Lewis base catalysts formation

The first use of chiral sulfoxides as Lewis-base catalysts in the allylation of aldehydes with allyltrichlorosilane was reported in 2003. The formation of the... 

Two patterns are possible in the activation mechanism by simple chiral Lewis base catalysts. One is through the activation of nucleophiles such as aUyltrichlorosilanes or ketene trichlorosilyl acetals via hypervalent silicate formation using organic Lewis bases such as chiral phosphoramides or A-oxides. " In this case, catalysts are pure organic compounds (see Chapter 11). The other is through the activation of nucleophiles by anionic Lewis base conjugated to metals. In this case, transmetal-lation is the key for the nucleophile activation. This type of asymmetric catalysis is the main focus of this section. 

The heterogeneously catalyzed disproportionation of 1,1,2,2-tetrachlorodimethyldisilane leads via oligo(chloromethylsilane)s to highly branched poly(chloromethylsilane)s. The disilane derived oligomer formation can be controlled by the nature of the Lewis base catalyst. 

Shibasaki has described the use of bifunctional catalysis in asymmetric Strecker reactions, using BlNOL-derived Lewis acid-Lewis base catalyst 160 (Equation 24) [114]. The aluminum complex had previously been shown to catalyze enantioselective cyanohydrin formation (Chapter 2, Section 2.9) [115]. In the proposed catalytic cycle, the imine is activated by the Lewis acidic aluminum while TMSCN undergoes activation by association of the silyl group with the Lewis basic phosphine oxide. Interestingly, the addition of phenol as a putative proton source was beneficial in facilitating catalyst turnover. The nature of the amine employed for the formation of the N-substituted aldimine proved to be vital for enantioselectivity, with optimal results obtained for N-fluorenyl imines such as 159, derived from aliphatic, unsaturated, and aromatic aldehydes (70-96% ee) [114],... 

The formed methylcyclohexane carbocation eliminates a proton, yielding 3-methylcyclohexene. 3-Methylcyclohexene can either dehydrogenate over the platinum surface or form a new carbocation by losing H over the acid catalyst surface. This step is fast, because an allylic car-bonium ion is formed. Losing a proton on a Lewis base site produces methyl cyclohexadiene. This sequence of carbocation formation, followed by loss of a proton, continues till the final formation of toluene. 

The Lewis acid-Lewis base interaction outlined in Scheme 43 also explains the formation of alkylrhodium complexes 414 from iodorhodium(III) meso-tetraphenyl-porphyrin 409 and various diazo compounds (Scheme 42)398), It seems reasonable to assume that intermediates 418 or 419 (corresponding to 415 and 417 in Scheme 43) are trapped by an added nucleophile in the reaction with ethyl diazoacetate, and that similar intermediates, by proton loss, give rise to vinylrhodium complexes from ethyl 2-diazopropionate or dimethyl diazosuccinate. As the rhodium porphyrin 409 is also an efficient catalyst for cyclopropanation of olefins with ethyl diazoacetate 87,1°°), stj bene formation from aryl diazomethanes 358 and carbene insertion into aliphatic C—H bonds 287, intermediates 418 or 419 are likely to be part of the mechanistic scheme of these reactions, too. 

Based on the same strategy, Denmark and coworkers developed a vinylogous aldol reaction using enolate activation with a catalyst derived from SiCl4 and dimeric phosphoramide 47 [24,25]. This strategy relies on the observation that not all Lewis acid - Lewis base interactions diminish the Lewis acidity [26-28]. Due to the formation of a pentacoordinated silicon cation (48), both the enolate and the substrate can be assembled in a closed transition state, giving rise to the observed high selectivities (Scheme 19) [29,30]. 

If olehn metathesis is to be conducted in solution, solvents of low Lewis-basicity will generally give the best results (CH2CI2 > toluene > THF). As discussed above, metathesis is initiated by the formation of a jt-complex between the metal and the alkene. Hence, other nucleophiles will compete with the alkene for these coordination sites and in some systems even THF can lead to complete deactivation of the catalyst [786]. Tungsten-based catalysts which can even metathesize allyl thioethers have, however, been described [787]. 

Lewis acid sites have empty orbitals able to accept electron density from the occupied orbitals of a Lewis base, in parallel with back-donation from the catalyst to the empty anti-bonding orbitals of the base [33]. This interaction leads to the formation of an activated acid-base adduct. In the case of alkanes activation may proceed by hydride abstraction [38]. Y and Beta are good examples of zeolites with Lewis acidity, often quite significant for catalysis [39, 40]. 

Cyanation of aldehydes and ketones is an important chemical process for C C bond formation." " Trimethylsilyl cyanide and/or HCN are commonly used as cyanide sources. The intrinsic toxicity and instability of these reagents are problematic in their applications. Acetyl cyanide and cyanoformates were used as cyanide sources in the enantioselective cyanation of aldehydes catalyzed by a chiral Ti complex and Lewis base (Scheme 5.31)." The Lewis base was necessary for the good yields and selectivities of these reactions. The desired products were obtained in the presence of 10mol% triethyl amine and 5mol% chiral titanium catalyst (Figure 5.14). Various aliphatic and aromatic aldehydes could be used in these reactions. 

The oxonium ylide mechanism requires a bifunctional acid-base catalyst. The validity of the oxonium ylide mechanism on zeolites was questioned459,461,464 because zeolites do not necessarily possess sufficiently strong basic sites to abstract a proton from the trimethyloxonium ion to form an ylide. It should, however, be pointed out, as emphasized by Olah,447,465 that over solid acid-base catalysts (including zeolites) the initial coordination of an electron-deficient (i.e., Lewis acidic) site of the catalysts allows formation of a catalyst-coordinated dimethyl ether complex. It then can act as an oxonium ion forming the catalyst-coordinated oxonium ylide complex (10) with the participation of surface bound CH30 ions ... 

Evidence was presented that cobalt precursors under the reaction conditions are transformed into cobalt carbonyls.31 Additives such as Lewis bases accelerate the formation of the catalyst.11 [CoH(CO)4] the key catalytic species was shown by infrared (IR) spectroscopy to be formed under hydroformylation conditions32 and was isolated in the reaction of [Co(CO)4]2 and hydrogen.33 [CoH(CO)4] dissociates carbon monoxide to create [CoH(CO)3] [Eq. (7.2)], which is capable of olefin com-plexation because of a ligand vacancy ... 

In acidic media, loss of a proton can give traces of methylene forms of type (609). Alternatively, a Lewis acid catalyst such as acetic anhydride may be used which involves formation of complexes of type (618) from which proton loss is facile. Such methylene bases can also react with electrophiles, gradually causing complete conversion of the heterocycle. 

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