For example, the Birch reduction of arylsilane 126 gives diene 127 in almost quantitative yield (equation 30).89, This reaction has been most widely used by the group of Landais, using asymmetric dihydroxylation of alkenes to effect the desymmetrization of cyclohexadienes.
Much indicates that glycosyl donors with silyl protective groups generally are more reactive than their alkylated counterparts presumably due to the O-silyl group being slightly less electron withdrawing than, e.g., a benzyl group. Explain why this bulky silyl group can be reversed under with fluoride but not H3O+. Determine the molecularity of the reaction. Because of the ability to make 5 or 6 bonds, the mechanism for the formation of silyl ethers could proceed in four ways: For each of the mechanisms proposed above. The silyl group was also varied as shown in Equation (11). Typically, silyl groups are added to alcohols when they might react with the reagent instead of the intended reaction. STEP 2: Form the Grignard reagent and perform the desired Grignard reaction which is now enabled because the group of the alcohol is protected.
Ring opening provides the allylation leaving a cationic silicon center and an alkenylgold intermediate (C) that then undergoes ring closure. As is always the case in silicon chemistry, the substitution reaction proceeds via pentacoordinated intermediates. The stabilization of intermediates and transition states with β-carbenium character by silicon may be explained as a σ–π-conjugative interaction between the silicon–carbon bond and the π-system.
This reaction generally works best for nonenolizable aldehydes and ketones <1995JOC7366>. This reaction has been used to prepare a range of silylimines for use in reductive coupling reactions Equation (37)) <1999JOC1958>. Provide a mechanism for the removal of the silyl ether. Sorry, your blog cannot share posts by email.
2000, 65, 7228. Fill in your details below or click an icon to log in: You are commenting using your WordPress.com account. Which mechanisms have five and six coordinate reactive intermediates. In 2007, Murakami and coworkers reported the transition metal-catalyzed reaction of 1,4-diphenylbuta-1,3-diyne with secondary silanes to produce siloles as well as spirocycles containing siloles partnered with a silafluorene. TBS-OTf is a very powerful silylating agent, and is capable of protecting primary, secondary, and teriary alcohols. However, the reactivity increase is further augmented when bulky silyl groups, that cause a conformational change to an axial-rich conformation, are present. A generic reaction scheme for a silyl protection of a hydroxy group on a carbohydrate. The alkyne could have alkyl, aryl, trimethylsilyl, or ester substituents,22 and 3 of the 21 reported examples (including those found only in the Supplemental) are given in Table 2 (entries 2-10, 2-11, and 2-12) and the general reaction is shown in Equation (10). These groups vary in reactivity providing orthogonal protection. However, the most convenient method for the preparation of trimethylsilylimines is the reaction of nonenolizable aldehydes with lithium hexamethyldisilazane. The kinetic data suggests that the reaction is second order in the nucleophilic alcohol. TBS is generally used as a first choice, but others are also used frequently. The most popular and commercially available silyl-protective groups are trimethylsilyl (TMS), triethylsilyl (TES), tert -butyldimethylsilyl (TBS), tert -butyldiphenylsilyl (TBDPS), triisopropylsilyl (TIPS) as well as the diol-protective groups DTBS and TIPDS (Figure 1). A widely used method of protecting hydroxyl groups in organic molecules is to turn them into silyl ethers.A protecting group is one which reduces the reactivity of the functional group it protects so that chemistry may be carried out elsewhere on the molecule without fear of interference from the protected group. The silyl groups found in these low-coordinate species often bear particularly large substituents such as t Bu or Ph groups. -TBS, TIPS, TBDPS groups are bulky enough to selectively protect primary alcohols in the presence of secondary and tertiary alcohols. Change ), You are commenting using your Facebook account. Provide a mechanism for the formation of a silyl ether. The 2-benzotriazolyl group behaves both as an activating group toward α-proton abstraction and as a good leaving group for β-elimination.
The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. In much of their work, an electrochemical Birch reduction was used, as shown in Scheme 35. Silyl ethers will be cleaved by nucleophiles (especially oxygen and fluoride) to form Si-O/F bonds – F– is therefore particularly effective because it is small and generally unreactive.
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