Reactivity of P-H Group of Phosphorus Based Compounds

Kolio D. Troev

• Publisher: Academic Press
• Year: 2017
• Print ISBN: 9780128138342
• eBook ISBN: 9780128138359
• Topic: Naturwissenschaften
• Subtopic: Chemie

Reactivity of P-H Group of Phosphorus Based Compounds bridges the gap between inorganic and organic phosphorus compounds, providing a basis to explore the myriad possibilities for synthesis of novel low and high molecular phosphorus-containing compounds. It covers well-documented reactions in detail, including: tautomerization, oxidation, reduction, alkylation, oxidation coupling, addition reaction to: carbon-carbon multiple bonds, Schiff base, isocyanates, nitriles, epoxides; addition to carbonyl group, Kabachnik- Fields reaction, cross-coupling reaction and more. In an accessible style complete with synthetic routes and figures, the resource then covers the reactivity of multiple P-H group members: phosphines, phosphine oxides, hypophosphorus acid, H-phosphinic acids and polys(alkylene H-phosphonate).This valuable coverage supports the advancement of research and applications in this area for scientists solving a scientific problem or starting a variety of new projects, such as a new reaction for the synthesis of biologically active compounds, new methods of polymer synthesis or a new methodology for polymer modification.- Describes the diverse reactivity of the phosphorus-hydrogen group, perhaps the most powerful in organic chemistry- Includes practical information for the synthesis of catalysts, biologically active substances, flame retardants, advance materials and polymer materials- Offers a visually-accessible guide to important reactions by an internationally recognized chemist

Chapter 1

Acidity and Tautomerization of P–H Group

Abstract

Acidity and tautomerization of P–H group are directly connected with its reactivity. Experimental pKa values for H-phosphonic acid and its esters provide a direct comparison between the strength of the P–H and P

OH types of acids and indicate the significant difference in their acidities. The substitution of one OH group in the molecule of H-phosphonic acid with an ethoxy group leads to an increase of the acidity of the remaining OH groups. Tautomerization is an important class of chemical reactions involving the interconversion of constitutional isomers (tautomers). Phosphinylidene compounds exist as an equilibrium mixture of two tautomeric forms, a tetracoordinated P(V) and a tricoordinated P(III) forms. The phosphinylidene compounds display prototropic (a hydrogen atom moves from one atom to another) tautomerism.

Keywords

Acidity; pKa values; tautomerization phosphine oxides; H-phosphinic acid (hypophosphorous acid); H-phosphonic acid

The target phosphorus compounds in this book are phosphines (primary and secondary phosphines) and phosphinylidene compounds, namely phosphine oxides (primary and secondary phosphine oxides [SPOs]), hypophosphorous acid (phosphinic acid), H-phosphinic acid, H-phosphonic acid, and poly(alkylene H-phosphonate)s. It is well documented that the hydrogen atom of P–H group of these compounds participates in the following reactions:

1. Tautomerization

2. Oxidation

3. Reduction

4. Alkylation

5. Oxidation coupling

6. Addition reaction to: Carbon–carbon multiple bonds, Schiff base, carbonyl group, Kabachnik–Fields reaction, isocyanates, nitriles, epoxides

7. Cross-coupling reaction

8. C

H activation (cross-dehydrogenerative coupling)

9. Transfer hydrogenation

These reactions make P–H group one of the most powerful functional groups in the chemistry. Due to these reactions, a huge number of compounds with different composition and structure are synthesized. In this book, these reactions are discussed. Acidity and tautomerization of P–H group are directly connected with its reactivity.

1.1 Acidity

Several general methods have been developed for the synthesis of organophosphorus compounds. The two most popular of these are: Reaction of an organometallic reagent with a phosphorus halide and reaction of a metal phosphide with an organic electrophile. In the second method, a metal phosphide is often prepared by deprotonation of the corresponding P–H-containing compound with a base (e.g., t-BuOK, EtONa, NaH, etc.). This is successful only if the base is obtained from a compound that is a weaker proton acid than the phosphine. Thus, there is a strong need for organic chemists to know the solution-phase pKa values of the P–H bonds in different organophosphorus compounds, both for scientific curiosity and for practical reasons [1]. Unfortunately, because phosphorus-centered anions are usually highly unstable species, it has been a formidable challenge to design experimental approaches to determine the pKa’s of organophosphorus compounds. Up to now, the solution-phase acidities of organophosphorus compounds remain almost entirely unknown except for PH₃ (pKa=29 in water); and there has not been a single pKa value for any P–H bond in the famous Bordwell scale of acidities. Fu and coworkers [2] have launched a program to systematically investigate how to utilize the modern quantum-chemical methods to acquire useful, quantitative data for realistic, solution-phase organic chemistry. In the first step of the program, they developed a generally applicable, ab initio protocol to calculate the pKa values of diverse organic acids in dimethyl sulfoxide (DMSO). Armed with the carefully benchmarked theoretical protocol, Fu et al. systematically calculated for the first time the pKa values of various types of organophosphorus compounds in DMSO (Table 1.1).

Table 1.1

Theoretical pKa Values in DMSO of Some Phosphorus-Containing Compounds [2]

Compound	pKa	Compound	pKa
PH3	24.1	(CH3)2P(O)H	26.9
CH3PH2	29.6	CH3O(CH3)P(O)H	22.0
CH3(Ph)PH	26.7	C6H5O(C6H5)P(O)H	17.9
Ph2PH	22.9	(CH3O)2P(O)H	18.4
Ph2P(O)H	20.6	(C6H5O)2P(O)H	9.0

The accuracy of these calculated values was estimated to be about 1.1 pKa units, which is sufficient for most practical applications. With these pKa values in hand, synthetic chemists can more rationally design the experimental conditions for the reactions that require the use of deprotonated organophosphorus compounds. The availability of these pKa values also allows us to study, for the first time, some interesting topics such as the substituent effects on the pKa values of various types of organophosphorus compounds. Only Issleib and Kummel once reported the experimental solution-phase acidities of six phosphines in tetrahydrofuran [3]. The P–H and As–H acidities of primary and secondary phosphines and arsines have been determined by metal–...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• About the Author
• Preface
• Chapter 1. Acidity and Tautomerization of P–H Group
• Chapter 2. Reactivity of P–H Group of Phosphines
• Chapter 3. Reactivity of P–H Group of Phosphine Oxides
• Chapter 4. Reactivity of P–H Group of Hypophosphorous Acid and Its Derivatives
• Chapter 5. Reactivity of P–H Group of H-Phosphinic Acid and Its Derivatives
• Chapter 6. Reactivity of P–H Group of H-Phosphonic Acid and Its Derivatives
• Chapter 7. Reactivity of P–H Group of Poly(alkylene H-phosphonate)s
• Index