HMDSO

Another illustrative example of the application of FTIR spectroscopy to problems of interest in adhesion science is provided by the work of Taylor and Boerio on plasma polymerized silica-like films as primers for structural adhesive bonding [15]. Mostly these films have been deposited in a microwave reactor using hexamethyldisiloxane (HMDSO) as monomer and oxygen as the carrier gas. Transmission FTIR spectra of HMDSO monomer were characterized by strong... 

Polar functional groups such as alcohols or phenols 11 or trimethylsilanol 4 are transformed by monofunctional silylating reagents Me3SiX 12 into their hpophilic and often volatile trimethylsilyl ethers 13 whereas water is converted into persilyl-ated water (=Me3SiOSiMe3, hexamethyldisiloxane, HMDSO, 7, b.p. 100 °C). The persilylation of phenols and, in particular, catechol (or hydroquinone) systems (Scheme 2.1) protects them efficiently against air oxidation even at temperatures of up to 180 °C. (cf, e.g., the silylation-amination of purine nucleosides with dopamine hydrochloride in Section 4.2.4)... 

Silylation of alcohols or phenols 11 with HMDS 2 (compared, e.g., with 22) to their silyl ethers 13 and of trimethylsilanol 4 with HMDS 2 to HMDSO 7 proceeds more slowly, because 2 silylates the alcohols or phenols 11 and 4 apparently via an kineticaUy less favored four-membered cychc transition state 27 (Scheme 2.4). 

Thus removal of water from classical rather inactive fluoride reagents such as tetrabutylammonium fluoride di- or trihydrate by silylation, e.g. in THF, is a prerequisite to the generation of such reactive benzyl, allyl, or trimethylsilyl anions. The complete or partial dehydration of tetrabutylammonium fluoride di- or trihydrate is especially simple in silylation-amination, silylation-cyanation, or analogous reactions in the presence of HMDS 2 or trimethylsilyl cyanide 18, which effect the simultaneous dehydration and activation of the employed hydrated fluoride reagent (cf, also, discussion of the dehydration of such fluoride salts in Section 13.1). For discussion and preparative applications of these and other anhydrous fluoride reagents, for example tetrabutylammonium triphenyldifluorosilicate or Zn(Bp4)2, see Section 12.4. Finally, the volatile trimethylsilyl fluoride 71 (b.p. 17 °C) will react with nucleophiles such as aqueous alkali to give trimethylsilanol 4, HMDSO 7, and alkali fluoride or with alkaline methanol to afford methoxytri-methylsilane 13 a and alkali fluoride. 

Finally, thermochemical data on the heat of hydrolysis of Me3SiCl 14 and of HMDS 2 in 1 M HCI to trimethylsilanol 6 or HMDSO 7 have been measured [30, 31]. 

Finally, reaction of SiCLi. 57 with HMDSO 2 affords 84% tetrakis(trimethyl-silyloxy)silane 140 and 16% hexakis(trimethylsilyloxy)disiloxane 141, both of which might be very interesting silylating agents, e.g., for silicon surfaces [72]. 140 is also obtained in 38% yield on reaction of SiCL. 57 with excess sodium tri-methylsilanolate 96 [73] (Scheme 3.14). 

The ready exchange of silyl groups is apparent from the reaction of equimolecu-lar amounts of trimethylsilyl formate 148 with N-triethylsilyhnethylamine 163 for 1 h at room temperature (Scheme 4.4) whereupon two layers separate the upper layer consists of HMDSO 7 and l,l,l-trimethyl-3,3,3-triethyldisiloxane 64 and the lower layer contains N-methylformamide 164 in almost quantitative yield [5]. 

Benzoic acids 194 react at 160 °C with 2 equivalents of anilines 192 in the presence of polyphosphoric acid trimethylsilyl ester (PPSE) 195 (which is prepared by reaction of P2O5 with HMDSO 7) to give the amidines 196 in 69-88% yield [30, 31] (Scheme 4.10)... 

Secondary amines such as dibenzylamine or 4-substituted piperidines are readily formylated in yields of up to 94% at room temperature by excess N,N-di-methylformamide (DMFj/MesSiCl 14/imidazole with formation of HCl and HMDSO 7 [32aj. 

