Professor Adrian Dobbs

This chapter will discuss the addition reactions of the hydrides of the Group IV elements to carbon–carbon double and triple bonds, namely hydrosilylation, hydrostannylation, hydrogermylation, and hydroplumbylation reactions of alkynes and alkenes.\ud Although there is no corresponding chapter in the first edition of Comprehensive Organic Synthesis, the material presented here will follow on from Chapters 3.9 and 3.12 and the reader is referred back to these.

A new electrochemical methodology has been developed for the preparation of a wide variety of functionalized orthoesters under mild and green conditions from easily accessible dithiane derivatives. The new methodology also offers an unprecedented way to access tri­(fluorinated) orthoesters, a class of compound that has never been studied before. This provides the community with a rapid and general method to prepare libraries of functionalized orthoesters from simple and readily available starting materials.

We report a new electrochemical supporting-electrolyte-free method for synthesizing ureas, carbamates, and thiocarbamates via the oxidation of oxamic acids. This simple, practical, and phosgene-free route includes the generation of an isocyanate intermediate in situ via anodic decarboxylation of an oxamic acid in the presence of an organic base, followed by the one-pot addition of suitable nucleophiles to afford the corresponding ureas, carbamates, and thiocarbamates. This procedure is applicable to different amines, alcohols, and thiols. Furthermore, when single-pass continuous electrochemical flow conditions were used and this reaction was run in a carbon graphite Cgr/Cgr flow cell, urea compounds could be obtained in high yields within a residence time of 6 min, unlocking access to substrates that were inaccessible under batch conditions while being easily scalable.

A two-step transition metal-free methodology for the C(sp(3))-C(sp(3)) functionalisation of saturated N-heterocyclic systems is disclosed. First, aminal derivatives are generated through the anodic oxidation of readily accessible carboxylic acids. Then, in the presence of BF3 center dot OEt2, iminium ions are unmasked and rapidly alkylated by organozinc reagents under flow conditions. Secondary, tertiary and quaternary carbon centers have been successfully assembled using this methodology. Such an approach is especially relevant to drug discovery since it increases C(sp(3))-functionalities rapidly within a molecular framework. As proof of concept, our methodology was applied to derivatization of peptides and an API.

The first detailed studies of intramolecular aza-Prins and aza-silyl-Prins reactions, starting from acyclic materials, are reported. The methods allow rapid and flexible access toward an array of [6,5] and [6,6] aza-bicycles, which form the core skeletons of various alkaloids. On the basis of our findings on the aza-Prins and aza-silyl-Prins cyclizations, herein we present simple protocols for the intramolecular preparation of the azabicyclic cores of the indolizidines and quinolizidines using a one-pot cascade process of N-acyliminium ion formation followed by aza-Prins cyclization and either elimination or carbocation trapping. It is possible to introduce a range of different substituents into the heterocycles through a judicial choice of Lewis acid and solvent(s), with halo-, phenyl-, and amido-substituted azabicyclic products all being accessed through these highly diastereoselective processes.

A detailed study on the synthesis and reactions of silylmethylcyclopropanes is reported. In their simplest form, these donor-only cyclopropanes undergo Lewis acid promoted reaction to give either cis- or trans-tetrahydrofurans, with the selectivity being reaction condition-dependant. The adducts themselves are demonstrated to be an important scaffold for structural diversification. The combination of a silyl-donor group in a donor-acceptor cyclopropane with novel acceptor groups is also discussed. [Display omitted]

The aza-Prins reaction is a widely employed and highly efficient method for the preparation of saturated nitrogen-containing heterocycles. Its major drawback has always been a lack of diastereoselectivity and the formation of racemic products. Herein, we address these problems and report, for the first time, the synthesis of both diastereomerically and enantiopure multiply substituted piperidines via the aza-Prins reaction. This method is widely applicable for natural product synthesis and is exemplified here by the synthesis of enantiopure pipecolic acid derivatives.

A new electrochemical method for the preparation of isocyanides from easily accessible aminotetrazole derivatives has been developed, which tolerates an unprecedented range of functional groups. The use of chemical, rather than electrochemical, oxidation to afford isocyanides was also demonstrated, which provides access to these compounds for those without electrosynthesis equipment. The practicality of scale-up using flow electrochemistry has been demonstrated, in addition to the possibility of using electrochemically generated isocyanides in further reactions.

