Trimethylsilyl halide

A number of approaches have been tried for modified halo-de-diazoniations using l-aryl-3,3-dialkyltriazenes, which form diazonium ions in an acid-catalyzed hydrolysis (see Sec. 13.4). Treatment of such triazenes with trimethylsilyl halides in acetonitrile at 60 °C resulted in the rapid evolution of nitrogen and in the formation of aryl halides (Ku and Barrio, 1981) without an electron transfer reagent or another catalyst. Yields with silyl bromide and with silyl iodide were 60-95%. The authors explain the reaction as shown in (Scheme 10-30). The formation of the intermediate is indicated by higher yields if electron-withdrawing substituents (X = CN, COCH3) are present. In the opinion of the present author, it is likely that the dissociation of this intermediate is not a concerted reaction, but that the dissociation of the A-aryl bond to form an aryl cation is followed by the addition of the halide. The reaction is therefore mechanistically not related to the homolytic halo-de-diazoniations. 

Trimethylsilyl halides can also be used for analogous reactions with arenediazo-nium tetrafluoroborates, as shown by Keumi et al. (1989). These authors treated 2-fluorenediazonium tetrafluoroborate in A/,Af-dimethylformamide or -acetamide with trimethylsilylchloride, -bromide, or -iodide in the presence of an excess of N-chlorosuccinimide, Af-bromosuccinimide, or methyl iodide, respectively, at 60 °C (Cl, Br) or at room temperature (I). The yields of the 2-halofluorenes were good in addition fluorene, the product of hydro-de-diazoniation, was obtained, if the reaction was run in tetrahydrofuran/Af,7V-dimethylformamide mixtures. The mechanism of these reactions, as well as that of the corresponding azido-de-diazoniation, is uncertain (see also Secs. 10.2 and 10.7). 

Reactions of Heterocyclic N-Oxides with Trimethylsilyl Cyanide, Trimethylsilyl Azide, Trimethylsilyl Isothiocyanate, and Trimethylsilyl Halides... 

Whereas the Prins-type cyclizations reported in this and the preceeding chapter were performed using stoichiometric amounts of Fe salts as Lewis acids, a breakthrough in the field of catalysis was reported in 2009 when the first iron-catalyzed Prins- and aza-Prins cyclization was reported. The catalytic system, which is obtained by combining catalytic amounts of an iron salt with trimethylsilyl halides as a halide source, is widely applicable and promotes the construction of substituted six-membered oxa- and aza-cycles (Scheme 33) [44]. 

FIGURE 6.36 Two mechanistic alternatives for the cyclization of para-tocophery lquinone 7 into o-QM 3 and subsequent reaction with acyl or trimethylsilyl halides. 

As an alternative method, poly(cyclodiborazane)s were prepared by the reaction of bis(silylimine)s with chlorodialkylboranes or with methyl dialkylborinates (scheme 18).32 This reaction proceeds via the condensation between V-silylimine and boron halide, eliminating trimethylsilyl halide followed by dimerization. However, the isolated polymer became insoluble after several hours of exposure under air, which resulted from the cross-linking reactions of unreacted trimethylsilyl groups to form trimerized hydrobenzamide derivatives. 

It is very likely, that this reaction occurs due to the equilibrium between trimethylsilyl halide and a nitrogen-containing nucleophile, which increases the electrophilicity of silyl Lewis acids. It should be noted that the configuration of stereocenters at the carbon atoms of the oxazine ring is partially distorted. Hence, it is assumed that the reaction proceeds through the intermediate cation B, which is partially isomerized into the stereoisomeric cation B through the open chain cation B". 

An alternative routeto closely related ring systems takes advantage of reactions which lead to the elimination of metal halides and trimethylsilyl halides (Scheme 5). The synthesis of (i<5) is but one example of many cyclodiphosph(III)-azanes which have been obtained by this general route. 

As already shown by Moore and co-workers9 and Tour and coworkers,10 addition of methyl iodide to a triazene resin at elevated temperature (110°) gives rise to aryliodides 17-1 (Nu = I) in excellent yields. We have shown that aryl halides 17-X (X = Cl, Br, I) are readily available by the action of lithium halides in the presence of an acidic ion-exchange resin or with the corresponding trimethylsilyl halide at room temperature.26 A mixture of acetic anhydride and acetic acid produces phenol acetates 17-OAc.26... 

A highly stereoselective vinylogous Pummerer rearrangement of a optically active sulfoxide has been shown to proceed by a [l,4]-migration of the sulfinyl oxygen atom via an intermediate intimate ion pair (Scheme 12).16 The reaction occurs with high enantioselectivity when o-sulfinyl-substituted benzyl carbanions are treated with trimethylsilyl halides. 

Trimethylsilyl halides will exchange halogen with Me3N BX3 adducts (9) and BF4" (80). Phosphorus pentahalides (9, 13), A12C16 (9), and AsC13 (9) act similarly on amine adducts. 

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]). 

Table 6 confirms that the Mannich reaction is usually quite. simple to achieve. The use of methyleneimmonium salts (condition E in Table 6), however, requires rather accurately controlled conditions, such as anhydrous solvents or a very low reaction temperature. - These reagents can be prepared in situ with good results - employing, for example, formaldehyde N,0-acetals in the presence of trimethylsilyl halide. The most frequently used solvent is acetonitrile, followed by dichloromethane, but it is important to remember that the type of solveni" -" - - - as well as the concentration of reactants may be critical for reaction yield or selectivity. 

Electrophilic reagents capable of bringing about Pummerer rearrangement include inorganic and organic aci, carboxylic anhydrides, acyl halides, isocyanates, carbodiimides, trimethylsilyl halides and triflate, sulfonyl and sulfenyl halides, phosphorus pentoxide and many typical Lewis acids such as boron... 

Carbon halides, CCI4 and CBr4 react with 62 in ether according to Eq. (84) with the formation of nitrogen and hydrazones (Me3Si)2N—N= CX2, as well as tetrakis(trimethylsilyl)tetrazene, trimethylsilyl halide, and... 

Apart from their properties as ligands, other aspects of the reactivity of phosphinite esters have been of interest. It has been shown that phosphinite esters (77) undergo the Michaelis-Arbuzov rearrangement to give the phosphine oxides (78) between room temperature and 80°C in the presence of trimethylsilyl halides, the reaction not needing the presence of any alkyl halide. The rearrangement proceeds even more efficiently at room temperature in the... 

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