Q7(c). SeO₂/H₂O₂, Pb(OAc)₄, OsO₄, periodate

Introduction. (i) and (iii) are two ways of adding two OH groups to a C=C, anti (via an epoxide) and syn (via an osmate ester); (ii) is an oxidative ring cleavage that keeps the cis geometry of the ring bonds; (iv) is a syn dihydroxylation followed by glycol cleavage.

Reading of the question: “NaSO₃H” is sodium hydrogen sulfite (NaHSO₃); “HIO₃” is read as periodic acid, HIO₄, the reagent that cleaves 1,2-diols.

(i) Maleic acid with H₂O₂/SeO₂

Scheme: Syn epoxidation and anti ring opening: net anti dihydroxylation.

SeO₂ and H₂O₂ form peroxyselenous acid, an electrophilic O-transfer agent like a peracid. It epoxidises the C=C (syn); in the aqueous acidic medium water opens the epoxide from the back side (anti). Net anti addition of two OH to a cis alkene gives the racemic (±)-tartaric acid, (2R,3R) + (2S,3S). (Syn reagents, OsO₄ or cold KMnO₄, give meso-tartaric acid from maleic acid.)

(ii) o-Phenylenediamine with Pb(OAc)₄

Scheme: Oxidative cleavage of the ring between the two N-bearing carbons.

Lead tetraacetate oxidises both amino groups (through the quinone di-imine and a dinitrene-like stage), and the C1–C2 bond between them breaks: each former C–N becomes a C#N. The product is cis,cis-mucononitrile. The two C=C bonds were ring bonds, which are necessarily cis; the cleavage does not touch them, so both double bonds are Z. (On heating or with iodine it isomerises to the cis,trans isomer.)

(iii) Cyclopentene with OsO₄, then aqueous Na₂SO₃

Scheme: Both C–O bonds form on the same face in one concerted step.

OsO₄ adds to the C=C in a concerted [3+2] cycloaddition, giving a cyclic osmate(VI) ester; sulfite reduces osmium and cleaves the Os–O bonds without touching the C–O bonds. Both OH are on the same face: cis-cyclopentane-1,2-diol, an achiral meso compound (1R,2S).

(iv) 3-Methyl-3-phenylcyclopropene: OsO₄, then periodate

Scheme: The diol carbons become two CHO groups on the quaternary carbon.

• X: OsO₄ adds syn to the strained, reactive cyclopropene C=C from the face anti to the larger phenyl group (the methyl face is less hindered). Bisulfite cleaves the osmate. X = cis-3-methyl-3-phenylcyclopropane-1,2-diol, both OH cis to CH₃ and trans to Ph. A mirror plane passes through C-3, Me and Ph, so X is achiral (meso); C-3 is pseudo-asymmetric. The minor diastereomer has both OH cis to Ph.
• Y: periodic acid forms a cyclic periodate ester with the cis-diol and cleaves the C1–C2 bond; both carbinol carbons become CHO. Y = 2-methyl-2-phenylpropanedial, CH₃C(Ph)(CHO)₂. The stereocentres of X disappear.

Conclusion. (i) (±)-Tartaric acid (anti addition, racemic); (ii) cis,cis-mucononitrile (both C=C Z); (iii) cis-cyclopentane-1,2-diol (syn, meso); (iv) X = cis-3-methyl-3-phenylcyclopropane-1,2-diol (syn, from the face anti to Ph; achiral), Y = 2-methyl-2-phenylpropanedial.

Application. Syn (OsO₄) versus anti (peracid, then hydrolysis) dihydroxylation is the standard way to choose the relative configuration of 1,2-diols, as in the synthesis of meso- and (±)-tartaric acids and of sugar-like polyols.