Some important synthetic reactions Regioselectivity in enol alkylations: Hydrogen atoms attached to carbon atoms in the α-position of carbonyl groups or ester groups are fairly acidic compared to ordinary alkyl hydrogen atoms. (Other groups such as nitro and cyano groups also produce such activation). Such –CH2- groups in the α-position of the activating groups are called active methylene groups. By treating with bases such as metal alkoxides in anhydrous alcohol, they form enolate anions which are stabilized by resonance:
CH3 CH2 C
O
CH3
C2H5O-
C2H5OHCH3 C
O
CH3CH CH3 CH C
O
CH3
Now if an alkyl cation is introduced (using an alkyl halide, for example), alkylation can take place at the position of the negative charge. In the above example, there are two α-carbon atoms which can take part in the enolate ion formation. Thus the negative charge may reside on any of these C-atoms or on the oxygen atom, and therefore alkylation may take place in any of these three positions.
CH3 C
O
CH3CHC4H9I
- I-CH3 C CH3CH
C4H9
O
1,3-diketones, diesters or β-keto esters substantially increase the acidity of the methylene group between the two keto groups and is of great application in organic synthesis. Malonic ester and acetoacetic ester are examples. Alkylation takes place specifically at this (doubly activated) carbon atom. On hydrolysis and heating, malonic acid derivatives decarboxylate to give the monocarboxylic acid, and the acetoacetic ester derivatives give corresponding hydrocarbons. One difficulty (or application!) of this reaction is that both C-alkylated and O-alkylated products may be formed. The yield of C-alkylated and O-alkylated products is found to depend on the acidity of the active methylene group, the solvent used and the alkylating agent. If the negative charge is stabilized on the more electronegative O-atom, O-alkylation takes place; otherwise C-alkylation takes place. For example:
Conditions favourable for C-alkylation Conditions favourable for O-alkylation
When the acidity of the active methylene group is low, such as when only one carbonyl group is present. In such cases, the concentration of the keto-tautomer is fairly high. C-alkylation is favoured by the use of non-polar solvents like benzene or carbon tetrachloride. In such cases, the solid enolate will not dissolve and the enolate anion is shielded by the metal cation in the crystal lattice, so that the alkylating agent attacks the C-atom. C-alkylation is favoured when the enolate oxygen is involved in intramolecular H-bonding. Thallium enolates of 1,3-diketones, in which both oxygen atoms are coordinated to the thallium atom, give almost 100% C-alkylation in heterogeneous system. Larger and more polarisable leaving groups on the alkylating agents favour C-alkylation. The order R–I > R–Br > R–Cl > R–OSO2OR is observed. Large primary alkyl halides, and unsaturated allyl and benzyl halides favour C-alkylation. Alkylation using p-nitro benzyl chloride gives a very high yield of C-alkylated product through a fast electron transfer mechanism.
When the acidity of the active methylene group is high, such as in phenols, β-keto esters, 1,3-dicarbonyl compounds. In such cases, the concentration of the enolate anion is fairly high. O-alkylation is favoured by the use of polar solvents like alcohol, water etc. in which the enolate ion is stabilized. The reaction is especially favoured by the polar aprotic solvent hexamethyl phosphoramide. O-alkylation is favoured by the use of alkoxides with large cations which are stabilized and have a tendency to dissociate from the anion. R4N+ > K+ > Na+ > Li+ Smaller leaving groups favour O-alkylation. This is explained by the hard base character of the smaller O-atom favoring a similar small leaving group. Saturated and small alkyl halides like CH3Cl and secondary alkyl halides favour O-alkylation.
Alkylating agents commonly used are the alkyl halides. Primary and secondary alkyl halides as well as allylic and benzylic halides can be used successfully. The mechanism involved is SN2. When there are two H-atoms on the active methylene group, dialkylation takes place if excess of alkylating agent is used. Use of tertiary alkyl halides as alkylating agents are not successful because alkene elimination predominates.
