Silicon compounds acylation

The similarities between organic tin and silicon compounds, particularly their abilities to stabilize positive charge on a -carbon atom, suggest that vinylic and allylic tin compounds should be useful substrates for Friedel-Crafts acylation reactions. However, very few examples of the acylation of stannanes under the action of Lewis acids have been reported, and this remains a field of Friedel-Crafts reactions not yet fully exploited. The acylation of 1,2-bis(tri-n-butylstannyl)ethylene in the presence of aluminum chloride offers a moderately yielding route to tributylstannyl-substituted enones, useful as precursors to 4,5-dialky lcyclopent-2-enones. 

Other, removable cation-stabilizing auxiliaries have been investigated for polyene cyclizations. For example, a sdyl-assisted carbocation cyclization has been used in an efficient total synthesis of lanosterol. The key step, treatment of (257) with methyl aluminum chloride in methylene chloride at —78° C, followed by acylation and chromatographic separation, affords (258) in 55% yield (two steps). When this cyclization was attempted on similar compounds that did not contain the C7P-silicon substituent, no tetracycHc products were observed. Steroid (258) is converted to lanosterol (77) in three additional chemical steps (225). 

Homogeneous catalysis by lin compounds is also of great indusirial importance. The use of SnCU as a Friedel-Crafts catalyst for homogeneous acylation, alkylation and cyclizaiion reactions has been known for many decades. The most commonly used industrial homogeneous tin catalysis, however, are the Sn(ll) salts of organic acids (e.g. acetate, oxalate, oleale, stearate and ocToate) for the curing of silicone elasloniers and, more importantly, for the production of polyurethane foams. World consumption of tin catalysts for the.se Iasi applications alone is over 1000 tonnes pa. 

The dicyclopentadienyl metal compounds undergo Friedel-Crafts alkylation and acylation, sulfonation, metalation, arylation, and formyla-tion in the case of ferrocene, dicyclopentadienyl ruthenium, and dicyclopentadienyl osmium, whereas the others are unstable to such reactions ( ). Competition experiments (128) gave the order of electrophilic reactivity as ferrocene > ruthenocene > osmocene and the reverse for nucleophilic substitution of the first two by n-butyl lithium. A similar rate sequence applies to the acid-catalysed cleavage of the cyclopentadienyl silicon bonds in trimethylsilylferrocene and related compounds (129), a process known to occur by electrophilic substitution for aryl-silicon bonds (130). 

The formation of a sol-gel porous material is through a hydrolysis-polycondensation reaction. An example is given in equation 1 with the methoxide of silicon (tetramethyl-orthosilicate, TMOS), but many other alkoxides, aryl oxides and acyl oxides can be used, as well as Si—N and Si—Cl compounds. 

The great reluctance of silicon-metal compounds to undergo carbonyl insertion reactions, familiar in analogous C-metal chemistry, is discussed in Section III,B. Entry 43 shows the reverse process a sila-acyl derivative (XIII), prepared by the reaction (14) of Eq. (48),... 

Problems with the selection of the stationary phase for the GC of acyl derivatives are similar to those associated with the GC of esters. In simple separations silicone and polyester phases are satisfactory but more complicated separations require special phases, low coatings, mixed phases, etc. Individual cases are discussed for specific types of compounds in Chapter 5. 

Carbodiimides are a unique class of reactive organic compounds having the heterocumu-lene structure R—N=C=N—R. They can be formally considered to be the diimides of carbon dioxide or the anhydrides of 1,3-substituted ureas, and they are closely related to the monoimides of carbon dioxide, the isocyanates. The substituent R can be alkyl, aryl, acyl, aroyl, imidoyl or sulfonyl, but nitrogen, silicon, phosphorous and metal substituted carbodiimides are also known. The unsubstituted carbodiimide HN=C=NH is isomeric with cyanamide, H2NCN. Mono substituted carbodiimides, generated in the thermolysis of 1-substituted tetrazoles, can be isolated at liquid nitrogen temperature but isomerize to the cyanamides at higher temperatures. ... 

