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Some Reactive Intermediates in Organo-Phosphorus Chemistry

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Some Reactive Intermediates in Organo-Phosphorus Chemistry SOME REACTIVE INTERMEDIATES IN ORGANO-PHOSPHORUS CHEMISTRY A Thesis submitted, in partial fulfilment of the conditions governing candidates for the degree of DOCTOR OP PHILOSOPHY in THE UNIVERSITY OF NEW SOUTH WALES by Ian D. Jenkins, B.Sc. July 1969 &'****» J v' * uii*»y>' (i) SUMMARY The decomposition of phenylphosphinic anhydride at room temperature to pentaphenylcyclopentaphosphine, phenylphosphine and phenylphosphonic anhydride has been shown to proceed with formation of the univalent phosphorus compound, phenylphosphini- dene. Pentaphenylcyclopentaphosphine reacts with phenylphosphinic acid under mild conditions to give phenylphosphine and phenyl­ phosphonic anhydride; the reaction appears to involve the unstable phenylphosphinic anhydride as an intermediate. The pentaphosphine reacts with phosphorous and hypophosphorous acids to give a large number of products including phosphine and phenylphosphonic acid. A convenient gravimetric method for the estimation of several trivalent phosphorus compounds, including phenylphosphine, by oxidation with tellurium tetrachloride, is reported. Tris(dimethylamino)phosphine reacts with tetraphenylcyclo- pentadienone to give a stable, 1:1 zwitterionic adduct containing a P-O-C bond. Pyrolysis of this adduct affords hexamethylphosphoric triamide, and a hydrocarbon, , tentatively assigned the structure 1,2,3,4,5»6,6a-heptaphenyl-lObH-benz[e]-as-indacene. The intermediacy of 5-carbena-1,2,3,4-tetraphenylcyclopenta-1,3- diene in this pyrolytic decomposition is discussed. Tris(dimethyl- amino )phosphine adds to 1,2,3,4-tetraphenylfulvene in a similar (ii) fashion to give a stable 1:1 adduct. Trimethylphosphite reacts with tetraphenylcyclopentadienone to give dimethyl-5-niethyl-2,3>4,5-tetraphenylcyclopenta-1,3-dien- 1-ol phosphate, the 2-ol phosphate isomer, and 1,2,3,4-tetraphenyl- fulvene. Evidence is presented for initial attack by trimethylphosphite on the carbonyl oxygen atom of the dienone to give a zwitterionic intermediate analogous to that obtained with the aminophosphine and this ketone. Tetraphenylcyclopentadienone undergoes base-catalysed Michael- type addition of methyl phenylphosphinate, dimethyl phosphonate and diphenylphosphine oxide, with formation of a carbon-phosphorus bond. Addition both a and p to the carbonyl group was observed, the more sterically demanding nucleophiles resulting in attack at the a position. Several unusual features in the mass spectra of cyclopentadiene systems are discussed. (iii) TABLE OF COBTEHTS Page INTRODUCTORY SECTION .......................... 0 0 0 0 0 0 1 Part A. Phenylphosphinidene from Phenylphosphinic Anhydride 17 Part B. The Reaction of Pentaphenylcyclopentaphosphine with Phenylphosphinic Acid ........ 0 9 0 0 0 0 26 Part C. Estimation of Phenylphosphine using Tellurium Tetrachloride .................... 0 9 0 0 0 0 35 Part D. The Reaction of Tris(dimethylamino)phosphine with Tetraphenylcyclopentadienone 0 9 9 0 0 0 38 Part E. The Reaction of Trimethylphosphite with Tetraphenylcyclopentadienone ........ 0 9 0 0 0 9 60 Part F. The Reaction of P(lV) Nucleophiles with Tetraphenylcyclopentadienone ........ 0 9 9 0 0 0 70 Part Gr. Mass Spectra of Cyclopentadiene Systems 0 9 9 0 0 0 173 EXPERIMENTAL SECTION .......................... 9 9 0 0 0 0 88 Part A ••• 9 9 0 0 0 0 9 0 0 0 0 0 0 0 0 0 0 0 90 3 0 0 0 0 0 0 9 0 0 0 0 0 0 0 0 0 0 9 0 0 9 98 C 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 9 9 0 0 9 0 113 3 9 0 0 0 0 0 0 0 0 0 0 0 0 0 9 0 9 0 9 0 0 114 ill 0 9 0 0 0 9 0 0 0 0 9 9 0 0 9 0 0 0 9 9 0 140 F 0 0 0 0 0 9 0 9 0 9 0 9 0 0 9 0 0 0 9 0 9 149 (iv) Page APPENDIX • •• ••• ••• • • • ••• • • # • • • 168 REFERENCES • •• ••• ••• • • • ••• ••• ••• 180 INDEX OF COMPOUNDS 190 ACKNOWLEDGEMENTS ... 192 INTRODUCTORY SECTION The rapid growth of organophosphorus chemistry over the last two decades can be attributed to its theoretical interest (for example, the recent studies of pseudo-rotation in P(V) structures), its wide practical applications (pharmaceutical preparations, insecticides and fungicides, plasticisers and stabilisers, surface active compounds, extractants, catalysts, fire resistant polymers, lubricating oil additives and additives conferring flame resistance 2 on fabrics, synthetic resins etc.) and, more recently, to the biological significance of these compounds. 2-Aminoethylphosphonic acid (AEPA) was isolated from protozoa of the sheep’s rumen in 1959^ and is distributed throughout the lower phyla in Protozoa, Porifera and extensively in molluscs. All known natural phosphonates appear to be closely related to or derived from AEPA. Although the biosynthesis of the C-Pbond, and indeed, its function in nature, has not yet been solved, the initial step is believed to be rearrangement of a phospho- enolpyruvate to the phosphonopyruvate. The breakdown of the phosphonates, involving cleavage of the very stable C-P bond, can be performed by a number of bacteria. In the organism Bacillus Cereus, AEPA undergoes transamination with formation of 2 2-phosphonoacetaldehyde which is then broken down by another 4 enzyme to orthophosphate and acetaldehyde. 