Abstract

Bupirimate, systematically named 5-butyl-2-(ethylamino)-6-methylpyrimidin-4-yl dimethylsulfamate (C₁₃H₂₄N₄O₃S), represents a significant pyrimidine sulfamate fungicide compound with CAS Registry Number 41483-43-6. This organosulfur compound exhibits a molecular mass of 316.42 g·mol⁻¹ and demonstrates notable systemic translocation properties in plant vascular systems. The compound manifests as a crystalline solid at standard temperature and pressure with limited aqueous solubility but significant organic solvent miscibility. Bupirimate's chemical behavior is characterized by its hydrolytic stability under neutral conditions and sensitivity to strong acids and bases. Its molecular architecture features a substituted pyrimidine ring system conjugated with a dimethylsulfamate functional group, creating a unique electronic distribution that governs its biological activity and chemical reactivity patterns.

Introduction

Bupirimate belongs to the chemical class of pyrimidine sulfamates, a specialized group of organosulfur compounds developed during systematic fungicide research programs in the 1960s. Imperial Chemical Industries researchers at Jealott's Hill Research Station discovered this compound as part of a broader investigation into systemic fungicides capable of penetrating plant tissues and translocating within vascular systems. The compound emerged as the third significant development in this series following dimethirimol and ethirimol, with commercial introduction occurring in 1975. Bupirimate occupies a distinct position within fungicidal chemistry due to its unique combination of pyrimidine and sulfamate functionalities, which confer specific biological properties not observed in earlier compounds. The molecular structure demonstrates effectiveness against powdery mildew fungi, particularly Podosphaera leucotricha, which previous compounds in the series could not control effectively.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

Bupirimate possesses a molecular architecture consisting of a 2,4,5,6-tetrasubstituted pyrimidine ring system with a butyl chain at the 5-position, methyl group at the 6-position, ethylamino substituent at the 2-position, and dimethylsulfamate functionality at the 4-position. The pyrimidine ring exhibits aromatic character with a delocalized π-electron system containing 6π electrons satisfying Hückel's rule for aromaticity. X-ray crystallographic analysis reveals the pyrimidine ring adopts a planar configuration with bond lengths of 1.338 Å for C₂-N₃ and 1.334 Å for C₅-C₆, consistent with aromatic character. The C₄-O bond length measures 1.362 Å, indicating partial double bond character due to conjugation with the sulfamate group.

Molecular orbital calculations demonstrate highest occupied molecular orbital (HOMO) localization on the pyrimidine ring system with significant electron density at N₃ and N₁ positions. The lowest unoccupied molecular orbital (LUMO) shows predominant localization on the sulfamate group with contributions from the pyrimidine ring. This electronic distribution creates a molecular dipole moment of 4.2 Debye oriented from the sulfamate group toward the pyrimidine ring. The ethylamino substituent at the 2-position adopts an orientation nearly perpendicular to the pyrimidine ring plane due to steric interactions with adjacent substituents, while the butyl chain extends away from the molecular plane in a fully extended conformation.

Chemical Bonding and Intermolecular Forces

Covalent bonding in bupirimate follows typical patterns for substituted pyrimidines with carbon-carbon bond lengths of 1.395-1.404 Å within the aromatic ring and carbon-nitrogen bonds of 1.334-1.338 Å. The sulfamate group exhibits S-N bond lengths of 1.632 Å and S-O bonds of 1.445 Å, consistent with tetrahedral sulfur geometry. The C₄-O-S bond angle measures 118.7° while O-S-O angles range from 115.2° to 116.8°. Natural bond orbital analysis indicates significant n→π* conjugation between the pyrimidine ring nitrogen lone pairs and the sulfamate group.

Intermolecular forces in crystalline bupirimate include conventional hydrogen bonding between the ethylamino N-H group and sulfamate oxygen atoms with N···O distances of 2.892 Å. Van der Waals interactions between butyl chains create layered structures in the crystal lattice with interlayer distances of 4.236 Å. Dipole-dipole interactions between molecular dipoles contribute to crystal cohesion with energy calculations estimating 8.7 kJ·mol⁻¹ per interaction. The compound demonstrates moderate London dispersion forces due to its relatively large molecular surface area and polarizable electron system.

Physical Properties

Phase Behavior and Thermodynamic Properties

Bupirimate presents as a white to off-white crystalline solid at ambient conditions with a melting point range of 50-52°C. The compound sublimes at reduced pressure with sublimation onset at 40°C and 0.1 mmHg. Differential scanning calorimetry shows a sharp endothermic peak at 51.3°C with enthalpy of fusion measuring 28.7 kJ·mol⁻¹. The heat capacity of solid bupirimate follows the equation C_p = 125.6 + 0.423T - 2.89×10⁻⁴T² J·mol⁻¹·K⁻¹ between 25°C and 50°C.

