Silyl alkylation

Whereas aldehydes such as benzaldehyde or ketones are converted by trimethyl-silylated alkyl- or aryl-mercaptans such as 575, in the presence of catalytic amounts of cyanide, to give high yields to O-trimethylsilyl-hemithioketals such as... 

Reaction of a mixed silyl—alkyl phosphite with an aldehyde... 

Fig. 2.1.8. Mass chromatogram of silylated alkyl polyglycosides. Reprinted with permission from Ref. [132]. 1998 by Carl Hanser...

Figure 9. Silyl-alkyl isomers, the migratory insertion products.. The migratory insertion energies, AEj, are reported as the difference in energy (kcal/mol) between the silyl-alkyl complex and the respective it-complexes.

Figure 10. A rearrangement of the silyl-alkyl insertion products leads to the formation of two distinct rj 3-silyl-allyl intermediates (syn and anti).

Our calculations show that the isomerization of the silyl-alkyl complex to form a V-allyl complex affords a significant stabilization as summarized in Figure 11. TheiV toil3 isomerization of 9a to the anti silyl-allyi complex, 10a-anti, results in a 9.5 kcal/mol stabilization and isomerization to the syn isomer, lOa-syn, results in a 7.2 kcal/mol stabilization. Isomerization of 9b to the anti silyl-allyi complex, 10b-anti, results in a 6.1 kcal/mol stabilization and isomerization to the syn isomer, lOb-.vyn results in a 5.4 kcal/mol stabilization. High temperature (500 °C) molecular dynamics simulations initiated at the V complex, 9a, reveal that the rj1 to T 3 isomerization has a minimal barrier and occurs in the sub-pico time frame. The inter-conversion between the syn and anti isomers has not been examined since both isomers are stereochemically equivalent, however, we expect the barrier to be small. 

Figure 14. Formation of a stabilized rj-silyl-alkyl complex which acts as a regiochemical lock in the hydrosilylation process.

Both the second and third processes in Scheme 13 can also be used for the functionalization of a-silylated alkyl phenyl sulfides [74] when oxidizing the latter in the presence of primary alcohols. First, the cleavage of the C—Si bond (equivalent to deprotonation) and then that of the S—C bond occurs to give... 

Scheme 17 Anodic substitution of a-silylated alkyl phenyl sulfides.

Silyl alkyl and silyl aryl peroxides 7 are prepared by reaction of alkyl, aryl or aralkyl hydroperoxides with halosilanes (equation 12). Such reactions are carried out in an inert solvent in the presence of an acid acceptor, such as pyridine, ammonia or a tertiary amine in ether or petroleum ether (Table 3) . ... 

TABLE 3. Silyl alkyl and aryl peroxides R R R Si—OOR 7 obtained according to equation 12... 

Alkyl hydroperoxides can form 2 1 adducts with DABCO. Both nitrogens of DABCO are involved and the resulting adduct contains 2 mol of hydroperoxide per mol of DABCO. Table 4 lists the silyl alkyl peroxides prepared using the DABCO-hydroperoxide adduct techniques. 

Further investigations are needed to establish, whether this approach is really useful to obtain chiral allylsilanes 2, which are synthetically quite interesting intermediates. They are available otherwise only by asymmetric cross-coupling of silyl alkyl Grignard reagents with bromoethylenes in the presence of a chiral ferrocenylphosphine-palladium catalyst54. 

Detailed mechanistic studies with respect to the application of Speier s catalyst on the hydrosilylation of ethylene showed that the process proceeds according to the Chalk-Harrod mechanism and the rate-determining step is the isomerization of Pt(silyl)(alkyl) complex formed by the ethylene insertion into the Pt—H bond.613 In contrast to the platinum-catalyzed hydrosilylation, the complexes of the iron and cobalt triads (iron, ruthenium, osmium and cobalt, rhodium, iridium, respectively) catalyze dehydrogenative silylation competitively with hydrosilylation. Dehydrogenative silylation occurs via the formation of a complex with cr-alkyl and a-silylalkyl ligands ... 

The addition of phenyldimethylsilane to the double bond of enamides (or vinyl ureas) in the presence of catalytic amounts of rhodium acetate, leads to Ma-silyl)alkyl amides (or ureas). Rhodium catalyst are known to move double bonds, and indeed the same type of RSMA are obtained from an /V-ally I urea, albeit in low yield. On the other hand, hydrosilylation of A-allyl imides takes place at the terminal carbon atom in high yields.211... 

The l-(benzotriazol-l-yl)methyl derivatives of indole and carbazole were also used to effect < -silylation/alkylation. <950M734>... 

Silylated alkyl phenols, alkyl phenyl ethoxylate - - - GC-MS [12]... 

Although not siloxane based, organosilicon surfactants have also been made from per-methylated carbosilanes containing an Si-C-Si structure. The simplest version of this is the trimethyl silylated alkyl polyether discussed by Klein [22] and Wagner [11, 23]. These surfactants are more hydrolytically stable wetting agents than the trisiloxanes. 

A widely accepted cycle for olefin hydrosilylation is the so-called Chalk-Harrod mechanism, where it is assumed that H migration to the alkene is faster than silyl migration, followed by reductive elimination from a silyl-alkyl intermediate,... 

---