A varity of synthetic uses of iodotrichlorosilane reagent in some organic reactions

Iodotrichlorosilane is readily available by mixing cheap , commercially available tetrachlorosilane with sodium iodide . Halogen analysis of the distillate indicates that the exchange of iodide ion practically stops at the first stage and yields iodotrichlorosilane under these conditions . for experimental purposes this mixure may be used in place of the isolated  ITCS .

1-The generation of carbonyl compounds from  acetals and ketals

The generation of carbonyl compounds from gem-diethers is an important process that is of continuing interest. Transacetalisation and hydrolysis with awide variety of acid catalysts  are the traditional methods of choice . iodotrichlorosilane can be used for the high yield cleavage of acyclic ketals , but is unsuitable for cyclic ketals whilst diiodosilane may be used at low temperatures but above zero it produces iodoalkanes.

We now report that ITCS readily cleavages both cyclic and acyclic acetals and ketals at room temperature .

The reactions with acetals are very fast as , even though ITCS readily cleavage esters and phenolic methyl ethers

Two equivalent of ITCS are required for efficient production of the carbonyl compounds .when one equivalent of ITCS  is used , then a α-iodoethers can be isolated

2-Uses of reductive properities of iodotrichlorosilane

2.1-Chemoselective reduction of alpha,beta   unsaturated ketones and nitriles

A tetrachlorosilane-sodium iodide  has been applied to many organic synthesis as an iodotrichlorosilane  (ITCS) equivalent , where beside the application to cleavage of ethers ; esters ,acetals and ketals, it can be used to introduce an acetamide or benzamide group into aromatic or aliphatic aldhydes to produce corresponding derivative  containing anitrogen function in the place of the oxygen function of its carbonyl group

Example: by using two equivalent of iodotrichlorosilane , β-phenyl α,β-unsaturated ketone can be reduced to the corresponding saturated ketones in high yield at room temp in acetonitrile solvent

 R: phenyl

The suggested mechanism of this reaction  involves 1,4 addition of iodotrichlorosilaneto enone to produce β-iodosilylenol ether  intermediate where β-iodo ketons can be isolated after hydrolysis ,the intermediate is attacked  by another molecule of ITCS to give another intermediate which have hydrogenated product by hydrolysis  

2.2- with some natural products

The versatility of iodotrichlorosilane (ITCS) ,(SiCl4/NaI in situ) as auseful reagent in organic synthesis is well established

Attempting to introduce an acetamide group in danthron  to produce an anthracene derivative containing a nitrogen function in the place of the oxygen function of its carbonyl group, we found that by using five equivalents of ITCS in presence of  CH3CN  and CH2Cl2 the resulting product  was anthralin

Since the main objective of the study is to establish the proposing mechanism of the reaction , it will be convenient to mention firstly some of the facts concerning the reaction under investigation these facts are :

1-by using 3 moles of ITCS with danthron under the same conditions ,the reaction does not proceed

2-under  the same conditions the reaction of anthraquinone with five equivalent  of ITCS does not proceed to  reduce  the carbonyl group

3- the reaction of carbonyl conjugated with an aromatic ring was found to be effected with some selectivity in the presence of  hydroxyl substituents in the ring . so , the order reactivity of carbonyl groups at C-9 and C-10 are not the same . the carbonyl at C-9 is less reactive for the reaction owing to unfavour  electronic factor and more sterically hindered .

Therefore, the suggested mechanism  may proceed as follow: in the scheme danthrone reacts with 3 equivalents of ITCS  to give intermediate (1) and two equivalents of HI .  the α-iodosilyl ether  (intermediate could be reduced by trichlorosilyl hydride ,which was produced by the reaction of ITCS with the liberated HI , to give intermediate (2) which could react with H2O during work up to give the product .

The mechanism of the reaction shows that , one mole of danthrone needs 5 equivalents of ITCS to be reduced to anthralin

 

2 Cl3SiI    +   2 HI           →      2 Cl3SiH   +    2 I2   

Nucleophilic substitution reactions

Nucleophile is defined as : anegative ion or a molecule that has at least one unshared pair of electrons. In reaction a nucleophile is areagent that seeks a positive centre e.g.hydroxide ion OH⁻ example of negative nucleophile  and H₂O as nucleophile with unshaired pair of electrons .

Nuc: + R-LG → R-Nuc + LG:

Arrangement of leaving group

 I⁻  > Br^- >  Cl⁻ > F⁻  in leaving group because I⁻  is very weak base (strong leaving group)

There are two nucleophilic substitution reactions
1-    SN1
2-   SN2

Kinetics of SN1

 

By taking this example

(CH3)3C-Cl  +  OH⁻   --------à (CH3)3C-OH + Cl⁻ 

the kinetics shows that the of formation of tert butyl alcohol is dependent only on the concentration of tert-butyl chloride.

So the rate equation for this substitution reaction is first order with respect to tert-butyl chloride and first orders overall

Rate  α  [(CH3)3 Cl]

Rate =  K[(CH3)3 Cl]

Because only tert- butyl chloride is involved in transition

state so this reaction is said to be unimolecular (SN1) Mechanism of SN1 reaction

The general form of the S_N1 mechanism is as follows:

Mechanism:

 

Kinetics of SN2

By taking this example

CH3-Cl + OH⁻   -------àCH3-OH + Cl⁻ 

The rate of reaction is determined by measuring the rate of disappearance of CH3Cl or OH⁻   from the solution , or rate of appearance of CH3OH or Cl⁻ 

The results of measurements  showed that ,

Rate α [CH3Cl][ OH⁻   ]

Rate =K[ CH3Cl][ OH⁻   ]

This reaction is second order (bimolecular reaction)

Bimolecular reactions means presence of two species in slow step (rate determining step)

Mechanism of SN2 reaction

 The general form of the S_N2 mechanism is as follows:

nuc: = nucleophile
X = leaving group (usually halide or tosylate)

Because of the backside attack of the nucleophile, inversion of configuration occurs.

The mechanism of SN2 reaction involves only one step through the transition state(concerted reaction)

The transition state involves the nucleophile and the substrate i.e. it is asecond order kinetics .the formation of the bond and breaking occur in asingle transition state i.e. it is called a concerted reaction