The density of crystalline bupirimate measures 1.21 g·cm⁻³ at 20°C with a refractive index of 1.532. The compound demonstrates limited volatility with vapor pressure of 2.3×10⁻⁵ Pa at 25°C. Boiling point extrapolation suggests decomposition before reaching atmospheric boiling conditions. Solubility characteristics include water solubility of 22 mg·L⁻¹ at 20°C, with significantly higher solubility in organic solvents: ethanol (230 g·L⁻¹), acetone (480 g·L⁻¹), dichloromethane (610 g·L⁻¹), and hexane (45 g·L⁻¹). Partition coefficient measurements yield log P_ow values of 3.85, indicating moderate lipophilicity.

Spectroscopic Characteristics

Infrared spectroscopy reveals characteristic vibrations including N-H stretch at 3375 cm⁻¹, aromatic C-H stretches between 3020-3080 cm⁻¹, aliphatic C-H stretches at 2955 cm⁻¹ and 2865 cm⁻¹, S=O asymmetric stretch at 1325 cm⁻¹, S=O symmetric stretch at 1155 cm⁻¹, and pyrimidine ring vibrations at 1585 cm⁻¹, 1560 cm⁻¹, and 1485 cm⁻¹. The C-N stretch appears at 1255 cm⁻¹ while S-N stretch vibrations occur at 915 cm⁻¹ and 885 cm⁻¹.

Proton nuclear magnetic resonance spectroscopy (¹H NMR, CDCl₃) shows signals at δ 0.92 (t, 3H, J = 7.3 Hz, CH₃-CH₂-), 1.25 (t, 3H, J = 7.1 Hz, NH-CH₂-CH₃), 1.30-1.38 (m, 2H, -CH₂-CH₂-CH₃), 1.58-1.65 (m, 2H, -CH₂-CH₂-), 2.35 (s, 3H, CH₃-C₆), 2.70 (t, 2H, J = 7.6 Hz, C₅-CH₂-), 3.15 (s, 6H, (CH₃)₂N-S), 3.40 (q, 2H, J = 7.1 Hz, NH-CH₂-), 5.20 (br s, 1H, NH), and 6.45 (s, 1H, pyrimidine H). Carbon-13 NMR displays signals at δ 13.8 (CH₃-CH₂-), 15.2 (NH-CH₂-CH₃), 22.1 (-CH₂-CH₃), 24.8 (C₅-CH₂-CH₂-), 27.9 (C₅-CH₂-), 33.5 ((CH₃)₂N-S), 38.2 (NH-CH₂-), 104.5 (C₅), 158.2 (C₂), 161.5 (C₄), 165.8 (C₆), and 172.4 (C=O).

Mass spectrometric analysis shows molecular ion peak at m/z 316 with major fragments at m/z 261 [M - C₄H₇]⁺, m/z 218 [M - C₄H₉NO]⁺, m/z 190 [C₈H₁₂N₃O]⁺, m/z 138 [C₆H₈N₃]⁺, and m/z 110 [C₅H₈N₂]⁺. UV-Vis spectroscopy reveals absorption maxima at 272 nm (ε = 12,400 M⁻¹·cm⁻¹) and 220 nm (ε = 8,700 M⁻¹·cm⁻¹) in ethanol solution.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Bupirimate demonstrates moderate hydrolytic stability with a half-life of 32 days at pH 7 and 25°C. Hydrolysis follows pseudo-first order kinetics with rate constants of 2.1×10⁻⁷ s⁻¹ (pH 5), 8.9×10⁻⁸ s⁻¹ (pH 7), and 3.2×10⁻⁶ s⁻¹ (pH 9) at 25°C. The activation energy for hydrolysis measures 86.4 kJ·mol⁻¹ at pH 7. Acid-catalyzed hydrolysis proceeds through protonation of the pyrimidine nitrogen followed by nucleophilic attack at the C₄ position. Base-catalyzed hydrolysis involves hydroxide ion attack at the sulfamate sulfur atom with subsequent breakdown to dimethylamine and the corresponding pyrimidinyl sulfate.

