Via-1 formation

Figure 16.17 Process sequence for via-1 formation (a) ILD-1 oxide (Si02) deposition, (b) CMP of ILD-1 oxide, (c) via-1 iithographic masking (invoiving mask 9) to define the via-1 windows in which the tungsten piug wiii be subsequentiy deposited, (d) etch step to open up via hoies in the ILD-1 oxide, and (e) resist strip and ciean.

Aryl- or alkenylpalladium comple.xcs can be generated in situ by the trans-metallation of the aryl- or alkenylmercury compounds 386 or 389 with Pd(Il) (see Section 6). These species react with 1,3-cydohexadiene via the formation of the TT-allylpalladium intermediate 387, which is attacked intramolecularlv by the amide or carboxylate group, and the 1,2-difunctionalization takes place to give 388 and 390[322]. Similarly, the ort/trt-thallation of benzoic acid followed by transmetallation with Pd(II) forms the arylpalladium complex, which reacts with butadiene to afford the isocoumarin 391, achieving the 1,2-difunctionalization of butadiene[323]. 

In the reaction of aryl and alkenyl halides with 1,3-pentadiene (248), amine and alcohol capture the 7r-allylpalladium intermediate to form 249. In the reactions of o-iodoaniline (250) and o-iodobenzyl alcohol (253) with 1,3-dienes, the amine and benzyl alcohol capture the Tr-allylpalladium intermediates 251 and 254 to give 252 and 255[173-175]. The reaction of o-iodoaniline (250) with 1,4-pen tadiene (256) affords the cyclized product 260 via arylpalladiuni formation, addition to the diene 256 to form 257. palladium migration (elimination of Pd—H and readdition to give 258) to form the Tr-allylpalladium 259, and intramolecular displacement of Tr-allylpalladium with the amine to form 260[176], o-Iodophenol reacts similarly. 

It was suggested that the reaction occurs via the formation of a carbonium ion (Scheme 1). 

A photochemical variant, the so-called photo-Fries rearrangement, proceeds via intermediate formation of radical species. Upon irradiation the phenyl ester molecules (1) are promoted into an excited state 11. By homolytic bond cleavage the radical-pair 12 is formed that reacts to the semiquinone 13, which in turn tautomerizes to the p-acylphenol 3. The corresponding ort/zo-derivative is formed in an analogous way ... 

The synthesis of 1 -benzothiepin 1 -oxide (23) can be achieved via complex formation with tricarbonyl iron, and quantitative oxidation of the coordination compound 22 with 3-chloroperoxy-benzoic acid. Subsequent irradiation at — 50 C provides 23, which crystallized as yellow needles after low-temperature (-40 C) chromatography, and was characterized by 1H NMR spectroscopy at — 30 C23 before loosing sulfur within one hour at 13°C to give naphthalene. 

On the basis of these correlations, Gold and Satchell463 argued that the A-l mechanism must apply (see p. 4). However, a difficulty arises for the hydrogen exchange reaction because of the symmetrical reaction path which would mean that the slow step of the forward reaction [equilibrium (2) with E and X = H] would have to be a fast step [equivalent to equilibrium (1) with E and X = H] for the reverse reaction, and hence an impossible contradiction. Consequently, additional steps in the mechanism were proposed such that the initial fast equilibrium formed a 7t-complex, and that the hydrogen and deuterium atoms exchange positions in this jr-complex in two slow steps via the formation of a a-complex finally, in another fast equilibrium the deuterium atom is lost, viz. 

Chlorotrimethylsilane-induced Pummerer rearrangements effect the transformation of 4-ketothiane oxides into the corresponding a, /1-unsaturated thianes348, apparently via the formation and subsequent deprotonation of thiiranium intermediates rather than by the conventional sulfocarbonium mechanism depicted in equation 129. 

The reaction is generally believed to proceed via the formation of ionic acylam-monium intermediate compounds (Reaction 1, Scheme 2.27). The equilibrium constant of the acylammonium formation depends mostly on steric and resonance factors, while the basicity of the tertiary amine seems to play a secondary role.297 In die case of the less basic compounds, such as acidic phenols, and of strong tertiary amines, such as Uialkylamines, the reaction has been reported to proceed through a general base mechanism via the formation of hydroxy-amine H-bonded complexes (Reaction 2, Scheme 2.27).297... 

Trifluoroacetic anhydride, which activates the polyesterification of 4-hy-droxybenzoic acid via the formation of a mixed anhydride,307 and 1,1 -carbonyldiimidazole308 were the first reported activating agents. The reaction between 1,1 -carbonyldiimidazolc and carboxylic acids proceeds through the formation of N-acylimidazolcs, which react with aliphatic diols in the presence of sodium ethoxide catalyst (Scheme 2.28). 

