View source for P4-t-Bu

{{DISPLAYTITLE:P<sub>4</sub>-''t''-Bu}}
{{Chembox
| Name = P<sub>4</sub>-''t''-Bu
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| ImageFile = P4-t-Bu Strukturformel.svg
| ImageSize = 250px
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| Section1 = {{Chembox Identifiers
| CASNo = 111324-04-0
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| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = 114S812CQB
| PubChem = 4339838
| SMILES = CC(C)(C)N=P(N=P(N(C)C)(N(C)C)N(C)C)(N=P(N(C)C)(N(C)C)N(C)C)N=P(N(C)C)(N(C)C)N(C)C
  }}
| Section2 = {{Chembox Properties
| Formula = {{chem2|(CH3)3C\sN\dP(\sN\dP(\sN(CH3)2)3)3}}
| C=22|H=63|N=13|P=4
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| Section3 = {{Chembox Hazards
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}}

'''P<sub>4</sub>-''t''-Bu''' is a readily accessible [[chemical]] from the group of neutral, [[peralkylated]] [[Steric effects#Steric hindrance|sterically hindered]] [[polyaminophosphazenes]], which are extremely strong [[Base (chemistry)|bases]] but very weak [[nucleophiles]], with the formula {{chem2|(CH3)3C\sN\dP(\sN\dP(\sN(CH3)2)3)3}}. "''t''-Bu" stands for [[Butyl group|''tert''-butyl]] {{chem2|(CH3)3C}}–. "P<sub>4</sub>" stands for the fact that this molecule has 4 [[phosphorus]] atoms. P<sub>4</sub>-''t''-Bu can also be regarded as tetrameric [[triaminoiminophosphorane]] of the basic structure {{chem2|H\sN\dP(\sNH2)3}}. The [[homologous series]] of P<sub>1</sub> to P<sub>7</sub> polyaminophosphazenes<ref>{{citation|author1=R. Schwesinger |display-authors=etal|periodical=[[Angew. Chem.]]|title=Wie stark und wie gehindert können ungeladene Phosphazene sein?|volume=105|issue=9|pages=1420–1422|date=1993|language=German|doi=10.1002/ange.19931050940}}</ref><ref name="Schwesinger" >{{citation|author=R. Schwesinger |display-authors=etal |periodical=[[Liebigs Ann. Chem.]]|title=Extremely strong, uncharged auxiliary bases; Monomeric and polymer-supported polyaminophosphazenes (P2-P5)|volume=1996|pages=1055–10081|date=1996|issue=7 |doi=10.1002/jlac.199619960705}}</ref> of the general formula <chem>[(R^1_2N)_3P=N-]_\mathit{x}-(R^1_2N)_{3\!-\mathit{x}}P=NR2</chem> with preferably methyl groups as R<sup>1</sup>, a [[methyl group]] or [[tert-butyl group|''tert''-butyl group]] as and even-numbered ''x'' between 0 and 6 (P<sub>4</sub>-''t''-Bu: R<sup>1</sup> = Me, R<sup>2</sup> = ''t''-Bu and ''x'' = 3)<ref name="Hupfield">{{Cite patent|country=US |number =6353075 |title=Polymerization of siloxanes |pubdate =2002-03-05|fdate =1999-12-09 |inventor =P. Hupfield, A. Surgenor, R. Taylor | assign=Dow Corning Ltd.}}</ref> has been developed by Reinhard Schwesinger; the resulting [[phosphazene]] bases are therefore also referred to as Schwesinger [[superbases]].<ref>{{citation|author1=J. Saame |display-authors=etal|periodical=[[J. Org. Chem.]]|title=Experimental basicities of superbasic phosphonium ylides and phosphazenes|volume=81|issue=17|pages=7349–7361|date=2016|doi=10.1021/acs.joc.6b00872|pmid=27392255}}</ref><ref>{{citation|author1=E.D. Nacsa |author2=T.H. Lambert|periodical=[[J. Am. Chem. Soc.]]|title=Higher-order cyclopropenimine superbases. Direct neutral Bronsted base catalyzed Michael reactions with α-aryl esters|volume=137|issue=32|pages=10246–10253|date=2015|doi=10.1021/jacs.5b05033|pmc=4617652|pmid=26131761|bibcode=2015JAChS.13710246N }}</ref>

