Hydroxyphosphonium salt

Mechanisms of competing reactions of Wittig reagents with substituted 2-amino- 1,4-naphthoquinones have been discussed73 and a study of the stereoselectivity of the indirect Wittig reaction of a 1,2-hydroxyphosphonium salt has led to the conclusion... 

Hydroxyphosphonium salts and l-(trimethylsiloxy)phosphonium salts are obtained219 from the addition of small-sized phosphines (PMe3 and PEt3) to a carbonyl group of ketones and aldehydes, in the presence of chlorotrimethylsilane or of the mixture ace-tone/bromine, which is a source of anhydrous HBr. 

A selection of AN values has already been given in Table 2-5 of Section 2.2.6 cf. also Table 6-6 in Section 6.5.1. The observed solvent-dependent P chemical shifts result mainly from the polarization of the dipolar P=0 group, induced by the interaction with electrophilic solvents A, particularly HBD solvents. The decrease in electron density at the phosphorus atom results in a deshielding proportional to the strength of the probe/solvent interaction. In solutions of protic acids, the P chemical shift of the 0-protonated triethyl hydroxyphosphonium salt is observed. Since Et3PO is very hygroscopic and therefore not very suitable from an experimental point of view, the use of (n-Bu)3PO instead of Et3PO as probe molecule has been recommended [250]. 

Surprisingly the phosphine oxide (40) is very soluble in water22 oxide (40) also forms stable hydroxyphosphonium salts with acids and stable 1 1 adducts with amines. 

For the synthesis of (all-E)-diatoxanthin [(all- )-94] the strategy C15 + C10 + C15 = C40 was selected. The Cio-dial 45 was first converted into the monophosphonium salt 100 and then reacted with the acetylenic Ci 5-end group 99 to give the acetylenic C25-hydroxyaldehyde 101 which was reacted with the Cia-hydroxyphosphonium salt 63 to (all-E,3R,3 R)-diatoxanthin (94) (Scheme 24). 

Triphenylphosphine can be quaternized by the vinyl bromide (60) to yield a phosphonium salt which on treatment with methyl-lithium followed by water gave the hydroxyphosphonium salt (61) stereospecifically. A... 

Several tests are available to determine whether equilibration of stereochemistry occurs in the course of oxaphosphetane decomposition (methods A-E, Scheme 7), but each method has some limitations. In method A, oxaphosphetane diastereomers are prepared independently by deprotonation of the )S-hydroxyphosphonium salts 27 or 28 with base (NaHMDS, NaNHj, KO-tert-Bu, etc.) (20). If each isomer affords a distinct oxaphosphetane 31 or 32 according to NMR analysis (usually, or H), then the solutions are warmed up to the decomposition temperature. Kinetic control is established if stereospecific conversion to the alkenes can be demonstrated from each diastereomer. A less rigorous version of this test is to perform the experiment only with isomer 27, the precursor of the cis-disubstituted oxaphosphetane 31 (21c). All known examples of significant (> 5%) stereochemical equilibration involve 31 and not the trans-disubstituted isomer 32 (20, 21c). A negative equilibration result with the cis diastereomer 31 can be assumed to apply to 32 as well. 

Method E is similar to method D in that it relies upon the nucleophilic cleavage of epoxides to generate stereochemically defined betaines. A two-step procedure is employed, starting with the reaction of a lithium phosphide with the cis or trans epoxides (36 or 37). The resulting alkoxy phosphines 34 or 35 are converted into betaines by direct alkylation with methyl iodide (19). This contrasts with method A where the betaine is generated by the deprotonation of a -hydroxyphosphonium salt, and method E may therefore produce a different population of betaine rotamers than does method A. This subtle difference may explain why method E proceeds with higher stereospecificity than does method A in the Trippett-Jones experiment (Table 7 entry 16,... 

There are some additional potential complications with the control experiments. Loss of stereochemistry in method D can be due to product equilibration induced by the phosphine additive as already mentioned. Furthermore, equilibration in method A or E can occur because of competing (reversible) (x-deprotonation to give the oxido ylide 38 or the derived hydroxy ylide 39 (21c). The latter problem can usually 1% avoided by lowering the temperature or by using a weaker base for the deprotonation of the )5-hydroxyphosphonium salt 27 or 28 (21c). Nevertheless, positive equilibration results cannot be attributed to retro-Wittig reaction unless (1) crossover is also demonstrated or (2) labeling results can rule out the intervention of 38 or 39. 

Hydroxyphosphonium salts from cyclic ethers and phosphines... 

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