Iron triflate

Table 2 Iron triflate-catalyzed glycosylation of alcohol 77 with acetate 1 under various conditions.

Table 3 Iron triflate-catalyzed glycosylation using acetate donor 1"...

Table 4 Iron triflate-catalyzed glycosylation using acetate 1 oxazoline 2 and different 2-acetamide-p-acetate donors 83-85 ...

Iron is also considered as a water-insensitive metal. However, reports on the catalytic behavior of iron triflate are contradictory. Bonnamour reported that iron triflate is an efficient catalyst for the typical Lewis acid-catalyzed reactions, but is inactive for the reactions carried in water, as solvent [62]. On the other side, Choi et al. indicated that iron triflate, in situ... 

TABLE IRON CATALYZED CROSS COUPLING REACTIONS OF GRIGNARD REAGENTS WITH (HETEROJARYL CHLORIDES OR -TRIFLATES... 

Sakakura and coworkers reported the catalytic action of iron(lll) triflate for the addition of acetylenes to olefins without the need for an inert gas atmosphere (Scheme 30) [43]. 

Choi and Sakakura et al. reported that iron(III) triflate, in situ formed from FeCls and triflic acid, efficiently catalyzes the intermolecular addition of carboxylic acids to various alkenes to yield carboxylic esters. The reaction is applicable to the synthesis of unstable esters, such as acrylates (Scheme 40) [50]. 

Other reactions described are those of imidazolides with Grignard reagents, which are carried out in the presence of the catalysts trimethylsilyl triflate, iron trichloride, or iron trisacetylacetonate ... 

A few further general examples of zinc catalytic activity or reactivity include the following. Other zinc-containing systems include a zinc phenoxide/nickel(0) catalytic system that can be used to carry out the chemo- and regioselective cyclotrimerization of monoynes.934 Zinc homoenolates have been used as novel nucleophiles in acylation and addition reactions and shown to have general utility.935,936 Iron/zinc species have been used in the oxidation of hydrocarbons, and the selectivity and conditions examined.362 There are implications for the mechanism of metal-catalyzed iodosylbenzene reactions with olefins from the observation that zinc triflate and a dizinc complex catalyze these reactions.937... 

Martin et al. have developed a unique series of capped tris(l,2-di-aminoethane) cages which can encapsulate divalent transition metal ions in a near octahedral geometry (28). The iron(II) complex with the ligand (NH2)2sar turns out to be a crossover system in solution [34], but the solid triflate salt is low-spin [43]. This is the only Fe(II) crossover system having 6 identical aliphatic nitrogen donors. 

Stille employed a Heck reaction on triflate 136 to install the acrylamide side chain in his synthesis of anthramycin and analogues, as illustrated below [79, 80]. Ironically, a Stille reaction was less efficient in this transformation. 

Acid-catalyzed dealkoxylation is particularly suitable for the preparation of highly reactive, cationic iron(IV) carbene complexes, which can be used for the cyclopropanation of alkenes [438] (Figure 3.11). Several reagents can be used to catalyze alkoxide abstraction these include tetrafluoroboric acid [457-459], trifluoroacetic acid [443,460], gaseous hydrogen chloride [452,461], trityl salts [434], or trimethylsilyl triflate [24,104,434,441,442,460], In the case of oxidizing acids (e.g. trityl salts) hydride abstraction can compete efficiently with alkoxide abstraction and lead to the formation of alkoxycarbene complexes [178,462] (see Section 2.1.7). 

Tetrathiotungstate complexes of iron(III)-tetraphenylporphyrin can be prepared from iron(III) tpp triflate they undergo slow spontaneous reduction to iron(II). " ... 

In 1993, Evans and co-workers examined phe-box 6, /-pr-box 45, and bu-box 3 ligands in the Diels-Alder reaction of cyclopentadiene 68 and 3-acryloyl-l,3-oxazolidin-2-one 69 using a weak Lewis acid such as copper(II) triflate." The results are summarized in Table 9.9. The reaction was carried out between —50 and —78 °C for 3-18 h and achieved selectivities of up to 98 2 (endo/exo) with an endo ee of >98% (using bu-box 3). Interestingly, the enantiomer produced in these reactions was the (25) configuration, compared to the (2K) isomer obtained with iron(III) and magnesium(II) as reported by Corey. This observed stereochemistry was explained by the chelation model of the copper(II) complex 74 (Fig. 9.23)... 

In addition to these commonplace substrates, only a few extraordinary educts have been used in iron-catalyzed DA reactions, such as the naphthoquinones investigated by Brimble and McEwen [75]. Whereas the application of FeCl3 and a chiral bisoxazoline ligand gave only a 25% yield and no chiral induction in the reaction of 2-acetyl-l,4-naphthoquinone with cyclopentadiene, the corresponding copper(II) triflate gave a 66% yield and moderate enantioselectivities (up to 50% ee). Another example was reported by Shibasaki s group in which the 2-alkoxy-l,3-butadiene 40... 

Two new tetraphenylporphyrin (TPP) catalysts have been reviewed.331 Cr(TPP)m triflate is highly regio- and stereo-selective in rearranging epoxides into aldehydes, while the iron perchlorate analogue affords the corresponding ketones. 

Reactions of complexes of 1,2-cycloheptadienes have received only cursory attention. 1,2-cycloheptadiene is readily displaced from bisftriphenyl-phosphine)platinum(O)118 [Eq. (54)], but no reagent has been found that will displace the allene from iron.119 Reaction of the iron complex with alcohol in the presence of base (e.g., 312 — 322) is typical of Fp+ complexes of acyclic allenes.131132 The thermal chemistry of 312 is unusual in its decomposition to 324 (Scheme 41). This is probably attributable to the presence of the triflate counterion, since the corresponding fluoroborate salt is stable when warmed to 40°C for 16 h.119 A mechanism to 324 via carbene complex 321 appears likely. 

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