Mechanism Challenge - 2

ANSWER

Consider the reactant, it contains two oxygen atoms (a great target for $TiCl_{4}$). We encounter the first question : Which one does it choose?

The oxygen atom (circled in blue) is much more open to attack when compared to the other (circled in red) which is bonded to a bulky silane group. With the first question out of the way we draw the first precursor :

The next step is quite simple, Silicon has a greater affinity for chlorine than titanium, however the Oxygen atom (circled in red) stopped it from reacting with it. Now that a charged system exists in the lower half, a chloride transfer occurs. This also helps in increasing the bond strength between Titanium and Oxygen (in blue) because of Titanium's increased Lewis acidity.

Again, the structure of the second precursor makes it evident to the reader. The negative charge shifts to form a carbonyl group. During the process the allyl system shifts and attacks the weakened epoxide system in an $S_{N}2$ manner to generate the third precursor :

The next step is hydrolysis. Before we continue with the organic reaction we mustn't forget about by-products. The following side reaction occurs because $SiMe_{3}$ is much smaller than the organic substrate:

$2SiMe_{3}Cl + H_{2}O \longrightarrow SiMe_{3}OSiMe_{3} + 2HCl$

With the aqueous hydrochloric acid, the Titanium substrate is removed. The molecule has been rotated using an axis parallel to the plane of the screen:

Again, don't forget about the by-products. This side reaction occurs simultaneously with the organic reaction given below.

$TiCl_{4} + 2H_{2}O \longrightarrow TiO_{2} + 4HCl$

Now protonation of the carbonyl group occurs, this increases the positive charge on the carbonyl system's carbon atom allowing the negatively charged Nitrogen atom in the azide system to attack it. One might assume that water would have already attacked the azide system to generate a substituted hydroxyl-amine, but this doesn't happen. The azide group can be considered as a bar of electrons that are inaccessible to many substrates, the protonation of the carbonyl system activates the electrophile allowing the azide system to attack.

A keen reader would now realise that the next step is the final one. The "carbonyl" oxygen atom pushes it's electrons back into the system to shift the bridging bond to the nitrogen atom. To complete its octet it expels thermodynamically stable nitrogen gas.

When redrawn we see that the stereochemistry has been accounted for in the mechanism.

Bro! @Vitthal Yellambalse we are confined with syllabus of JOE ADVANCED , so we don't seem to know every reagent and mechanisms but we would like to try ....

You can help us by mentioning some of the different names of the reactions and cyclisations that we may learn 1st ( If we don't know and then apply!) That are part of this mechanism .