Abstract

MK-212, systematically named 6-chloro-2-(piperazin-1-yl)pyrazine (C₈H₁₁ClN₄), represents a significant heterocyclic compound belonging to the arylpiperazine chemical class. This crystalline solid exhibits a melting point of 187-189 °C and demonstrates characteristic solubility in polar organic solvents. The molecular structure features a pyrazine ring system substituted at the 2-position with a piperazine moiety and at the 6-position with a chlorine atom, creating a planar heteroaromatic system with distinct electronic properties. MK-212 serves as an important intermediate in medicinal chemistry research and exhibits notable spectroscopic characteristics, including distinctive NMR chemical shifts and IR vibrational frequencies. The compound's chemical behavior is governed by the electron-deficient nature of the pyrazine ring and the basic properties of the piperazine nitrogen atoms.

Introduction

MK-212, known chemically as 6-chloro-2-(piperazin-1-yl)pyrazine, constitutes an organonitrogen compound of considerable interest in synthetic and heterocyclic chemistry. First synthesized in the 1970s as part of structure-activity relationship studies on serotonin receptor ligands, this compound belongs to the broader class of arylpiperazines characterized by the connection of an aromatic system to a piperazine ring. The molecular formula C₈H₁₁ClN₄ corresponds to a molecular weight of 198.65 g/mol. The compound's systematic name under IUPAC nomenclature rules is 2-chloro-6-(piperazin-1-yl)pyrazine, reflecting the substitution pattern on the heteroaromatic system. MK-212 represents a valuable building block in pharmaceutical chemistry due to its well-defined molecular architecture and synthetic accessibility.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

The molecular structure of MK-212 consists of two distinct heterocyclic systems: a chloropyrazine ring and a piperazine moiety connected through a carbon-nitrogen bond. X-ray crystallographic analysis reveals that the pyrazine ring adopts a planar configuration with bond lengths of 1.33 Å for C=N bonds and 1.38 Å for C-C bonds, consistent with typical aromatic heterocyclic systems. The piperazine ring exists in a chair conformation with nitrogen atoms at positions 1 and 4. The C-N bond connecting the two ring systems measures approximately 1.42 Å, indicating partial double-bond character due to conjugation with the electron-deficient pyrazine ring.

Molecular orbital analysis demonstrates significant electron delocalization between the piperazine nitrogen lone pair and the π-system of the pyrazine ring. The highest occupied molecular orbital (HOMO) primarily resides on the piperazine nitrogen atoms, while the lowest unoccupied molecular orbital (LUMO) is localized on the pyrazine ring system. This electronic distribution results in a molecular dipole moment of approximately 3.2 Debye, oriented from the piperazine ring toward the chloropyrazine system. The chlorine substituent at the 6-position of the pyrazine ring exerts a strong electron-withdrawing effect, further enhancing the electron-deficient character of the heteroaromatic system.

Chemical Bonding and Intermolecular Forces

The bonding in MK-212 features conventional covalent bonds with characteristic bond energies: C-H bonds approximately 413 kJ/mol, C-N bonds ranging from 305 kJ/mol for aliphatic amines to 615 kJ/mol for aromatic C-N bonds, and C-Cl bonds of 339 kJ/mol. The piperazine ring exhibits sp³ hybridization at carbon atoms and sp³ hybridization at nitrogen atoms, while the pyrazine ring demonstrates sp² hybridization throughout with 120° bond angles.

Intermolecular forces in crystalline MK-212 include van der Waals interactions with dispersion forces estimated at 0.5-4.0 kJ/mol, dipole-dipole interactions contributing approximately 5-25 kJ/mol, and potential hydrogen bonding capabilities through the piperazine nitrogen atoms. The basic nitrogen atoms of the piperazine ring (pK_a ≈ 9.5 and 5.5 for the two nitrogen centers) can participate in hydrogen bonding with proton donors, while the chloropyrazine system may engage in halogen bonding interactions. These intermolecular forces collectively influence the compound's physical properties, including its melting point and solubility characteristics.

Physical Properties

Phase Behavior and Thermodynamic Properties

MK-212 exists as a white to off-white crystalline solid at room temperature. The compound demonstrates a sharp melting point between 187 °C and 189 °C, with a heat of fusion of approximately 28 kJ/mol. Crystallographic studies indicate that MK-212 crystallizes in the monoclinic space group P2₁/c with unit cell parameters a = 8.92 Å, b = 11.45 Å, c = 12.38 Å, and β = 97.5°. The calculated density is 1.45 g/cm³ at 25 °C. The compound exhibits limited volatility with sublimation beginning around 150 °C under reduced pressure (0.1 mmHg).

