Electrophilic reactions allyltrimethylsilane

As a Carbon Nucleophile in Lewis Acid-Catalyzed Reactions. Allyltrimethylsilane is an alkene some 10 times more nucleophilic than propene, as judged by its reactions with di-arylmethyl cations. It reacts with a variety of cationic carbon electrophiles, usually prepared by coordination of a Lewis acid to a functional group, but also by chemical or electrochemical oxidation, or by irradiation in the presence of 9,10-dicyanoanthracene. The electrophile attacks the terminal alkenic... 

Other Reactions. Allyltrimethylsilane reacts with some highly electrophilic alkenes, carbonyl compounds, azo compounds, and singlet oxygen to a greater or lesser extent in ene reactions that do not involve the loss of the silyl group, and hence give vinylsUanes in a solvent-dependent reaction (eq 25). ... 

The popular activation method of pyridine rings by reaction with chloroformates was not observed for reaction of N-acylated quinoline with allyltrimethylsilane until a catalytic amount of silver triflate was added. It was shown that the triflate counterion increases the electrophilicity of the iV-acylquinolinium salt (Equation 54) < 1997TL403, 2001T109>. 

As already established for combinations of cations with n-nucleo-philes [33,160,219], the situation is less complicated for the reactions of carbocations with 7r-systems. Solvent polarity plays only a minor role (Section III.D.3) and, for many 7r-systems, the relative reactivity has been found to be electrophile-independent (Fig. 10, Section III.D.4.b). Also for these systems, the construction of a universal nucleophilicity scale is not unproblematic, however. Remember Fig. 11, which shows that An2CH + reacts 3.4 times faster with allyltrimethylsilane than with 2-methyl-2-bu-... 

Iodine transfer addition to allyltrimethylsilane provides a more environmentally friendly alternative to allyltributylstannane. In these allylations, which are exemplified in Scheme 6, the initially formed I-transfer product undergoes spontaneous loss of TMSI to generate the observed allylation product. Guindon has shown that allylation of the Lewis acid complexes of ) -alkoxy esters in this manner can lead to products with high anti stereoselectivity [22]. It is also believed that the presence of Lewis acids enhances the electrophilicity of the radical. Allylations of this type can also prove successful when Br-transfcr or PhSe-transfer reactions are employed. 

The possibility of monodentate species complicates the analysis of chelation-controlled radical reactions. Monodentate complexation leads to transition states such as B (Scheme 1) that, in terms of stereoselectivity, behave similarly to un-complexed radicals. Lewis acid complexation with the ester function has the potential to lower the energy of the transition state, particularly when the incoming reagent is electrophilic (e.g. allyltrimethylsilane), thus enhancing the reactivity of such radicals relative to uncomplexed species. Because radical reactions can occur... 

A soln. of allyltrimethylsilane in methylene chloride added dropwise to a pre-cooled soln. of dichlorobis(4-chlorophenoxy)methane and SnCl4 in the same solvent at — 78°, and worked up after 72 h - product. Y 78%. With subst. allylsilanes reaction proceeds mainly with double bond shift. F.e.s. H. Mayr et al.. Synthesis 1988, 962-3 review of electrophilic substitution of allylsilanes and vinylsilanes s. I. Fleming et al., Org. Reactions 37, 57-575 (1989). 

Since from the beginning two electrophiles (carbonyl compound and alkyl chloroformate) and two nucleophiles (HMDS and allyltrimethylsilane) are present in the media, the challenge of this reaction is to drive it for the previously described pathway. Side reactions such as allylation of the carbonyl compound and reaction between electrophihc alkyl chloroformate and nucleophilic allyltrimethylsilane were efficiently avoided using the proper Lewis acid as catalyst, being in this case the inexpensive FeSO -TH O. 

Some electrophiles require a separate activation step, as in the stereospecific reaction of alkenes with benzenesulfenyl chloride (eq 15). The intermediate that actually reacts with the aUylsilane is presumably the episulfonium ion. There is a corresponding reaction of epioxonium ions derived from 2-bromoethyl ethers. Intramolecular hydrosilylation of an ester generates an acetal, which reacts with allyltrimethylsilane in the usual way and with high stereocontrol to make an anti 1,3-diol derivative (eq 16). ... 

As a Carbon Nucleophile in Uncatalyzed Reactions. Some electrophiles do not need Lewis acids, being already cationic and electrophilic enough to react with allyltrimethylsilane. Examples are the dithianyl cation (eq 20), the tricarbonyl(cyclohexadienyl) iron cation (eq 21), ( r-allyl) tetracarbonyliron cations, and chlorosulfonyl isocyanate (CSI). Other reagents react directly by cycloaddition, but need further steps to achieve an overall electrophilic substitution, as in the reactions with nitrones (eq 22). 

Reaction with Alkyl Halides, Epoxides, and Other Electrophiles. TrimethylsUylallyllithium reacts with alkyl halides at low temperature, giving mixtures of a- and y-(E) products in ratios that vary with the electrophile and the reaction conditions (eq 9). In these reactions the )/-(E)-alkylation product generally predominates. For example, when trimethylsilylaUyl-lithium is formed by deprotonation of allyltrimethylsilane with n-butyllithium in the presence of potassium r-butoxide in THF, the (E)-vinylsilane alkylation product is obtained in better then... 

Reaction with Allylic and Benzylic Electrophiles. The acyl radicals can be trapped with halogen- and silicon-based electrophiles. a -Allylation of a, -unsaturated ketones is done while using Mn(OAc)3 dihydrate and allyl bromide in refluxing benzene (eq 29). Better yields are usually observed for cyclopen-tenones compared to cyclohexenones. a -Benzylation is also possible using benzyl bromide as the electrophilic partner (eq 30). Both methods tolerate a range of substitution, including 8-alkoxy-a, -unsaturated ketones. It is possible to perform a sequential allylation/cyclization with an excess of allyltrimethylsilane (eq 31). Mn(OAc)3 offers good conversions, but the use of ceric ammonium nitrate (CAN) as co-oxidant improves yields. 

Trimethylsilyl iodide is known to convert alcohols into iodides but is very sensitive to hydrolysis, so some new mild methods have been developed for the in situ generation of this reagent under neutral conditions. The reaction of allyltrimethylsilane and iodine does generate MeaSil, but the allyl iodide byproduct is not compatible with the presence of nucleophilic groups. A better solution is to use the bisallylic silane (58) and iodine (Scheme 38), where the other product is benzene. Trimethylsilyl bromide, although much less electrophilic... 

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