Hf Lewis Acids

Minkwitz et al.604 have prepared the hexafluorometalates of cations 308. The reaction of a,a-dichloromethyl methyl ether in HF-Lewis acid solution at —78°C leads to the formation of chloro cation 308-C1, whereas at -65°C fluoro derivative 308-F is isolated as a result of chlorine-fluorine exchange [Eq. (3.80)]. Interestingly, the chlorine atom and the methyl group are trans in the hexafluoroantimonate salt of cation 308-C1, whereas the fluorine atom and the methyl group are cis in cation 308-F. The arrangement of the C—O—C—H atoms is nearly planar with F/Cl—C—O—C torsion angles of 2.84° (308-F) and 179.0° (308-C1). The C Obond distances (1.224 and 1.479 A for cation 308-F, and 1.252 and 1.517 A for cation 308-C1) reveal dominant oxonium ion character. 

Table 2. Hq values for HF, H2SO4, and some HF-Lewis acid mixtures...

The salts rapidly hydroly2e to form NF and O2, and react with glass at temperatures above 85°C to form NF and SiF (21). The NF" 4 salts are stable in dry atmospheres to 200°C, but rapidly decompose above 300°C to yield NF, F2, and the corresponding Lewis acid. Therefore, these salts are solid sources of NF and F2, free of atmospheric contaminants and HF. 

The great electron-pair acceptor capacity (Lewis acidity) of SbFs has been utilized in the production of extremely strong proton donors (Brpnsted acids, p. 48). Thus the acidity of anhydrous HF is substantially increased in the presence of SbF ... 

Note that Lewis acidity decreases, whereas Brpnsted acidity increases, going down the table. There is no contradiction here when we remember that in the Lewis picture the actual acid in all Brpnsted acids is the same, namely, the proton. In comparing, say, HI and HF, we are not comparing different Lewis acids but only how easily F and 1 give up the proton. The effect discussed here is an example of a symmetry factor. For an extended discussion, see Eberson, L. in Patai The Chemistry of Carboxylic Acids and Esters, Wiley NY, 1969,... 

Apart from the alkyl halide-Lewis acid combination, two other sources of carbo-cations are often used in Friedel-Crafts reactions. Alcohols can serve as carbocation precursors in strong acids such as sulfuric or phosphoric acid. Alkylation can also be effected by alcohols in combination with BF3 or A1C13.37 Alkenes can serve as alkylating agents when a protic acid, especially H2S04, H3P04, and HF, or a Lewis acid, such as BF3 and A1C13, is used as a catalyst.38... 

Traditionally, the production of LABs has been practiced commercially using either Lewis acid catalysts, or liquid hydrofluoric acid (HF).2 The HF catalysis typically gives 2-phenylalkane selectivities of only 17-18%. More recently, UOP/CEPSA have announced the DetalR process for LAB production that is reported to employ a solid acid catalyst.3 Within the same time frame, a number of papers and patents have been published describing LAB synthesis using a range of solid acid (sterically constrained) catalysts, including acidic clays,4 sulfated oxides,5 plus a variety of acidic zeolite structures.6"9 Many of these solid acids provide improved 2-phenylalkane selectivities. 

Lewis acid catalysts can also effect dealkylation, i.e. the reaction is reversible. Thus ethylbenzene (22) with BF3 and HF, is found to disproportionate ... 

Lewis acids (the effective nucleophile here may be RC=0 or in some cases RCOC1 is formed by the action of A1C13 on the original anhydride), and also by acids themselves. This latter is promoted by strong acids, e.g. H2S04, HF, as well as by Lewis acids and may involve formation of acyl cations through protonation ... 

Antimony pentachloride is a reactive Lewis acid that can be used for Friedel-Crafts reactions and some other Lewis-acid-catalyzed reactions. The HF-SbF5 system is known as magic acid, and carbocations are stabilized in this medium.353 By using the HF-SbF5 system, alkylation of acetophenone (a relatively unreactive aromatic compound) has been achieved (Scheme 87). 

The acidity of liquid HF is high enough that it can function as an acid catalyst in many instances. The cation characteristic of the solvent, H2F+, is generated when HF reacts with a strong Lewis acid that is capable of forming stable fluoride complexes. The reactions with BF3 and AsF5 are typical ... 

Lewis bases having two or more lone-pair-bearing atoms can exhibit a form of isomerism in H-bonding. This can be illustrated most simply with carbon monoxide, which has sigma-type lone pairs both on C and on O, and thus is expected to form n—>-cr H-bonds of either OC HA or CO HA type with a hydride-bearing Lewis acid. For example, complexation of CO with HF gives rise to distinct OC- -HF or CO- -HF complexes.39 The two possible isomeric forms are shown in Fig. 5.3, both bound by rather typical H-bond energies ... 

Table 5.8 summarizes the NBO descriptors of the net charge transfer from Lewis base to Lewis acid (Qcf), change in covalent-bond polarization (A/Ah), and (P)NBO overlap of n0 with bond (Sna) and antibond (Sna>) orbitals of the Lewis acid. The entries in Table 5.8 show the unfavorable diminution of l/w /5nal and reduced charge transfer as the Lewis acid changes from polar HF to apo-lar CH4. These NBO descriptors can also be closely correlated with quantities in Table 5.7, showing their mutual dependence on the strength of n-a donor-acceptor interaction. 

In the case of A = F, there is apparently no local minimum corresponding to the HO- HF isomer, and instead the proton transfers to form HOH- F- as the only stable equilibrium species. However, in the case of the weaker HA Lewis acids, stable HO- HA structures are found. Figure 5.7 displays optimized structures of these complexes and the dominant n-cr interaction in each case, while Table 5.9 summarizes energetic and structural properties of these complexes for... 

Figure 5.26 (a) The isomeric anticooperative open (HF)3 structure (fully optimized), and (b) the leading np interaction with one of the two equivalent Lewis-acid monomers (with the second-order stabilization energy in parentheses). The net binding energy is 7.92 kcal mol-1. 

Hydrosilylation of fi-hydroxy ketones.1 (3-Silyloxy ketones (2), prepared by silylation of (3-hydroxy ketones with 1 under the usual conditions (DMAP or Py), on treatment with a Lewis acid form a mixture of siladioxanes, 3a and 3b, which on desilylation with HF is converted into a mixture of anti- and syn-diols (4). The... 

Cyclization of mixed acetals (13,300).4 This reaction is a particularly useful route to eight-membered cyclic ethers (oxocanes) and provides the first practical route to a natural oxocene, (- )-laurenyne (3), from an optically active mixed acetal 1. Thus cyclization of 1 followed by O-desilylation affords 2 as the only cyclic product. Remaining steps to 3 involved C-desilylation, for which only HF/pyridine is useful, introduction of unsaturation into the C2-side chain, and extension of the C8-side chain. Exploratory studies showed that unsaturation at the p- or y-positions to the cite of cyclization of 1 prevent or retard cyclization with a wide variety of Lewis acids. The cyclization is apparently more tolerant of substitution in the terminator position, C3-Q, of the oxocene. 

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