Highly Diastereoselective Preparation of Tertiary Alkyl Thiocyanates en Route to Thiols by Stereoinvertive Nucleophilic Substitution at Nonclassical Carbocations

Formally Stereoretentive SN1 Reactions of Homoallylic Tertiary Alcohols Via Nonclassical Carbocation

We present a stereoretentive nucleophilic substitution of homoallylic tertiary alcohols via the formation of a nonclassical cyclopropyl carbinyl (CPC) carbocation intermediate. This strategy enables the creation of highly congested tertiary centers with preserved stereocontrol, addressing the typical challenges of carbocation instability and reactivity in SN1 mechanisms. The stabilization of the CPC intermediate is crucial for achieving precise regio- and stereoselectivity, significantly enhancing the utility of SN1-type mechanisms in complex molecule synthesis.

Stereospecific molecular rearrangement via nucleophilic substitution at quaternary stereocentres in acyclic systems

Nucleophilic substitution at tetravalent (sp3) carbon is a fundamental transformation in organic synthesis, essential for creating carbon-carbon and carbon-heteroatom bonds. While the mechanism of the SN2 reaction is well understood, achieving stereochemical control in SN1-type reactions remains extremely challenging due to the complexity of successive carbocation intermediates. Here we present a strategy for preparing complex molecular skeletons via stereospecific SN1 at a quaternary stereocentre in acyclic systems. By leveraging neighbouring group participation, we facilitate the selective formation of a unique cyclopropylcarbinyl cation intermediate that undergoes selective nucleophilic substitution with high diastereoselectivity and complete inversion of configuration at a distant position from the original carbocation via molecular rearrangement. This methodology has been applied to generate homoallylic tertiary fluorides, bromides, chlorides, ethers, thiocyanates and azides, demonstrating its applicability in accessing diverse functional groups with exceptional diastereoselectivities. This transformation opens new avenues for constructing complex molecular architectures through precise stereocontrol of C-C bond cleavage at a quaternary stereocentre in acyclic systems.

Stereoinvertive SN1 Through Neighboring Group Participation

Neighboring group participation, the assistance of non-conjugated electrons to a reaction center, is a fundamental phenomenon in chemistry. In the framework of nucleophilic substitution reactions, neighboring group participation is known to cause rate acceleration, first order kinetics (SN1), and retention of configuration. The latter phenomenon is a result of double inversion: the first one when the neighboring group displaces the leaving group, and the second when a nucleophile substitutes the neighboring group. This powerful control of stereoretention has been widely used in organic synthesis for more than a century. However, neighboring group participation may also lead to inversion of configuration, a phenomenon which is often overlooked. Herein, we review this unique mode of stereoinversion, dividing the relevant reactions into three classes with the aim to introduce a fresh perspective on the different modes of stereoinversion via neighboring group participation as well as the factors that control this stereochemical outcome.

Stereoselective Nucleophilic Halogenation at CF3-Substituted Nonclassical Carbocation

CF3-substituted cyclopropyl carbinol derivatives undergo regioselective and diastereoselective nucleophilic halogenation at the quaternary carbon center to provide acyclic products as a single diastereomer. The selectivity of the substitution is rationalized by the formation of a nonclassical cyclopropylcarbinyl cation intermediate, reacting at the most-substituted carbon center. Tertiary alkyl chlorides, bromides, and fluorides adjacent to a stereogenic C-CF3-motif are diastereomerically pure and can be obtained in few catalytic steps from commercially available alkynes.