Acyl halides free from traces of phosphorus or sulfur compounds may be made from carboxylic acids and silicon tetrachloride or oxalyl halides (method 339). 

In the benzoin condensation, one molecule of aldehyde serves as an electrophile. If a carbanion is generated from protected cyanohydrins, a-aminonitriles or dithioacetals, it can react with electrophiles such as alkyl halides, strongly activated aryl halides or alkyl tosylates to form ketones. Amongst other electrophiles which are attacked by the above carbanions are heterocyclic A -oxides, carbonyl compounds, a,p-unsaturated carbonyl compounds, a,3-unsaturated nitriles, acyl halides, Mannich bases, epoxides and chlorotiimethyl derivatives of silicon, germanium and tin. 

More sensitive than silicon-derived compounds are dialkoxystannanes. They are formed in good yields on treatment of the respective diols with dibutyltin oxide. The particular advantage offered by this protecting technique is that it allows for regioselective acylations and alkylations of diol structures. Two examples are given in Scheme 56. 

The acyl complex of reaction (i) undergoes decarbonylation above its melting pont (182-184°C) to give the corresponding compound containing a rhenium-silicon bond . 

Carbon monoxide insertion into zirconium(IV)-silicon bond has been reported . The trimethylsilyl derivative Zr(/7 -C5H5)2(SiMe3)Cl was carbonylated under CO pressure (about 7 atm) in diethyl ether, giving the following silyl-acyl compound ... 

Triazole is fairly resistant to A -alkylation under neutral conditions, however both acylations and alkylations involving iV-anions occur readily, but mixtures of 1- and 2-substituted products are often obtained/ Trimethylsilylation produces 2-trimethylsilyl-1,2,3-triazole, and this can be alkylated unambiguously at N-1, with concomitant loss of silicon/ An equilibrium mixture of A-acetyl-1,2,3-triazoles contains predominantly the 2-acetyl isomer. The 2//-tautomer is also dominant in 1,2,3-triazole itself, which may reflect unfavourable adjacent imine lone-pair/lone-pair interactions in the 1//-isomer and is in agreement with calculations that suggest that the 2//-isomer is more aromatic." Heating in sulfolane at 150 °C converts the A-acyl compounds into oxazoles in a synthetically useful transformation. ... 

N-(l-Alkoxyalkyl)-aniides or -carbamates (2 X = OR), most frequently used as stable precursors for A -acyliminium ions, are usually prepared by one of the following routes (equations 7-13). For five- or six-membered cyclic cases a simple acid-catalyzed solvolysis of the hydroxy compound provides the alk-oxy derivative (equation 7). A silicon-assisted approach involves the TMSOTf-catalyzed reaction of bis(trimethylsilyl)formamide with aldehydes (equation 8). /V-(l-Trimethylsilyloxyalkyl)formamides are thus formed in good yields, which on TMSOTf-catalyzed solvolysis lead to the /V-( 1-alkoxyalkyl)form-amides. A third method is based on the NaBH4 reduction of imidates (equation 9), and has proved useful for a total synthesis of the insect poison pederine. Addition of reactive acid derivatives to imines constitutes another method (equations 10 and 11). Acylation with acid chlorides followed by treatment with ethanol in the presence of base leads to N-(l-alkoxyalkyl)amides. A one-step protocol using diethyl dicarbonate provides the corresponding carbamates. 2... 

Stabilization of a lipid membrane onto a solid support by covalent attachment also provides the physical stability necessary for the development of practical sensors. An oriented membrane can be prepared by allowing self-assembly of individual amphiphilic molecules onto a solid surface through either the reaction of terminal silane moieties with a hydroxylated surface to form a silyl ether [33,34], or by the reaction of sulfur-terminated compounds (alkylthiols or disulfides) with gold surfaces [35,36]. A variety of species, both with and without polar head groups, have been deposited onto surfaces such as glass, quartz, silicon, and gold [37-39]. These include phospholipids, fatty acids, and fatty amines which were synthetically altered so as to contain either a silyl chloride or a thiol moiety at the terminus of the acyl chain [40]. Both monolayers... 

---