A widely used procedure for the selective alkylation of an aldehyde or ketone involves the initial reaction of the carbonyl compound with a secondary amine to form an intermediate enamine. Typical examples are given below:
O
+ (CH3)2NHCaCl2
ether
N(CH3)2
(52%)
(CH3)2CH-CO-CH(CH3)2 + (CH3)2NHTiCl4
benzene, 25oC(CH3)2C=C-CH(CH3)2
N(CH3)2
|
(72%)
NH
N
N
(CH3)2CH-CH2-CHO +K2CO3
(CH3)2CH-CH2-CH-|
Ndistill
(CH3)2CH-CH=CH-
(74%) The resonance structure of the enamines indicate that the C-atom β to the N bears considerable negative charge, making it susceptible to attack by electrophiles. Just as in the case of enolates, this makes the hydrogen atom on the carbon atom α- to the carbonyl group acidic and susceptible to alkylation using alkyl halides.
(Since the enamine group can be removed and the product converted back to the carbonyl compound by refluxing with dilute acids, it is also used as a protecting group for carbonyl compounds in organic synthesis.) In the alkylation of an unsymmetrical ketone, the product of reaction at the less substituted α-carbon is formed predominantly. For example,
O
NH
N N+p-CH3C6H4SO3H
Benzene+
(85% of the product)
(15% of the product)
Doering explains this to be due to the reduced interaction between the lone pair on the N-atom and the π-electrons in the more substituted isomer due to steric interaction between the methyl group and the methylene group α to the nitrogen. (For effective formation of the enamine, the p-orbitals of the double-bond must overlap with the p-orbital containing the lone pair on the nitrogen). But the success rate of direct C-alkylation is not as good as that in enolates since the N-atom is more polarizable and considerable amount of N-alkylated products are also formed. But the N-alkylated products very often convert to the C-alkylated product by the intramolecular or intermolecular transfer of the alkyl group from N to the C-atom. This fact makes it synthetically useful for C-alkylation.
A modification of the original procedure, in which a primary amine is treated with the enolisable aldehyde or ketone to form an imine, is found to increase the yield of C-alkylated product very much. The imine is deprotonated on treatment with Grignard reagent or lithium diisopropyl amide. The metal salts on treatment with alkyl halides give good yields of the C-monoalkylated product.
CHO+ t-C4H9-NH2
benzene
reflux CH=N-C4H9-t
CH-N-C4H9-t MgBr_ +
C2H5MgBr, THFreflux
C6H5CH2Cl
H3O+CHO
C6H5(80%) With imines derived from methyl ketones, alkylation takes place selectively at the methyl group at low temperature.
O NC4H9O
LiN(iso-C3H7)2
DME, -60oCCH3I, H3O+
Alkylations similar to those discussed above have also been carried out using hydrazones and oximes of carbonyl compounds also with high region and stereoselectivity. The Mitsunobu Reaction [Ref: http://users.ox.ac.uk/~mwalter/tutorial_web/year2/phsu/phsuindex.shtml] It is common when carrying out substitution reactions upon alcohols to first convert the hydroxyl group into a better leaving group (i.e. Cl, Br, I, OTs, OMs) and then react with a suitable nucleophilic species. This approach requires two distinct stages. However, a very mild phosphorous mediated reaction – the Mitsunobu Reaction – enables the direct ‘one-pot’ conversion of an alcohol into the corresponding ether or ester. In this reaction, triphenylphosphine and diethylazo dicarboxylate (DEAD) are combined with the alcohol which serves as the electrophile and to this mixture added the nucleophile. The mechanism first involves addition of the phosphine to the rather weak N=N p-bond to afford an anion that is stabilised by one of the ester units. The nitrogen nucleophile thus generated then abstracts the proton from the alcohol’s hydroxyl group to generate an alkoxide. As observed in the formation of chlorides from the corresponding alcohols using triphenylphosphine-CCl4, the formation of strong P–O bonds is also employed in this reaction.
