Question 1 - start each question on a new page.

(a)         The synthesis of disaccharide 4 from protected mannosyl donor 1 and glucose derivative 2 is

shown below.

Explain why it is challenging to form the glycosidic bond in 3 with the given          (2)

stereochemistry.

The synthesis of 3 proceeds via tethered intermediate A, which is formed from       (6)

the reaction of 1 and 2 in the presence of DDQ.

●         Give the structure of A.

●         Draw a curly arrow mechanism for the transformation of A into 3.

Note: you may abbreviate structures, provided you define your abbreviations.

Outline a strategy for the multistep conversion of 3 into 4, including the                  (4)

required reagents and a brief description for each of the steps.

(b)         Partially protected dipeptide 7 can be synthesised by solution phase peptide synthesis from

appropriately protected amino acids 5 and 6, as outlined below.  In this synthesis, BOP is the coupling reagent and R1 – R4 are protecting groups.

Draw a curly arrow mechanism for the reaction between 5 and 6 in the presence     (4)

of BOP and a base.

Identify suitable protecting groups R1, R2, R3 and R4, and briefly explain why         (4)

you have chosen each of these groups.

Question 2 - start each question on a new page.

(a)         Briefly outline the information about a reaction mechanism that can be gained

from its entropy of activation, considering reactions both in the gas phase and in solution.

(b)         Draw the lowest energy conformation for each of the compounds, 1-3, shown below.

(c)         Mechanisms (1)-(3), shown below, have been suggested for the acid-catalysed hydrolysis of compound 4.

Give the names for mechanisms (1) and (3) and describe an experiment which could help distinguish between them.

Describe an isotope labelling experiment which could help distinguish      between mechanisms (1) and (2). Your answer should give the structure of the labelled compound, suggest an analytical method which could be used to determine the distribution ofthe label in the products, and interpret the     results in terms ofreaction mechanism.

(d)    Substituted (X) benzophenones react with MeLi in diethyl ether to produce the      corresponding alcohols. The correlation ofkinetic data for this reaction with Hammett Q bYLYme!eL2 8Y^e b = 0.o寸.

An electrochemical cell has been designed to permit the use of nuclear magnetic resonance (NMR)       spectroscopy to study the in-situ conversion of ascorbic acid, 1, into dehydroascorbic acid, 2.  The cell  comprises a platinum wire working electrode, platinum wire counter electrode and silver wire reference electrode in a 0.2 mol dm−3 H2 SO4 in D2O electrolyte solution.

Redraw the structures of 1 and 2 and, on both structures, indicate the oxidation state               (2)

changes that accompany the conversion of 1 to 2.  Give the values of m and n.

Figure 1 compares 100 mV s−1 cyclic voltammograms measured in the electrochemical cell    outside an NMR instrument (in the absence of an applied magnetic field) and in-situ inside an NMR instrument (in the presence of a 14 T magnetic field).

(i)          Calculate the time taken to complete one cyclic voltammetry sweep.                          (2)

(ii)         Giving your reasons, deduce if the conversion of 1 to 2 is reversible or                      (2)

irreversible.

(iii)        Predict how the cyclic voltammetry of 1 measured outside an NMR instrument         (3)

would change upon increasing the concentration of H2 SO4 to 2.0 mol dm−3 . State any assumptions that you make.

(iv)        It is postulated that the application of a magnetic field induces stirring in the             (3)

electrochemical cell.  Explain, with justification, whether the cyclic voltammetry data shown in Figure 1 supports this hypothesis.

Figure 2 shows chronoamperometry measured when the same electrochemical cell was used and a potential of 1.2 V vs Ag wire was applied to the working electrode in a        solution of 0.1 mol dm−3 of 1 in electrolyte solution.

Figure 2

Calculate the Faradaic efficiency of the reaction for a cell volume of 400 μL and a final concentration of 21 mmol dm−3 of 2.

Figure 3 shows a Pourbaix diagram for iron, with the water half reactions indicated as dotted lines.

(4)

Figure 3

Based on the diagram, explain why an iron wire cannot be used to replace platinum as           (4)

the working electrode in the electrochemical cell.  You can assume that

E (V vs SHE) = E (V vs Ag wire) + 0.15 V.

Question 4 - start each question on a new page.

(a)          Some reactivity associated with zirconium alkene complexes is described below.

(i)          Give the overall electron count and oxidation state for each of complexes 1           (2) and 2 shown below.

(ii)         For   the   multistep   reaction   shown   above   there   are   three   possible

intermediates that are formed on the pathway from 1 to 2.  Draw each of these and briefly describe the type of transformation occurring in each step.

(iii)        Using suitable orbital overlap diagrams, explain why 2 forms a stable

complex with an ethene ligand bound, but complex 1 does not.  In your answer account for the bond length in complex 2, 1.46 Å, that differs from that in free ethane, 1.34 Å .

(iv)        The   complex   Zr(η5-C5H5)2(PMe3)(n2-H2C=CHMe),   3,   has   a  bound

propene ligand.  Explain why, in the 1H NMR spectrum of 3, two isomers are observed.