==Preparation==
The [[convergent synthesis]] of P<sub>4</sub>-''t''-Bu<ref>{{cite thesis|first=Vinayak|last=Gupta|date=2010|degree=PhD|title=New synthetic methods for biologically active aromatic heterocycles|publisher=Iowa State University|doi=10.31274/etd-180810-2033|doi-access=free}}</ref> is derived from [[phosphorus pentachloride]] (1) and leads in branch [{{EquationNote|A}}] to the well-characterized aminotris via the non-isolated chlorine (dimethylamino)phosphonium chloride (2)<ref name="Schwesinger" /> via [(Dimethylamino)phosphonium tetrafluoroborate (3) and further via [{{EquationNote|A2}}] to the liquid iminotris (dimethylamino) phosphorane<ref>{{Cite patent|country = EP|number =0921128 |title=Process of preparing iminotris(dimethylamino)phosphorane |pubdate =2002-09-25|fdate =1998-12-03 |inventor =T. Nobori et al. | assign=Mitsui Chemicals, Inc.}}</ref>(4)

{{NumBlk2|:|
{{NumBlk2||<math chem>\begin{matrix}{}\\
\underbrace\ce{PCl5}_{(1)}\ \ce{->[\ce{Me2NH}]} 
\underbrace{\left[ \ce{(Me2N)3\overset{\oplus}P-Cl . \overset{\ominus}Cl} \right]}_{(2)}\ \ce{->[1.\ \ce{NH3}][2.\ \ce{NaBF4}]}\ 
\underbrace\ce{(Me2N)3\overset{\oplus}P-NH2 . \overset{\ominus}BF4}_{(3)}
\\{}\end{matrix}</math>|A1|FmtN=['''[[#%i|%l]]''']|RawN=y}}
{{NumBlk2||<math chem>
\underbrace\ce{(Me2N)3\overset{\oplus}P-NH2 . \overset{\ominus}BF4}_{(3)}\ \ce{->[\ce{KOMe}]}\ 
\underbrace\ce{(Me2N)3P=NH}_{(4)}
</math>|A2|FmtN=['''[[#%i|%l]]''']|RawN=y}}
|A|FmtN=['''[[#%i|%l]]''']|RawN=y}}
and in branch [{{EquationNote|B}}] with phosphorus pentachloride and ''tert''-butylammonium chloride to ''tert''-butylphosphorimide trichloride (5)<ref>{{citation |author=R. Schwesinger |author2=J. Willaredt |author3=H. Schlemper |author4=M. Keller |author5=D. Schmitt |author6=H. Fritz|periodical=[[Chem. Ber.]]|title=Novel, Very Strong, Uncharged Auxiliary Bases; Design and Synthesis of Monomeric and Polymer-Bound Triaminoiminophosphorane Bases of Broadly Varied Steric Demand|volume=127|issue=12|pages=2435–2454|date=1994|doi=10.1002/cber.19941271215}}</ref>

