Giese Radical Conjugate Addition

How it works: the radical chain mechanism

The Giese reaction runs as a radical chain, and understanding the three-step propagation cycle is the key to controlling it. Consider an alkyl iodide R–I reacting with methyl acrylate under Bu₃SnH/AIBN.

• Initiation: AIBN decomposes thermally near 65–80 °C, extruding N₂ and giving two isobutyronitrile radicals. These abstract hydrogen from Bu₃SnH to generate the chain-carrying tributyltin radical, Bu₃Sn•.
• Atom abstraction: Bu₃Sn• abstracts the halogen from R–X (the strong Sn–X bond is the thermodynamic driving force), releasing the nucleophilic carbon radical R•.
• Addition (the C–C bond-forming step): R• adds to the terminal (β) carbon of the acrylate. Addition occurs at the β-position because it places the new odd electron α to the carbonyl, where it is stabilized by conjugation. This forms a resonance-stabilized α-carbonyl radical.
• Hydrogen transfer: that stabilized radical abstracts H from another molecule of Bu₃SnH, giving the saturated product and regenerating Bu₃Sn• to carry the chain forward.

The selectivity for conjugate addition over simple reduction of R• to R–H hinges on concentration and polarity matching. A nucleophilic alkyl radical adds fastest to the electron-poor alkene, and keeping [Bu₃SnH] low (slow syringe-pump addition) ensures R• meets the acceptor before it is prematurely quenched by tin hydride.

Polarity matching and reactivity

Giese framed radical addition in terms of polar effects, and this predictive model is why the reaction is reliable. Alkyl radicals are nucleophilic — their SOMO is relatively high in energy — so they pair best with the low-lying LUMO of an electron-poor alkene. The rate of addition of a primary alkyl radical to acrylonitrile or an acrylate is orders of magnitude faster than to a simple, electron-rich olefin.

• Fast acceptors: acrylates, acrylonitrile, vinyl ketones, vinyl sulfones, maleimides, and α,β-unsaturated aldehydes.
• Radical stability order: the ease of generating R• follows tertiary > secondary > primary, mirroring C–X bond strengths and radical stabilization.
• Captodative and heteroatom-stabilized radicals (e.g. α-amino or α-oxy radicals) are especially good donors and are central to modern variants.

This SOMO–LUMO complementarity is the radical analog of nucleophile–electrophile pairing in ionic Michael chemistry, and it is what lets chemists predict which fragment becomes the radical and which becomes the acceptor.

Conditions, reagents, and tin-free alternatives

The classic conditions use a radical precursor (alkyl halide, xanthate, or selenide), Bu₃SnH as the H-atom donor and chain carrier, and a small amount of initiator — AIBN (thermal, ~70 °C) or triethylborane/O₂ (which works even at −78 °C). Reactions are typically run in benzene, toluene, or degassed solvents to exclude O₂.

The major drawback is organotin toxicity and difficult purification, so a large body of tin-free chemistry has grown up:

• Tris(trimethylsilyl)silane, (TMS)₃SiH, is a lower-toxicity H-atom donor that behaves much like Bu₃SnH.
• Photoredox catalysis: a visible-light photocatalyst (Ru or Ir polypyridyl, or an organic dye) generates R• from carboxylic acids, redox-active esters, boronic acids, or alkyl halides at room temperature. A Hantzsch ester frequently serves as the terminal H-atom or electron source.
• Decarboxylative Giese: abundant carboxylic acids — or their N-hydroxyphthalimide (NHPI) “redox-active esters” — lose CO₂ to give R• directly, an approach popularized by MacMillan, Baran, and others in the 2010s.

These modern variants keep the mechanistic logic identical — a nucleophilic radical adds to an electron-poor alkene — while replacing tin and offering mild, functional-group-tolerant conditions.

Scope, stereochemistry, and limitations

The Giese reaction shines on complex, densely functionalized substrates precisely because radicals are indifferent to nearby polar groups. Free hydroxyls, ketones, and even unprotected amines usually survive. It is a favorite for elaborating sugars and nucleosides, where anomeric radicals add to acceptors with useful facial bias.

Stereochemistry is governed by the geometry of the radical and the transition state rather than by a chiral base. Cyclic and ring-fused radicals often add with good diastereoselectivity set by steric approach and, in pyranosyl systems, by an anomeric radical effect. Enantioselective variants now exist using chiral Lewis acids or chiral photoredox/NHC co-catalysts to control the prochiral face during either addition or the H-transfer step.

Limitations to plan around:

• Reduction competes with addition: if the radical is quenched by the H-donor before it finds the alkene, you simply get R–H. Low H-donor concentration and excess acceptor suppress this.
• Polymerization/oligomerization: if the product radical adds to a second alkene before H-transfer, telomers form; electron-poor acceptors that give stabilized (slow-adding) radicals minimize this.
• Electron-rich or 1,2-disubstituted alkenes add sluggishly and are generally poor acceptors.

Applications: why it matters

Because it stitches together fragments with minimal protecting-group overhead, the Giese addition is heavily used in total synthesis and, increasingly, in medicinal chemistry for late-stage functionalization.

• C(sp³)–C(sp³) coupling: decarboxylative Giese reactions convert cheap carboxylic acids into radicals that install alkyl groups onto acrylate-type acceptors, forging saturated linkages that are hard to make by cross-coupling.
• Carbohydrate and nucleoside chemistry: anomeric radicals from glycosyl halides add to acrylates to build C-glycosides that resist enzymatic hydrolysis.
• Complex-molecule synthesis: radical conjugate additions build quaternary centers and bridge rings in terpene and alkaloid syntheses where ionic methods clash with sensitive functionality.
• Photoredox library synthesis: visible-light Giese reactions run in parallel at room temperature, making them ideal for rapidly diversifying drug-like scaffolds.

In short, the reaction offers a broadly useful, functional-group-tolerant way to make the very C–C bonds — especially C(sp³)–C(sp³) — that ionic and organometallic methods struggle with.

A brief history

Radical additions to alkenes were known from the 1930s–1940s (Kharasch’s anti-Markovnikov HBr addition), but they were plagued by uncontrolled polymerization and seen as messy. In the early 1980s, Bernd Giese reframed the chemistry: by quantifying rate constants and articulating the role of polar effects in radical addition, he showed how to run clean, predictable intermolecular C–C bond formations using nucleophilic radicals and electron-poor acceptors under tin-hydride chain conditions.

His work — captured in the influential 1986 monograph Radicals in Organic Synthesis: Formation of Carbon–Carbon Bonds — established the modern discipline of synthetic radical chemistry. The 2010s photoredox and decarboxylative renaissance then removed the tin bottleneck and made the Giese reaction one of the most practiced radical transformations in industry and academia.

Giese addition versus the classic ionic Michael addition

Feature	Giese (radical)	Michael (ionic)
Nucleophile / donor	Carbon radical (R•)	Stabilized carbanion / enolate
Reactive alkene	Electron-poor (acrylate, enone)	Electron-poor (enone, acrylate)
Functional-group tolerance	High — ignores OH, ketone	Lower — base/nucleophile sensitive
Chain carrier	Bu₃Sn• or catalyst turnover	None (stoichiometric)
Typical conditions	AIBN 80 °C, or photoredox at rt	Base, protic or aprotic solvent