Alkenes hydrogen-terminated

Scheme 20 Hydrosilylation of an alkene by hydrogen-terminated Si(l 11) surface.

In 1993, Linford firstly reported a quite useful method to prepare monolayers of alkyl chains by thermal hydros-ilylation of hydrogen-terminated silicon surfaces [25]. Alkyl chains are covalently bound to Si surface by Si-C bonds. This thermal hydrosilylation could be attributed to a free-radical process with 1-alkene. First, a diacyl peroxide initiator was used to produce free radicals. However, at higher temperature, only hydrogen-terminated silicon and a neat solution of 1-alkene or 1-alkyne can form Si-C linkages [26]. Furthermore, lately it is found that such Si-C covalent links can be observed even in dilute solutions of 1-alkenes [27]. In that case, the density of monolayer packing strongly depends on the reaction temperature. 

Fig. 59. Molecular modification of semiconductor silicon surfaces. Removal of the oxide generates a hydrogen-terminated layer that reacts with a range of molecular functional groups including alkenes. 

In general, the rate of alkene hydrogenation is typically ordered as follows terminal>di-substituted>tri-substituted >tetra-substituted. In fact, this allows terminal or di-substituted olefins to be hydrogenated selectively in the presence of tri- or tetra-substituted ones. Additionally, the rate of hydrogenation of al-kynes is much slower than that of alkenes, although the cis-alkene intermediate... 

Therefore, surface modification strategies for the formation of direct silicon-carbon bonds require, first, a special pre-treatment of the silicon surface to prevent oxidation and, second, an activation of the silicon surface for subsequent reaction with organic moieties. This has been achieved by treatment of the silicon surface with hydrofluoric acid to generate a hydrogen-terminated Si(lll) surface, which can further react with unsaturated co-functionahzed alkenes in the presence of UV irradiation or by thermal activation [27,44,45]. Using this method, carboxylic acid modified silicon substrates have been successfully generated and coupled to thiol modified ONDs via a polylysine/sulfosuccinimidyl 4-(M-maleimidomethyl)-cyclohexane-l-carboxylate couphng (Fig. 12). 

Cl and C2 are unequal. However, the larger isotope effect at Cl implies more advanced binding at this carbon atom in the transition state (19). Note too the inverse ot-secondary ( h/ d) isotope effects at the hydrogens bonded to Cl and C2. These carbons rehybridize from sp toward sp as the addition proceeds, so that h/ d < 0 is expected. The greater effect at H-C versus H-C2 is also consis tent with greater carbene binding to the alkene s terminal C in the transition state. 

Linford, M. R., Fenter, P., Eisenberger, P. M. and Chidsey, . E. D. Alkyl monolayers on silicon prepared from 1-alkenes and hydrogen-terminated silicon. Journal of the American Chemical Society 117, 3145 (1995). 

The site-selective reduction of a,co-dienes, in which the two double bonds are differentiated only by the presence of an allylic substituent, is a challenging task. A pentamethylyttrocene complex that is extremely selective in hydrogenating terminal alkenes exhibits good selectivity in this reaction 142... 

Aqueous biphasic catalysis is also used in homogeneous hydrogenations.117-119 In new examples Ru clusters with the widely used TPPTN [tris(3-sulfonatophenyl) phosphine] ligand120 and Rh complexes with novel carboxylated phosphines121 were applied in alkene hydrogenation, whereas Ru catalysts were used in the hydro-genation of aromatics. Aerobic oxidation of terminal alkenes to methyl ketones was carried out in a biphasic liquid-liquid system by stable, recyclable, water-soluble Pd(II) complexes with sulfonated bidentate diamine ligands.124... 

CoH(BH4)(PCy3)2] in benzene solution hydrogenates terminal alkenes and styrene faster than internal alkenes or dienes. Isomerization of terminal alkenes caused a rapid deterioration of the reaction rate.69 An ort/io-metallated triphenyl phosphite complex, for which the structure (20) was proposed, gave a turnover number of more than 300 in the hydrogenation of 1-butene. Isomerization was not observed in this case.70... 

The nitrosyl complex [Rh(NO)(PPh3)3] (36) catalyzes the hydrogenation of both 1-hexene and cyclohexene in dichloromethane as solvent and was also found to add deuterium to cyclohexene without H/D scrambling.138 A further study extended the range of substrates hydrogenated to internal alkenes, to conjugated and non-conjugated dienes, activated alkenes and terminal and internal alkynes.139... 

