Substitution of E-Halostyrenes by MeS
– 257
discard the concerted mechanism, since the electron-donating group in the aromatic ring makes the styryl C=C double bond less electrophilic, and for that reason, the nucleophilic attack would be also less favored.
Therefore, the retention of the stereochemistry and the lower reactivity of the pmethoxy derivative compared with the unsubstituted substrate are not able to distinguish between a concerted and a stepwise mechanism. Only the element effect
unequivocally points to an addition-elimination process in which the nucleophilic
attack is rate-determining.
Based on the previous discussion, the most likely mechanism for the vinylic substitution of E- and Z-bromostyrenes Z Z
1 can be represented as follows (Scheme
38.9).
H
Br
Ph
H
MeS
MeS
H
H
Ph
Br
H
Br
Ph
H
SMe
H
H
Ph
Br
SMe
H
SMe
Ph
H
H
H
Ph
SMe
slow
- Br
Z-1
Z-2
slow
- Br
E-1
6
E-2
Scheme 38.9
There are two additional points that deserve further discussion. First, the experimental data indicate that the reactions are very fast in dipolar aprotic solvents
like HMPA, DMSO or DMF, which are excellent cation solvators. The combination of the ability of the solvent in removing the cation from the media, with an
g
excellent nucleophile (in fact the MeS
– will be
–
naked in the presence of such solvents) is ideal to accelerate the process. The addition of solvents that are unable to
enhance the reactivity of the nucleophile, either protic (see for example the effect
of the addition of CD 3 OD in Table 38.1 or aprotic like CHCl 3 , will result in an appreciable decrease of the substitution rate.
The second question we should comment is the higher reactivity of the Z-iso- Z Z
mers compared with the E-isomers in all cases studied. Possibly the steric interaction between the vicinal phenyl group and bromine atom is responsible for a less
stable (higher in energy) ground state, more prone for a nucleophilic attack that
will reduce the steric strain (Scheme 38.10).
These additional points are also in full agreement with the stepwise mechanism
proposed above in Scheme 38.9.
– 257
discard the concerted mechanism, since the electron-donating group in the aromatic ring makes the styryl C=C double bond less electrophilic, and for that reason, the nucleophilic attack would be also less favored.
Therefore, the retention of the stereochemistry and the lower reactivity of the pmethoxy derivative compared with the unsubstituted substrate are not able to distinguish between a concerted and a stepwise mechanism. Only the element effect
unequivocally points to an addition-elimination process in which the nucleophilic
attack is rate-determining.
Based on the previous discussion, the most likely mechanism for the vinylic substitution of E- and Z-bromostyrenes Z Z
1 can be represented as follows (Scheme
38.9).
H
Br
Ph
H
MeS
MeS
H
H
Ph
Br
H
Br
Ph
H
SMe
H
H
Ph
Br
SMe
H
SMe
Ph
H
H
H
Ph
SMe
slow
- Br
Z-1
Z-2
slow
- Br
E-1
6
E-2
Scheme 38.9
There are two additional points that deserve further discussion. First, the experimental data indicate that the reactions are very fast in dipolar aprotic solvents
like HMPA, DMSO or DMF, which are excellent cation solvators. The combination of the ability of the solvent in removing the cation from the media, with an
g
excellent nucleophile (in fact the MeS
– will be
–
naked in the presence of such solvents) is ideal to accelerate the process. The addition of solvents that are unable to
enhance the reactivity of the nucleophile, either protic (see for example the effect
of the addition of CD 3 OD in Table 38.1 or aprotic like CHCl 3 , will result in an appreciable decrease of the substitution rate.
The second question we should comment is the higher reactivity of the Z-iso- Z Z
mers compared with the E-isomers in all cases studied. Possibly the steric interaction between the vicinal phenyl group and bromine atom is responsible for a less
stable (higher in energy) ground state, more prone for a nucleophilic attack that
will reduce the steric strain (Scheme 38.10).
These additional points are also in full agreement with the stepwise mechanism
proposed above in Scheme 38.9.