Iminophosphorane catalysts

A DFT study of the origins of stereoselectivity in the aldol reaction of bicyclic amino ketones (20) with aromatic aldehydes has been reported (Scheme 18). ° Base-catalysed direct aldolization of a-alkyl-a-hydroxy trialkyl phosphonoacetates with aldehydes proceeds via a fully substituted glycolate enolate intermediate formed by a [l,2]-phosphonate-phosphate rearrangement. High enantioselectivity can be achieved by the application of chiral iminophosphorane catalysts. 

Reduction of azides is a classical approach to primary amine synthesis. Treatment of 17 with sodium azide in DMF or in THF/H2O mixtures in the presence of phase transfer catalysts effects a quantitative conversion to the corresponding polymeric azide, 27. Recently the reduction of azides to primary amines via hydrolysis of iminophosphoranes produced by interaction of the azide with triethyl phosphite was reported.30 Application of this technique to the azidomethyl polymer, 27, as shown below, failed to produce a soluble polyamine. 

Employing iminophosphoranes to protect a group labile under alkaline conditions can lead to a dramatic increase in yield. This is exemplified by the transformation of allylic azide 31 into the corresponding iminophos-phorane 32 shown in Scheme 16. Hydrolysis under basic conditions leads finally to 4-amino-3-hydroxycyclohexa-l,5-diene-l-carboxyclic acid (33) in 80% yield. However, when the same azide (31) is converted with a Lindlar catalyst, via allylic amine 34 into carboxylic acid 33, only 0-30% yields are found as a consequence of the low stability of the allylic amine [93JCR(S)148]. 

An interesting l,3-oxazol-2-one synthesis of 126 starts from propargyl alcohol 125, CO2, and primary amines with n-butylphosphane as a catalyst (Scheme 50). It is not yet clear if the phosphane reacts by formation of an iminophosphorane followed by an aza-Wittig reaction with CO2 (90TL1721). 

Related studies were recently reported by Contel, Urriolabeitia et al. using organogold(III) iminophosphorane complexes as catalysts for the addition of... 

The metathesis of symmetrical aliphatic carbodiimides is also catalyzed by tungsten imido complexes above 140 Other carbodiimide metathesis catalysts include Cr(ll)/Si02/ iminophosphoranes, imido circonocenes, guanidine supported titanium complexes and group 14 amide complexes. ... 

Ylides Coordinated to Metals. - Complexes of iminophosphoranes have attracted attention due to their potential application as catalysts for the polymerisation of olefins. A recent contribution to this field reports the coordination of sterically hindered iminophosphorane (73) towards titanium and zirconium tetrachlorides. Treatment of TiCb with (73) led to the phosphoranato complex (74) whereas ZrCU was unreactive. However, treatment of Zr(NMe2)4 with (73) led to complexes (75) and (76) (Scheme 23). The structures of ligand (73) and... 

Iminophosphoranes (117), (118) and their polymer-supported analogues (119), (120) (Scheme 24) have been found to catalyse the acylation of primary alcohols with enol esters with high yields and selectivities. Moreover, these ylide based catalysts appear more tolerant of sensitive functional groups than alternative Lewis base reagents. ... 

Vinyl acetate or 2-propenyl acetate, toluene, Cp 2Sm(THF)2, rt, 3 h, 88-99% yield. Other esters can also be prepared by this method. Iminophosphorane bases also serve as excellent transesterification catalysts with vinyl acetate (74-99% yield). ... 

Polystyrene supported formamide 64 was more reactive than 65 for the allylation of aldehydes with allyltrichlorosilane to afford 69 (Scheme 10.13) [201], Good yields after 24 h could be obtained only when an excess of catalyst was used. Recycling of the catalyst was possible without appreciable loss of activity for three runs, but only when agitation was carried out with an automatic shaker. The yield decreased dramatically for the third cycle when a magnetic stirrer was used. A similar phenomenon was detected when iminophosphorane 67 was studied for different... 

A novel iminophosphorane-based [P2N2] rhodium complex representing a very innovative catalyst system was prepared and was used to reduce C=0 and C=N bonds [191]. 

Iminophosphorane base ligands (also known in the literature as phosphazenes or phosphinimes, R3P=NR ) have been scarcely used in metal-catalyzed reactions in water. We have recently reported a highly active novel catalyst for the Huisgen 1,3-dipolar cycloadditions in pure water, based on a l,3,5-triaza-7-phosphaadamantane (PTA)-iminophosphorane Cu(I) complex (6) [29]. This complex represents one of the few examples of an isolated and crystallographically characterized... 

We have recently reported that the air-stable and hydrosoluble iminophosphorane copper(I) complex 6 is also active in CuAAC of 1-iodoalkynes in aqueous media, under mild and aerobic conditions according to click laws and displaying a broad substrate scope and functional compatibility [29] (see Scheme 15.8). It is important to note the following catalytic features (i) catalyst 6 was the first example of an isolated and crystallographically characterized copper(I) catalyst active for cycloaddition of 1-iodoalkynes with azides, to give 5-iodo-1,2,3-triazoles exclusively, (ii) The presence of a free thio moiety in the substrate does not deactivate the catalyst, a fact generally observed in CuAAC for functionalized substrates... 

The most versatile catalyst in this three-component transformation is the iminophosphorane copper(I) complex (6), which is also active under very mild reaction conditions (pure water as solvent, at room temperature, and under air conditions) with both terminal and internal 1-iodoalkynes [40]. The latter, is an unprecedented catalytic reaction that is also efficient for a wide array of 1-iodoalkynes and organic bromides (Scheme 15.10). Both electron-withdrawing and electron-donating substituent groups are tolerated, although a longer reaction time is required for the internal iodoalkynes when compared with their terminal counterparts. 

A series of neutral ehiral bis(guanidine)iminophosphoranes (139, R = Me, Bu, Br, benzhydryl) have been prepared, and their hydrohalide salts have been characterized by X-ray crystallography. They act as organosuperbase catalysts for enantioselective (g)... 

By treatment of cyclic and acyclic proazaphosphatranes with benzyl azide, a new class of highly selective acylation catalysts has been synthesized. These iminophosphorane bases, (3) and (4), catalyse the transesteriflcation of primary, but not secondary, alcohols... 

Alkylamino)-6-aryl-3-phenyl-2-thioxo-2,3-dihydrothiazolo[4,5-d]pyrimidin-7(6H)-ones (I R = alkyl, substituted benzyl R1 = H, F) 141 were easily synthesized via a tandem aza-Wittig reaction. Treatment of iminophosphorane 139 with aromatic isocyanates gave carbodiimides 140 (same Rl), which reacted with fluoro-substituted alkylamines to provide the title compoimds 141 (Scheme 64) in 65-87% isolated yields using sodium ethoxide as catalyst [98],... 

Review articles containing material of mechanistic interest cover iminophosphoranes, the chemistry of P=X species,and the regio- and stereochemical control of nucleophilic substitution in cyclophosphazanes/ The kinetics of the series substitution of PsNjQg with CF3CH2OH using Bu4NBr as phase-transfer catalyst have been studied in a two-phase system, following the concentration of all of the species from Clg to CIq. The hydrolysis of the cyclic trimer in aqueous THF has been followed by P NMR, and several intermediate species (12), (13), and (14) identified. Other... 

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