Succinimide is readily silylated by HMDS 2 to the N-silylated product 201, which seems, however, to be in equilibrium with the O-silylated derivative 202 a (cf the closely related reactive center in persilylated uridine 3) and reacts after 6-10 days at 24 °C with one equivalent of primary or secondary amines such as morpholine to give the crystalline colorless cyclic acylamidine 203 and HMDSO 7, even in the absence of any protective gas [33] (Scheme 4.12). The reaction is much faster on heating to 120 °C under argon. At these temperatures 201 and 202 a, and possibly also the acylamidine 203, are apparently partially O-silylated by HMDS 2 to the very sensitive 2,5-bis(trimethylsilyloxy)pyrrole 202b or to 2-tri-... 

As yet, a number of experiments have failed to convert ureas 205 such as N-phenylurea or imidazolin-2-one by silylation amination with excess amines R3NHR4 such as benzylamine or morpholine and excess HMDS 2 as well as equivalent amounts of NH4X (for X=C1, I) via the silylated intermediates 206 and 207 in one reaction step at 110-150°C into their corresponding guanidines 208 with formation of NH3 and HMDSO 7 [35] (Scheme 4.13). This failure is possibly due to the steric repulsion of the two neighbouring bulky trimethylsilyl groups in the assumed activated intermediate 207, which prevents the formation of 207 in the equilibrium with 206. Thus the two step Rathke-method, which demands the prior S-alkylation of 2-thioureas followed by amination with liberation of alkyl-mercaptans, will remain one of the standard syntheses of guanidines [21, 35a,b,c]. 

The oxime 299 is silylated in the presence of catalytic amounts of TMSOTf 20 to 300, which affords, via the Beckmann fragmentation intermediate 301 and alkylation with allyltrimethylsilane 82, 66% of the seco nitrile 302 [101, 102] (Scheme 4.39). Tris(trimethylsilyl) ketenimine 303 reacts with aldehydes such as benzaldehyde in the presence of Bp3-OEt2, via the aldol adduct 304, to give the unsaturated nitriles 305, in 99% yield, and HMDSO 7 [103]. 

On thermolysis of bis(trimethylsilyl) malonate 337 at 160°C in the presence of P4O10 carbon suboxide 339 is formed in 54% yield, via 338 two equivalents of tri-mefhylsilanol 4 are also formed and react in situ with P4O10 to give polyphosphoric acid trimethylsilylester (PPSE) 195 [118] (Scheme 4.44). Pyrolysis of trimethylsilyl 2,2-dimefhylmalonate at 700°C gives dimethyl ketene and HMDSO 7 [118a]. 

AU these results indicate that silylated amides and, in particular, silylated lactams such as 388 will react with methyl or ethyl cyanoacetate or malonate and malodinitrile in the presence of HMDS 2 (to convert the leaving group MeaSiOH 4 into HMDSO 7) via the O-silylated forms such as 384b or 389 to give similar products such as 385 and HMDSO 7 (Scheme 4.54). 

N-Acetals of aldehydes can be readily prepared by reaction of aldehydes with tri-methylsilylated secondary amines. Thus, formaldehyde is converted by diethylami-notrimethylsilane 146, in 55% yield, into the silylated 0,N-acetal 422, which reacts with a further equivalent of 86 to give 90% of the N,N-acetal 423 and 94% hexa-methyldisiloxane 7 [41, 42]. On heating of diethylamine with formaldehyde and HMDS 2, 22% 422, 70% of the N,N-acetal 423, HMDSO 7, and ammonia are obtained [42] (Scheme 5.10). 

N-Acetals such as 429 react with silylated amines such as 294 at ambient temperature or on gentle heating in the presence of trimethylsilyl iodide 17 in diethyl ether to afford, e.g., the N,N-acetal 453 in 88% yield and HMDSO 7 [53]. The silylated tetrazole 454 reacts on heating to 160 °C with the 0,N-acetal 422 to... 

Benzaldehyde reacts with formamide and MesSiCl 14 on heating to give, via 435, the N,N-acetal 469, which reacts in situ with p-toluenesulfinic acid, in high yields, to give 470 [58]. The analogous reaction of excess a,)9-unsaturated ahphatic primary amide with aliphatic aldehydes in the presence of TMSOTf 20 in 1,2-di-chloroethane at 25 °C affords the unsaturated N,N-acetals in high yield [58 a]. Benzaldehyde also condenses with excess HMDS 2, in the presence of catalytic amounts of ZnCl2, via 471, to 472 and HMDSO 7 [59] (Scheme 5.21). 

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