Quasiphosphonium salts

In this context, it was suggested [53] that the reaction involved a tertiary carbocation intermediate that was in equilibrium with quasiphosphonium salt. The elimination of HC1 from the latter, leads to the formation of 2,5-dihydro-l,2-oxaphosphole derivatives. Macomber [54], however, has shown that bromination of optically pure... 

Figure 1.6 A. Trimethyl(phenyl)phosphonium chloride (a typical quaternary phos-phonium salt). B. Examples of quasiphosphonium salts.

Attack on Halogen. Predictably, the 3-bromo-3-cyano-imides (56) and (57) form quasiphosphonium salts (58) with aryl phosphites, phosphonites, and phosphinites.47 In the case of (56) these salts cyclize to the oxazaphosphorane (59), which is in equilibrium with the iminophosphorane (60), depending on the phosphorus ester used. 

An additional approach toward the preparation of tertiary phosphines is by the reduction of more highly coordinated phosphorus species, particularly phosphine oxides [0=PR3] and phosphine sulfides [S=PR3] (see Section 5.2), but also phosphonium salts [ILtP+X"] and quasiphosphonium salts [R3P-YR +X ] (see Sections 4.2 and 4.4). Numerous reducing agents have been used to accomplish these conversions, including hexachlorodisilane [CbSi-SiCft], trichlorosilane [HSiCft], phenylsilane [PhSiHs], and lithium aluminum hydride [LiAlH4]. 

In some instances, relatively mild nucleophilic agents are capable of generating tertiary phosphines from phosphonium or quasiphosphonium salts (see Section 4.4). Reagents such as KCN, aUcoxide salts, or even tertiary amines are capable of attacking certain substituents to remove them from phosphorus leaving the tertiary phosphine with retention of configuration at phosphorus. ... 

Using P NMR, the intermediacy of the quasiphosphonium ion bearing the phosphorus-oxygen linkage has been demonstrated experimentally. In the absence of added acid, the same product formation occurs, but the intermediate quasiphosphonium species has not been detected experimentally. However, evidence of an intermediate oxyphosphorane (see Section 5.3) has been demonstrated under these conditions, as it has under acidic conditions, and such an intermediate has independently been shown to provide the usual products of the reaction. Evidence is available in other systems for the equilibrium of oxyphosphoranes with oxygen-containing quasiphosphonium salts. 

Evidence for the formation of intermediate quasiphosphonium salts comes from the isolation of the ion 436 (A = B = EtO R = H, R = R = Me E = SPh) as the hexa-chloroantimonate and further evidence stems from reactions with cyclic esters of the (alka-l,2-dienyl)phosphonic acids. The reactions of the l,3,2-dioxaphosph(V)olanes 444, of known geometry, with CI2, Br2 and RSCl or RSeCl " are highly stereoselective and would be expected to proceed through the quasiphosphonium salts 445 such salts have been isolated from other reactions. The 1,2-oxaphosph(V)ol-3-enes 446 have been isolated (66-75%) when EY is RSCl or RSeCl, but in the former case, were accompanied by 449 (65-73%) Phosphonium salts 436 (A = B = alkyl) have been obtained from reactions of propadienyldialkylphosphineoxides. ... 

Spiro(5,5)phosphonia-undecane chloride (12) was produced in low yield by the reaction of dimethylurea with the little exploited, readily available, tetrakis(hydroxymethyl)-phosphonium salts <81PS(10)147>. Tetrathiodibenzo quasiphosphonium salts (24 R = Dop X = PFe") have been produced by the reaction of silylated dithiocatechol (123) with PF5 <73CC144>. Spiro(5,5)phosphazenes (20) may be made directly from trichlorophosphine and diaminophosphazene, H2NPR2=PR2NH2,... 

Chlorospiro-phosphoranes, under the action of Lewis acids such as SbCls, may be converted to the quasiphosphonium salt, for example (16) <88Zaac(561)49>. Hydroxyphosphorane (42 R = OH, Y = CMej) is converted to the spiroquasiphosphonium triflate (14) by the action of trifluoro-methanesulphonic acid <78JA5229> this in turn can be reduced by LAH to the phosphoranide (42 R = electron pair, Y = CMcj). 

Interestingly, the reaction of diallq l[2-(5-chloro-2-hydro)q henyl)-2-phenylethenyl]phosphine oxides with thionyl chloride afforded quasiphosphonium salts, 2,2-diallqrl-6-chloro-4-phenyl-2/f-l,2-benzoxa-phosphinin-2-onium chlorides via cyclization (Scheme 53). ... 

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