Therefore, the alkoxide thus formed attacks the positively charged phosphorous centre and in a concerted manner displaces a second nitrogen anion that is stabilised by one of the ester units. An alkoxyphosphonium cation is generated. This is an SN2 reaction at the phosphorous centre. The stabilised nitrogen anion acts as a base upon the nucleophile, abstracting a proton and therefore activating it for subsequent nucleophilic attack in a substitution reaction at the alcohol derivatives a-carbon centre. Phosphine oxide and the reduced form of DEAD are the by-products of this reaction. Overall, the stable C-O bond is broken, however, this energetically unfavourable process is offset by formation of very strong P=O and N-H bonds. Since the original C-O bond is broken in the Mitsunobu reaction via an SN2 mechanism, inversion of configuration at that carbon centre occurs and indeed, this route has been employed to generate either ethers or esters with inversion of configuration.
The Mitsunobu reaction is an extremely versatile reaction, as well as mediating the efficient conversion of alcohols into ethers and esters, it can be utilised to afford alkyl halides and is a common approach to highly reactive iodides.
1,3-dipolar cycloaddition in the construction of rings: This is a reaction which is similar to the Diels-Alder reaction, but is used for the construction of five-membered heterocyclic ring systems. Instead of the 1,3-diene in DA reaction, a 1,3-dipole which is a three-atom system in which at least one is a hetero atom and having a zwitterionic structure is used. Altogether, there will be four π-electrons in three parallel p-orbitals in this system. A few examples of such commonly used 1,3-dipoles are given below:
HC≡N+–CH2‾ HC≡N+–NH‾ HC≡N+–O‾
nitrile ylides nitrile imines nitrile oxides
H2C=N+=N‾ HN=N+=N‾ diazo alkanes azides
The 2 π-electron ‘ene’ component (which is usually known as the dienophile in DA reaction) is called the ‘dipolarophile’ in this reaction. It is a compound containing a double bond or a triple bond. A five-membered ring system is formed through a 6 π-electron pericyclic reaction.
a+b
c- ab+
c-
d e
ab
c
d e
A typical example is the well known reaction between diazomethane (the 1,3-dipole) and an α,β-unsaturated ester (the dipolarophile) to form a pyrazoline derivative.
CH2 N+ N-
CH2 C COOCH3
CH3
H2C
H2C
N
N
C COOCH3
CH3
diazomethane
methyl methacrylate 1,3-dipolar cycloadditions provide a versatile route to a wide variety of five-membered heterocyclic compounds. In most reactions the 1,3-dipole is not isolated but generated in situ in presence of the dipolarophile (remember diazotization and coupling of aniline to form an azo dye). In most cases the dipolarophile is an alkene or alkyne, but other compounds with heteroatom multiple bonds like imines, nitriles and carbonyl compounds have also been used.
Nitrile oxides can be conveniently prepared in situ by treating α-chloro oximes with bases, or by the dehydration of primary nitro compounds using phenyl isocyanate. They can then be used for further 1,3-dipolar cycloadditions.
NO
NO
CH3CH2NO2
C6H5NCO
benzeneCH3 C N+ O- CH3
CH3
CH3
CH3CH3
CC6H5
Cl
NOH(C2H5)3N
ether, 20oCC N+ O-
C6H5C CH
C6H5
C6H5
C6H5
The products formed in the above reactions are valuable synthetic intermediates. On catalytic hydrogenation they give 1,3-amino alcohols and on hydrolytic reduction they give β-hydroxy ketones. The β-hydroxy ketones may be dehydrated to get α,β-unsaturated ketones.
