Attack on Halogen

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. 

Boigegrain and B. Castro, Tetrahedron Letters, 1975, 3459 R. Boigegrain and B. Castro, Tetrahedron, 1976, 37, 1283 R. Boigegrain, B. Castro, and B. Gross, Tetrahedron Letters, 1975, 3947. 

Attack on Halogen. The effect of changing the phosphorus nucleophile used on the reactions with a-halogenosulphones that give the dehalogenated sulphone (52) has 

Diazadiphosphetidines form the halogenophosphonium salts (53) on treatment with high-valent chlorides of tin, phosphorus, and antimony,  

Attack on Halogen. The expected ethynylphosphonates, e.g. (64), are the products of the reaction of bromoacetylenes with dialkyl phosphite anions.  

2-Dihalogenoethyl cyanides undergo dehalogenation to acrylonitrile on reaction with either secondary or tertiary phosphites, presumably through initial attack of phosphorus on halogen. Surprisingly, bis(diphenylphosphinyl)-amine (65) is the product of the reaction of sodium diphenylphosphide and 

5-tetrabromobenzene in liquid ammonia. u-Dihalogenobenzenes are known to undergo dehalogenation with metal phosphides, and in this case initial attack of phosphorus on bromine followed by reaction with ammonia seems a likely mechanism. 

The factors controlling the site of attack of halogenophosphonium cations on ambident anions have been investigated for (66) and shown to be quite 

The reactions of iV -chloroguanidine and trichloromethyl isocyanate with tertiary phosphites give the salt (67) and the phosphonate (69) respectively. 

Nucleophilic Attack on Halogen.—Tris(dimethylamino)phosphine and carbon tetrachloride has been used to prepare C-glycosides (38) which are precursors for C-nucleosides. Carbon tetrachloride and the silylaminophosphines (39) can give different products, (40) or (41), depending on the reaction conditions.  

The mechanism of the reaction of tertiary phosphites with halogeno-acetylenes has been investigated by two groups of workers. - Initial attack of phosphite could be on carbon to give the anion (78), which can eliminate halide, or on halogen to give the ion pair (79) which leads to the same intermediate (80). In both cases an Arbusov reaction would give the isolated product (81). 

A much faster reaction for bromides than chlorides usually suggests attack on halogen, since bromine is more readily attacked by nucleophilic phosphorus. However, for phenyl acetylenes (82 = Ph) these rates 

Simpson and Burt have studied the same reactions in the presence of various amounts of ethanol and have plotted graphs of phosphonate (81 R = Ph) and phenyl acetylene produced against moles of alcohol added. Acetylene in the product reached a maximum (around 60%) when two moles of ethanol were added and stayed fairly constant beyond this, which suggests that the attack-on-halogen contribution to the mechanism is approximately 60%. The rest of the reaction presumably follows some other mechanism and the authors suggest the addition-elimination route (79) in view of the isolation of the phosphonate (83) from the reaction of tri(isopropyl) phosphite with the bromoacetylene (84). 

The reaction of triethyl phosphite with the acetal of 1-bromopropion-aldehyde to give the vinylphosphonate (97) has been reported. The mechanism is thought to involve initial conversion into the vinyl ether (98) followed by attack of phosphite on halogen. A similar reaction with 1-bromopropionaldehyde gives the phosphate (99). 

The first isolation of an intermediate from the bromination of a phosphite has been achieved with the cyclic phosphite (100). N.m.r. and conductance studies on the reaction mixture kept at 0 °C support structure (101) for the intermediate, which on warming to room temperature gives the Arbusov product (102). 

The reaction of phosphorous esters and amides with polyhalogeno-methanes has been studied and used in a synthesis of phosphates. The reaction of diphenyl phosphorochloridate (103), produced in situ from diphenyl phosphonate and carbon tetrachloride, has been studied with various diamines.  

A new route to monoalkyl phosphates, using ethylthio protecting groups (104) and reaction with iodine, has been described.  

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B. Nucleophilic Attack on Halogen.- (/ )-( +)-2,2-dimethylpropan( H)ol has been converted to the chloride with inversion of configuration using triphenylphosphine and carbon tetrachloride. The corresponding reaction using carbon tetrabromide gave the bromide with considerable racemiza-tion. ... 

In contrast to the IIT mechanism, the clement effect on the Arens path is expected to be A (I) > A (Br) > A (C1) as is also the case with S 2 reactions of alkyl halides (Table 25) . This reactivity order has been observed in the reactions of l-halo-2-(2-thienyDacetylenes with sulphides and thiolates in methanol-water in which the C—X bond is presumably broken in the rate-determining step by attack on halogen (see Section Il.C.l.d). In both 5 n2 and halogen abstraction reactions, the magnitude of the element effect is quite large (Table 25). 

PTC has been used for the reactions of carbanions with CCI4, hex-achloroethane, etc. (for review, see Ref 45). These processes consist of nucleophilic attack on halogen atom (halophilic reaction) and result in halogenation of the carbanions. Depending on the structure of the carbanion and other factors, halogenated product often enters further reactions (eq. 95). 

Nucleophilic Displacement at Acetylenic Carbon.— Among studies of nucleophilic displacement at acetylenic carbon are the reactions of phosphines and amines with halogenoacetylenes. Using phosphines, displacement occurs by competitive attack on halogen and carbon the general order of reactivity in substitution at carbon by phosphine nucleophiles is sp <- sp > sp. In... 

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