Silylation Chapter deriv

No chapter on alkylation methodology can hope to cover all facets of the topic, let alone make reference to all papers which bear on the subject. Here, attention has been focused on the most commonly used types of derivatives and procedures which, owing to their reliability, have found wide application. Since esterification with alcohols in the presence of acid catalysts is dealt with in Chapter 2, the method is considered here only with respect to simple glycosidation of sugars and the formation of benzyl ethers from substances related to catechol. Silyl ether derivatives are dealt with in Chapter 4. 

Unfortunately, space does not permit a full review, so that many examples and important applications of derivatives in GC—MS have been omitted. However, some mass spectrometric properties and applications as well as detailed derivative preparation instructions are given in the specialist chapters on Esterification (Chapter 2), Acylation (Chapter 3), Silylation (Chapter 4), Formation of cydic derivatives (Chapter 7), and also in the respective chapters of the first edition of the handbook [12]. 

In this chapter we will first discuss the synthesis, chemistry and physical properties of silicon-containing thiophene monomers, oligomers and polymers. We will first summarize the synthesis of silylated thiophene derivatives. The use of selective silylation reactions allows the design of thiophene monomers and oligomers with various structural features. Some of the properties of these new conjugated structures will be discussed. 

Fleming has shown (2) that the cuprate reagent (Chapter 8) derived from dimethylphenylsilyl lithium and copper(t) cyanide (molar ratio 2 1) adds regioselectively in an overall syn manner to terminal alkynes, the silyl moiety becoming attached to the terminal carbon atom (variation in reagent... 

The currently accepted mechanism of the alkali metal-mediated Wurtz-type condensation of dichlorosilanes is essentially that outlined in COMC II (1995) (chapter Organopolysilanes, p 98) which derived from studies by Gautier and Worsfold,42 and the groups of Matyjaszewski43 and Jones,22,44,45 a modified polymerization scheme of which is included here. The mechanism was deduced from careful observations on the progress of polymerizations in different solvents (such as those which better stabilize anions and those which do not), at different temperatures,44 with additives, and with different alkali metal reductants. Silyl anions, silyl anion radicals,42 and silyl radicals28,46,47 are believed to be involved, as shown in Scheme 3. 

Because of the polyfunctional nature of carbohydrates, protective-group strategy plays an important role in synthetic methodology involving this class of compounds. In the present Chapter, results are described from a study of the utility of N-trimethylsilyl- and N-tert-butyldimethylsilyl-phthalimide for the selective silylation of primary hydroxyl groups in carbohydrates. Also described, is a new, facile method for cleavage of acetals and dithioacetals in carbohydrate derivatives the method involves treatment of the derivatives with a dilute solution of iodine in methanol. 

The best alternatives to enamines for conjugate addition of aldehyde, ketone, and acid derivative enols are silyl enol ethers, Their formation and some uses were discussed in Chapters 21 and 26-28, but these stable neutral nueleophiles also react very well with Michael acceptors either spontaneously or with Lewis acid catalysis at low temperature,... 

Besides the initiation with the vinyl ether adducts, trimethylsilyl halides in conjunction with oxolane [135] or a carbonyl compound [136-141] also provide an interesting method of end-functionalization. As discussed in Chapter 4, Section V.E.2 (also Figure 9 therein), the a-end group is (CH3)3SiO—, derived from the silyl compound, to be converted into the hydroxyl group [140,141], Depending on the structure of the carbonyl compounds, it is either secondary (from aldehyde) [136-139] or tertiary (from ketone) [137,138,140,141], both of which are difficult to obtain from the vinyl ether adducts (note that the adduct of AcOVE leads to a primary alcohol [30,31]). 

Since the required polymer is a functionalized polystyrene, the most sensible approach would be to co-polymerize styrene and some 3,4-dihydroxystyrene, perhaps protected as an acetal or a silyl derivative. A proportion of the benzene rings in the polystyrene would have the correct functionalization and the crown ether could be built on to them by passing a large excess of a suitable reagent, such as one of those we discussed in Problem 2 of this chapter or in the main text (p. 1456), deprotecting as required. A potassium salt would be used as a base in the final cyclization to take advantage of complexation by the crown ether. The various methods of polymerizing styrene (radical, anionic, etc.) are described in the chapter (pp. 1459-62). 

With ketones we come to the problem of regioselectivity, and the situation from chapter 3 is that methyl ketones 98 and ketones with one primary and one secondary alkyl group, particularly cyclic ketones such as 103 give the less substituted lithium enolate 97 or 102 by kinetically controlled deprotonation with LDA, and the more substituted silyl enol ether 99 or 104 on silylation under equilibrium conditions. Either derivative (lithium enolate or silyl enol ether) may be used to make the other, e.g. 96 and 100. 

Modern developments have included allyl groups functionalised at the ends, particularly those derived from silylated enals such as 72. The application shown here creates a cyclopentenone 74 by combination of the allyl nickel 73 with an alkyne and CO rather in the style of the Pauson-Khand reaction (chapter 6).18... 

The leaving group X may be only an OH group, inserted by hydroxylation of a silyl enol ether 136 (chapter 33) formed by conjugate addition of a silyl cuprate (chapters 9 and 10) to an enone 135 and protected by the robust silyl group TBDMS (f-BuMe2Si-) 137 for reaction with the lithium derivative of 117. 

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