{{Chembox |ImageFileL1 = UnsubstitutedPhosphole.png |ImageFileR1 = Phosphole_B3LYP_6-31G.png |PIN = 1''H''-Phosphole<ref name=iupac2013>{{cite book |title= Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013 (Blue Book) |publisher= The Royal Society of Chemistry |date= 2014 |location= Cambridge |page= 146 |doi= 10.1039/9781849733069-00130 |isbn= 978-0-85404-182-4}}</ref> |Section1={{Chembox Identifiers |CASNo = 288-01-7 |CASNo_Ref = {{cascite|correct|CAS}} | Beilstein = 969375 | ChEBI = 33134 |PubChem = 164575 |ChemSpiderID = 144273 |SMILES = P1C=CC=C1 |InChI = InChI=1S/C4H5P/c1-2-4-5-3-1/h1-5H }} |Section2={{Chembox Properties |C=4 | H=5 | P=1 }} |Section8={{Chembox Related | OtherCompounds = Pyrrole, bismole, arsole, stibole; phosphorine}} }}

'''Phosphole''' is the organic compound with the chemical formula {{chem|C|4|H|4|P|H}}; it is the phosphorus analog of pyrrole. The term phosphole also refers to substituted derivatives of the parent heterocycle. These compounds are of theoretical interest but also serve as ligands for transition metals and as precursors to more complex organophosphorus compounds.

'''Triphosphole''', {{chem|C|2|H|3|P|3}}, is a heterocycle with 3 phosphorus atoms.

'''Pentaphosphole''', {{chem|P|5|H}}, is a cyclic compound with 5 phosphorus atoms.

==Structure and bonding== Unlike the related 5-membered group 15 heterocycle pyrrole the aromaticity of phospholes is diminished, reflecting the reluctance of phosphorus to delocalise its lone pair.<ref>{{cite journal|author1=D. B. Chesnut |author2=L. D. Quin |title=The important role of the phosphorus lone pair in phosphole aromaticity|journal=Heteroatom Chemistry|volume=18|pages=754–758 |year=2007|issue=7 |doi=10.1002/hc.20364}}</ref> The main indication of this difference is the pyramidalisation of phosphorus. The absence of aromaticity is also indicated by the reactivity of phospholes.<ref>{{cite journal|title=Phosphaphenalenes: An Evolution of the Phosphorus Heterocycles|journal=Synlett|year=2016|author1=Philip Hindenberg|author2=Carlos Romero-Nieto|doi=10.1055/s-0035-1562506|volume=27|issue=16 |pages=2293–2300}}</ref>

Contrariwise, deprotonation at phosphorus gives the highly aromatic phospholyl {{sic}} anion.<ref name=NoncoordRev/>{{rp|434}}

==Preparation== The parent phosphole was first described in 1983, prepared by low-temperature protonation of lithium phospholide.<ref name=Mathey>{{cite journal |title=Proton [1,5] shifts in P-unsubstituted 1H-phospholes. Synthesis and chemistry of 2H-phosphole dimers |author1=Claude Charrier |author2=Hubert Bonnard |author3=Guillaume De Lauzon |author4=Francois Mathey |journal=J. Am. Chem. Soc. |year=1983 |volume=105|issue=23 | pages=6871–6877 |doi=10.1021/ja00361a022|bibcode=1983JAChS.105.6871C }}</ref> Pentaphenylphosphole was reported in 1953.<ref>''A Guide to Organophosphorus Chemistry'' Louis D. Quin '''2000''' John Wiley & Sons {{ISBN|0-471-31824-8}}</ref> One route to phospholes is via the McCormack reaction, involving the addition of a 1,3-diene to a phosphonous chloride (RPCl<sub>2</sub>) followed by dehydrohalogenation.<ref>{{OrgSynth |author= W. B. McCormack |title= 3-Methyl-1-Phenylphospholene oxide |collvol= 5 |collvolpages= 787 |year= 1973 |prep= CV5P0787}}</ref> Phenylphospholes can be prepared via zirconacyclopentadienes by reaction with PhPCl<sub>2</sub>.<ref>{{OrgSynth |author1= Paul J. Fagan |author2= William A. Nugent |title= 1-Phenyl-2,3,4,5-Tetramethylphosphole |collvol= 9 |collvolpages= 653 |year= 1998 |prep= CV9P0653}}</ref> Alternatively, phospholes can be generated from addition to a (conjugated) diyne. Radical or strongly basic conditions add phenylphosphine in conjugate across the two triple bonds,<ref name=NoncoordRev>{{cite journal|doi=10.1021/cr00084a005|journal=Chemical Reviews|year=1988|volume=88|title=The organic chemistry of phospholes|first=F.|last=Mathey|pp=429&ndash;453|publisher=American Chemical Society}}</ref>{{rp|431}} and electron-poor alkynes add to organophosphite esters to give phospholes.<ref name=NoncoordRev/>{{rp|432}} :400px|Phospholes

