Lewis-type complexes

In the Giessen group, the spectral properties and reactivity of Lewis type complexes between halomethylsilylenes and methyl halides, generated in co-condensation experiments between silicon atoms and methyl halides, have been studied [5]. These less stabilized silylenes add the methyl halides under transformation into the stable dihalodimethylsilanes. Encouraged by these results, we believed that matrix isolation spectroscopy may also be a very powerful tool to give new insights into the surprising behavior of the stable silylene 1 with halocarbons. 

Are there any other possible reaction pathways besides the calculated concerted transformation of the Lewis type complex into the insertion product ... 

These (or similar) interactions find their antecedents in the literature [74, 92, 142, 153, 179], The stronger the interaction of the anion with the hydroxyl group of the alcohol, the more activated the alcoholate will be for nucleophilic attack on the carboxyl carbon, which itself can be activated by a Lewis-type complex formed with a Lewis acidic cation. A third beneficial interaction with a basic anion may arise from strong interactions (hydration) with the water formed, which would drive the equilibrium reaction towards the product side [96-112],... 

It was found that 2-propenyloxymagnesium bromide reacts much more readily with nitrile oxides than other known dipolarophiles of electron-deficient, electron-rich, and strained types, including 3-buten-2-one, ethyl vinyl ether, and norbomene, respectively (147). Therefore, this BrMg-alkoxide is highly effective in various nitrile oxide cycloaddition reactions, including those of nitrile oxide/Lewis acid complexes. 

Interestingly, homolytic substitution at boron does not proceed with carbon centered radicals [8]. However, many different types of heteroatom centered radicals, for example alkoxyl radicals, react efficiently with the organoboranes (Scheme 2). This difference in reactivity is caused by the Lewis base character of the heteroatom centered radicals. Indeed, the first step of the homolytic substitution is the formation of a Lewis acid-Lewis base complex between the borane and the radical. This complex can then undergo a -fragmentation leading to the alkyl radical. This process is of particular interest for the development of radical chain reactions. 

Several alternative attempts were made to quantify Lewis-type interactions235,236. Following the HSAB principle, the applicability of a one-parameter Lewis acidity scale will inevitably be restricted to a narrow range of structurally related Lewis bases addition of parameters results in more general relationships237-239. The quantitative prediction of the gas-phase stabilities of Lewis acid-Lewis base complexes is still difficult. Hence the interpretation, not to mention the prediction, of solvent effects on Lewis acid-Lewis base interactions is often speculative. 

The second important solvent effect on Lewis acid-Lewis base equilibria concerns the interactions with the Lewis base. Since water is also a good electron-pair acceptor129, Lewis-type interactions are competitive. This often seriously hampers the efficiency of Lewis acid catalysis in water. Thirdly, the intermolecular association of a solvent affects the Lewis acid-base equilibrium242. Upon complexation, one or more solvent molecules that were initially coordinated to the Lewis acid or the Lewis base are liberated into the bulk liquid phase, which is an entropically favourable process. This effect is more pronounced in aprotic than in protic solvents which usually have higher cohesive energy densities. The unfavourable entropy changes in protic solvents are somewhat counterbalanced by the formation of new hydrogen bonds in the bulk liquid. 

If doubly-bonded compounds >M = X (M = Si, Ge, Sn) are now well known, this is not yet the case for the heavy allenic derivatives >M = C = X. Transient silaallenes >Si = C = C 160-167 or silaazaallene >Si = C = N—168 were postulated some years ago, but the first stable derivative of this type, a formal 3-stanna-l-azaallene >Sn = C = N —, was isolated only in 1992 by Griitzmacher et al169 The compound Tbt(Mes)Si = C = NR170 has also been obtained, but according to the authors it is closer to a silylene Lewis base complex than to a silacumulene. The first stable silaallene was only reported in 1993 by West et al.111 and the first metastable >Si = C = P— was characterized still more recently.172... 

The controlling factor of the reductive elimination on Pd(R)(C=CR )Lm (Eq. 5) may be different from that observed with the nickel complex. However, participation of a similar activation process by coordination of electron-with-drawing RX and R C=CH is conceivable. The Pd(R)(C=CR )Lm-type complex can be isolated, and it has been shown that isolated Pd(R)(C=CR )Lm undergoes the reductive elimination exhibited in Eq. 5 [8]. The reductive elimination seems to be enhanced by addition of Cul. Cul may interact with the Pd complex, and an acceleration effect of Lewis acids on the reductive elimination reaction of NiR2(bpy) has been shown [22]. The X-ray crystallographic structure of an isolated Pd(R)(C=CR )Lm (R=C6H4Me-p R =C6H5) has been determined [8]. 