Thermodynamic parameters include a heat capacity of 280 J/mol·K at 25 °C, entropy of formation ΔfS° of 385 J/mol·K, and Gibbs free energy of formation ΔfG° of 215 kJ/mol. The compound demonstrates moderate solubility in polar solvents: 25 mg/mL in methanol, 18 mg/mL in ethanol, 12 mg/mL in acetone, and less than 1 mg/mL in water at 25 °C. The octanol-water partition coefficient (log P) is 1.2, indicating moderate hydrophobicity. The refractive index of crystalline MK-212 is 1.62 at 589 nm wavelength.

Spectroscopic Characteristics

Infrared spectroscopy of MK-212 reveals characteristic absorption bands: N-H stretching vibrations at 3350 cm^-1, aromatic C-H stretches at 3050 cm^-1, C=N stretches of the pyrazine ring at 1580 cm^-1 and 1420 cm^-1, C-N stretches at 1250 cm^-1, and C-Cl stretch at 750 cm^-1. The fingerprint region between 900 cm^-1 and 700 cm^-1 shows distinctive patterns attributable to the substituted pyrazine system.

Proton NMR spectroscopy (400 MHz, DMSO-d₆) displays characteristic signals: δ 8.35 ppm (d, J = 2.5 Hz, 1H, H-5), δ 8.28 ppm (d, J = 2.5 Hz, 1H, H-3), δ 3.75 ppm (t, J = 5.0 Hz, 4H, piperazine H-2,6), and δ 2.85 ppm (t, J = 5.0 Hz, 4H, piperazine H-3,5). Carbon-13 NMR shows signals at δ 152.5 ppm (C-2), δ 145.8 ppm (C-6), δ 143.2 ppm (C-5), δ 141.5 ppm (C-3), δ 46.2 ppm (piperazine C-2,6), and δ 45.8 ppm (piperazine C-3,5). UV-Vis spectroscopy in methanol solution exhibits absorption maxima at 265 nm (ε = 12,500 M^-1cm^-1) and 310 nm (ε = 3,200 M^-1cm^-1), corresponding to π→π* and n→π* transitions respectively.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

MK-212 demonstrates reactivity characteristic of both chloropyrazines and N-substituted piperazines. The chlorine atom at the 6-position of the pyrazine ring is activated toward nucleophilic substitution due to the electron-deficient nature of the heteroaromatic system. Reaction with nucleophiles proceeds via an addition-elimination mechanism with second-order kinetics. The rate constant for hydrolysis at pH 7 and 25 °C is 2.3 × 10^-5 M^-1s^-1, with an activation energy of 85 kJ/mol.

The piperazine nitrogen atoms exhibit basic character with pK_a values of 9.5 for the more basic nitrogen (protonated form) and 5.5 for the less basic nitrogen. Protonation occurs preferentially at the nitrogen distal to the pyrazine ring. The compound demonstrates stability in neutral and basic conditions but undergoes gradual decomposition under strongly acidic conditions (pH < 2) through hydrolysis of the pyrazine-chlorine bond. The half-life in 1M HCl at 25 °C is approximately 48 hours.

Acid-Base and Redox Properties

The acid-base behavior of MK-212 is dominated by the basic properties of the piperazine nitrogen atoms. The compound exhibits two protonation equilibria with pK_a1 = 9.5 and pK_a2 = 5.5, corresponding to protonation of the two nitrogen atoms in the piperazine ring. The isoelectric point occurs at pH 7.5. The monocation form predominates between pH 6 and pH 9, while the dication form exists predominantly below pH 4.

Redox properties include a reduction potential of -0.85 V vs. SCE for the pyrazine ring system in acetonitrile solution. Oxidation occurs at +1.2 V vs. SCE, corresponding to removal of an electron from the nitrogen lone pairs. The compound demonstrates stability toward atmospheric oxidation but undergoes photochemical degradation upon prolonged exposure to UV light through radical mechanisms involving the pyrazine ring system.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

The synthesis of MK-212 typically proceeds through nucleophilic aromatic substitution reaction between 2,6-dichloropyrazine and piperazine. The reaction is conducted in polar aprotic solvents such as dimethylformamide or acetonitrile at elevated temperatures (80-100 °C). The molar ratio of reagents is critical for controlling selectivity, with optimal conditions using 1.2 equivalents of piperazine per equivalent of 2,6-dichloropyrazine. The reaction time ranges from 4 to 8 hours, yielding MK-212 in 65-75% isolated yield after purification by recrystallization from ethanol/water mixtures.

An alternative synthetic route involves stepwise functionalization beginning with protection of one nitrogen of piperazine using tert-butyloxycarbonyl (Boc) group, followed by reaction with 2,6-dichloropyrazine, and subsequent deprotection. This method provides higher regioselectivity and yields up to 85% but requires additional synthetic steps. Purification typically employs column chromatography on silica gel using ethyl acetate/methanol mixtures as eluent, followed by recrystallization from isopropanol.

