Lewis acids adducts

In important recent work, Shriver has demonstrated that electrophiles can promote the migration of alkyl groups to coordinated CO. Lewis acid adducts of metal acyl complexes are isolated [37]. Thus it is possible that electrophilic species might also facilitate the generation of catalyst-bound formyls. 

Charge-transfer complexes involving one-center n acceptor orbitals (n-n, a-n, and 7t-n Lewis-base-Lewis-acid adducts) have been discussed in Sections 3.2.10 and 3.6. Many CT complexes involving a acceptors have been illustrated for H-bonds (n-a ) and dihydrogen bonds (a-o ), as well as for the Cg H6 -B (7t-a ) example above. In the remainder of this section we shall therefore focus on CT... 

Reactions of Co-ordinated CO and Lewis Acid Adducts. The details of the preparation and properties of [Me3SiFe(COSiMe3XCO)3]2 (see Vol. 1, p. 139) have been published.The reaction of FeiCO) and MeC H N=PPh, in THF yields [Fe(CO)4(CNCftH4Me)], [Fe(CO)3(CNC6H4Me2], and Ph3PO (o-, m-, and p-isomers all isolated ). This is a rare example of deoxygenation of co-ordinated CO without M—C bond cleavage. The p-Me complexes can also be obtained from the reaction of Fe2(CO)g and p-MeC H N PPhj in benzene at room temperature. 

The ionic nature of ClFsO Lewis acid adducts was established by vibrational (33-35, 38, 58, 300) and F NMR (61) spectroscopy. 

In marked contrast, because of the nucleophilicity of the stable amino- and phos-phino-carbenes, numerous examples of carbene-Lewis acid adducts (reverse ylides) (Scheme 8.19) have been reported, whereas a very few examples of normal ylides are known. 

The lower stability of (phosphino)(silyl)carbene-group 13 (IIIA) element Lewis acid adducts compared to that of the corresponding imidazol-2-ylidene adducts is easily rationalized by considering the inferior ability of phosphorus to stabilize a positive charge in the ot-position. In other words, once complexed, the carbene... 

In a recent report, protonation of a bridging carbonyl oxygen has been described by Shriver and co-workers (62a). The compound is proposed to have structure (12b), and represents an extension of Shriver s previous studies on Lewis acid adducts of bridging carbonyls (49-51). The compound is prepared under rigorously anhydrous conditions and the protonation of the O atom is ascribed to the enhanced basicity of O in the bridged bonding mode (4). 

After the formation of a 1 1 substrate - Lewis acid adduct, the rearrangement proceeds in two steps, beginning with the cleavage of the ester bond and the release of formyl chloride in situ, which, in turn, acts as a formylating agent, introducing... 

SiMe3, x = 1-2 R = Me, x = 1-3), have also been prepared from the reaction of HCl(g) with the alkyl complexes (286). Reaction with Au(PPh3)Cl results in the formation of [Ru(N)ClMe3] and Au(PPh3)Me from [Ru(N)Me4], in a manner analogous to that of the HC1 reaction (286). In contrast to the related osmium complexes, Lewis acid adducts of the ruthenium nitrides are thermally unstable. 

TABLE 9. Parameters (bond length in pm, angle in deg) of Lewis-acid adducts to tetrylenes A <- ER2 with acceptor atom A... 

Table 2. Lewis Acid Adducts of Cyanide Complexes Compound Reference...

Even Lewis acid adducts of the acyl metalate 11 (Section 8.3.1.1, Scheme 8) proved effective in generating benzannulation products. The reaction of a trimethylsilyloxy chromium carbene complex with 3-hexyne to give the respective semi-silylated hydroquinone tricarbo-nylchromium complex in 35 % yield [30a] and the use of titanoxycarbene complexes [30b] constitute such examples. Thiocarbene complexes form hydrothioquinones in a Lewis acid supported benzannulation reaction [32]. 

Carbon monoxide shows very little basicity or nucleophilicity within Figure 1.11 Representative p-block chemistry, although some Lewis acid adducts are known, e.g. rc-iigands H3B-CO. In contrast, there are many thousands of known complexes of... 

CpTi(AsF6)3 S02 and CpTi(SbF6)3-S02 are Lewis acid adducts of CpTiFj. CpTi(AsF6)3-S02 and CpTi(SbF6)3-S02 decompose in solution at room temperature and slowly at -50 °C, but are nonexistent in the solid state. ... 

Infrared spectroscopy has been used for many years to probe acid sites in zeolites. Typically, strong bases such as ammonia or pyridine are adsorbed, and the relative or absolute intensities of bands due to Lewis acid adducts or protonated Bronsted acid adducts are measured. The basicity of ammonia or pyridine is however much stronger than that of most hydrocarbon reactants in zeolite catalysed reactions. Such probe molecules therefore detect all of the acid sites in a zeolite, including those weaker acid sites which do not participate in the catalytic reaction. Interest has recently grown in using much more weakly basic probe molecules which will be more sensitive to variations in acid strength. It is also important in studying smaller pore zeolites to use probe molecules which can easily access all of the available pore volume. 

Early examples of catalysis by lewis acid adducts involving manganese... 

Key Words Lewis acid adducts, Radical oxidations, Epoxidation, Hydrogen peroxide, Bond dissociation energy, Catalyst durability, Methyltrioxorhenium, Cross-bridged cyclam, Mn(IV), Late transition metal. Propylene oxide. Titanium silicalite (TS-1) catalyst, Ethylanthrahydroquinone/H2 process, Polyoxometallates, Mn(IV) catalyst. Hydrogen abstraction. Rebound mechanism, Isotopic label, t-BuOOH, Peroxide adduct. 2008 Elsevier B.v. 

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