2-Phosphonoacetaldehyde appears to be stable in aqueous solution at high and low pH values, but at pH5 and 90° it breaks 5 down to acetaldehyde and phosphate." The breakdown of phosjhonic to phosphoric compounds via a metaphosphate-type intermediate has been the subject of intensive study for many years, because of the suspected role of metaphosphate in the reactions of the 6k biological phosphorylating agent, ATP. Conversion of 2-phos- phonoacetaldehyde to acetaldehyde and phosphate at pH5 can bs visualised as proceeding via the hypothetical metaphosphate 'l). 0 - / HO —> HO C X H (1) h3p°4 + CH^CHO There is some analogy for this in the conversion of phosphono-amino g- acids to phosphate in the presence of ninhydrin and a base. " Here also, a system of the type ”0-P(0)(0R)-X-Y=Z^ is the precursor to metaphosphate formation. Breakdown to metaphosphate is a sinple heterolytic fragmentation (X=Y-Z~ being eliminated) of the t^pe 7 reviewed by Grob and Schiess . The original aim of this work was to study suspected inier- mediates of the metaphosphate type (l), as well as the lower valency - 3 - forms (2) (metaphosphite, see ref. 8) and (3) (phosphinidenes). 0 (1) (2) (3) These three species appear to be highly reactive and whenever they are suspected intermediates, polymeric compounds (RPO^)^, (RPO)^ and (RP) can usually be isolated. These polymers are themselves quite reactive and attempts to ’trap' (1 - 3) are often complicated by reaction between the trapping agent and the polymers to give the same products expected for the monomeric units. Although species of type (1) are probably formed in several of the reactions described in this thesis, the only attempt to generate (1) per se was by photolysis (u.v.) of 2-phenyl-1,3»2- dioxaphospholane which could theoretically break down to phenyl- metaphosphonate (1, R = Ph) and ethylene. No reaction was observed however, when a Vfo solution of this ester was irradiated in ether for six hours. The elimination of hydrogen chloride from a phosphinic chloride, RPH(0)C1, seemed to offer a ready route to (2). The phosphinic chloride was to be prepared from the corresponding amide, RPH(o)NR2. All attempts to prepare phenylphosphinic morpholinamide however, led to the isolation of the bis-morpholinium salt of phenylphosphonic anhydride and pentaphenylcyclopenta- - 4 - phosphine. Moreover, phenylphosphinic anhydride was found to decompose at room temperature to give phenylphosphonic anhydride, pentaphenylcyclopentaphosphine and phenylphosphine. * It seemed more than likely that phenylphosphinidene (3, R = Ph) was involved in these decompositions and most of the work described in this thesis arose from attempts to prove the existence of this fascinating species. Phosphinidenes are carbene analogues. Phenylphosphinidene has received the most attention of univalent phosphorus compounds, 11 but has been studied very little when compared with nitrenes. 1 2 Pluck and Issleib have claimed the existence of phenylphosphinidene and its dimer in (PhP)^ melts and solutions of these melts in 4 31 benzene or tetrahydrofuran. Their evidence was based on P n.m.r. spectra which showed small peaks at +47.2 (assigned to PhP-)» +26.1 (assigned to PhP=PPh) p.p.m. as well as the main absorption at +2.0 p.p.m. (H^PO^ = 0.0 p.p.m.). The presence of such large amounts of phosphinidene (detectable by the relatively insensitive 0 -1 P n.m.r.) seems dubious, especially in view of the recent work of Schmidt et al.^ who observed no phosphinidene (m/e 108) in the mass spectrum of pentaphenylcyclopentaphosphine at low ionisation energies ( <10 e.v., 150°) and concluded that the fragments PhP and ^2^2 were f>orme<i by electron impact and not thermally in the melt. The trimeric species Ph^P^ appeared to be formed thermally (and not by electron impact) however, since its intensity relative to Ph(rP[r+ decreased by a factor of 5 after minutes (150°, 20 e.v.). j j - 5 - * This they attributed to the higher volatility of Ph^P^. Schmidt et al. point out that some of the reactions attribut­ able to phenylphosphinidene can also be described in terms of diradicals. Irradiation of (PhP),- at low temperature resulted in a red colour which slowly disappeared as the temperature was raised. The sample was paramagnetic. The e.s.r. signal was assigned to the open chain pentaphosphine diradical, PhP-(PPh)^-PPh. 14 The reaction of cyclopolyphosphines with 2,3-dimethyl-1,3-butadiene (catalysed by u.v. light) to give the tetrahydrophosphorin (4) was explained in terms of such diradical species. Heating (RP) with dienes gives both (4) and the phospholene (5) and was presumed to occur via the Ph^P^ ring.
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