==Reactivity== Phospholes undergo different cycloaddition reactions; coordination properties of phospholes are also well studied.<ref>{{cite journal|title=Phospholes – development and recent advances|journal=Mendeleev Communications|year=2013|author1=Almaz Zagidullin|author2=Vasily A. Miluykov|doi=10.1016/j.mencom.2013.05.001|volume=23|issue=3 |pages=117–130}}</ref>

The behavior of the secondary phospholes, those with P&minus;H bond, is dominated by the reactivity of this bond.<ref name=Mathey/> They readily rearrange by migration of H from P to carbon 2, followed by dimerisation. The corresponding anions are strong acids, not protonating in water.<ref name=NoncoordRev/>{{rp|448}}

Most phospholes are tertiary, typically P-methyl or P-phenyl. The weak aromaticity of these phospholes is manifested in their reactivity: for example, phospholes are basic at P, and serve as ligands,<ref name=Mathey/> although they are less basic than divinylphosphines and quaternize slowly.<ref name=NoncoordRev/>{{rp|p=439}} With strong dienophiles (e.g., electrophilic alkynes) they undergo Diels–Alder reactions and "upon oxidation, sulfurization, quaternization, or complexation of the phosphole lone pair, the reactivity of the dienic system sharply increases as expected".<ref name=NoncoordRev/>{{rp|441}} P&minus;C bonds remain intact in most reactions, but 7-phosphanorbornadiene oxides eliminate the corresponding phosphonous anhydride to give the benzene.<ref name=NoncoordRev/>{{rp|443}}

λ<sup>5</sup> coordination at P is also possible, although orbital overlap with the adjacent π orbitals means that such substituents tend to migrate to the adjacent carbon atoms.<ref name=NoncoordRev/>{{rp|pp=433-434}} Phospholes react with nucleophilic acids to give the corresponding phospholene, as though they were protonated at the 2-carbon, but in fact the process is an oxidative addition to phosphorus to give a λ<sup>5</sup> phosphorane, followed by hydrogen migration.<ref name=NoncoordRev/>{{rp|439}}

Electrophilic substitution onto phospholes is difficult and rare. Organolithium compounds can displace a substituent from P in "clean nucleophilic attacks at phosphorus" or add in conjugate to give the corresponding phospholene.<ref name=NoncoordRev/>{{rp|pp=440,444}} Friedel-Crafts acylation occurs to phospholes coordinated to =Mo(CO)<sub>5</sub>, but not Vilsmeier-Haack formylation.<ref name=NoncoordRev/>{{rp|445}}

2,5-Diphenyl phospholes can be functionalised by deprotonation followed by P-acylation then a 1H, 2H, 3H phospholide equilibrium resulting in a 1:3 shift of the acyl group.<ref>{{cite journal |author1=Magali Clochard |author2=Joanna Grundy |author3=Bruno Donnadieu |author4=François Mathey |name-list-style=amp |title=A straightforward synthesis of 3-acylphospholes |journal=Organic Letters |year=2005 |volume=7 |issue=20 |pages=4511–4513 |doi=10.1021/ol051816d |pmid=16178571}}</ref>

Phospholes can also be turned into β-functional phosphabenzenes via functionalisation by imidoyl chloride and insertion.<ref>{{cite journal |author1=Grundy, J. |author2=Mathey, F. |name-list-style=amp |year=2005 |title=One-Pot Conversion of Phospholide Ions into β-Functional Phosphinines |journal=Angewandte Chemie International Edition |volume=44 |issue=7 |pages=1082–1084 |doi=10.1002/anie.200462020 |pmid=15662672}}</ref> In general, phosphole oxides are stable only when pentasubstituted, otherwise forming a dimer with substantial angle strain at P.<Ref name=NoncoordRev/>{{rp|442}}

==See also== * Benzophosphole * Metallole * Organophosphorus compound * Phosphorine, {{chem|C|5|H|5|P}}

==References== {{Reflist|30em}}

{{Simple aromatic rings}}

Category:Phosphorus heterocycles Category:Five-membered rings Category:Substances discovered in the 1980s