Cyclopropane Chemistry: A Comprehensive Review of Synthesis Strategies, Mechanistic Paradigms, and Functional Applications

1. Introduction: Structural Singularity and Electronic Nature

In the vast expanse of organic chemistry, cyclopropane ($C\_3H\_6$) stands as a singular entity, challenging classical geometric intuition with its unique electronic properties. As the smallest cycloalkane, its existence defies the ideal tetrahedral carbon theory. It serves not only as a versatile synthetic intermediate but also as a "privileged scaffold" in medicinal chemistry and material science.1

1.1 Bonding Theory and Ring Strain

In the cyclopropane molecule, three carbon atoms form an equilateral triangle, compressing the bond angles to $60^{\\circ}$, far from the ideal $109.5^{\\circ}$ of an $sp^3$ hybridized carbon. This extreme geometric compression results in significant angle strain (Baeyer Strain). Additionally, the rigid planar structure forces the hydrogen atoms on adjacent carbons into a fully eclipsed conformation, introducing substantial torsional strain (Pitzer Strain).2

Experimental data indicates that the total Ring Strain Energy (RSE) of cyclopropane is approximately 27.5 kcal/mol (115 kJ/mol).4 To mitigate this strain, the carbon atoms do not utilize traditional $\\sigma$ bonds along the internuclear axis.

Banana Bonds Model:
To maximize orbital overlap efficiency while reducing angle strain, the hybrid orbitals of the carbon atoms project outward from the ring rather than directly at the neighboring nuclei. According to the Coulson-Moffitt model, the carbon-carbon bonds possess high $p$-character (approaching $sp^5$), while the C-H bonds exhibit high $s$-character (approx. $32\\%$, closer to $sp^2$). This results in electron density being concentrated outside the bond axis, forming "bent" or "banana" bonds,.
Walsh Orbital Model:
The Walsh model offers another robust explanation, assuming $sp^2$ hybridization for the carbons. Two $sp^2$ orbitals bind to hydrogens, while the third points toward the ring center. The unhybridized $p$ orbitals overlap within the ring plane to form delocalized bonding orbitals. This model successfully explains cyclopropane's UV absorption spectra and its ability to conjugate with $\\pi$ systems,.

1.2 Physicochemical Properties and Reactivity

Cyclopropane's unique electronic structure endows it with properties intermediate between alkanes and alkenes:

1. Super-Alkene Character: Due to the high electron density and high energy state of the bent bonds, cyclopropane can react with electrophiles similarly to alkenes. It can undergo proton-induced ring opening or halogen addition.1
2. Acidity and Electronics: The high $s$-character of the C-H bonds renders cyclopropane protons significantly more acidic ($pK\_a \\approx 46$) than typical alkanes ($pK\_a \\approx 50$).1
3. Spectral Features: In NMR spectroscopy, the magnetic anisotropy of the ring current shifts proton signals significantly upfield (typically $\\delta$ 0–1 ppm), a diagnostic feature for this skeleton.

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2. Cyclopropane in Medicinal Chemistry: Bioisosteres and Conformational Restriction

In modern drug discovery, cyclopropane is a precision tool for modulating pharmacokinetics (PK) and pharmacodynamics (PD).5

2.1 Bioisosteric Strategies

1. Gem-dimethyl Bioisostere:
Replacing a gem-dimethyl group (e.g., in an isopropyl or tert-butyl group) with a cyclopropyl ring often maintains hydrophobicity and steric bulk while altering metabolic stability. The strong C-H bonds (~106 kcal/mol) of cyclopropane resist cytochrome P450-mediated oxidation, prolonging drug half-life.5
2. Alkene and Phenyl Isosteres:
Cyclopropane mimics the electronic properties of alkenes without their chemical instability. It also serves as a 2D mimic or saturated bioisostere for phenyl rings, improving solubility and increasing the fraction of $sp^3$ carbons ($Fsp^3$), which correlates with higher clinical success rates.5
3. Basicity and Permeability:
Incorporating a cyclopropyl group adjacent to an amine can lower the amine's $pK\_a$ via inductive effects, reducing ionization at physiological pH and thereby improving membrane permeability and reducing hERG channel liability.5

2.2 Conformational Restriction

Cyclopropane's rigidity can "lock" a molecule into its bioactive conformation, minimizing entropy loss upon binding to a target protein. This strategy is widely used to enhance potency and selectivity, as seen in dopamine receptor ligands.5

2.3 Case Studies in Drugs and Agrochemicals

Saxagliptin:
A DPP-4 inhibitor for type 2 diabetes. Its core features a cis-methanopyrrolidine scaffold (a cyclopropane fused to a pyrrolidine). This rigid structure prevents intramolecular nucleophilic attack (cyclization) that would otherwise occur, ensuring chemical stability.8
Grazoprevir:
An HCV NS3/4A protease inhibitor containing a highly functionalized vinyl cyclopropane macrocycle. The synthesis involves complex stereocontrol, often achieved via ring-closing metathesis or asymmetric cyclopropanation.10
Lenvatinib:
A multi-kinase inhibitor for thyroid and liver cancer, utilizing a cyclopropane carboxamide to fill hydrophobic pockets and modulate metabolic properties.12
Pyrethroids:
Synthetic analogues of natural pyrethrins (e.g., Deltamethrin) feature a dimethylcyclopropane carboxylic acid core. The stereochemistry (cis vs. trans) is critical for insecticidal activity and mammalian toxicity.13

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3. Classical Carbene Transfer: The Simmons–Smith Reaction

Since its discovery in 1958, the Simmons–Smith reaction has been the standard method for alkene cyclopropanation.14

3.1 Mechanism and Stereochemistry

The classic reagent is generated from diiodomethane ($CH\_2I\_2$) and a Zinc-Copper couple ($Zn-Cu$), forming the organozinc carbenoid $IZnCH\_2I$.

