Electrophiles ether complexes

Alkyl silyl ethers are cleaved by a variety of reagents Whether the silicon-oxygen or the carbon-oxygen bond is cleaved depends on the nature of the reagent used Treatment of alkoxysilanes with electrophilic reagents like antimony tri-fluonde, 40% hydrofluonc acid, or a boron tnfluonde-ether complex results in the cleavage of the silicon-oxygen bond to form mono-, di-, and tnfluorosiloxanes or silanes [19, 20, 21) (equations 18-20)... 

In cyclopropanations with electrophilic carbene complexes, yields of cyclopropanes tend to improve with increasing electron density of the alkene. As illustrated by the examples in Table 3.5, cyclopropanations of enol ethers with aryldiazomethanes often proceed in high yields. Simple alkyl-substituted olefins are, however, more difficult to cyclopropanate with diazoalkanes. A few examples of the cyclopropanation of enamines with diazoalkanes have been reported [650]. 

The reaction of acceptor-substituted carbene complexes with alcohols to yield ethers is a valuable alternative to other etherification reactions [1152,1209-1211], This reaction generally proceeds faster than cyclopropanation [1176], As in other transformations with electrophilic carbene complexes, the reaction conditions are mild and well-suited to base- or acid-sensitive substrates [1212], As an illustrative example, Experimental Procedure 4.2.4 describes the carbene-mediated etherification of a serine derivative. This type of substrate is very difficult to etherify under basic conditions (e.g. NaH, alkyl halide [1213]), because of an intramolecular hydrogen-bond between the nitrogen-bound hydrogen and the hydroxy group. Further, upon treatment with bases serine ethers readily eliminate alkoxide to give acrylates. With the aid of electrophilic carbene complexes, however, acceptable yields of 0-alkylated serine derivatives can be obtained. 

Table 4.13. Preparation of cyclic ethers by intramolecular 0-H insertion of electrophilic carbene complexes.

Ethers can react with electrophilic carbene complexes to yield oxonium ylides, which usually undergo either elimination reactions or 1,2-alkyl shifts to yield products of a formal carbene C-O bond insertion (Figure 4.11) [1020,1255-1259]. 

If chiral catalysts are used to generate the intermediate oxonium ylides, non-racemic C-O bond insertion products can be obtained [1265,1266]. Reactions of electrophilic carbene complexes with ethers can also lead to the formation of radical-derived products [1135,1259], an observation consistent with a homolysis-recombination mechanism for 1,2-alkyl shifts. Carbene C-H insertion and hydride abstraction can efficiently compete with oxonium ylide formation. Unlike free car-benes [1267,1268] acceptor-substituted carbene complexes react intermolecularly with aliphatic ethers, mainly yielding products resulting from C-H insertion into the oxygen-bound methylene groups [1071,1093]. 

Xenon difluoride reacts with alcohols to form unstable alkoxyxenon fluoride intermediates. Alkoxyxenon fluorides react as positive oxygen electrophiles when boron trifluoride-diethyl ether complex is used as a catalyst. However, these alkoxyxenon fluorides react as apparent fluorine electrophiles with proton catalysts (hydrogen fluoride generated in situ).49... 

Vinyl epoxides react with Pd(0) to yield electrophilic allyl complexes which can convert alcohols and phenols into allyl ethers (Scheme 4.80). These alkylations usually yield 2-alkoxy-2-vinylethanols, and if the Pd-mediated etherification is performed in the presence of a chiral, enantiomerically pure diphosphine, enantiomeri-cally enriched ethers may be obtained (Scheme 4.80) [356, 357]. 

The chemistry of iron vinylidene complexes is dominated by the electrophilicity of the carbon atom adjacent to the iron organometallic unit. While addition of water leads to an acyl complex (i.e., the reverse of the dehydration shown in equation 10), addition of an alcohol leads to a vinyl ether complex. Similarly, other iron vinyl complexes can be prepared by the addition of thiolate, hydride, or an organocuprate (Scheme 33). " The nucleophilic addition of imines gave enaminoiron intermediates that could be further elaborated into cyclic aminocarbenes. This methodology has been used to provide access to /3-lactams and ultimately penicillin analogs, and good diastereoselectivities were observed (6 1-15 1) (Scheme 34). 04 Iso, vinylidene complexes are intermediates in cyclizations of alkynyl irons with substituted ketenes, acid chlorides, and related electrophiles an example is shown (equation 11). These cyclizations led to the formation of a series of isolable and characterizable cyclic vinyl iron complexes. 

