Triflate, Magnesium

In 2012, Chi et al. disclosed an oxidative y-addition of enals to tri-fluoroacetophenone for the synthesis of unsaturated 5-lactones 114 under NHC catalysis. Scandium triflate/magnesium triflate as relatively strong Lewis acid cocatalysts were found to be effective for enantiocontrol involving the relatively remote enal y-carbon (Scheme 20.49). 

Magnesium triflate and zinc triflate are outstanding catalysts for the intro duction of the thioketal group for the protection of the ketone function [126] The reaction of a variety of ketones with ethane 1,2-dithiol in the presence of these triflates proceeds under mild conditions to form the corresponding thioketals in high yield (equation 62)... 

Desimoni et al. have shown that the use of magnesium perchlorate or magnesium triflate, and three chiral to(oxazolines) and two equivalents of achiral auxiliary ligands such as water or tetramethylurea, induces a strong change of the enantiofacial selectivity with >94% ee in the... 

Recently, very effective asymmetric conjugate addition of 1,3-dicarbonyl compounds to nitroalkenes has been reported, as shown in Scheme 4.10. The reaction of ethyl acetoacetate with nitrostyrene is carried out in the presence of 5 mol% of the preformed complex of magnesium triflate and chiral bis(oxazoline) ligands and a small amount ofW-methylmorpholine (NMM) to give the adduct with selectivity of 91%. The selectivity depends on ligands. The effect of ligands is presented in Scheme 4.10.63... 

Sibi and Chen demonstrated for the first time that relative and absolute stereocenters of both a and fJ> carbons can be controlled in the intermolecular addition trapping experiments (Scheme 55) [149]. Magnesium and copper Lewis acids performed better than zinc. The use of 30 mol % of chiral Lewis acid gave higher selectivities than the stoichiometric amounts for both magnesium and copper. Interestingly, copper triflate gave better selectivities with... 

As well as alkenylstannanes [106-108], other classes such as a-heteroatom-substituted alkyltributylstannanes [109] and, more importantly, allylic stannanes [110, 111] also undergo these Sn-Cu transmetalations. Otherwise difficult to prepare, allylic copper reagents may, however, be obtained by treatment of allylic stannanes (produced in turn from organolithium, magnesium, or zinc organometallics) with Me2CuLi LiCN. They enter into cross-coupling reactions with alkyl bromides [110] or vinyl triflates (Scheme 2.52) [111]. 

The diastereoselective cycloaddition of 2-phenyl-4-dimethylamino-l-thia-3-azabuta-l,3-diene with a choice of dienophiles and in the presence of a Lewis acid provides a convenient route to 5,6-dihydro-4//-l,3-thiazines <2002TL6067, 2004T1827>. The more stable /ra r-adducts are produced exclusively. The approach using (4A)-3-acryloyl-4-benzyloxazolidin-2-one 198 provides access to the chiral 5,6-dihydro-4//-l,3-thiazine 199 <2004T1827>. The exceptional level of selectivity is only achieved when magnesium bromide is used. The chiral auxiliary was removed by reaction with lithium benzoxide to give the benzyl ester 200, and reaction with catalytic amount of samarium triflate and methanol provides the methyl ester 201 (Scheme 21). 2-Substituted-5,6-dihydro-l,3-thiazines are conveniently synthesized from nitriles or thiocyanates and 4-mercapto-2-methylbutan-2-ol to produce... 

Elsewhere, Faita et al. (438) bound the Evans chiral auxiliary to Wang or Merrifield resin for use as a dipolarophile in cycloadditions with C,N-diphenyl-nitrone. Yields on both resins are significantly reduced in comparison to the solution phase reaction (43-20% compared to 95%) but are unaffected by addition of magnesium perchlorate or scandium triflate catalyst. A one-pot process has been reported by Hinzen and Ley (439) that oxidizes secondary hydroxylamines to the... 

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)... 

Takacs, J.M., Lawson, E.C., Reno, M.J., Youngman, M.A. and Quincy, D.A. (1997) Enantioselective Diels—Alder reactions Room temperature bis(oxazoline)-zinc, magnesium, and copper triflate catalysts. Tetrahedron Asymmetry, 8, 3073—3078. 

A combination of chiral oxazoline 15 and magnesium triflate was employed by Ji and Barnes in the asymmetric addition of keto esters and malonates to ni-troalkenes (Scheme 13) [44]. 

Since the addition of dimethylpyrazole to an isocyanate is reversible by heating at 60°C, the product (10) is known as a blocked isocyanate . Now, it has been shown that magnesium triflate, Mg(OTf)2, is highly active as a catalyst in the reaction at 60 °C between an acid and a blocked isocyanate to give an amide. Thus blocked hexyl isocyanate (10) and heptanoic acid (11) when heated in equimolar quantities in CHCI3 at reflux for 1 h in the presence of 1 mol% Mg(OTf)2 gave a quantitative yield of IV-heptyloctamide (12) with evolution of CO2.6... 

---