Aldehydes, reaction with phosphonium salts

Finally, aldehyde 49 was converted in a Wittig reaction with phosphonium salt 45, affording the methyl ester of EPA (6) with the correct all Z-double bonds. Saponification then produced EPA (6). 

The olefin metathesis of 3-hydroxy-4-vinyl-l,2,5-thiadiazole 112 and a McMurry coupling reaction (Ti3+ under reductive conditions) of the aldehyde 114 were both unsuccessful <2004TL5441>. An alternative approach via a Wittig reaction was successful. With the use of the mild heterogenous oxidant 4-acetylamino-2,2,6,6-tetramethyl-piperidine-l-oxoammonium perfluoroborate (Bobbitt s reagent), the alcohol 113 was converted into the aldehyde 114. The phosphonium salt 115 also obtained from the alcohol 113 was treated with the aldehyde 114 to give the symmetrical alkene 116 (Scheme 16) <2004TL5441>. 

The first three retrosynthetic cleavages are formation of the C7-C8 aldol by intramolecular chromium-Reformatsky reaction of linear precursor 51, esterification between northern and southern half building blocks and Wit-tig reaction of phosphonium salt 52 known from Mulzer s work [85] and northern half precursor 53. The final disconnections were placed at the C2-C3 aldol in 51 (again to be formed by chromium-Reformatsky reaction, here between bromoacetimide 56 and aldehyde 57) and the C14-C15 bond by alkylation of acetoacetate 54 with neryl bromide 55. 

Conjugated yne-2-ynylidene 1,3-dithioles 228 were synthesized via Wittig reactions of phosphonium salts 225 with 3-phenyl-substituted propargyl aldehydes 227 (R = H, = Ph). Diaryl-substituted derivatives 228 were prepared in Horner-Wittig reactions of phosphonates 226 with ketones 225 (R =R = Ph or -02NC6H4) (Equation 13) <2004CL1190>. 

The synthetic strategy leading to formation of the unsymmetrical rt-exTTF 805 involved two successive olefina-tion reactions of phosphonium salts 800 and 801 with glyoxal through two possible routes, A and B. In route A, the aldehyde 802 was further reacted with 801, while in route B the aldehyde 803 was reacted with phosphonium salt 800. Since yields in both approaches did not exceed 40%, the phosphonate 804 was reacted with the aldehyde 802 to give 805 in 84% yield (Scheme 121) <1995TL1645>. 

Spheroidene (97) was synthesized [16,17] by a Wittig reaction of 67 with apo-8 -neuro-sporenal (74) and sodium methoxide (Scheme 21). The aldehyde 74 was prepared by the Wittig reaction of phosphonium salt 26 with the Cio-dialdehyde 20 and subsequent Horner-Emmons reaction with the phosphonoester 28. Reduction of the ester group with LiAlH4 and oxidation with Mn02 gave 74. 

The Wittig reaction of 91 with crocetindialdehyde (536) and lithium methoxide resulted in the Cso-aldehyde 92. Lycoxanthin (109) was obtained by a Wittig reaction of 92 with phosphonium salt 15 and BuLi to give the carotenoid ester 93 (which is not known to occur in Nature) followed by reduction with LiAlH4 to give 109 in an overall yield of 25 % referred to 92 (Scheme 26). 

This route is especially convenient because no over-alkylation of the anion of acetonitrile occurs. Over-alkylation can be a problem in attempts to methylate the anion of diethyl cyano-methylphosphonate (4) directly a mixture of unalkylated, monoalkylated and dialkylated products in a ratio of 1 2 1 is formed. The same problem arises with the alkylation of triethyl phosphonoacetate (11). For the preparation of a Ca-ester synthon, an alternative method to the propionitrile route is used (Scheme 7). This method has been used in the synthesis of labelled Cio-central units, described in the next Section. The starting material is acetic acid (9) which is converted into ethyl bromoacetate (10) as described above (Scheme 3). The ethyl bromoacetate (10) is reacted with triphenyl phosphine in a nucleophilic substitution reaction the phosphonium salt is formed (yield 97%). The phosphonium salt is deprotonated in a two-layer system of dichloromethane and an aqueous solution of NaOH. After isolation, the phosphorane 22 is reacted at room temperature with one equivalent of methyl iodide (19) the product consists mainly of the monomethylated phosphonium salt (>90%) which is deprotonated with NaOH, to give the phosphorane 23 in quantitative yield relative to phosphorane 22, and 23 is reacted with the aldehyde in dichloromethane. The ester product 12 can subsequently be reduced to the corresponding alcohol and reoxidized to the aldehyde 8. An alternative two-step sequence for this has also been used. First, the ester 12 is converted into the A -methyl-iV-methoxyamide (16) quantitatively by allowing it to react with the anion of A, 0-dimethylhydroxylamine as described above (Scheme 5). This amide 16 is converted, in one step, into the aldehyde 8 by reacting it with DIB AH in THF at -40°C [46]. 