{{NumBlk2|:|<math chem>\underbrace\ce{PCl5}_{(1)}\ \ce{->[t\text{-}\ce{Bu-\overset{\oplus}{NH3}.\overset{\ominus}{Cl}}]}\ \underbrace{t\text{-}\ce{Bu-N=PCl3}}_{(5)}</math>|B|FmtN=['''[[#%i|%l]]''']|RawN=y}}
The reaction [{{EquationNote|C}}] of excess (4) with (5) yields the hydrochloride of the target product P<sub>4</sub>-''t''-Bu (6) in 93% yield
{{NumBlk2|:|<math chem>\scriptstyle\begin{matrix}{}\\
\underbrace\ce{7(Me2N)3P=NH}_{(4)}\ +\ \underbrace{t\text{-}\ce{Bu-N=PCl3}}_{(5)}\ \ce{->}\ 
\underbrace{\ce{[(Me2N)3P=N]3P=N\overset{\oplus}H-}t\text{-}\ce{Bu . \overset{\ominus}{Cl}}}_{(6)}\ \ce{->[\ce{NaBF4}]}\ 
\underbrace{\ce{[(Me2N)3P=N]3P=N\overset{\oplus}H-}t\text{-}\ce{Bu . \overset{\ominus}BF4}}_{(7)}\ \ce{->[\ce{KOMe / NaNH2}]}\ 
\underbrace{\ce{[(Me2N)3P=N]3P=N\overset{\oplus}H-}t\text{-}\ce{Bu}}_{(8)}
\\{}\end{matrix}</math>|C|FmtN=['''[[#%i|%l]]''']|RawN=y}}

which is also converted into the tetrafluoroborate salt (7) from which the free base (8) can be obtained almost quantitatively with [[potassium methoxide]]/[[sodium amide]]<ref name="Schwesinger" /> or with [[potassium amide]] in liquid [[ammonia]].<ref name="e-EROS" >{{citation |author=R. Schwesinger |author2=Y. Kondo|periodical=E-EROS Encyclopedia of Reagents for Organic Synthesis|title=Phosphazene Base P<sub>4</sub>-''t''-Bu|date=2010|doi=10.1002/047084289X.rp150.pub2|isbn=978-0471936237}}</ref> The transfer of the hygroscopic and readily water-soluble hydrochlorides and the liquid free bases into the tetrafluoroborates, which are difficult to solubilize in water, facilitate the handling of the substances considerably.

{{NumBlk2|:|[[File:Konvergente P4-t-Bu-Synthese.svg|700px|Konvergente P<sub>4</sub>-''t''-Bu-Synthese]]|D|FmtN=['''[[#%i|%l]]''']|RawN=y}}

The relatively uncomplicated convergent synthesis with easily accessible [[reactants]] and very good yields of the intermediates make P<sub>4</sub>-''t''-Bu an interesting phosphazene superbase.<ref name="Fluka" >{{cite web|title=Strong and Hindered Bases in Organic Syntheses|website=sigmaaldrich.com|publisher=Sigma-Aldrich|url=http://www.sigmaaldrich.com/chemistry/chemical-synthesis/learning-center/chemfiles/chemfile-2001-2003/vol-3-no-1.html|format=PDF; 1,2&nbsp;MB|accessdate=2016-12-20}}</ref>

== Properties ==
P<sub>4</sub>-''t''-Bu is one of the strongest neutral [[nitrogenous base]]s with an extrapolated [[pKa|p''K''<sub>a</sub>]] value of 42.1 in [[acetonitrile]] and is compared to the strong base [[1,8-Diazabicycloundec-7-ene|DBU]] with a pKa value of 24.3 by 18 orders of magnitude more basic.<ref name="Schwesinger" /> The compound is very soluble in non-polar solvents, such as [[hexane]], [[toluene]] or [[tetrahydrofuran]], and is usually commercially available as a 0.8 to 1 molar solution in hexane.<ref name="Fluka" /> Already in weakly acidic media [[protonation]] produces the extremely delocalized and soft P<sub>4</sub>-''t''-Bu-H cation and causes besides a very strong solubilization effect also an extreme acceleration of addition reactions even at temperatures below -78&nbsp;°C.

P<sub>4</sub>-''t''-Bu owes its extraordinarily high [[basicity]] with low [[nucleophilicity]] to its very high [[steric hindrance]] and the involvement of many donor groups in conjugation with the spatially demanding structure of the cation formed by protonation.

