Proazaphosphatranes, reactions

Proazaphosphatrane, P(RNCH2CH2)3N, is an efficient catalyst for the Henry reaction, and various ketones give nitro-aldols by the reaction with nitromethane and other nitroalkanes (Eq. 3.20).21... 

Recently, Verkade and coworkers have reported the successful synthesis of /3-hydroxy-nitriles from carbonyl compounds in a reaction promoted by strong nonionic bases, such as proazaphosphatrane types. The reaction occurs in the presence of magnesium sulfate. 

The effect of substituents on C(2) of vinyl bromide and on the amine of the palladium catalyst in the. S n reaction with secondary amines was examined. The catalyst formed from the reaction of Pd2(dba)3 with proazaphosphatrane (12) gave the best yields (>91%) of enamine.16 The. S n product formed with primary amines rearranged to give the imine as the product in yields of >86%. 

When aliphatic nitro compounds are used instead of aldehydes or ketones, no reduction occurs, and the reaction has been referred to as a Tollens reaction (see 16-43). However, the classical condensation of an aliphatic nitro compound with an aldehyde or ketone is usually called the Henry reaction or the Kamlet reaction, and is essentially a nitro aldol reaction. A variety of conditions have been reported, including the use of a silica catalyst, Mg—A1 hydrotalcite, a tetraalkylam-monium hydroxide,proazaphosphatranes, " or an ionic liquid.A solvent free Henry reaction was reported in which a nitroalkane and an aldehyde were reacted on KOH powder. Potassium phosphate has been used with nitromethane and aryl aldehydes. The Henry reaction has been done using ZnEt2 and 20%... 

Proazaphosphatrane ligands, such as 420, partnered with palladium species generated highly active catalysts for Buchwald-Hartwig reactions. An example of such transformations was illustrated when 26 was able to react with 413 to afford 414 [149]. 

Other phosphorus containing superbases are phosphines and phosphorus yhdes. The most detailed studied class is Verkade s proazaphosphatranes (cyclic azaphosphines). Because of the extraordinary basicity and relatively weak nucleophilicity of nonionic proazaphosphatranes, they have been found to be efficient catalysts and promoters for many reactions... 

The successful synthesis of p-hydroxynitriles was reported in good to excellent yields from aldehydes and ketones in a simple reaction that is promoted by proazaphosphatrane bases. The reaction occurs in the presence of magnesium salts, which activate the carbonyl group and stabilize the enolate thus produced [68] (Scheme 5.46). 

Proazaphosphatrane bases were used for palladium catalysed Stille reactions of aryl chlorides. These bases efficiently catalyse the coupling of electronically diverse aryl chlorides with an array of organotin reagents. The catalyst system based on benzyl (Bn)-proazaphosphatrane is active for the synthesis of sterically hindered biaryls. The use of the proazaphosphatrane allows room temperature coupling of aryl bromides and it also permits aryl triflates and vinyl chlorides to participate in Stille coupling [69] (Scheme 5.47). 

The variable basicity of (187 R = Me) due to the flexible transannular bond is believed to play an important role in the superior catalytic activity of this compound for the cyclotrimerization of phenylisocyanate <93AG(E)896>. The relative stability of (188) has been attributed to its partially annulated structure (P—N 3.250 A) <92JA3129>. The reaction of the proazaphosphatrane (189) with methyl iodide gave both the annulated —S—Me and open N+—Me regioisomers <93JA1660>. 

Proazaphosphatrane (155), a strong nonionic base, catalysed the reaction of ethyl isocyanoacetate with aromatic and aliphatic aldehydes to afford trrms-oxazoline 156 with... 

Bulky, electron-rich imino-proazaphosphatranes are another relatively new and novel Ugand class for coupling reactions of aryl chlorides (Equation 2.32)... 

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