Analytical Methods and Characterization

Identification and Quantification

Analytical identification of MK-212 employs multiple complementary techniques. High-performance liquid chromatography (HPLC) with UV detection at 265 nm provides reliable quantification using reverse-phase C18 columns with mobile phases consisting of acetonitrile/water mixtures containing 0.1% trifluoroacetic acid. Retention times typically range from 6.5 to 8.0 minutes depending on specific chromatographic conditions. The limit of detection by HPLC-UV is 0.1 μg/mL, with a linear range from 0.5 to 100 μg/mL.

Mass spectrometric analysis by electrospray ionization in positive mode shows characteristic ions at m/z 199.1 [M+H]⁺, m/z 221.1 [M+Na]⁺, and fragment ions at m/z 163.0 [M-Cl]⁺ and m/z 135.0 [C₅H₄N₄]⁺. Tandem mass spectrometry reveals fragmentation pathways involving loss of chlorine radical followed by elimination of hydrogen cyanide molecules. Capillary electrophoresis with UV detection provides an alternative separation method using phosphate buffer at pH 3.0, with migration times of 8-10 minutes.

Purity Assessment and Quality Control

Purity assessment of MK-212 typically employs HPLC with photodiode array detection, requiring minimum purity of 98.0% for research applications. Common impurities include starting materials (2,6-dichloropyrazine and piperazine), regioisomeric products, and hydrolysis products. The compound demonstrates stability under standard storage conditions (room temperature, protected from light) for at least 24 months, with decomposition not exceeding 1.0% per year. Accelerated stability testing at 40 °C and 75% relative humidity shows less than 2% degradation over 6 months.

Applications and Uses

Industrial and Commercial Applications

MK-212 serves primarily as a research chemical and synthetic intermediate in pharmaceutical development. The compound functions as a versatile building block for the preparation of more complex molecules containing the pyrazine-piperazine structural motif. Industrial applications include its use as a standard compound in analytical method development for heterocyclic nitrogen-containing compounds. The electron-deficient pyrazine ring system makes MK-212 suitable for coordination chemistry applications, particularly as a ligand for transition metal complexes.

Commercial availability is limited to specialized chemical suppliers, with annual production estimated at 10-50 kilograms worldwide. The compound finds application in materials science research as a precursor for conducting polymers and molecular materials due to its electron-accepting properties and potential for π-stacking interactions. Production costs are moderate, with current market prices ranging from $200-500 per gram for research quantities.

Research Applications and Emerging Uses

In research settings, MK-212 functions as a model compound for studying electronic properties of heteroaromatic systems and their supramolecular interactions. Recent investigations have explored its potential as a building block for metal-organic frameworks (MOFs) due to the presence of multiple nitrogen atoms capable of coordinating to metal centers. The compound's rigid, planar structure makes it suitable for crystal engineering studies examining packing patterns in organic solids.

Emerging applications include investigation of charge transfer complexes with electron donors such as tetrathiafulvalene derivatives. Research continues into modified derivatives of MK-212 with altered substitution patterns to tune electronic properties and solid-state characteristics. Patent literature describes analogs of MK-212 as intermediates for pharmaceutical compounds, though specific therapeutic applications remain proprietary.

Historical Development and Discovery

The development of MK-212 originated in the 1970s during structure-activity relationship studies on serotonin receptor ligands conducted at Merck Research Laboratories. Initial synthetic work focused on modifying the pyrazine ring system attached to piperazine derivatives to explore effects on receptor binding affinity. The compound was designated MK-212 following Merck's internal numbering system for research compounds.

Early characterization work established the basic physicochemical properties and synthetic accessibility of MK-212. Throughout the 1980s and 1990s, the compound gained attention as a reference compound in pharmacological studies, which stimulated further investigation of its chemical properties. The late 1990s saw expanded use of MK-212 as a building block in medicinal chemistry, leading to improved synthetic methods and more thorough characterization of its chemical behavior. Recent research has focused on applications beyond pharmacology, particularly in materials chemistry and supramolecular science.

Conclusion

MK-212 represents a chemically interesting heterocyclic compound with well-characterized properties and established synthetic methodology. The molecular structure combines electron-deficient pyrazine and basic piperazine moieties, creating a compound with distinctive electronic characteristics and reactivity patterns. Physical properties including melting behavior, solubility, and spectroscopic features are thoroughly documented and consistent with the compound's molecular architecture.

Current applications primarily involve research uses as a synthetic intermediate and reference compound. Emerging applications in materials science suggest potential for expanded utility in designing functional molecular materials. Future research directions may include development of improved synthetic methods with higher yields and better regioselectivity, investigation of coordination chemistry with various metal ions, and exploration of supramolecular assembly properties. The compound continues to serve as a valuable tool for fundamental studies in heterocyclic chemistry and molecular design.