A variation of the Wittig reaction was described [14, 15] in which a-hydroxysulfonates (13) were used, instead of aldehydes, to react with phosphonium salts. Polymers with only traru-vinylene units were formed (Scheme 5). 

Since the above phase transfer catalysed reactions worked particularly well with phosphonium salts prepared from benzyl halides, it was considered of interest to investigate reactions using phosphonium salts (4) prepared from chlorome thy late d polystyrenes, that is reactions where the supported species was the alkyl halide (see Scheme 2). Both linear and crosslinked chloromethylated polystyrenes reacted smoothly with triphenylphosphine to give polymers with residues (4). ° An alternative way of preparing the linear polymer is by copolymerisation of styrene and the salt (5). When the salts (4) were treated with various aldehydes in methylene chloride and... 

In general, nonstabilized and semistabilized phosphonium ylides are prepared by deprotonation of the corresponding phosphonium salts with strong bases that are incompatible with aldehydes or ketones. However, some studies have shown that these phosphonium ylides can be generated in the presence of aldehydes by treatment of the corresponding phosphonium salts with a weaker base such as l,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) [100], NaOH, LiOH [101, 102], KOH [103], or K2CO3 [49, 104, 105] (Scheme 20). In addition, these bases promote the one-pot Wittig reaction of phosphonium salts with aldehydes in a number of solvents such as toluene, tetrahydrofuran, dimethyl sulfoxide, isopropanol, and water. 

A related example disclosed recently by McNulty et al. is a one-pot Wittig reaction of aldehydes with phosphonium salts in the presence of 10 mol% of morpholine, L-proline or p-toluenesulfonamide and 2.0 equiv. of NaHCOs (Scheme 55) [227]. This reaction gives high E selectivity. A rapid and reversible condensation of the aldehyde with the amine (derivative) catalyst has been proposed to form an iminium or an imine intermediate that is subjected to olefination with the in situ generated phosphonium ylides, though a base-catalyzed pathway is not ruled out. It has been confirmed that an N-sulfonyl imine can be formed quantitatively from the corresponding aldehyde and sulfonamide under the reaction conditions. 

Tetrabutylphosphonium and triphenylchlorophosphonium chlorides were reported to act as catalysts for the addition of diethylzinc to aldehydes (aromatic, heteroaromatic or aliphatic) and the dehydration of aromatic and aliphatic aldoximes to nitriles, respectively. Catalysis was so efficient that both types of reactions occurred at room temperature, even with phosphonium salts in pol)Tner-bonded form. 

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 Wittig reaction an aldehyde or ketone is treated with a phosphorus ylid (also called a phosphorane) to give an alkene. Phosphorus ylids are usually prepared by treatment of a phosphonium salt with a base, and phosphonium salts are usually prepared from the phosphine and an alkyl halide (10-44) ... 

The ylide obtained from (methyl)triphenylphosphonium bromide reacts with morpholine derivatives 597 to give phosphonium salts 598 which upon treatment with -butyllithium are converted to new ylides 599. In a reaction with aldehydes, ylides 599 form iV-(l,3-disubstituted allyl)-morpholines 602 (Scheme 94) <1996AQ138>. Another less common nucleophile that can be used for substitution of the benzotriazolyl moiety in Af-(a-aminoalkyl)benzotriazoles is an adduct of iV-benzylthiazolium salt to an aldehyde which reacts with compounds 597 to produce adducts 600. Under the reaction conditions, refluxing in acetonitrile, salts 600 decompose to liberate aminoketones 601 <1996H(42)273>. 