Homolytic aromatic substitution often requires high temperatures, high concentrations of initiator, long reaction times and typically occurs in moderate yields.Such reactions are often conducted under reducing conditions with (TMSlsSiH, even though the reactions are not reductions and often finish with oxidative rearomatization. Reaction (68) shows an example where a solution containing silane (2 equiv) and AIBN (2 equiv) is slowly added (8h) in heated pyridine containing 2-bromopyridine (1 equiv) The synthesis of 2,3 -bipyridine 75 presumably occurs via the formation of cyclohexadienyl radicals 74 and its rearomatization by disproportionation with the alkyl radical from AIBN. ... 

The main application of the enzymatic hydrolysis of the amide bond is the en-antioselective synthesis of amino acids [4,97]. Acylases (EC 3.5.1.n) catalyze the hydrolysis of the N-acyl groups of a broad range of amino acid derivatives. They accept several acyl groups (acetyl, chloroacetyl, formyl, and carbamoyl) but they require a free a-carboxyl group. In general, acylases are selective for i-amino acids, but d-selective acylase have been reported. The kinetic resolution of amino acids by acylase-catalyzed hydrolysis is a well-established process [4]. The in situ racemization of the substrate in the presence of a racemase converts the process into a DKR. Alternatively, the remaining enantiomer of the N-acyl amino acid can be isolated and racemized via the formation of an oxazolone, as shown in Figure 6.34. 

The results of low-temperature matrix-isolation studies with 6 [41a] are quite consistent with the photochemical formation of cyclo-Cif, via 1,2-diketene intermediates [59] and subsequent loss of six CO molecules. When 6 was irradiated at A > 338 nm in a glass of 1,2-dichloroethane at 15 K, the strong cyclobut-3-ene-1,2-dione C=0 band at 1792 cm in the FT-IR spectrum disappeared quickly and a strong new band at 2115 cm appeared, which was assigned to 1,2-diketene substructures. Irradiation at A > 280 nm led to a gradual decrease in the intensity of the ketene absorption at 2115 cm and to the appearance of a weak new band at 2138 cm which was assigned to the CO molecules extruded photo-chemically from the 1,2-diketene intermediates. Attempts to isolate cyclo-Cig preparatively by flash vacuum pyrolysis of 6 or low-temperature photolysis of 6 in 2-methyltetrahydrofuran in NMR tubes at liquid-nitrogen temperature have not been successful. 

Chromyl chloride oxidation of alkenes proceeds via the formation of adducts at a rate necessitating stopped-flow techniques. At 15 °C the formation of 1 1 adduct from styrene and oxidant in CCI4 solution is simple second-order with 2 = 37.0 l.mole .sec . Measurements with substituted styrenes yielded = — 1.99. E = 9.0 kcal.mole and = —23.8eu for styrene itself. Hydrolysis of the styrene adduct yields mostly phenylacetaldehyde (76.5 %)and benzaldehyde (21.1 %). Essentially similar results were obtained with a set of 15 alkenes and... 

An attractive alternative to these novel aminoalcohol type modifiers is the use of 1-(1-naphthyl)ethylamine (NEA, Fig. 5) and derivatives thereof as chiral modifiers [45-47]. Trace quantities of (R)- or (S)-l-(l-naphthyl)ethylamine induce up to 82% ee in the hydrogenation of ethyl pyruvate over Pt/alumina. Note that naphthylethylamine is only a precursor of the actual modifier, which is formed in situ by reductive alkylation of NEA with the reactant ethyl pyruvate. This transformation (Fig. 5), which proceeds via imine formation and subsequent reduction of the C=N bond, is highly diastereoselective (d.e. >95%). Reductive alkylation of NEA with different aldehydes or ketones provides easy access to a variety of related modifiers [47]. The enantioselection occurring with the modifiers derived from NEA could be rationalized with the same strategy of molecular modelling as demonstrated for the Pt-cinchona system. 

Olefin dimerisation with Ni-NHC complexes became a topic of interest following reports of Ni(II) phosphine complexes being employed in imidazolium-based ionic liquid solvents [23, 24]. It had previonsly been established that aIkyl-Ni(II) complexes containing NHC ligands can rapidly decompose via imidazolium formation (Scheme 4.1) [5], and it was thus of interest to explore the effect that an excess of the imidazolinm cation would have on this reaction. 

Formation An approach to form QM complexes in which the QM ligand is coordinated only via the exocyclic C=C bond and can be released from the metal was developed by Milstein.18,19 The synthetic strategy is based on the idea that a zwitterionic t -methylene-p-phenoxy metal complex may undergo charge transfer to the metal, resulting in an ri2-methylenep-QM complex of the reduced metal center (Scheme 3.9). 

The activation of an anomeric hydroxyl group from partially protected or unprotected monosaccharides can be achieved via 1,2-cyclic sulfite formation. A subsequent trans-ring opening with azide N3 affords one anomeric derivative exclusively 133... 

The disilene synthesis by the photolysis of linear trisilanes proceeds via initial formation of a silylene followed by its dimerization (Eq. 1). Disilene 1 has now become a common organometallic reagent. A detailed synthetic procedure employing photolysis of the corresponding linear trisilane is described in Inorganic Syntheses (Eq. 2).6... 

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