Photochemical degradation follows first-order kinetics with a half-life of 15 hours under simulated sunlight. Major photodegradation pathways include N-S bond cleavage, dealkylation of the ethylamino group, and oxidation of the butyl chain. The compound demonstrates stability toward oxidation by common oxidants but undergoes rapid cleavage with strong reducing agents. Thermal decomposition begins at 180°C with elimination of dimethylamine followed by fragmentation of the pyrimidine ring.

Acid-Base and Redox Properties

Bupirimate exhibits weak basic character with protonation occurring primarily at the pyrimidine N₃ position. The conjugate acid has a pK_a of 3.2, indicating the compound exists predominantly in its neutral form under environmental conditions. The ethylamino group shows negligible basicity due to conjugation with the aromatic system. Redox properties include a one-electron reduction potential of -1.23 V versus standard hydrogen electrode, indicating moderate susceptibility to reduction. Oxidation potential measures +1.56 V, suggesting resistance to oxidation under ambient conditions.

Electrochemical studies reveal irreversible reduction waves at -1.45 V and -1.85 V corresponding to sequential reduction of the pyrimidine ring. Oxidation occurs irreversibly at +1.70 V involving the sulfamate group. The compound demonstrates stability across a pH range of 4-9 with optimal stability at pH 6-7. Buffer capacity studies show minimal buffering action with a buffer index of 0.003 mol·L⁻¹ per pH unit.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

Laboratory synthesis of bupirimate typically begins with 4,6-dihydroxy-2-methylthiopyrimidine, which undergoes sequential functionalization. The synthetic pathway involves O-alkylation at the 4-position using dimethylsulfamoyl chloride in the presence of base, followed by nucleophilic displacement of the 2-methylthio group with ethylamine. Butylation at the 5-position employs butyl lithium or butyl magnesium bromide under anhydrous conditions.

A representative procedure involves reacting 4,6-dichloro-2-methylthiopyrimidine (1.0 equiv) with dimethylsulfamoyl chloride (1.1 equiv) in anhydrous tetrahydrofuran using sodium hydride (1.2 equiv) as base at 0°C for 2 hours. The intermediate undergoes selective displacement at the 4-position to give 4-chloro-6-hydroxy-2-methylthiopyrimidine dimethylsulfamate. Subsequent reaction with ethylamine (2.0 equiv) in ethanol at reflux for 4 hours replaces the 2-methylthio group. Finally, butylation using butyllithium (1.1 equiv) in tetrahydrofuran at -78°C followed by quenching with water affords bupirimate in overall yields of 45-50% after recrystallization from hexane-ethyl acetate.

Industrial Production Methods

Industrial production utilizes a more efficient route starting from barbituric acid derivatives. The process involves condensation of ethylurea with diethyl butylmalonate followed by chlorination and sulfamoylation. Key steps include cyclization under acidic conditions, selective chlorination at the 4-position using phosphorus oxychloride, and reaction with dimethylsulfamoyl chloride in the presence of tertiary amine bases.

Process optimization focuses on reaction temperature control between 0-5°C during sulfamoylation to minimize diester formation and solvent selection to facilitate product isolation. Typical production scales operate at 500-1000 kg batches with reaction times of 8-12 hours for the cyclization step and 4-6 hours for sulfamoylation. Purification employs fractional crystallization from isopropanol-water mixtures yielding technical grade bupirimate with purity exceeding 95%. Waste streams contain primarily inorganic salts and solvent residues, which undergo distillation recovery and biological treatment.

Analytical Methods and Characterization

Identification and Quantification

Chromatographic methods provide primary means for bupirimate analysis. Gas chromatography with flame ionization detection employs DB-5 capillary columns (30 m × 0.25 mm × 0.25 μm) with temperature programming from 100°C to 280°C at 10°C·min⁻¹. Retention time typically measures 12.4 minutes with detection limits of 0.05 mg·L⁻¹. High-performance liquid chromatography with UV detection at 272 nm utilizes C18 reverse-phase columns with acetonitrile-water (70:30) mobile phase at 1.0 mL·min⁻¹ flow rate, providing retention times of 6.8 minutes and detection limits of 0.02 mg·L⁻¹.

Mass spectrometric detection enhances specificity with characteristic ions at m/z 316 [M]⁺, 261 [M - C₄H₇]⁺, and 218 [M - C₄H₉NO]⁺. Liquid chromatography-tandem mass spectrometry achieves detection limits of 0.001 mg·L⁻¹ using multiple reaction monitoring transitions 316→261 and 316→218. Validation parameters include linearity range of 0.01-10 mg·L⁻¹ (r² > 0.999), accuracy of 95-105%, and precision with relative standard deviation less than 5%.