$2 CH\_2I\_2 \+ 2 Zn(Cu) \\rightarrow 2 IZnCH\_2I$
The reaction proceeds via a concerted "butterfly-type" transition state, transferring the methylene group to the alkene. This mechanism ensures stereospecificity: the geometry of the starting alkene (cis or trans) is strictly retained in the cyclopropane product,.

3.2 Furukawa Modification

Furukawa improved the reaction in 1966 by using diethylzinc ($Et\_2Zn$) to generate the carbenoid via halogen exchange:

$Et\_2Zn \+ CH\_2I\_2 \\rightarrow EtZnCH\_2I \+ EtI$

This homogenous method is milder, more reproducible, and compatible with various solvents, making it the preferred choice for complex synthesis.12

3.3 Directing Effects and Chiral Control

Hydroxyl Directing Effect:
In allylic alcohols, the hydroxyl group coordinates to the zinc reagent, directing cyclopropanation to the syn-face. This effect is exploited in the stereoselective synthesis of polyol natural products.15
Charette Asymmetric Modification:
André B. Charette developed chiral dioxaborolane ligands derived from tartaric acid or amino acids. These ligands create a chiral pocket around the zinc carbenoid, enabling highly enantioselective cyclopropanation (ee > 90%) of allylic alcohols.16

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4. Metal-Catalyzed Decomposition of Diazo Compounds

For introducing substituted carbenes (aryl, vinyl, ester), transition metal catalysis of diazo compounds ($R\_2C=N\_2$) is the premier strategy.17

4.1 Rhodium (Rh) Catalysis: Donor/Acceptor Carbenes

Dirhodium tetracarboxylates ($Rh\_2(O\_2CR)\_4$) are highly efficient catalysts.

• Donor/Acceptor Carbenes: Developed largely by the Davies group, using diazo compounds with both electron-donating (e.g., aryl, vinyl) and withdrawing (e.g., ester) groups stabilizes the metal-carbene intermediate, suppressing dimerization and enhancing selectivity.18
• Chiral Catalysts: $D\_2$-symmetric catalysts like $Rh\_2(DOSP)\_4$ induce high enantioselectivity (>90% ee) by creating a defined chiral environment.18

4.2 Copper (Cu) and Ruthenium (Ru) Systems

• Cu-Box: Copper complexes with chiral Bisoxazoline (Box) ligands act as Lewis acids. The bulky ligands shield the metal center, forcing the alkene to approach in a specific orientation, typically favoring trans-isomers with high enantiocontrol.20
• Ru-Pheox: Iwasa's Ruthenium-Phenyloxazoline (Pheox) catalysts are notable for their stability (water-tolerant) and ability to drive high cis-selectivity in certain cyclopropanations.23

4.3 Cobalt (Co) and Iron (Fe): Metalloradical Catalysis (MRC)

Unlike the 2-electron pathways of Rh/Cu, Cobalt(II) porphyrins catalyze reactions via a single-electron radical mechanism.

• Mechanism: The Co(II) center activates the diazo compound to form a Co(III)-carbene radical. This radical adds to the alkene to form a $\\gamma$-carbon radical, which then rapidly cyclizes.25
• Utility: This mechanism allows the cyclopropanation of electron-deficient alkenes (e.g., acrylates), a class of substrates typically inert to electrophilic Rh/Cu carbenes.27

4.4 Gold (Au) Catalysis: Ohno Rearrangement

Gold catalysis offers a diazo-free route via the cycloisomerization of enynes. Propargylic esters undergo 1,2-acyloxy migration (Ohno/Rautenstrauch rearrangement) to generate vinyl gold carbenes, which are subsequently trapped by alkenes to form cyclopropanes.29

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5. Nucleophilic and Ylide Pathways

5.1 Johnson–Corey–Chaykovsky Reaction

This classic reaction uses sulfur ylides to cyclopropanate electron-deficient alkenes (Michael acceptors).

• Reagents: Dimethylsulfoxonium methylide (thermodynamic, "soft") prefers 1,4-addition to enones followed by ring closure to form cyclopropanes. Dimethylsulfonium methylide (kinetic) typically targets carbonyls to form epoxides 31, [31],.
• Stereoselectivity: The reaction generally favors the formation of trans-cyclopropanes.31

5.2 MIRC (Michael-Initiated Ring Closure)

Beyond sulfur ylides, MIRC strategies employ various carbon nucleophiles (e.g., halo-malonates) that undergo conjugate addition to an acceptor followed by intramolecular displacement of a leaving group. This is a powerful method for synthesizing highly substituted, electron-deficient cyclopropanes,.