NO
CH3
CH3
CH3
H2
Ni
NH OH
CH3 CH3
CH3
NH2 OH
CH3 CH3
CH3
H2
H2OO OH
CH3 CH3
CH3
O
CH3
CH3
CH3
Like the DA reaction, 1,3-dipolar cycloadditions are both regio- and stereo-selective. Although reaction of an unsymmetrical 1,3-dipole with an unsymmetrical dipolarophile could apparently give two products, in general, one is formed predominantly or even exclusively. In reactions with geometrically isomeric alkenes, the ‘cis rule’ holds (cis isomer gives cis product and trans isomer gives trans product; the steric disposition of the substituents about the double bond is retained in the adduct). It appears that in the addition of nitrones and nitrile oxides, the oxygen attacks the more highly substituted carbon atom (see starting material shown in the reaction scheme given above). Like the DA reaction, [3+2] cycloadditions are reversible (cf. the retro DA reaction). It is now generally agreed that, like the DA reaction, 1,3-dipolar cycloadditions are also concerted 6 π-electron reactions and take place through a five-membered cyclic transition state in which the two components approach each other in parallel planes.
Latent polarity of carbon atoms in a compound: Whenever carbon atoms are attached to hetero atoms in functional groups, a polarity develops, usually with the negative charge on the hetero atom and the positive charge on the carbon atom. The effect of this polarity propagates through the carbon chain and affects any further reaction which the compound may undergo. This imaginary pattern of alternating positive and negative charges on each C-atom is known as the latent polarity, and the information is very handy in selecting reagents for producing new C-C bonds.
If conjugated double bonds are present, the polarity is really transmitted along the chain through the mobile π-electrons. It must be noted that nucleophylic reagents will attack the C-atoms with a latent positive charge while electrophylic reagents are required to attack the C-atoms with a latent negative charge. For example, to introduce a methyl group at the carbonyl carbon, a nucleophylic reagent like CH3MgBr is used, but to introduce a methyl group at the α-carbon, an electrophylic reagent like CH3Br is used.
CH3MgBr ≡ ‾CH3 +MgBr CH3Br ≡ +CH3 Br‾
nucleophylic alkyl electrophylic alkyl If two functional groups are present which reinforce the latent polarities, they are said to be consonant. If the two functional groups disagree in the latent polarities produced by them, they are said to be dissonant. We very commonly come across β-keto esters, β-dicarbonyl compounds, α,β-unsaturated carbonyl compounds etc. in synthetic chemistry because they are all consonant systems. When dissonant groups are present, we cannot find suitable disconnections and synthons based simply on the natural polarity of the carbonyl group or its derivatives.
O O O
O-
Ph Ph+ +
+
-Ph Ph+
+ +-
-
consonant groups dissonant groups Umpolung To synthesise such dissonant carbonyl groups, we will therefore require synthons equivalent to a carbonyl function (ie. which can be converted to a carbonyl group later), but with opposite polarity. The German word umpolung is used to describe situations of this sort in which a system of opposite polarity to that normally associated with the required functional group must be used. A great deal of research has been done to develop such synthons. A common example of the application of such a reagent is seen in the benzoin condensation in which the equivalent of a “benzoyl anion” is used. (Note that in the ordinary benzoylation such as the Schotten-Baumann reaction, the benzoyl cation attacks the –OH or –NH2 group and not the C-atom.) The benzoyl anion (umpolung) is produced with the help of the cyanide ion.
In the above example, the negative charge on the C-atom is stabilized by resonance between the O-atom and the –CN group. Another example is provided by thioketals (or 1,3-dithianes) where the negative charge on C-2 is stabilized by the two electronegative sulfur atoms. Note that on hydrolysis using aqueous mercuric ions, 1,3-dithianes liberate the keto compound and is therefore the synthetic equivalent to a carbonyl with negative charge on the C-atom, as shown in the following synthesis of benzyl phenyl ketone.
SH
SH
S
S
S
S
S
S
+ OHC-C6H5
H+
C6H5
H C4H9Li
C6H5
Li C6H5-C-O+
C6H5CH2Br
C6H5
H2C-C6H5H2O / Hg2+
C6H5CH2-CO-C6H5
umpolung
benzyl phenyl ketone Aliphatic nitro compounds can also generate umpolungs which are synthetic equivalents of amines, as is shown in the next example which is a synthesis of 1-methyl-2-phenyl ethanamine.