Purity Assessment and Quality Control

Quality control specifications for technical grade bupirimate require minimum purity of 95.0% with maximum limits for specific impurities: 4,6-dihydroxy-2-ethylaminopyrimidine (1.0%), 4-chloro-2-ethylamino-6-methylpyrimidine (0.5%), and bis(dimethylsulfamate) derivative (0.8%). Determination employs area normalization in gas chromatography or high-performance liquid chromatography. Residual solvent limits include methanol (<0.5%), tetrahydrofuran (<0.2%), and hexane (<0.1%).

Stability testing under accelerated conditions (40°C, 75% relative humidity) shows less than 2% degradation over 6 months. Forced degradation studies identify major degradation products as 5-butyl-2-ethylamino-6-methylpyrimidin-4-ol (hydrolysis product) and N-desethyl bupirimate (oxidative dealkylation product). Shelf life determinations estimate 36 months for properly stored material in original packaging at temperatures below 30°C.

Applications and Uses

Industrial and Commercial Applications

Bupirimate serves primarily as a fungicidal active ingredient in agricultural and horticultural formulations. Commercial products typically contain 25-30% active ingredient in emulsifiable concentrate or wettable powder formulations. Application rates range from 0.5-1.0 kg·ha⁻¹ for field crops and 0.1-0.3% concentration for horticultural uses. The compound demonstrates particular efficacy against powdery mildew diseases in apples, pears, stone fruits, cucurbits, roses, strawberries, gooseberries, currants, raspberries, hops, and beet crops.

Formulation technology often combines bupirimate with other fungicides or insecticides to broaden the spectrum of activity. Market analysis indicates annual production volumes of approximately 500-700 metric tons worldwide, with major manufacturing facilities in Europe and Asia. Economic assessments show production costs of $25-30 per kilogram with market prices of $40-50 per kilogram for technical grade material. The compound represents approximately 2-3% of the global fungicide market by value.

Research Applications and Emerging Uses

Research applications utilize bupirimate as a model compound for studying systemic transport in plants due to its xylem-mobile characteristics. Studies investigate structure-activity relationships in pyrimidine sulfamates, particularly examining the effects of alkyl chain length and sulfamate substitution patterns on biological activity and physicochemical properties. The compound serves as a synthetic intermediate for preparing radiolabeled analogs using carbon-14 or sulfur-35 for metabolic and environmental fate studies.

Emerging research explores potential applications in materials science, particularly as a building block for liquid crystalline compounds due to its rod-like molecular structure and polar end groups. Patent analysis shows ongoing development of bupirimate analogs with modified alkyl chains and sulfamate groups for improved environmental compatibility and reduced mammalian toxicity. Recent investigations examine coordination chemistry with transition metals, revealing potential applications in catalyst design and molecular recognition systems.

Historical Development and Discovery

The discovery of bupirimate emerged from systematic research at Imperial Chemical Industries' Jealott's Hill Research Station during the 1960s. Researchers sought fungicides capable of penetrating plant tissues and moving systemically to combat established infections. Initial discoveries included dimethirimol (1968) and ethirimol (1970), which demonstrated efficacy against various mildew species but showed limitations against apple powdery mildew.

Bupirimate represented the third generation in this series, first synthesized in 1972 and commercially introduced in 1975. The key innovation involved incorporating a sulfamate group at the 4-position of the pyrimidine ring, which enhanced systemic movement and improved activity against resistant mildew strains. Development work focused on optimizing the alkyl substituents, ultimately selecting the butyl chain for optimal balance of activity, mobility, and environmental persistence. The compound received regulatory approval in multiple countries throughout the late 1970s and remains in use today despite the development of newer fungicide classes.

Conclusion

Bupirimate represents a significant achievement in fungicide chemistry, combining a pyrimidine ring system with a sulfamate functional group to create a compound with unique systemic properties and biological activity. Its molecular architecture features careful balance of lipophilic character from the butyl chain and polar functionality from the sulfamate group, enabling both penetration into plant tissues and mobility within vascular systems. The compound demonstrates moderate environmental persistence and favorable toxicological profile compared to many contemporary fungicides.

Future research directions include development of more environmentally benign analogs with reduced persistence, investigation of non-agricultural applications in materials science, and exploration of mechanism of action at the molecular level. Ongoing synthetic efforts focus on creating analogs with improved selectivity and reduced potential for resistance development. The fundamental chemistry of bupirimate continues to provide insights into structure-activity relationships for systemic fungicides and design principles for compounds requiring specific distribution patterns within biological systems.