Acyclic Phosphoranes

Trifluorophosphoranes, (4a-c), were obtained by the oxidative addition of (2) to (3a-c). At room temperature only (4a) showed dynamic behaviour in solution but at — 5 °C pseudorotation was slow enough to detect /(PFax) and /(PFax) coupling by P NMR. X-ray crystallographic analysis of (4a-c) confirmed the expected tbp geometry with two axial fluorine substituents. All 

Organophosphorus Chemistry, Volume 32 The Royal Society of Chemistry, 2002 

HBr and reduction of (4b) with LiAlH4 gave either (6) or (7) according to the reaction conditions. An equilibrium exists between (4a) or (4b) and the starting materials as demonstrated by the trapping of (3a) with tetrachloro-o-benzoqui-none (8) to form (9). 

Pentacoordinate compounds of antimony and bismuth are becoming more prominent as illustrated by the work of Sharutin et al. For example, penta-arylantimony (lOab) and pentaphenylbismuth (16) ary late organotin halides (e.g. RsSnCl (11) and R2SnBr2 3)) to form aryltin derivatives RaSnAr (12), R2SnArX (14) or R2SnAr2 (15), in 78-95% yields.  

A series of tetraarylantimony P-diketonates (18) have been prepared by the reaction of (lOab) with P-diketones, P-ketoesters or diethyl malonate (17a-g). Furthermore, the reaction of (lOab) with phenols (19) and (21), gave penta-coordinate aryloxytetraarylantimony compounds (20) and (22), which were characterised by IR and X-ray crystallography. Both compounds were found to have the tbp configuration with the aryloxy groups in apical positions. 

Acyclic Phosphoranes - Ylides (3) containing the P-F bond react with alcohols (4) to form stable alkoxydialkyldifluorophosphoranes (S) which were isolated by distillation and characterised by multinuclear n.m.r.  

Finally in this section, McAuliffe et al. have provided a route to new and existing transition metal complexes of Ni, Fe and Mn by the reaction of coarse-grain metal powders with halophosphoranes as exemplified by the formation of (19) from nickel powder and (18).  

A theoretical study of model apically-substituted and equatorial ly-substituted acyclic phosphoranes, H PX, revealed a ligand apicophilicity in the order, Ct CN F CeCH H CHg OH  

On the experimental front, values have been used to esti- 

It has also been shown by 31P n.m.r., that pseudorotation of the oxyphosphoranes (4) and (5) involves intermediate structures with a diequatorial disposition of the dioxaphospholene ring, thus violating the ring-strain rule16. 

Acyclic phosphoranes. - A number of difluorotris(perfluoro-alkyl)phosphoranes (7) have been prepared by electrochemical fluor-ination of trialkylphosphine oxides (6) in anhydrous HF17. The phosphoranes are conveniently converted into the perfluoroalkyl-phosphine oxides (8) by reaction with hexamethyldisiloxane and the phosphoranes are regenerated by treatment of (8) with HF. 

The pioneering work of Denney et ai19 on the synthetic utility of oxyphosphoranes has been thoroughly exploited by Evans et al. in demonstrating that diethoxytriphenylphosphorane promotes mild and efficient cyclodehydration of diols (e.g. 11) to cyclic ethers (e.g. 13) via the cyclic phosphorane (12)20 21. Simple 1,2-, 1,4-, and 1,5- diols afford good yields of the cyclic ethers but 1,3-propanediol and 1,6-hexandiol give mainly 3-ethoxy-l-pro-panol and 6-ethoxy-l-hexanol respectively whereas tri- and tetra-substituted 1,2-diols afford the relatively stable 1,3,2- diox-phospholanes. In some instances (e.g. 14), ketones (e.g. 16) are formed by a synchronous 1,2-hydride shift within (15). The synthetic utility has been extended to diethoxyphosphoranes supported on a polystyrene backbone22. 

In a related study, pentaphenylantimony (21) was found to decompose at room temperature in toluene and in the presence of catalytic amounts of cupric acetate to form triphenylantimony and a 4 1 ratio of diphenyl to benzene , jn butyl alcohol as solvent, however, the products were triphenylantimony (0.43 mole), benzene (1.67 mole), ethanoic acid (0.07 mole) and butyl phenyl ether (0.44 mole) together with Sb(0Ac)3 and cuprous acetate. The mechanisms of these reactions were not discussed. 

Diphenylfluorophosphine (22) reacts with piperonal (23) to form the difluorophosphorane (27) via (24)-(26). The structure of the product was determined by a combination of mass spectrometry, multinuclear n.m.r. and X-ray analysis. Although alkyltrifluoroalkoxyphosphoranes are thermally unstable above 0°C, the reaction of alkyltetrafluorophosphoranes (28) with trimethylalkoxysilanes (29) at 20°C 

The Mitsxmobu reaction is also not formally the province of pentaco-ordinate phosphorus chemistry but the mechanism of this synthetically useful reaction received considerable attention during the yearl5-19 Since such a discussion inevitably includes the possible involvement of pentaco-ordinate intermediates prior to the final dealkylation ( or deacylation) step, it is pertinent to summarise the current findings on this subject. 