P<sub>4</sub>-''t''-Bu is an extremely hygroscopic solid which is thermally stable up to 120&nbsp;°C and chemically stable to (dry) [[oxygen]] and bases.<ref name="e-EROS" /> Traces of water and protic impurities can be eliminated by addition of [[bromoethane]]. The base is both very hydrophilic and very lipophilic and can be recovered easily and almost completely from reaction mixtures by the formation of the [[Common-ion effect|sparingly soluble]] tetrafluoroborate salt.

Because of its extremely weak Lewis basicity, the [[cation]] of P<sub>4</sub>-''t''-Bu suppresses typical side reactions of metal organyls (such as [[aldol condensation]]s) as can be caused by [[lithium amide]]s such as [[lithium diisopropylamide]] (LDA).<ref name="Pietzonka" >{{citation|surname1=T. Pietzonka, D. Seebach|periodical=[[Angew. Chem.]]|title=Die P4-Phosphazenbase als Teil eines metallfreien Initiatorsystems für die anionische Polymerisation von Methacrylsäuremethylester|volume=105|issue=5|pages=741–742|date=1993|language=German|doi=10.1002/ange.19931050514|bibcode=1993AngCh.105..741P}}</ref>

== Applications ==
The neutral superbase P<sub>4</sub>-''t''-Bu is superior to ionic bases if those are sensitive to oxidation or side reactions (such as acylation) when they cause solubility problems or Lewis acid catalysed side reactions (such as [[aldol reaction]]s, epoxy ring opening etc).

The [[dehydrohalogenation]] of n-alkyl bromides yields the alkene, such as the reaction [[1-bromooctane]] with P<sub>4</sub>-''t''-Bu which yields [[1-Octene|1-octene]] almost quantitatively (96%) under mild conditions, compared to the [[potassium tert-butoxide]]/[[18-Crown-6|18-crown-6]] system with only 75% yield.<ref>{{citation |author=R. Schwesinger |author2=H. Schlemper|periodical=[[Angew. Chem.]]|title=Peralkylierte Polyaminophosphazene – extrem starke neutrale Stickstoffbasen|volume=99|issue=11|pages=1212–1214|date=1987|language=German|doi=10.1002/ange.19870991134|bibcode=1987AngCh..99.1212S}}</ref>

[[Alkylation]]s on weakly acidic methylene groups (e.g. in the case of [[carboxylic ester]]s or [[nitrile]]s) proceed with high yield and selectivity. For example, by the reaction of 8-phenylmenthylphenylacetate with iodoethane in the presence of P<sub>4</sub>-''t''-Bu only the monoethyl derivative in the ''Z'' configuration is obtained in 95% yield.<ref>{{citation |author=A. Solladié-Cavallo |author2=A.G. Csaky |author3=I. Gantz |author4=J. Suffert|periodical=[[J. Org. Chem.]]|title=Diastereoselective Alkylation of 8-Phenylmenthyl Phenylacetate: Aggregated Lithium Enolate versus "Naked" Enolate|volume=59|issue=18|pages=5343–5346|date=1994|doi=10.1021/jo00097a041}}</ref>

: [[File:Ethylierung von 8-Phenylmenthyl-phenylacetat.svg|200px|Monoethylierung von 8-Phenylmenthyl-phenylacetat]]

[[Succinonitrile]] reacts with [[iodoethane]] in the presence of P<sub>4</sub>-''t''-Bu in 98% yield to give the tetraethyl derivative without undergoing a [[Thorpe-Ziegler reaction]] to form a cyclic α-[[ketonitrile]].<ref name="e-EROS" />

: [[File:Alkylierung von 1,2-Dinitrilen.svg|242x242px|Tetraalkylierung von 1,2-Dinitrilen]]

[[Trifluoromethylation]] of [[ketone]]s (such as [[benzophenone]]) is also possible at room temperature in good yields up to 84% with the inert [[fluoroform]] (HFC-23) in the presence of P<sub>4</sub>-''t''-Bu and [[tris(trimethylsilyl)amine]].<ref>{{citation |author=S. Okusu |author2=K. Hirano |author3=E. Tokunaga |author4=N. Shibata|periodical=[[ChemistryOpen]]|title=Organocatalyzed trifluormethylation of ketones and sulfonyl fluorides by fluoroform under a superbase system|volume=4|pages=581–585|date=2015|issue=5 |doi=10.1002/open.201500160|pmc=4608523 |pmid=26491635}}</ref>

