Phosphazenes reactions with aldehydes

The aza-Wittig reaction of phosphazenes 3 with a,P-unsaturated aldehydes gives 3-azatrienes, which on heating are converted to pyridines 4 <96TL(37)6379>. 

Further reactions of P-hydridocyclotriphosphazenes (See Section IV,E) have been described. They undergo insertion reactions with aldehydes, ketones, isothiocyanates and electrophilic olefins. Addition of sulfur or oxidation with KMn04 gives thioxo-or oxo-cyclotriphos-phazene derivatives which are methylated at the chalcogen to afford methylthio- and methoxy- cyclotriphosphazenes (36). Hydridocyclo-phosphazenes can also be oxidized to give symmetric and unsymmetric bis(cyclotriphosphazenyl) oxides (35). 

The olefination product was obtained in 70% yield in reaction with aromatic as well as ahphatic aldehyde using phosphazene bases or KOH-TBAB. Both aldehydes gave the -isomer predominantly. Cinnamaldehyde gave the corresponding diene with lower yields. 

Reactions of phosphazenes (22) and (23) with aldehydes, ketones and nitriles provide a convenient route to new heterocycles containing l,3-oxaza-4-phospha-2-phosphorine (24) and l,3-diaza-4-phospha-2-phosphorine (25) rings respectively (Scheme 3) ... 

N-vinylic phosphazenes react with isocyanates to yield N-vinylic carbodiim-ides. The reaction of isothiocyanates and an N = P moiety has been used to prepare oxazole derivatives The reactivity of the — N = P moiety in synthetic procedures has also been demonstrated by the aza-Wittig reaction with aldehydes " ... 

In a novel principle of 1,3-diazine formation phosphazenes like 20 containing an amidine moiety undergo an aza-WiTTiG reaction with a,p-unsaturated aldehydes followed by oxidative electrocyclic ringclosure to pyrimidines 21 [141] ... 

The reaction between a carbanion derived from alkyl 3,5-bis(trifluoromethyl)phenyl sulfones and aldehydes affords, with good yields and stereoselectivities, the corresponding 1,2-disubstituted alkene through the Julia-Kocienski olefination reaction. This one-pot protocol can be performed using the phosphazene base at —78 °C and has been successfully used in a high yielding and stereoselective synthesis of various stilbenes such as resveratrol [47] (Scheme 5.28). 

The reaction of 1 -trimethylsilylnaphthalene with pivaldehyde in the presence of 20 mol% Bu-P4 base proceeded smoothly at room temperature to give the alcohol in 91% yield. Other phosphazene bases with weaker basicities, such as Bu-P2 base and BEMP, showed no catalytic activity. As one of the conventional strong organic bases, DBU was found to be inactive. Caesium fluoride (CsF) was then examined as a fluoride anion donor, but no carbon-silicon bond cleavage was observed. Reactions with other aldehydes have been examined that with benzaldehyde was found to proceed somewhat slowly at room temperature. Other aryl aldehydes with electron-donating groups were also employed as electrophiles and the reactions proceeded smoothly at room temperature [57] (Table 5.5). 

A-Vinylic phosphazenes, are useful building blocks, that have been used in Aza-Wittig reactions with unsaturated aldehydes to form 3-azatrienes through a [2 -I- 21-cycloaddition-cycloreversion sequence. " The presence of an alkyl substituent in position 3 of A-vinylic phosphazenes increases the steric interactions, and [4 + 2] periselectivity (1,4 addition) is observed." Other Aza-Wittig reactions include the reaction of iminophosphorane (60) with aromatic isocyanates to obtain, inter alia, useful carbodiimides for the selective synthesis of pyrimidones." Also the iminophosphorane (61) was reacted with furan-2-carbaldehyde, thiophene-2-carbaldehyde, furan-3-carbaldehyde, and thiophene-3-carbaldehyde to give, depending on temperature and aldehyde, trans imines or mixtures of trans and cis imines." " The... 

The oxidation of the alcohol was performed with supported perruthenate (8.48, Fig. 8.46) to produce clean aldehydes 8.91 after filtration. The Henry reaction was performed in the presence of a commercially available, supported strong base 8.92 and an excess of volatile nitroalkenes, giving clean nitroalcohols 8.93 after filtration and evaporation. The reaction mixtures from the trifluoroacetylation/elimina-tion steps were purified with commercially available amino PS resin 8.58 to scavenge the trifluoroacetates and with acidic ion-exchange resin 8.76 to remove the TEA-derived salts. Again, the nitrostyrenes 8.94 were obtained cleanly after filtration and evaporation. Cycloaddition with isocyanoacetate was promoted by the commercially available, supported guanidine base 8.95, while the subsequent N-alkylation of the pyrroles 8.96 was performed with an excess of halide in the presence of the commercially available, supported phosphazene 8.97. In this case, the excess halide was removed by treatment with supported 8.58, and filtra-... 

As shown in Scheme 8.2, chiral P-hydroxy-a-amino adds can be obtained by the Mukaiyama-type aldol reaction of aldehydes with glycine-derived enol silyl ethers using cinchona-based quaternary ammonium salts. In 2004, Castle and coworkers [9] found that dnchona-based quaternary ammonium salts such as 13 are also able to catalyze the dired aldol readion of aldehydes with the glydne donor 14 in the presence of a phosphazene base such as BTTP (t-butyliminotri(pyrrolidino)phos-... 

Vinyl phosphazenes have been found to undergo tandem Aza-Witting/l,6-elect-rocyclization as illustrated by the reaction of 564 with tricarbonyl compound 565 to give 566 (Scheme 94) (95T3683). The reaction of 564 with aliphatic, heteroaryl, or aryl aldehydes first produced an intermediate imine 567 that ultimately cyclized to produce imidazol[l,5-a]pyridines 568 in good (65-82%) yields. 

The synthetic applications of o-halobenzyl sulfones as precursors of 1,3- and 1,5-zwitter ionic synthons, have been investigated and their sulfonyl carbanions, generated by means of the phosphazene base Et-P2 reacted with different electrophiles, such as alkyl halides and aldehydes. When the alkylation was performed with ethyl bromoacetate, orf/io-substituted cinnamates were obtained after a subsequent p-elimination of sulfmic acid. When the aldol reaction was performed with paraform aldehyde, vinyl sulfones were obtained. The... 

Najera and co-workers reported the application of 3,5-bis(trifluoromethyl)phenyl (BTFP, 9) sulfones in the modified Julia olefmation to the synthesis of various stilbenes such as resveratrol. This one-pot protocol can be performed using KOH at room temperature or the phosphazene base P4-t-Bu at -78 °C, and given a high yielding and good stereoselectivity. For example, the sulfone 85, prepared from 84 by two-steps reaction, was reacted with aldehyde 86 in the presence of P4-/-Bu in THF finished the alkene 87 in 74% yield with E/Z ratio of 98 2. 

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