Alkoxy silanes, reactivity

Two-part room temperature condensation cure. Silicone can be formulated into two-part systems [3,12,14,33] that prevent the reactive groups from coming into contact before they are needed. The reactions in these systems are based on the condensation of a silanol group with an alkoxy silane group, catalyzed by organo-tin compounds (Scheme 9). 

Multifunctional (meth)acrylate alkoxysilanes were developed, a new class of reactive compounds. Compared with commercially available organo(alkoxy)silanes having reactive C=C bonds in the organic substituent, the new compounds can be varied to a much higher degree. The main improvements are ... 

The accelerated rate for alcoholysis with le, which was observed for the 10 % Pd/C catalytic system, was also seen with the Mn(CO)sBr catalyst. Reactions of le with primary, secondary or tertiary alcohols resulted in moderate yields of the corresponding silyl ketals after 2 h (Table 8 and 9). When mono-alkoxy silane from 3-hydroxy butyrate (lg) was treated with homoallyl alcohol in the presence of Mn(CO)sBr as the catalyst under the standard conditions, 76 % of the silyl ketal was obtained. These silyl ethers possess neighboring carbonyl groups that can participate in the reaction by forming a more reactive pentacoordinated silicon center upon addition of the silane to the metal center.. 

Gas/vapor phase modifications. Many inorganic membrane materials display functional groups that have chemical affinity to selected chemical agents. A well known example is a gamma-alumina membrane which has hydroxyl groups on the surfaces of the alumina crystallites. These hydroxyl groups present on the pore walls and the macroscopic surface of the membrane can act as the reactive sites for modifications of the pore structure with a chemical agent such as the diversified family of silane compounds (chloro- or alkoxy>silanes). 

Primers for adhesive bonding chemically functionalize the substrate surface to provide pathways for chemical bonding with a selected silicone cure system. The increase of chemical bond improves adhesion durability. The main disadvantage of priming the substrate is the addition of an extra step in the whole process of adhesive application. The primers are usually Silane adhesion promoters, a reactive alkoxy silane molecule, oligomer, or a mixture of two or more different silanes. 

Where the matrix polymer undergoes a polymerisation, or crosslinking process, during composite formation then it is fairly simple to choose a suitable polymer reactive group (e.g., a carbon=carbon double bond). However, with polymers such as the polyolefins, this can be difficult. Highly reactive functionalities, such as azides, are sometimes used in this case. It is also possible to pre-functionalise a part of the polymer matrix, by grafting on a species such as an acid, anhydride or alkoxy-silane, in a separate process. 

The study of the reactivity of trialkylsilyl radicals in solution has been placed on a firmer foundation by the measurement of absolute rate constants for some reactions of triethylsilyl radicals (generated by the reactions of tert-butoxyl radicals with triethylsilane), with some organic halides and benzil/ These data show for example, the greater reactivity of Et3Si in halogen abstraction than that of trialkyltin radicals. Kinetic isotope effects (kn/fco) for the insertion of photochemically generated dimethylsilylene and methylphenylsilylene into Si-H and 0-H single bonds are about 1.3 and 2.1 - 2.3, respectively. The preferred mechanism for insertion of silylenes into O-H bonds is shown in equation (1). Other workers haye shown that dimethylsilylene inserts preferentially into O-H bonds of alcohols compared with S-H bonds in silanes or Si-O bonds in alkoxy-silanes. ... 

A frequently asked question is whether the silicon needs to have three surface reactive groups. The tri-functionaUty arose initially because the vinyl trichlorosilane (and hence the trialkoxy derivatives) was readily prepared. It is more difficult and expensive to make less functional molecules such as vinyl dimethyl mono-alkoxy silane. There is no definitive information on how such compounds might compare with the trialkoxy ones. 

Each of these silanes consist of two functionalities the moisture-reactive functionality (usually alkoxy), and the organic functionality. It is believed that this... 

The very different reactivity toward allylation of a,P-unsaturated esters and acyl-silanes [132] is due to stabilization of the allyl cation intermediates by the alkoxy group in the former class of substances, in contrast to those derived from acylsilanes which are expected to be less stable than 3-monooxyallyl cations. In the 1,1-dioxy substituted cations two donor atoms are bonded to the acceptor site, but in the 1-oxy-3 -silyl congeners a rf,a-antagonism (captodative) exists. 

In the vast majority of silane surface treatment applications, the alkoxy groups of trialkoxysilanes are hydrolyzed to form silanol-containing species. The silanol-containing species are highly reactive intermediates which are responsible for bond formation with the substrate. Hydrolysis of trialkoxysilane alkoxy groups... 

The Lewis acid catalyzed conjugate addition of allylsilanes (140) to (142) and allylstannanes (154) and (155) to ot,0-enones, described by Sakurai,68a,68b is highly efficient and experimentally simple in contrast to the allylcuprate additions. Various substituents can be incorporated into the allylsilanes (allylstannanes), e.g. alkoxy, alkoxycarbonyl and halogen, some of which are incompatible with cuprate reagents 69 In addition, Heathcock and Yamamoto report that diastereoselectivity is correlated to the alkene geometry of both the allylmetals and the acceptor units for example, allylation of ( )-enones (143) and (146) affords predominantly the syn adducts (144) and (147), while (Z)-enone (149) gives predominantly the anti adduct (150 Scheme 25).680 On the other hand, with cyclohexen-2-one the (Z)-silane (141) affords predominantly the threo adduct (152), while (142) affords erythro adduct (ISS).686 The more reactive allylstannanes (154) and (155) also afford similar diastereoselectivity.68e,f... 

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