: [[File:Trifluormethylierung von Benzophenon.svg|350px|Trifluormethylierung von Benzophenon mit Fluoroform]]

[[Intramolecular cyclization]] of ''ortho''-alkynylphenyl ethers leads in the presence of P<sub>4</sub>-''t''-Bu under mild conditions without metal catalysts to substituted [[benzofuran]]s.<ref>{{citation |author=C. Kanazawa |author2=K. Goto |author3=M. Terada|periodical=[[Chem. Commun.]]|title=Phosphazene base-catalyzed intramolecular cyclization for efficient synthesis of benzofurans via carbon-carbon bond formation|pages=5248–5250|date=2009|doi=10.1039/B913588J |issue=35|pmid=19707635 }}</ref>

: [[File:Cyclisierung zu Benzofuranen.svg|300px|Cyclisierungen mit P<sub>4</sub>-''t''-Bu zu Benzofuranen]]

Due to its extreme basicity it was suggested early on that P<sub>4</sub>-''t''-Bu should be suited as an [[initiator element|initiator]] for [[anionic polymerization]]. With the ethyl acetate/P<sub>4</sub>-''t''-Bu initiator system, [[poly(methyl methacrylate)]] (PMMA) with narrow [[polydispersity]] and molar masses up to 40,000 g·mol<sup>−1</sup> could be obtained in [[Tetrahydrofuran|THF]].<ref name="Pietzonka" />

: [[File:Anionische Polymerisation von MMA.svg|400px|Anionische Polymerisation von MMA zu PMMA]]

Anionic polymerization of [[Ethylene oxide]] with the initiator system [[n-Butyllithium]]/P<sub>4</sub>-''t''-Bu yields well-defined [[Polyethylene glycol|Polyethylene oxide]]s with low [[Dispersity|polydispersity]].<ref>{{citation |author=B. Eßwein |author2=M. Möller|periodical=[[Angew. Chem.]]|title=Polymerisation von Ethylenoxid mit Alkyllithiumverbindungen und der Phosphazenbase "t Bu-P4"|volume=108|issue=6|pages=703–705|date=1996|language=German|doi=10.1002/ange.19961080620|bibcode=1996AngCh.108..703E}}</ref>

Cyclic [[siloxane]]s (such as [[hexamethylcyclotrisiloxane]] or [[decamethylcyclopentasiloxane]]) can also be polymerized with catalytic amounts of P<sub>4</sub>-''t''-Bu and water or [[silanol]]s as initiators under good molecular weight control to thermally very stable polysiloxanes having decomposition temperatures of >450&nbsp;°C.<ref name="Hupfield" /><ref>{{citation |author=P.C. Hupfield |author2=R.G. Taylor|periodical=J. Inorg. Organomet. Polym. Mater.|title=Ring-opening polymerisation of siloxanes using phosphazene base catalysts|volume=9|issue=1|pages=17–34|date=1999|doi=10.1023/A:1021429320083|s2cid=91275737}}</ref> Because of its extreme basicity, P<sub>4</sub>-''t''-Bu eagerly absorbs water and carbon dioxide, both of which inhibit anionic polymerization. Heating to temperatures >100&nbsp;°C removes CO<sub>2</sub> and water and restores the anionic polymerization. The extreme [[hygroscopy]] of the phosphazene base P<sub>4</sub>-''t''-Bu as a substance and in solutions requires a great effort for storage and handling and prevents its broader use.

== References ==
{{Reflist}}

: [[Category:Phosphorus compounds]]
: [[Category:Nitrogen(−III) compounds]]
: [[Category:Non-nucleophilic bases]]
: [[Category:Superbases]]
: [[Category:Tert-butyl compounds]]