Tetrachlorosilane-A Versatile Reagent in Organic Synthesis

ABSTRACTA recent increased attention is witnessed to the use of tetrachlorosilane (TCS), a cheap industrial intermediate, as a versatile reagent in organic synthesis. For example, TCS was used as an effective dehydrating agent, trans-silylating reagent and as a starting substrate for in situ preparation of other useful stoichiometric reagents in synthetic organic chemistry. Of these reagents TCS-NaI (Iodotrichlorosilane, ITCS), TCS-NaN3 (azidochlorosilanes) and TCS in combination with ZnCl2, Zn, KCN, Na2S and others. Some selected synthetic applications of TCS as dehydrating agent as well as one of its based "in situ" reagents (TCS-NaN3) are briefly highlighted herein.


Tetrachlorosilane (TCS) as a dehydrating agent:
TCS is known as a dehydrating agent either in pyridine, dichloromethane in presence or absence of triethylamine or in absolute ethanol.

TCS mediated amide bond formation
TCS acts as a simple and efficient dehydrating reagent for the formation of the amides [1][2][3] and hydrazides 4 from carboxylic acids by using pyridine as solvent.
or or H This methodology was also extended to peptide synthesis through the reaction of N-protected amino acid with amino ester to afford dipeptides. 5A002]

General
Among a range of Lewis acids, TCS was successeful as an alternative to TiCl 4 as a Lewis acid catalyst in cyclisation of enamine-ketone and in the subsequent dehydration leading to the alkaloid julandine. 9This success suggested that TCS may find application in other reactions as an alternative to TiCl 4 .
TCS, in the presence of triethylamine was used for the activation of tertiary amides, this concept was applied for the dehydration of N-alkyl N-acyl alanines leading to mesoionic oxazolones 10 and extended for the synthesis of imidazoheterocycles such as the cycloadenine derivatives and benzimidazoles through cyclodehydration reactions.A Japanese patent reported the dehydration of primary amides to the corresponding nitriles by TCS in the presence of amines. 13For examole, 5-cyano-4-methyl oxazole was prepared in high yield through the reaction of 5-cabamoyl-4-methyloxazole with TCS and triethylamine in N-methylpyrolidine as solvent.

TCS/Ethanol
TCS reacts with dry ethanol with exclusion of moisture to produce a regent combination with the general formula Si(OEt) n Cl 4-n .nHCl. 14This system was found to be an efficient reagent for achieving many cyclotrimerization reactions of active methylene ketones.TCS-ethanol induced self condensation of ketones to yield tri-or hexasubstituted benzene has been reported, 15 thus, treatment of cycloalkanones with TCS in ethanol gave the triannulated benzene.
Similarly, a variety of aryl methyl ketones, [16][17][18][19][20][21][22] under the same conditions, gave good yields of 1,3,5-triaryl benzenes.This methodology was extended for a direct synthesis of conjugated star polyaromatics and organometallics as well, such as symmetrically substituted arenes 23 1,3,5-C 6 H 3 R 3 where R = (C 5 H 4 )Mn(CO) 3 and (C 5 H 4 )Fe(C 5 H 5 ).Instead of the expected hexasubstituted benzene 2,4,6-trimethyl-1,3,5-triphenylbenzene, reaction of proiophenone with TCS-EtOH gave a novel compound , which proposed as a result of three aldol type condensations of four molecules of propiophenone. 24 3 Dimmers of aryl methyl ketones, ß-methyl chalcones were isolated in some cases in lower amount. 25,26By optimizing the reaction conditions, ß-methyl chalcones were isolated as major products.we have reported a new approach to the stereoselective synthesis of ß-methyl chalcones through the reaction of aryl methyl ketones with TCS in absolute ethanol under milder conditions. 27ß-methyl chalcones were isolated as major products along with minor amounts of symmetrical 1,3,5-triaryl benzenes.Substituted acetophenones having strongly electron-withdrawing groups such as nitro group gave the ß-methyl chalcones only.Thus, 3 or 4-nitroacetophenones gave exclusively the corresponding ß-methyl chalcones in good The above mentioned convenient synthesis of ß-methyl chalcones was efficiently used for a selective synthesis of unsymmetrical branched triarylbenzenes through the reaction of ßmethyl chalcones with variety of aryl methyl ketones in the presence of TCS-ethanol mixture. 28,29 Compared with other synthetic routes to unsymmetrical triarylbenzenes, this method considered as an extremely efficient.An attractive and convenient route to branched fictionalized benzenoid compound has been reported by us through successive reactions of some cyclic ketones with aryl methyl ketones mediated by TCS-ethanol reagent.The reactions of cyclic ketones e.g.cyclohexanone, α-tetralone with nitro-or methoxyacetophenone have been examined.Some examples of the products from these condensations are listed below.

