P4-t-Bu

Key takeaways

P4-t-Bu belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect P4-t-Bu to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of P4-t-Bu from memory before moving on to harder problems.

Reference excerpt

P4-t-Bu is a readily accessible chemical from the group of neutral, peralkylated sterically hindered polyaminophosphazenes, which are extremely strong bases but very weak nucleophiles, with the formula (CH3)3C−N=P(−N=P(−N(CH3)2)3)3. "t-Bu" stands for tert-butyl (CH3)3C–. "P4" stands for the fact that this molecule has 4 phosphorus atoms. P4-t-Bu can also be regarded as tetrameric triaminoiminophosphorane of the basic structure H−N=P(−NH2)3. The homologous series of P1 to P7 polyaminophosphazenes of the general formula [ ( R 2 1 N ) 3 P = N − ] x − ( R 2 1 N ) 3 − x P = NR 2 {\displaystyle {\ce {[(R_{2}^{1}N)_{3}P=N-]_{\mathit {x}}-(R_{2}^{1}N)_{3\!-{\mathit {x}}}P=NR2}}} with preferably methyl groups as R1, a methyl group or tert-butyl group as and even-numbered x between 0 and 6 (P4-t-Bu: R1 = Me, R2 = t-Bu and x = 3) has been developed by Reinhard Schwesinger; the resulting phosphazene bases are therefore also referred to as Schwesinger superbases.

Preparation The convergent synthesis of P4-t-Bu is derived from phosphorus pentachloride (1) and leads in branch [A] to the well-characterized aminotris via the non-isolated chlorine (dimethylamino)phosphonium chloride (2) via [(Dimethylamino)phosphonium tetrafluoroborate (3) and further via [A2] to the liquid iminotris (dimethylamino) phosphorane(4)

and in branch [B] with phosphorus pentachloride and tert-butylammonium chloride to tert-butylphosphorimide trichloride (5)

The reaction [C] of excess (4) with (5) yields the hydrochloride of the target product P4-t-Bu (6) in 93% yield

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 or with potassium amide in liquid ammonia. 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.

The relatively uncomplicated convergent synthesis with easily accessible reactants and very good yields of the intermediates make P4-t-Bu an phosphazene superbase.

Properties P4-t-Bu is one of the strongest neutral nitrogenous bases with an extrapolated pKa value of 42.1 in acetonitrile and is compared to the strong base DBU with a pKa value of 24.3 by 18 orders of magnitude more basic. 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. Already in weakly acidic media protonation produces the extremely delocalized and soft P4-t-Bu-H cation and causes besides a very strong solubilization effect also an extreme acceleration of addition reactions even at temperatures below -78 °C. P4-t-Bu owes its 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. P4-t-Bu is an extremely hygroscopic solid which is thermally stable up to 120 °C and chemically stable to (dry) oxygen and bases. 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 sparingly soluble tetrafluoroborate salt. Because of its extremely weak Lewis basicity, the cation of P4-t-Bu suppresses typical side reactions of metal organyls (such as aldol condensations) as can be caused by lithium amides such as lithium diisopropylamide (LDA).

Applications The neutral superbase P4-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 reactions, epoxy ring opening etc). The dehydrohalogenation of n-alkyl bromides yields the alkene, such as the reaction 1-bromooctane with P4-t-Bu which yields 1-octene almost quantitatively (96%) under mild conditions, compared to the potassium tert-butoxide/18-crown-6 system with only 75% yield. Alkylations on weakly acidic methylene groups (e.g. in the case of carboxylic esters or nitriles) proceed with high yield and selectivity. For example, by the reaction of 8-phenylmenthylphenylacetate with iodoethane in the presence of P4-t-Bu only the monoethyl derivative in the Z configuration is obtained in 95% yield.

Succinonitrile reacts with iodoethane in the presence of P4-t-Bu in 98% yield to give the tetraethyl derivative without undergoing a Thorpe-Ziegler reaction to form a cyclic α-ketonitrile.

Trifluoromethylation of ketones (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 P4-t-Bu and tris(trimethylsilyl)amine.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with P4-t-Bu

Start with the simplest possible case. Write down what P4-t-Bu claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to P4-t-Bu before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about P4-t-Bu ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of P4-t-Bu

In research: P4-t-Bu appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses P4-t-Bu in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: P4-t-Bu is common in secondary-school and first-year university syllabi. It links to neighbouring topics Dimethylamino compounds, Nitrogen(−III) compounds, Non-nucleophilic bases, so understanding it makes those chapters shorter.
In everyday life: Look for P4-t-Bu outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study P4-t-Bu in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what P4-t-Bu means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain P4-t-Bu out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is P4-t-Bu in simple terms?

P4-t-Bu is a readily accessible chemical from the group of neutral, peralkylated sterically hindered polyaminophosphazenes, which are extremely strong bases but very weak nucleophiles, with the formula (CH3)3C−N=P(−N=P(−N(CH3)2)3)3. "t-Bu" stands for tert-butyl (CH3)3C–. "P4" stands for the fact th…

Why does P4-t-Bu matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study P4-t-Bu?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on P4-t-Bu.