Li
NO2
2 C4H9Li
THFN+
O-
O-Li+ CH-NH2
umpolung
1) C6H5CH2Br
2) H2O
NO2
CH2C6H5reduction
NH2
CH2C6H5
Yet another example for umpolung generation is by the use of epoxides, as shown in the following example.
A further example is provided by the use of α-haloketones or esters. Perhaps the simplest solution to the problem of mismatched latent polarities is to use reagents with strategically placed heteroatoms. The leaving group ability of bromide overrides the inherent latent polarity imposed by the carbonyl group.
O
OO
O
BrO
BrO
+
reagent synthon
+NaOH
Compounds with opposite charges on adjacent atoms are called ylides. These are also used to generate umpolungs. For example: phosphorus ylides and sulfur ylides.
C = PR3
R1R2
C - +PR3
R1R2
phosphorane form ylide form
(CH3)2S+ - CH2
a sulfur ylide
Electrochemical reduction and oxidation reactions.
Cathodic reduction of organic halogen, nitro and carbonyl compounds. Reductive coupling reactions. Electrolytic reduction of functional groups have been tried using different cathodes like mercury and lead, in which the organic compounds are ‘electronated’ at the cathode metal surface. The results are often similar to that of metal-ammonia reductions. Stereochemical information shows that generally the unhindered ketones yield the more stable isomer of the alcohol, while strained or sterically hindered ketones are reduced to mixtures of alcohols in which the less stable isomer may predominate.
The reductions of aliphatic ketones to alcohols electrolytically are believed to follow the following mechanism.
CH3
H
OCH3
H
OCH3
H
OH
CH3
H
OHCH3
H
OH
H
e- / cathode.
.
e- / cathode
H+
H+
.
..
The above mechanism would indicate that protonation of the radical ion has to be sufficiently rapid for the formation of the product. Thus highly acidic solutions have to be used in electrolytic reductions. In aprotic or neutral solutions, dimerisation of the radical ion will be a competing reaction, especially in the case of aromatic ketones in which the radical is stabilized and has a fairly long lifetime. This results in the formation of pinacols in considerable quantities. These are known as reductive coupling reactions.
CH||O
2 CH|
OH
*+ CH
|OH
*
CH|
OH
CH
OH|
pinacol Another example of reductive coupling reaction is discussed below. The reduction potentials required for α-halogenated carbonyl compounds are found to be significantly less negative than either halogenated compounds or carbonyl compounds alone. Therefore halide ion elimination easily occurs and reductive coupling of the unhalogenated members occur.
The stereochemical course of the reaction is determined by some combination of two factors. If the intermediate radical anion has a sufficient lifetime to adopt the more stable configuration before it is protonated, the product will be the more stable isomer. However, reliable information on the situation is not available at present. The alternative view that protonation of the radical anion is faster than conformational inversion appears to be more probable at present. A transformation similar to Clemmensen reduction can be accomplished electrolytically if a lead cathode is used. But if a mercury cathode is used, reductive dimerisation occurs.
O
Pb cathode
H2SO4, H2O
97% Aliphatic nitro compounds are also readily reduced to radical anions electrochemically. In the presence of a proton source, N-alkyl hydroxyl amines or amines are obtained based on the fate of the intermediate nitroso compound (see figure). If a primary or secondary nitrosoalkane is formed, isomerisation to an oxime is possible. In acid media and elevated temperatures, the indicated elimination becomes possible to produce an imine, which is then reduced to an amine. In alkaline media and at low temperatures, reduction to the hydroxylamine is favoured. Alkyl hydroxylamines are usually stable to further reduction.