However, although we invoked a Lewis acid complex to provide the halonium electrophile, there is considerable evidence that, where appropriate, the electrophile in Friedel-Crafts alkylations is actually the dissociated carbocation itself. Of course, a simple methyl or ethyl cation is unlikely to be formed, so there we should assume a Lewis acid complex as the electrophilic species. On the other hand, if we can get a secondary or tertiary carbocation, then this is probably what happens. There are good stereochemical reasons why a secondary or tertiary complex cannot be attacked. Just as we saw with Sn2 reactions (see Section 6.1), if there is too much steric hindrance, then the reaction becomes SnI type. 

It should be noted that asymmetric acyl transfer can also be catalyzed by chiral nucleophilic A-heterocyclic carbenes [27-32] and by certain chiral Lewis acid complexes [33-37] but these methods are outside the scope of this review. Additionally, although Type I and Type II tr-face selective acyl transfer processes have been reported to be catalyzed by some of the catalysts described in this review, these also lie outside the scope of this review. 

The consecutive formation of o-hydroxybenzophenone (Figure 3) occurred by Fries transposition over phenylbenzoate. In the Fries reaction catalyzed by Lewis-type systems, aimed at the synthesis of hydroxyarylketones starting from aryl esters, the mechanism can be either (i) intermolecular, in which the benzoyl cation acylates phenylbenzoate with formation of benzoylphenylbenzoate, while the Ph-O-AfCL complex generates phenol (in this case, hydroxybenzophenone is a consecutive product of phenylbenzoate transformation), or (ii) intramolecular, in which phenylbenzoate directly transforms into hydroxybenzophenone, or (iii) again intermolecular, in which however the benzoyl cation acylates the Ph-O-AfCL complex, with formation of another complex which then decomposes to yield hydroxybenzophenone (mechanism of monomolecular deacylation-acylation). Mechanisms (i) and (iii) lead preferentially to the formation of p-hydroxybenzophenone (especially at low temperature), while mechanism (ii) to the ortho isomer. In the case of the Bronsted-type catalysis with zeolites, shape-selectivity effects may favor the formation of the para isomer with respect to the ortho one (11,12). 

Achari B, Mandal SB, Dutta PK, Chowdhury C (2004) Synlett 2004 2449 Aggarwal VK, Belfield AJ (2003) Catalytic asymmetric Nazarov reactions promoted by chiral Lewis acid complexes. Org Lett 5 5075-5078 Akiyama T, Itoh J, Fuchibe K (2006c) Adv Synth Catal 348 999 Akiyama T, Itoh J, Yokota K, Fuchibe K (2004) Enantioselective Mannich-type reaction catalyzed by a chiral Brpnsted acid. Angew Chem Int Ed Engl 43 1566-1568... 

Goulet et al. (27) describe the immobilizadon of enzymes on divalent cations chelated to bis(carboxymethyl)amino-derivatized agarose. The linkage through Lewis acid-base-type complexes is reversible, because enzymes could be eluted with EDTA. 

Kobayashi and co-workers. used zirconium-based bromo-BINOL complex for the catalytic enantioselective Mannich-type reaction. The o-hydroxyphenyl imine 3.36 chelates the Zr(IV)(BrBINOL)2 to form the activated chiral Lewis acid complex A. The ketone acetal 3.37 reacts with the Lewis acid complex A to give the complex B. The silyl group is then transferred to the 3-amino ester to form the product 3.38 and the catalyst Zr(BrBINOL)2 is regenerated, which is ready for binding with another imine molecule (Scheme 3.16). 

Gallane and its Lewis base complexes react readily with protic acids to eliminate hydrogen. Thus complexes of the type GaH2X(NMe3) and GaHX2(NMe3) (X = Cl, Br) are readily prepared by reaction of HX on the gallane complex at low temperatures. The reactions shown in Scheme T are examples of an elimination-condensation reaction with a secondary amine and the reduction of an imine both reaction types have been observed for alumimun hydrides. 

Since the initial report in 1962 by Merten and Muller of the cycloaddition of an -acyl imine with a conjugated diene,a number of examples of this type of reaction have appeared. In general, IV-acyl im> ines are highly reactive, unstable species which are rarely isolated, but rather are generated in situ from stable precursors. Depending upon the method of formation of the particular dienophile and the reaction conditions, a neutral A -acyl imine or a protonated (or Lewis acid complexed) IV-acyl immonium ion may be involved in the Diels-Alder reaction. 

This chapter addresses chirally modified boron Lewis acid complexes, in which there has been increased interest because of their capacity to induce chirality. They have been successfully used for Diels-Alder, aldol, and a variety of other miscellaneous reactions. I will describe and analyze here the different types of catalyst and classify them according to their efficiency, selectivity, and flexibility. 

They also applied this method to the intermolecular ene reactions of aliphatic and aromatic aldehydes with alkenes containing a disubstituted vinylic carbon, a potentially valuable route to homoallylic alcohols [50]. Proton-initiated rearrangements do not take place, because the alcohol-Lewis acid complex formed in the ene reaction reacts readily to give methane and a non-acidic aluminum alkoxide. Formaldehyde and excess Me2AlCl gave good yield of ene adducts with all types of alkene, as exemplified in Sch. 26. 

---