Saturated nitrogen-containing heterocycles are the basis of a plethora of natural products, including alkaloids and aza-sugars, and a recent survey also demonstrated that the piperidine ring is the most abundant nitrogen-containing heterocycle in FDA-approved pharmaceutically active and commercially available drugs, being found in 72 drugs. The aza-Prins and related reactions have emerged as a rapid and highly efficient method for the preparation of 6-membered nitrogen-containing heterocycles. This chapter reviews recent developments in the aza-Prins reaction, particularly focussing on reports since 2000.

The spiroketal moiety is an important substructure within many biological natural products. One method to access them is via the oxidative cyclisation of a pendant hydroxyl group on to a pre-formed pyran. However access to such precursors has hitherto been challenging and requires multistep syntheses frequently with considerable protecting group manipulation. Herein we report a novel and high yielding method for the preparation of hydroxydi- and hydroxytetra-hydropyrans, as spiroketal precursors, utilizing a novel double-Prins cyclisation approach.

The growing impetus to develop greener and more cost-efficient synthetic methods has prompted chemists to look for new ways to activate small organic molecules. Among them, electrosynthesis is one of the greenest and cheapest since it is possible to perform redox reactions without the need for any chemical reagents. Even though electrosynthesis is on the verge of a resurgence, it is far from being a new discipline. In fact, organic electrosynthesis was popularised by Manual Baizer in the early 60s while working at Monsanto. In this article, we will review the major, as well as the most recent, achievements in industrial organic electrosynthesis.

The first total synthesis of an important C13 butyl rubber oligomer is reported. The structure of the oligomer, which is an important and potentially toxic extractable and leachable component of elastomeric closures, is confirmed by synthesis for the first time. The method described is scalable, making large quantities of the oligomer available for the first time for AMES toxicity studies. The challenging synthesis commences with isophorone and the key steps of the synthesis involve the development of highly novel dithoacetal chemistry, cuprate addition and Tebbe olefination. The first total synthesis of important C13 butyl rubber oligomers is reported. The structure of the oligomer, which is an important extractable and leachable component of elastomeric closures, is confirmed for the first time. The 10‐step synthetic sequence starting from isophorone is robust and scalable, making large quantities of the oligomer available for the first time.

We describe the development of a novel low-cost small-footprint 3D-printed electrosynthesis continuous flow cell system that was designed and adapted to fit a commercially available Electrasyn 2.0. The utility and effectiveness of the combined flow/electrochemistry system over the batch process was demonstrated in the development of an improved and supporting-electrolyte-free version of our anodic methoxymethylation of alcohols.

A novel carbonylative approach to the synthesis of functionalized 1H-benzo[d]imidazo[1,2-a]imidazoles is presented. The method consists of the oxidative aminocarbonylation of N-substituted-1-(prop-2-yn-1-yl)-1H-benzo[d]imidazol-2-amines, carried out in the presence of secondary nucleophilic amines, to give the corresponding alkynylamide intermediates, followed by in situ conjugated addition and double-bond isomerization, to give 2-(1-alkyl-1H-benzo[d]imidazo[1,2-a]imidazol-2-yl)acetamides. Products were obtained in good to excellent yields (64-96%) and high turnover numbers (192-288 mol of product per mol of catalyst) under relatively mild conditions (100 °C under 20 atm of a 4:1 mixture of CO-air), using a simple catalytic system, consisting of PdI (0.33 mol %) in conjunction with KI (0.33 equiv).

A new electrochemical methodology has been developed for the generation of oxycarbonyl radicals under mild and green conditions from readily available hemioxalate salts. Mono- and multi-functionalised γ-butyrolactones were synthesised through exo-cyclisation of these oxycarbonyl radicals with an alkene, followed by the sp3–sp3 capture of the newly formed carbon-centred radical. The synthesis of functionalised valerolactone derivatives was also achieved, demonstrating the versatility of the newly developed methodology. This represents a viable synthetic route towards pharmaceutically important fragments and further demonstrates the practicality of electrosynthesis as a green and economical method to activate small organic molecules.