Tetrachlorosilane -Sodium Azide (TCS-NaN 3 ):
Hergs and Stark 34 have reported that silicon tetrachlorosilane reacts with sodium azide in acetonitrile at room temperature to form an equilibrated mixture chloroazidosilanes according to the molar ratio of added sodium azide.The ratios (%) of chloroazidosilanes in that reaction were determined by 29 Si-NMR spectroscopy.Increasing proportions of sodium azide shift the equilibrium towards the higher azides as shown in the following table: A separation and further characterization of the silanes Si(N 3 ) n Cl 4-n ; (n = 1-4 ) was not attempted because of the explosive nature of these compounds.The above reaction was applied for the synthesis of 4,4`,7,7`-tetramethyl-2,2-diazido-1,2,3-dioxasila-5-cycloheptene which used for the chemical vapor deposition of quartz.
A novel conversion of aldehydes to nitriles in one-pot reaction was reported on treatment the aldehyde with SiCl 4 /NaN 3 at room temperature. 35etones are converted to the corresponding tetrazole derivatives in nearly quantitative yield by their reaction with TCS/NaN 3 system (major triazidochlorosilane).The reaction was widely applicable for different ketonic compounds.It has succeeded with acyclic, cyclic, oxacyclic, azacyclic and aromatic ketones. 37The reaction of acetophenone derivatives was found to give a mixture of two isomeric tetrazoles that indicates the preferential migration of aryl group.The reaction was further applied to the α,β-unsaturated ketones 37 as well as dienones 38 giving regiospecifically substituted tetrazole derivatives with preferential migration of the aryl or alkyl rather than alkenyl groups in arylidines of acetophenones.However, in case of arylidines of alicyclic ketones such that of cycloocatnone and benzosuberone, as proved by 2D NMR studies, alkenyl migration was favourable. 38 A new mild one-step method for the conversion of the primary acid amides to 5substituted tetrazoles in nearly quantitative yields employing TCS-NaN 3 reagent has been reported. 65Thus, acid amides on treatment with TCS/NaN 3 in situ in acetonitrile under reflux, yielded 5-aryltetrazole derivatives in high yields. 39,40Cl Tetrazole-containing amino acid derivatives were also prepared through the reaction of TCS-NaN 3 reagent with N-acetyl-amino acid esters. 41,42inally, TCS still finds an update interest in the scientific community [44][45][46][47][48][49][50] which will be followed in a forthcoming review.
In conclusion, we have presented herein an overview on the role of the inexpensive and readily available tetrachlorosilane as an efficient and versatile reagent in synthetic organic chemistry.It is expected that its use, together with a number of convenient "in situ"preparations, will continue to expand and find many additional applications.

36 SiCl 4
Under similar conditions, aldehydes were converted into acid azides in the presence of active manganese dioxide.

43 TCS
Carboxylic acid chlorides react with the TCS-NaN 3 reagent to give tetrazolinones and/or carbamoyl azides in good yields presumably through the intermediacy of isocyante derivatives.