Reaction with a first aldehyde transforms 176 into the vinylphosphonium chloride 177, which for practical reasons is subjected to an anion-exchange process, leading to the phosphonium salt 178. From this, phenyllithium treatment liberates the allenic phosphorane 179, an intermediate that has previously been used to prepare allenes from aldehydes [69], in the present case providing the products 180. The same protocol has also been applied to o-alkynylbenzaldehydes to yield allenes of interest as model compounds for the study of Schmittel and Myers-type cyclization reactions [70]. 

The migration of a C=C bond to form a C=N bond was also observed with hydro-xylamine [78, 79], hydrazine [80, 81] and primary amines [82]. The /f iminylphos-phine oxide formed in the reaction may serve as a Wittig reagent in the presence of a base to react with a ketone or an aldehyde leading to ,/fun saturated alkenyl-imines 153 (Scheme 10.74). The phosphorus group can be a phosphonium salt as well as a phosphonate. 

These types of compounds can also be formed via a Wittig reaction [19]. Triphenylphosphine can be quatemized with a suitable alkyl iodide, and the resultant perfluoroalkylated phosphonium salt will react with aldehydes to give fluorinated alkenes which are easily hydrogenated (Scheme 3.4). This methodology has recently been expanded to the formation of perfluoroalkylated pyridines [20],... 

Tellurophosphoranes, obtained through a transylidation reaction between tellurenyl halides and phosphoranes, react with aldehydes to give the expected vinylic tellurides as an E Z isomeric mixture (method a). One other methodology involves the treatment of equimolar amounts of phenyl tellurenyl bromide and phosphonium salts with t-BuOK followed by an aldehyde (method b). Under these lithium-salt-free conditions, (Z)-vinylic tellurides are the main products. ... 

By 1989 Mukaiyama had already explored the behaviour of phosphonium salts as Lewis acid catalysts. It was possible to show that the aldol-type reaction of aldehydes or acetals with several nucleophiles and the Michael reaction of a,j3-unsatu-rated ketones or acetals with silyl nucleophiles gave the products in good yields with a phosphonium salt catalyst [116]. In addition, the same group applied bisphosphonium salts as shown in Scheme 45 in the synthesis of ]3-aminoesters [117]. High yields up to 98% were obtained in the reaction of A-benzylideneaniline and the ketene silyl acetal of methyl isobutyrate. Various analogues of the reaction parteers gave similar results. The bisphosphonium salt was found to be superior to Lewis acids like TiCl and SnCl, which are deactivated by the resulting amines. 

Furthermore, phosphonium salts have been applied as catalysts in the TMSCN addition to aldehydes [118] and ketones [119]. Methyltriphenylphosphonium iodide [118] was fonnd to be a reasonably active catalyst for the addition of TMSCN to aldehydes at room temperatnre by the gronp of Plumet. In general, the yields varied between 70% and 97% in 24 h, depending on the aldehyde, applied in the reaction (Scheme 46). However, the salt did not support the addition of TMSCN to ketones, with one exception, when the highly reactive cyclobutanone was applied in the reaction [120]. 

Finally, achiral phosphonium salts have been applied as Lewis acid catalysts in some other reactions. The examples will be listed here but not discussed in more detail. Phosphonium salts have been used as catalysts for the A,A-dimethylation of primary aromatic amines with methyl alkyl carbonates giving the products in good yields [123]. In addition acetonyltriphenylphosphonium bromide has been found to be a catalyst for the cyclotrimerization of aldehydes [124] and for the protection/ deprotection of alcohols with alkyl vinyl ethers [125, 126]. Since the pK of the salt is 6.6 [127-130], the authors proposed that, next to the activation of the phosphonium center, a Brpnsted acid catalyzed pathway is possible. 

After these results had established the feasibility of generating and utilizing a carbohydrate phosphorane, the two systems that had been reported earlier were examined in order to determine if similar conditions would allow them to undergo the Wittig reaction. The ylide derived from phosphonium salt I condensed with both benz-aldehyde and U-chlorobenzaldehyde to produce good yields of olefinic products Villa and Vlllb. The ylide derived from phosphonium salt II also was successfully condensed with benzaldehyde, but the yield of IX was only 30 , presumably because of its extremely poor solubility even in an HMPA-THF solvent mixture. Both of these systems supported the tenet that it was possible to use unstabilized carbohydrate phosphoranes if the conditions are proper and if the g-oxygen is attached to the carbohydrate through another set of bonds. 

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