Scheme 13.1 Reaction of a hydrogen terminated surface with an alkene to generate surface mono-layers (a) monolayer formation by reaction of silicon with H2C=CH(CH2)90Ph(C6H40Me)2, (b, c) monolayer derivatisation to give a surface layer of tethered magnetic TiW5 polyoxometalate clusters.4...

An alternate route to formation of alkyl monolayers is via Lewis acid catalyzed reactions of alkenes with the hydrogen terminated surface. In this approach, a catalyst such as ethyl aluminum dichloride is used to mediate the hydrosilylation reaction of an alkene (or alkyne), resulting in the same type of product as in the case of the photochemical or thermal reactions. This type of reaction is well known based on molecular organosilane chemistry and has also been used successfully to alkylate porous silicon [31]. Although this route has been shown to work on H/Si(lll), the resulting monolayers are found to have lower coverages than those achieved using the photochemical or thermal approach [29], Another concern with this approach is the possibility of trace metal residues from the catalyst that could adversely affect the electronic properties of these surfaces (even when present at levels below the detection limit of most common surface analysis techniques). 

Linford MR, Fenter P, Eisenberger PM, Chidsey CED (1995) Alkyl Monolayers on Silicon Prepared from 1-Alkenes and Hydrogen-Terminated Silicon, J Am Chem Soc 117 3145—3155... 

We have already seen in Section 2.2.2 that metal-alkyl compounds are prone to undergo /3-hydride elimination or, in short, /3-elimination reactions (see Fig. 2.5). In fact, hydride abstraction can occur from carbon atoms in other positions also, but elimination from the /8-carbon is more common. As seen earlier, insertion of an alkene into a metal-hydrogen bond and a /8-elimination reaction have a reversible relationship. This is obvious in Reaction 2.8. For certain metal complexes it has been possible to study this reversible equilibrium by NMR spectroscopy. A hydrido-ethylene complex of rhodium, as shown in Fig. 2.8, is an example. In metal-catalyzed alkene polymerization, termination of the polymer chain growth often follows the /8-hydride elimination pathway. This also is schematically shown in Fig. 2.8. 

Hydroboration. When these two reagents are mixed in THF, hydrogen is evolved and a solution of a violet titanium-boron complex forms. This complex catalyzes the hydroboration of alkenes with LiBH4 to form lithium alkylboro-hydrides, which are converted to alcohols by NaOCHj and H2O2. The relative reactivity of alkenes is terminal > cyclic > internal. The reaction involves anti-Markownikoff addition. 

In the hydroformylation of terminal alkene, hydrogenation proceeds and a considerable amount of alkane is formed, together with 0x0 alcohols (Eq. 11.77). 

The complexes [RCo(CO)2 P(OMe)3 2] (R = Me and MeCO) show a marked catalytic activity (450 turnovers h ) for hydrogenation of terminal alkenes under ambient conditions, while more-hindered alkenes are reduced slowly. Cobalt complexes such as [CoH2(PR3)3] and [CoHN2(PPh3)3] are also active catalysts for alkene hydrogenation. 

The iron cluster [ ( ri -C5H5)Fe( jL3-CO) 4] catalyzes the selective hydrogenation of alkynes to alkenes at 100-130 °C and 6.8-68 atm of hydrogen. Terminal alkynes are selectively hydrogenated to alkenes even in the presence of alkenes or internal alkynes. ... 

Alkenes can be hydroformylated " by treatment with carbon monoxide and hydrogen over a catalyst. The most common catalysts are cobalt carbonyls (see below for a description of the mechanism) and rhodium complexes, " but other transition metal compounds have also been used. Cobalt catalysts are less active than the rhodium type, and catalysts of other metals are generally less active. " Commercially, this is called the 0x0 process, but it can be carried out in the laboratory in an ordinary hydrogenation apparatus. The order of reactivity is straight-chain terminal alkenes > straight-chain internal alkenes > branched-chain alkenes. With terminal alkenes, for example, the aldehyde unit is formed on both the primary and secondary carbon, but proper choice of catalyst and additive leads to selectivity for the secondary product " or primary... 

Scanning probe lithography on metal or silicon substrates is a well known technique and can be supported by a self-assembled monolayer (SAM) [1,2], Such monolayers are of great interest e.g. for passivation of silicon surfaces [3]. Covalently bound monolayers by Si-C bonds that are formed by the reaction of 1-alkenes and a hydrogen terminated silicon surface [4,5], are known to show high thermal [6] as well as chemical stability [3,7]. 

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