Organic Chemistry-Help Designing A Synthesis Reaction

Starting with 4-bromo-1-butanol, create a synthesis for 5-methyl-1,5-nonanediol using any reagents.

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Looking at the product, which has the structure HO(CH2)4CCH3OH(CH2)3CH3, we recognize that there is a tertiary alcohol moeity at carbon #5. Recall that these can be synthesized by the nucleophilic addition of a Grignard reagent to a ketone followed by aqueous workup, according to the following general scheme:

Preparation of Grignard: RBr + Mg (in ether) ==> RMgBr
Preparation of tertiary alcohol:
RMgBr + R'COR'' ==> RR'R''COMgBr ==dilute H3O+==> RR'R''COH + MgBrOH

Now, the hydrocarbyl groups R, R', and R'' can be all the same or they can be different, such that there can be up to three different possibilities for the reactants of this particular step. Note, however, that if we disconnect between carbons #4 and #5 of the parent nonane chain in our desired product (numbering starts at the end with the OH group), we arrive at 2-hexanone, CH3CO(CH2)4CH3, as the ketone and 4-bromo-1-butanol, HO(CH2)4Br as the precursor to the Grignard and also our required starting material.

2-hexanone is a relatively inexpensive, commercially available reagent, so this poses no problem. However, we would encounter a problem if we tried to prepare a Grignard directly from 4-bromo-1-butanol. It turns out that in addition to being a nucleophile, a Grignard is also a strong base and the dominant reaction by far would be deprotonation of any acidic compounds (i.e. OH groups) in solution plus any associated side reactions:

HO(CH2)4MgBr + HO(CH2)4Br ==> HO(CH2)3CH3 + BrMgO(CH2)4Br
Possible side reaction: BrMgO(CH2)4Br ==> THF + MgBr2

The way around this problem is to convert the OH into an inert derivative using a protecting group, carry out the reaction, and then remove the protecting group later. A very popular class of OH protecting group are the trialkylsilyl groups (-OSiR3), introduced by reacting the OH with a trialkylsilyl chloride (R3SiCl) in the presence of a non-nucleophilic base:

Br(CH2)4OH ==Me3SiCl, pyridine==> Br(CH2)4OSiMe3 + pyridine*HCl

The Grignard step can now be carried out:

Me3SiO(CH2)4Br ==Mg, ether==> Me3SiO(CH2)4MgBr
==CH3CO(CH2)4CH3==> Me3SiO(CH2)4CCH3O(MgBr)(CH2)3CH3
==dilute H3O+==> Me3SiO(CH2)4CCH3OH(CH2)3CH3

Finally, we use tetrabutylammonium fluoride (known as TBAF) to remove the protecting group and yield the final product:

Me3SiO(CH2)4CCH3OH(CH2)3CH3 ==TBAF==> HO(CH2)4CCH3OH(CH2)3CH3