Zirconium triflate

Collins and coworkers applied the bis(tetrahydroindenyl)zirconium triflate 32, which is used as a polymerization catalyst, to the asymmetric Diels-Alder reaction [50] (Scheme 1.61). A remarkable solvent effect was observed - although only a low optical yield was obtained in CH2CI2, high optical purity (91% ee) was realized in 2-nitropropane by use of only 1 mol% of the catalyst. The catalyst is also effective for crotonoyloxazolidinone, giving the cycloadduct in 90% ee. 

Similarly, scandium triflate (Sc(OTf)3), zirconium triflate (Zr(OTf)4) and titanium chloro(tris)triflate (TiCl(OTf)3) were also used for the orf/zo-acylation of phenols and 1-naphthols using acid chlorides . ... 

Regioselective Friedel-Crafts reaction with unsymmetrical alkynes was reported. A reaction of phenylacetylene with p-xylene afforded gem-diarylethylene (71) selectively (Equation 30) [35]. Zirconium triflate showed much higher catalytic activity than that of chloride. Hf-catalyzed reaction of benzene with 1-phenylpropane in ionic liquid produced 1,1-diphenylpropene (72) in excellent yield with excellent regioselectivity (Equation 31) [36]. 

The cyclopentadienyl triflate complexes of zirconium and titanium 51 and 52 (Figure 3.7) are also active catalysts [51]. Their activity has been tested in a wide variety of dienes and dienophiles. It is noteworthy that even at low catalyst loadings, rate accelerations between 10 and > 10 times have been observed. No special precautions were taken to dry the solvents or the substrates, in contrast with the traditional Lewis acids which require either predried solvents or high catalyst loadings. 

However, this catalyst is not effective for less reactive aromatics such as o-nitrotoluene. In such cases, hafnium(IV) and zirconium(IV) triflates are excellent catalysts (10 mol%) for mononitration of less reactive aromatics. The catalysts are readily recycled from the aqueous phase and reused (Eqs. 2.3 and 2.4).12... 

NMR studies at low temperature provided evidence for the formation of a mono-cationic oxazoline—zirconocene complex in which the substrate is a bidentate ligand and one triflate is still coordinated to the zirconium [4],... 

Although in the recent years the stereochemical control of aldol condensations has reached a level of efficiency which allows enantioselective syntheses of very complex compounds containing many asymmetric centres, the situation is still far from what one would consider "ideal". In the first place, the requirement of a substituent at the a-position of the enolate in order to achieve good stereoselection is a limitation which, however, can be overcome by using temporary bulky groups (such as alkylthio ethers, for instance). On the other hand, the ( )-enolates, which are necessary for the preparation of 2,3-anti aldols, are not so easily prepared as the (Z)-enolates and furthermore, they do not show selectivities as good as in the case of the (Z)-enolates. Finally, although elements other than boron -such as zirconium [30] and titanium [31]- have been also used succesfully much work remains to be done in the area of catalysis. In this context, the work of Mukaiyama and Kobayashi [32a,b,c] on asymmetric aldol reactions of silyl enol ethers with aldehydes promoted by tributyltin fluoride and a chiral diamine coordinated to tin(II) triflate... 

Friedel-Crafts alkylation has been used in an important synthesis of aryl C-glycosides, which are potent anti-tumor agents, from glycosyl fluorides (equation 99)65 661. The reaction takes place rapidly in dichloromethane, at room temperature using a novel zirconium complex and silver perchlorate combination catalyst. A similar alkylation has been performed by replacing the aromatic compound with either a silyl enol ether or an allylic compound using silver triflate as the catalyst662,663. 

A chiral zirconium bis(triflate) complex has been found to catalyze asymmetric Diels-Alder reactions (Eq. 25) [25]. 

R-X = alkenyl and atyl halides and triflates R M = alkenyl and aryl zinc, aluminum, zirconium, or boron and tin... 

The zirconium(IV) complex 27, with one and one t -allyl unit, upon heating, undergoes facile intramolecular rearrangement to a 3-allylmetallacyclobutane complex 28, which after treatment with silver(I) triflate releases allylcyclopropane (29) in quantitative yield. ... 

The aim of this contribution is to present data on the preparation of catalysts containing as embedding species a large family of eolloids such as colloids of ruthenium, platinum, or palladium-gold alloys and triflate derivatives such as lanthanum and silver triflate or tert-butyldimethylsilyltrifluoromethanesulfonate (BMSTM). Silica, zirconia and tantalum oxides were used as carrier. All these preparations considered the polymeric sol-gel route using as starting materials silicon, zirconium or tantalum alcoxides. 

From the above structural analysis it becomes clear that metal triflates with charge-to-size ratios greater than "3" (i.e. greater than that of the smallest lanthanide lutetium) should be more effective nitration catalysts. We considered that the group IV metals hafnium (r4+ = 0.78 A, Z/r = 5.13) and zirconium (r4 = 0.79 A, Z/r = 5.06) might be suitable for such a purpose. In line with this reasoning we noted that hafnium(IV) triflate has been shown to be an effective catalyst for Friedel-Crafts acylations and alkylations where the corresponding lanthanide salts were less active.20... 

Hafnium and zirconium mononitrate tris(triflate), [M(H20)x(N03)](0Tf)35 were prepared from their tetrachlorides in analogous fashion to the lanthanide salts. Much to our delight these (deliquescent) salts displayed A, nitrate stretching frequencies in their IR spectra at 1651 and 1650 cm1 respectively. Armed with this pleasing information and with a specific programme aim of nitrating o-nitrotoluene (ONT) to dinitrotoluenes (DNTs) with these catalyst types, (catalytic quantities of ytterbium(III) triflate were essentially ineffective for this transformation) hafnium(IV) and zirconium(IV) triflate were prepared as their hydrated salts via metathesis of their tetrachlorides with silver triflate in water. 

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