15988 10.1002/chem.202002506 32618025 7756809 450px|center 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.Yoshifuji Masaaki Shibata Manabu Toyota Kozo Miyahara Ikuko Hirotsu Ken Reaction of dichlorocarbene with 1-(2,4,6-tri-t-butylphenyl)-1-phosphaallenes Heteroatom Chemistry June 1994 5 3 195–200 10.1002/hc.520050302 350px|center 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.Fernández Israel Cossío Fernando P. Bickelhaupt F. Matthias Aromaticity and Activation Strain Analysis of [3 + 2] Cycloaddition Reactions between Group 14 Heteroallenes and Triple Bonds The Journal of Organic Chemistry 1 April 2011 76 7 2310–2314 10.1021/jo102572x 21388217 350px|center Reaction with transition metal complexes right 200 Crystal structure showing phosphaallene-transition metal coordination. Crystal structure of Phosphaallene-W bond.png Phosphaallene-tungsten complex displaying ɳ1 coordination. Crystal structure of Phosphaallene-Pt bond.png Red cartouche Phosphaallene-platinum complex displaying ɳ2 coordination. 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. 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.Nixon John F. Coordination chemistry of compounds containing phosphorus-carbon multiple bonds Chemical Reviews November 1988 88 7 1327–1362 10.1021/cr00089a015 Decomposition right 200 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. 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%. 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. The phosphorus-bound H of the product can be used in catalyst-free hydrophosphination reactions and in the synthesis of bulky phosphine ligands. 420px|center 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. 400px|center 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. 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.Yoshifuji Masaaki Toyota Kozo Inamoto Naoki Hirotsu Ken Higuchi Taiichi Nagase Shigeru Structures of Phosphaethylenes and a 1-Phosphaallene Containing Phosphorus in Lower Coordination State Phosphorus and Sulfur and the Related Elements December 1985 25 3 237–243 10.1080/03086648508072740 The crystal structure also showed a bent geometry with the P=C=C bond angle at 168.0(3)°. 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.Eichler Barrett West Robert Chemistry of group 14 heteroallenes Advances in Organometallic Chemistry 2000 46 1–46 10.1016/S0065-3055(00)46001-6 978-0-12-031146-0 Davidson Peter J. Harris David H. Lappert Michael F. 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 1976 21 2268 10.1039/DT9760002268Grützmacher Hansjörg Freitag Stefanie Herbst-Irmer Regine Sheldrick George S. Investigations of the Structure and Reactivity of a Stannaketenimine Angewandte Chemie International Edition in English April 1992 31 4 437–438 10.1002/anie.199204371Chen Xiaodan Li Zhongshu Frenking Gernot Fernández Israel Zhao Lili Grützmacher Hansjörg Bent Phosphaallenes With "Hidden" Lone Pairs as Ligands Chemistry – A European Journal 12 June 2019 25 33 7912–7920 10.1002/chem.201900645 30927503 88481667 350px|center 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). 290px|center 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. 300px|center 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. References Category:Alkene derivatives Category:Organophosphanes Category:Heteroallenes