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6. Titanium-Mediated Synthesis: The Kulinkovich Reaction

Discovered in 1989, this reaction transforms esters directly into cyclopropanols using Grignard reagents and catalytic $Ti(OiPr)\_4$.32

6.1 Mechanism: Titanacyclopropane

The key intermediate is a low-valent titanacyclopropane species (generated from ligand exchange with the Grignard). The ester inserts into the Ti-C bond, and subsequent rearrangement/elimination yields the cyclopropanol. This effectively acts as a 1,2-dicarbanion equivalent [36],,.

6.2 Variants

• Kulinkovich-de Meijere: Uses amides to synthesize cyclopropylamines.33
• Kulinkovich-Szymoniak: Uses nitriles to produce aminocyclopropanes.32

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7. Photoredox and Radical Chemistry

Visible light photoredox catalysis has introduced "Radical-Polar Crossover" (RPCC) strategies.

7.1 RPCC Mechanism

A radical (generated via decarboxylation or halide abstraction) adds to an alkene. The resulting radical intermediate is then reduced (Single Electron Transfer) to a carbanion, which displaces a leaving group to close the ring.

• Application: This allows the use of stable precursors like carboxylic acids or redox-active esters to synthesize complex cyclopropanes under mild conditions.34
• 4CzIPN: An organic photocatalyst frequently used in these metal-free transformations.36

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8. Biocatalysis: Engineering Heme Proteins

Engineered heme proteins (e.g., Cytochrome P450, Myoglobin) represent the frontier of asymmetric cyclopropanation.

8.1 Directed Evolution

Frances Arnold's group demonstrated that by mutating axial ligands (e.g., Cys to Ser) in P450 enzymes, the redox potential can be tuned to favor carbene transfer over oxidation. These biocatalysts can accept diazo compounds and transfer carbenes with exquisite stereocontrol (>99% ee) and high turnover numbers (TON), often exceeding synthetic catalysts.38

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9. Materials Science Applications

9.1 Polymers

Incorporating cyclopropane into polymer backbones increases rigidity and glass transition temperature ($T\_g$). It also allows for the tuning of crystallinity and mechanical properties, such as tensile strength.42

9.2 Liquid Crystals

Fluorinated cyclopropanes are used in liquid crystal displays (LCDs). The combination of the rigid ring and electronegative fluorine atoms allows for precise tuning of dielectric anisotropy and dipole moments, essential for Vertical Alignment (VA) display technologies.43

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10. Industrial Scale-Up and Safety

10.1 Handling Diazo Reagents

The explosion hazard of diazo compounds is a major industrial bottleneck. Continuous Flow Chemistry allows for the in situ generation and immediate consumption of hazardous intermediates (like diazomethane or diazoacetates), maintaining a low inventory of active energetic material and ensuring safety [46],.

10.2 Process Optimization

In the manufacturing of Saxagliptin, early routes using Simmons-Smith chemistry were replaced or optimized to avoid unstable intermediates and heavy metal waste. Alternative coupling agents (like T3P) and optimized cyclopropanation protocols (using mesylates) were developed to streamline the synthesis.8

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11. Conclusion

Cyclopropane synthesis has evolved from the classic zinc-carbenoids of Simmons and Smith to the sophisticated chiral pockets of Rh/Cu catalysts, and now into the realms of photoredox and biocatalysis. Future developments lie in electro-organic synthesis 16, and further expansion of the enzymatic toolbox, promising greener and more efficient access to this strained yet essential motif.

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Appendix: Comparison of Key Methodologies

Methodology	Key Reagents	Mechanism	Advantages	Limitations
Simmons–Smith	$Zn(Cu)$/$Et\_2Zn$, $CH\_2I\_2$	Concerted Electrophilic	Stereospecific, hydroxyl-directing, no explosion risk	Stoichiometric metal waste, sensitive to sterics
Rh/Cu Catalysis	Diazo + Rh/Cu	Metal Carbene (Polar)	High enantioselectivity, low catalyst loading	Diazo safety, cost of Rh, requires stabilized carbenes
Co/Fe MRC	Diazo + Co/Fe Porphyrin	Metalloradical	Works on electron-deficient alkenes, earth-abundant metals	Radical side reactions, substrate specific
Corey–Chaykovsky	Sulfur Ylide + Base	MIRC / Nucleophilic	Good for enones, trans-selective	Limited to Michael acceptors, sulfide waste
Kulinkovich	Grignard + $Ti(OiPr)\_4$	Titanacyclopropane	Direct ester-to-cyclopropanol conv., cheap reagents	Grignard handling, functional group tolerance
Photoredox (RPCC)	Photocatalyst + Light	Radical-Polar Crossover	Mild, unique bond disconnections (e.g. decarboxylative)	Light penetration (scale-up), catalyst cost
Biocatalysis	Engineered P450	Heme Iron Carbene	Green, aqueous, perfect stereocontrol	Enzyme engineering required, substrate scope

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