(CH3)3C--NO2
Hg cathodeHCl
(CH3)3C--NO-
O-+
. H+
(CH3)3C--NOH
O-+
.
e-
(CH3)3C--NOH
O-
..(CH3)3C--N=O
2e-
2H+(CH3)3C--NH--OH
N-alkyl hydroxylamine80-90%
tertiary nitrocompound
But if a primary nitro compound is used,
C6H5--CH2--NO2
N-alkyl hydroxylamineprimary nitrocompound
C6H5--CH2--NH--OH e-C6H5--CH2--NH--OH
H+ H2O
C6H5--CH2--NHH+
C6H5--CH2--NH2
primary amine In the case of halogenated compounds, elimination of the halogen atom with or without dimerisation may occur on electrolytic reduction at the cathode metal surface. The mechanism may be as given below. The formation of an intermediate free-radical accounts for the lack of stereospecificity in these reductions and the partial or complete racemisation has been observed in various C-X bond reductions. But in situations that retard isomerisation of the free-radical intermediate, optically active products are obtained.
A few examples of such reductions are given below:
Br COCH2CH2CH2Cl COCH2CH2CH2ClHg cathode
96% In the above example, the bromide ion is always a better leaving group than chloride. Further, the radical ion is stabilized by resonance in the benzene ring and the carbonyl function which is conjugated to it. Hence the bromine is eliminated in preference to chlorine.
C6H5C6H5
CH3 Br
C6H5C6H5
CH3 HHg cathode
optically active In the second example, the two phenyl groups and the tight cyclopropane ring system prevents ring flipping and isomerisation of the free radical. Therefore the stereochemistry of the molecule is retained.
CH3 CH2Br CH3 CH2
Hg cathode
2
Hg
64% The above is an example where reductive elimination is followed by dimerisation.
C2H5 C2H5
H I
C2H5 C2H5
H H
C2H5 H
H C2H5
Hg cathode+
cis, 30% trans, 70%
Conversion of an alkene to a trans-diol – the Prevost reaction The addition of acyl hypoiodites to olefins is thought to be an ionic reaction in which the reagent attacks the olefin to form an iodonium ion that is in turn attacked by the carboxylate ion, resulting in overall trans addition.
If two molar equivalents of the silver salt and only one equivalent of iodine is used in the above reaction, an intermediate complex is produced (known as Simonini complex). If the resulting mixture is heated in an anhydrous inert solvent like anhydrous benzene, a trans diol ester is obtained which can be saponified to obtain a trans diol (see next page). This sequence of reactions is known as the Prevost procedure. In short, treatment of a (cyclic) alkene with two equivalents of a silver salt of carboxylic acid and one equivalent of iodine, heating in an anhydrous inert solvent followed by hydrolysis gives the trans diol. This is the Prevost procedure.
CH3
I
CH3
I
O
O
C6H5
CH3O
O
C6H5
CH3CH3
OH
OH
C6H5COO
C6H5COOAg
C6H5COOAgC6H5COO-
+
C6H5COO
Simonini complex
C6H5COO
hydrolysis
trans diol trans diester Conversion of an alkene to a cis-diol – the Woodward hydroxylation reaction A useful modification of the Prevost reaction, called the Woodward hydroxylation procedure, utilizes an olefin with silver acetate and iodine in moist acetic acid. In the second stage of this reaction, an oxonium intermediate is produced in the presence of water. This undergoes hydrolysis (rather than a second neucleophylic displacement) to form a hydroxyl acetate. Subsequent saponification leads to a diol, whose stereochemistry corresponds to cis addition of the two hydroxyl groups from the more hindered side of the doublebond. The direction of this cis hydroxylation is of particular interest since it is opposite to that using osmium tetroxide or potassium permanganate which attack from the less hindered side.
CH3
I
CH3
I
O
O
CH3
CH3O
O
CH3
CH3
OH
OH CH3O
OH
O
CH3
CH3OCOCH3
OH
CH3COO
CH3COOH+
hydrolysis
cis diolon more hindered side
H2O
H+
H2O H
OH|
-H++H
In short, treatment of a (cyclic) alkene with two equivalents of a silver salt of carboxylic acid and one equivalent of iodine, heating in a moist acidic solvent followed by hydrolysis gives the cis diol from the more hindered side. This is the Woodward procedure.
