In 2016, the Nobel Prize in Chemistry was jointly awarded to J.-P. Sauvage, J. F. Stoddart and B. L. — Organic Chemistry Chemistry Question
Synthesis and study of a molecular motor
In 2016, the Nobel Prize in Chemistry was jointly awarded to J.-P. Sauvage, J. F. Stoddart and B. L. Feringa for the "design and synthesis of molecular machines". Within this area, the synthesis of a new type of molecular motor which allows the conversion of chemical energy to mechanical energy is examined. The synthetic sequence leading to tris(indazolyl)borate ligand Q will then be detailed and the redox properties of the whole molecular motor will finally be addressed.
Draw the structure of the first reaction intermediate resulting from the action of KOH .
Model Answer
The first reaction intermediate is the enolate species (or any of the resonance structures of this enolate species).
[Is the base regenerated after each aldol condensation sequence?]
Model Answer
Yes: the base is regenerated after each aldol condensation sequence. However, an excess of KOH is usually used in such reactions.
[Suggest a reagent to perform the next step of the synthesis of the intermediate F.]
Model Answer
The addition of an organolithium reagent or a Grignard reagent, such as p-methylphenylmagnesium bromide or p-methylphenyllithium.
[Determine the mechanism of the nucleophilic substitution step in the synthesis.]
Model Answer
Unimolecular nucleophilic substitution SN1.
[Draw the structure of intermediate F.]
Model Answer
The structure of F is as shown in the solution key.
[Draw the structure of the next intermediate in the synthesis leading to the molecular motor.]
Model Answer
The structure of the intermediate is as shown in the solution key.
A metallic cluster is a structure involving at least three metal atoms linked via metal-metal bonds. Give the oxidation state of the ruthenium atoms in the cluster Ru3(CO)12.
Model Answer
Ru0 (the oxidation state is 0).
Write the electronic configuration of ruthenium in the free ion corresponding to G.
Model Answer
Ru2+: (1s)2(2s)2(2p)6(3s)2(3p)6(4s)2(3d)10(4p)6(5s)0(4d)6, which can be abbreviated as [Kr](5s)0(4d)6.
Write a balanced equation for the formation of complex G starting from F and Ru3(CO)12.
Model Answer
3 F + Ru3(CO)12 → 3 G + 6 CO (evolution of carbon monoxide gas is observed).
Draw the compound which is formed as an intermediate in the first step (H → J).
Model Answer
The intermediate is a carboxylic acid chloride (with -COCl group replacing the -COOH group on the indazole ring).
Draw the Lewis structure of the 3-methylbutylnitrite reagent involved in the second step (J → K).
Model Answer
The Lewis structure of 3-methylbutylnitrite is drawn with all lone pairs on nitrogen and oxygen atoms.
Select appropriate experimental conditions for the third step (K → M) from the following list:
[ ] NaBH4 in ethanol/water (vol. 50/50)
[ ] LiAlH4 in diethyl ether
Model Answer
LiAlH4 in diethyl ether (since the reduction of the ester group to alcohol requires a stronger reducing agent than NaBH4).
In the fourth step (M → N), the hydroxy group is converted into a better leaving group in view of the nucleophilic substitution step (N → P). Suggest a reagent to treat alcohol M.
Model Answer
Alcohol M is treated with HBr in acetic acid to convert the hydroxy group into a bromide leaving group.
[Draw the structure of intermediate N or P.]
Model Answer
The structure of the intermediate is as shown in the solution key.
What is the role of DBU in this reaction? Choose the appropriate answer from the following list:
[ ] nucleophile
[ ] electrophile
[ ] Bronsted base
[ ] Bronsted acid
[ ] oxidizing agent
[ ] reducing agent
Model Answer
Bronsted base (DBU acts as a non-nucleophilic sterically hindered base to deprotonate the indazole).
Write a balanced equation of the reaction accounting for the formation of potassium tris(indazolyl)borate ligand Q starting from P and KBH4.
Model Answer
3 P + KBH4 → Q + 3 H2 (evolution of dihydrogen gas is observed).
Give the oxidation state of the iron center in each of the ferrocene groups.
Model Answer
FeII (iron is in the +2 oxidation state).
The four iron centers should be oxidized selectively. Order the standard potential of an appropriate oxidant (named "Ox") regarding to the ones of the ruthenium and iron ions.
Model Answer
E(Fe3+/Fe2+) < E(Ox/Red) < E(Ru3+/Ru2+)
Fill in the scheme given below with the oxidation state of each metallic center and the charge of the complex after selective oxidation of the four iron centers. [VISUAL]
Model Answer
The metallic centers after selective oxidation are four Fe(III) atoms and one Ru(II) central atom, which gives the complex an overall charge of 4+.