Phosphonium salts secondary reactions

The desired building block 204 was obtained by partial reduction of the triple bond in 218, followed by successive protection of the primary alcohol, oxidation of the secondary hydroxy group to the ketone, deprotection of the alcohol, Grignard reaction with vinylmagnesium bromide (65) and acetylation. The C2o-building block 204 was converted, with triphenylphosphine hydrobromide, into the phosphonium salt 219, reaction of which with the Cio-dialdehyde 45 gave C.p. 450 (203) Scheme 48). 

The addition of P—H bonds across a carbonyl function leads to the formation of a-hydroxy-substituted phosphines. The reaction is acid-cataly2ed and appears to be quite general with complete reaction of each P—H bond if linear aUphatic aldehydes are used. Steric considerations may limit the product to primary or secondary phosphines. In the case of formaldehyde, the quaternary phosphonium salt [124-64-1] is obtained. 

In the reactions characterized in the general sense above, phenyl and benzyl were the organic residues bound to phosphorus. It is not clear which of these groups is removed in the LAH dealkylation step. It should also be noted that as tetraquaternary phosphonium salts, these species do not qualify as crown analogs in the strictest sense because of the absence of lone pair electrons on phosphorus. Furthermore, the quaternary phosphorus can resist oxidation much better than secondary or tertiary phospho-... 

The cyclic phosphonium salts 140,141,143,145, and 146 so obtained are evidence for the mechanism of the oxaphospholic cyclization and especially for the main role of the tertiary carbocation formation during the process. The additional data which support this assumption, come from the investigation of the same reaction, but with different substrate, i.e., dimethyl(l,2-hexadienyl)phosphine oxide 147. In this case, the reaction mechanism involved formation of secondary carbocation that gives oxaphosphole product 148 only in 10% yield (Scheme 60) [124],... 

The reaction of triphenylphosphine with 2-acylamino-2-(chloro)acetophenones gives the salts (113), which may be cyclized to the oxazolylphosphonium salts (114).113 Similarly, the halogenocarboxamides (115) react readily with triphenylphosphine to give the enamide phosphonium salts (116), which on treatment with primary or secondary amines give the salts (117).114... 

The reactions of phosphine with a-branched aldehydes follow a different pathway. They lead to the formation of stable, heterocyclic, secondary phosphines of the following type, J With suitable dialdehydes, spirocyclic phosphonium salts are obtained these are very difficult to prepare by... 

Secondary reactions in the alkaline hydrolysis of phosphonium salts... 

Side Reactions. There are three secondary reactions which can be observed during cyanation substitutive hydrogenation (Reaction 5) (8, 17, 18, 19) aryl dimerization (Reaction 6) (8, 17, 18, 19), and catalytic formation of phosphonium salts (Reaction 7) (14). 

Before the TPS ether is cleaved with TBAF, secondary alcohol 7 has to be protected as methyl ether. TBAF is a reagent to cleave every silyl ether. Most other functional groups are not affected (see Chapter 2). In the next two steps the conversion of alcohol 33 into mesylate 34, which is a good leaving group, and then into iodide 35 in a Finkelstein type reaction occurs. Acetone is the solvent of choice, because Nal is better soluble in it than NaOMs and consequently reaction equilibrium is forced to the product side. Direct transformation from an alcohol to an iodide is possible with PPha and I2 in an Appel-like reaction, but in some cases this reaction fails. Final procedure is the generation of phosphonium salt 8. 

Carbonyl compounds are capable of undergoing another completely different sort of reaction, in which the carbonyl oxygen is replaced, not by a heteroatom as we saw above, but by an sp carbon atom. When triphenylphosphine is treated with a secondary aliphatic halide, a normal substitution reaction occurs, resulting in the replacement of the halide by the phosphorus compound. This phosphonium salt adduct may then be treated with a very strong base, such as phenyllithium, to produce an ylid. An ylid is a compound in which there is a positive charge adjacent to a negative charge. Write down this reaction sequence. 

Phosphorus ylids are prepared by treatment of phosphonium salts with bases such as phenyllithium, butyllithium. dimsyl anion, or potassium f-butoxide. They are not isolated but used directly after preparation because they are sensitive to oxygen and moisture. The requisite phosphonium salts are available in great variety from the reaction of triphenylphosphine and primary or secondary alkyl halides, usually bromides [24]. 

In contrast to the difficulty experienced in the hydrolysis of (5), the phosphonium salts (6) were all easily hydrolyzed by 5% sodium bicarbonate, 5% sodium carbonate or 5% sodium hydroxide to give the a-fluoroester. These results are summarized in Table II. The increased positive charge on phosphorus in(6) compared to (5) facilitates attack by the hydrolysis reagent and allows the overall scheme to succeed. Thus, a-fluoroesters can be readily prepared via alkylation and hydrolysis of (4). The reaction is not as general with (4) (secondary halides did not react), but for primary and activated halides, the reaction does provide a convenient, facile, easily scaled up synthesis for a variety of a-fluoroesters. 

Tetraarylphosphonium salts undergo the Heck reaction. Benzamides are produced when the phosphonium salts are treated with a secondary amine under CO. ... 

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