Past exam of the chemistry course of the year 4 of the natural sciences course of the University of Cambridge / 2022 / Paper 1 / 14

Product 5 is 1-methylcyclohexan-1-ol. The tertiary ring C–H bond of methylcyclohexane is replaced by C–OH. This is a single constitutional product; the two ring paths at that carbon are equivalent, so no new stereocenter is created there.

The manganese–oxo species abstracts the relatively weak tertiary hydrogen, then hydroxyl rebound converts the carbon radical into the tertiary alcohol. The unfunctionalized catalyst has no crown-ether receptor to preassociate the ammonium substrate. Its selectivity therefore mainly follows intrinsic C–H reactivity: the tertiary site of methylcyclohexane outcompetes the secondary C–H sites of the linear chain. The cationic ammonium group also electronically deactivates nearby sites. This explains the dominant product under the idealized selectivity described in the question.

Product 6 is a remotely oxidized ketone of the undecylammonium chain, retaining the ammonium group and its counterion. Number from the ammonium-bearing carbon as C1. Crown-mediated recognition positions the remote C8–C9 region near the Mn–oxo center. A representative major-site product is the C8 ketone,Its adjacent possible regioisomer is the C9 ketone $[{\mathrm{H}}_3{\mathrm{N}}^{+}-(\mathrm{C}{\mathrm{H}}_2{)}_8-\mathrm{C}(\mathrm{O})-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_3]\mathrm{B}{\mathrm{F}}_4^{-}$. The precise single-site assignment is a limitation discussed below.

The crown ether binds the primary ammonium end through several N–H···O hydrogen bonds. This host–guest complex holds a folded portion of the chain close to the manganese oxidant, so the effective local concentration of a remote secondary C–H bond can overcome the intrinsic advantage of a free tertiary C–H bond. Such recognition-driven reversal of substrate preference is experimentally established.^[ref]

The key intermediates are the crown-bound ammonium–Mn=O assembly, a remote carbon radical after H abstraction, and the rebound secondary alcohol. A second oxidation removes the remaining hydrogen on the carbinol carbon to give the ketone. The overall oxygen-equivalent count isThus one ketone requires two equivalents of peroxide, explaining a maximum near 50% ketone yield with one equivalent and near-complete conversion with two.

The primary remote-oxidation study reports preference for C8 and C9, not a universally unique carbon. The schematic catalyst drawing and the oxygen budget alone cannot distinguish those positions quantitatively. The “single product” wording is therefore treated as an idealized remote-site assignment; a uniquely verified C8:C9 ratio is not invented.^[ref]

Ba${}^{2+}$ competes strongly for the crown ether receptors. With the recognition sites occupied, the undecylammonium chain can no longer be effectively preorganized near manganese. The reaction should move back towards oxidation of methylcyclohexane to 1-methylcyclohexan-1-ol, with suppression of the selectively formed remote amino ketone.

The single available peroxide equivalent is then sufficient for tertiary C–H hydroxylation, whereas a secondary-chain ketone would require two oxidations. Competitive binding constants and metal-induced structural changes would determine the exact conversion and residual selectivity; neither a numerical yield nor complete inhibition of all oxidation follows merely from adding two equivalents of barium salt.

Without a competing hydrocarbon and without a crown receptor, catalyst 1 can still oxidize accessible secondary C–H bonds in the chain. Expect a less site-selective mixture of secondary oxidation products, predominantly ketones when two peroxide equivalents are effectively used, rather than the recognition-selected remote product pattern of catalyst 2.

The strongly electron-withdrawing, protonated ammonium end makes nearby C–H bonds less reactive, so the distribution need not be statistically uniform over every methylene. More distant secondary positions are comparatively favorable, while primary terminal C–H oxidation is generally harder. alcohol intermediates, incomplete conversion and unproductive peroxide consumption are also possible. Exact regioisomer ratios or a guaranteed quantitative yield cannot be predicted from the supplied conditions.