The regioselectivity of nitration of toluene with nitronium salts has been successfully altered by their prior complexation with crown ethers. Complexation of NO2 BFJ by 18-C-6 crown ether substantially altered the selectivity in nitration of toluene and benzene as reported by Elsenbaumet and Wasserraan [128]. Similar effect was observed with polyethylene oxides. Savoie al. reported isolation of the IS-C-b-NOa BFJ complex and its characterization [129], Masci carried out the yet most detailed study on the effect of crown ethers on the selectivity of electrophilic aromatic nitration [130]. 

Intermolecular hetetolysis of H2 is exemplified by the protonation of ethers by cationic electrophilic H2 complexes often containing electron-withdrawing CO (Equations (9) and (10)). ... 

Generation of Electrophilic Cations. Complexation of Et2AlCl to ketones and aldehydes activates the carbonyl group toward addition of a nucleophilic alkyl- or allylstannane or allylsilane. Et2AlCl has been used to initiate Beckmann rearrangements of oxime mesylates. The ring-expanded cation can be trapped intermolecularly by enol ethers and cyanide and in-tramolecularly by alkenes (eq 5). ... 

The mechanism of organolithium addition to naphthyl oxazolines is believed to occur via initial complexation of the alkyllithium reagent to the oxazoline nitrogen atom and the methyl ether to form chelated intermediate 17. Addition of the alkyl group to the arena 7t-system affords azaenolate 18, which undergoes reaction with an electrophile on the opposite face of the alkyl group to provide the observed product 4. The chelating methyl... 

The synthesis of alkoxy amines 2 by addition of organometallic reagents to the C-N double bond of oxime ethers 1 is plagued by the propensity for proton abstraction a. to the C-N double bond, the lability of the N-O bond and the poor electrophilicity of the oxime ethers. Therefore, frequently no products, undesired products or complex mixtures are obtained. The result depends on the substrate, organometallic reagent, solvent, temperature and additives1 6. 

The electrophilic carbene carbon atom of Fischer carbene complexes is usually stabilised through 7i-donation of an alkoxy or amino substituent. This type of electronic stabilisation renders carbene complexes thermostable nevertheless, they have to be stored and handled under inert gas in order to avoid oxidative decomposition. In a typical benzannulation protocol, the carbene complex is reacted with a 10% excess of the alkyne at a temperature between 45 and 60 °C in an ethereal solvent. On the other hand, the non-stabilised and highly electrophilic diphenylcarbene pentacarbonylchromium complex needs to be stored and handled at temperatures below -20 °C, which allows one to carry out benzannulation reactions at room temperature [34]. Recently, the first syntheses of tricyclic carbene complexes derived from diazo precursors have been performed and applied to benzannulation [35a,b]. The reaction of the non-planar dibenzocycloheptenylidene complex 28 with 1-hexyne afforded the Cr(CO)3-coordinated tetracyclic benzannulation product 29 in a completely regio- and diastereoselective way [35c] (Scheme 18). 

As mentioned earlier, metal complexation not only allows isolation of the QM derivatives but can also dramatically modify their reactivity patterns.29o-QMs are important intermediates in numerous synthetic and biological processes, in which the exocyclic carbon exhibits an electrophilic character.30-33 In contrast, a metal-stabilized o-QM can react as a base or nucleophile (Scheme 3.16).29 For instance, protonation of the Ir-T 4-QM complex 24 by one equivalent of HBF4 gave the initial oxo-dienyl complex 25, while in the presence of an excess of acid the dicationic complex 26 was obtained. Reaction of 24 with I2 led to the formation of new oxo-dienyl complex 27, instead of the expected oxidation of the complex and elimination of the free o-QM. Such reactivity of the exocyclic methylene group can be compared with the reactivity of electron-rich enol acetates or enol silyl ethers, which undergo electrophilic iodination.34... 

There are some cases where o-substitution occurs to the almost total exclusion of any p-attack. These commonly arise from complexing of the substituent already present with the attacking electrophile so that the latter is steered into the adjacent o-position. Thus when the ether l-methoxy-2-pheny ethane (58) is nitrated with nitrating mixture, 32 % o- and 59 % p-isomers are obtained (quite a normal distribution) but nitration with N205 in MeCN results in the formation of 69 % o-and 28% p-isomers. This preferential o-attack in the second case is believed to proceed ... 

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