The reaction of hexafluoroisopropyl benzenesulphenate (36) with phosphinites (37b) and phosphonites (37c) gave, as expected, hexafluoroisopropoxyphos-phoranes (38b,c), but with the phosphite (37d) the intermediate phenylthio-phosphorane (39) showed unusual stability and did not react with a second mole of (36). The variable-temperature F n.m.r. of (39) revealed a barrier to pseudorotation, — 50.4 kJ mol (12.0 kcal mol ), whereas the tri- 

The Mitsonubo reaction between triphenylphosphine and diethylazodi-carboxylate (49) has been shown to be a source of diaryloxy- or dialkoxy-phosphoranes (51) via (50), which contradicts a recent proposal that the key intermediate in the reaction is an 0,A-phosphorane. 

Bis(trifluoromethyl)nitroxyl (52) reacts with CF3PX2 (X = F, Cl, Br, or CN) at —70 °C to afford oxidative addition products (53), although with X = Br or CN the products are unstable. Substituted vinyl phosphites (54a—c) are readily chlorinated to form thermally stable phosphoranes (55a—c), which in turn may be converted into (57) via (56).  

Bis(hexafluoroisopropoxy)triphenylphosphorane (38a) alkylates aromatic amines to form (58), which on heating with triethylamine give (59). The reaction has been employed to introduce the trifluoromethyl group into the desired position of heterocycles, since on using or/Ao-substituted anilines the alkylation products (60a—c) eliminate HF and then cyclize to form (61a—c).  

A report has also appeared on the alkylation of aldehydes and ketones by polyfluoroalkoxyphosphoranes (62), which in the cases of paraformaldehyde, aromatic or unsaturated aldehydes, and hexafluoroacetone gives polyfluoro-alkyl-containing acetals [e.g., (63)]. 

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Acyclic phosphoranes, ArnP(OR)5 n with n = 0 - 3 have been shown to hydrolyse by an SN1(P) mechanism for n. = 1, 2 and 3 but by an S 2(P) or addition-elimination mechanism for n. = O23. This duality of mechanism is analogous to the classical S l vs S 2 mechanisms observed for solvolysis reactions at tetrahedral carbon. 

The trifluoromethylphosphoranes (CF3)2PMe3 and (CF3)3PMe2 have been obtained as stable, unreactive, white solids from the reactions of tetramethyl-lead with the corresponding chlorophosphoranes.23 Pseudorotation of (CF3)3PMe2 is slow on the n.m.r. time-scale at 100 °C, indicating a very considerable difference in apicophilicity between methyl and trifluoromethyl groups. Among other acyclic phosphoranes prepared are (23),24 (24),25 and (25).26... 

All MO calculations, both semiempirical and ab initio, have demonstrated that the barrier to aZ>3h-C4v-D3h interconversion, the Berry pseudorotation process, is small for acyclic phosphoranes (see also Section 2.1.). Estimates of 1.4 (extended Hiickel MO34 ), 4.8 (ab initio35 ) and 3.5 kcal/mol (CNDO/231 ) have been obtained for PFS. The barrier computed from the ab initio calculation increases to 8.5 kcal/mol if the 3c -orbitals are not included. The turnstile rotation mechanism traverses a much higher energy barrier 10.0 (extended Hiickel MO), 18.1 (ab initio) and 9.1 kcal/mol (CNDO/2). On the basis of these calculations, the Berry pseudorotation mechanism must be the preferred explanation of pentatopal isomerisation in acyclic phosphoranes (see Section 2.1.). 

Acyclic phosphoranes bearing a fluoroalkyl side chain (75), undergo intramolecular Wittig reactions when heated, producing cyclic benzoates. Shen and Gao have reported a stereoselective synthesis of trifluoromethylated a-chloro-a,P-unsaturated esters and nitriles by employing trifluoromethylated phosphoranes (scheme 13). 

The second condition is not unreasonable if bond-breaking with the leaving group is concerted with bond-forming with the nucleophile, since octahedral symmetry will never be attained (Scheme 26). The hydrolysis of acyclic phosphoranes may proceed by both an S l mechanism or an 8 2 mechanism via a hexacoordinate species (Lowther and Hall, 1985). However, the identity of this species, reactive intermediate or transition state, has not been confirmed. 

Acyclic phosphoranes can be utilised to make ketones (12.317,12.318), while non-cyclic derivatives will yield heterocyclic rings (12.319-12.321). Phosphoranes form essential intermediates in the conversion of diketones to acetylenes (12.322). 

G.-V. Rdschenthaler Introduction Acyclic phosphoranes Monocyclic phosphoranes Bicyclic and polycyclic phosphoranes Phosphatranes... 

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