Conversion of the carbonyl oxygen to a methylene - the Wittig reaction The reaction between a phosphorane or phosphonium ylide and a carbonyl compound to give an alkene and a phosphine oxide is known as Wittig reaction. The reaction is easy to carry out and proceeds under mild conditions. The phosphonium ylides can be prepared from alkyl halides and triphenyl phosphine.
R1
R2
R1
R2
R1
R2CH-X + (C6H5)3P (C6H5)3P+ CH
X
C6H5Li(C6H5)3P=C
where R1 and R2 may be alkyl groups or hydrogen. Phosphoranes are resonance stabilized structures in which there is some overlap between carbon p-orbitals and phosphorus d-orbitals.
R1
R2C=O + (C6H5)3P=OCH2=P(C6H5)3
R1
R2C=CH2 +
The reactions with carbonyl compounds are carried out in ether medium. Unlike other methods of producing alkenes, Wittig reaction gives alkenes in which the position of the double bond is unambiguous. Reaction with carbonyl compound takes place by the attack of the carbanionoid carbon of the ylide on the electrophylic carbon of the carbonyl group, which gets converted into the alkene and phosphine oxide through a four-membered cyclic intermediate. The driving force is provided by the formation of very strong P-O bonds. R1
R2C=O + (C6H5)3P=OCH2=P(C6H5)3
R1
R2C=CH2 ++ + R1
R2C----O
CH2--P(C6H5)3
+
De-oxygenation of epoxides to alkenes This can be effected in a number of ways. Two of these involve selenium reagents. Reaction of the epoxide with triphenylphosphine selenide [(C6H5)3P=Se] and trifluoroacetic acid or with potassium selenocyanate (KSeCN) gives the corresponding alkene with retention of configuration of substituent groups on the epoxide. Both conversions are believed to proceed by extrusion of selenium from the derived episelenide.
OH C6H5
HC6H5
O
C6H5
HC6H5
SeCNH
SeO
C6H5 C6H5
H H
N
HC6H5
C6H5
HSeC6H5 H
H C6H5
Se
HC6H5
H C6H5
KSeCN
methanol / water60oC
NCO
100%
Olefin metathesis [Ref: Premamoy Ghosh. Polymer Science and Technology – Plastics, Rubbers, Blends and Composites, 2nd edn. Tata McGraw-Hill, New Delhi. (2002)] Olefin metathesis refers to reactions in which the atom groups attached to double bonds are exchanged between molecules, as indicated in the following examples.
The metatheses reactions do not take place spontaneously. They are catalysed by transition metal complexes, especially carbene complexes of molybdenum and ruthenium. Chemistry professors Robert H. Grubbs of California Institute of Technology (CIT) and Richard R. Schrock at Massachusetts Institute of Technology (MIT) did pioneering work in developing these catalysts, which are therefore known as Grubbs and Schrock catalysts. The mechanism involves initiation and propagation steps similar to photochemical reactions. This is illustrated below. The reaction being considered is:
R - CH = CH2
R - CH = CH2
(R1)2 C=M R - CH
R - CH+
CH2
CH2 This reaction may be envisaged as taking place in the following manner. First the metal catalyst reacts with the olefin to produce a reactive metal-carbene complex of the olefin.
R - CH = CH2 R - CH+
CH2
C(R1 )2M = C(R1 )2 Mcatalyst metal-carbene
complex intermediate The complex intermediate then reacts with the olefin to form further reactive forms of the catalyst.
The above steps are then repeated producing more of the product and regenerating the metal catalyst.
R - CH = CH2 +R - CH
product
M = CH2
CH2
M CH2
Olefin metathesis reactions are being used for the synthesis of block co-polymers with very special properties. Since propene is a chemical of great industrial demand, petroleum industries convert ethane and 2-butene into propene through metathesis reactions.
A 1.5-inch-thick rod of polydicyclopentadiene resin prepared with ruthenium technology is impenetrable to 9-mm bullets.
