A QM Study of the Reaction Mechanism of Diazotransfer
QM Magic Class | Chapter 24
1,2,3-Triazole is a common heterocycle in medicinal chemistry, providing us novel structures with a wide range of biological activities. 1,4-Disubstituted 1,2,3-triazoles are versatile (Figure 1): they have been used as part of the pharmacophores; as bioequivalent for specific functional groups; as peptidomimetic to improve metabolic stability against proteases; and as linkers for multispecific drugs, such as PROTAC®, drug conjugates, and so on .
1,2,3-triazoles are usually constructed with [3 + 2] cycloaddition reaction, including the Cu(I) catalyzed Click Reaction (Figure 2) developed by Professor Sharpless. The required azido compounds are highly reactive, yet usually associated with toxic and explosive properties. As such, safer methods for their generation and handling are highly desirable in MedChem discovery efforts.
In 2019, Jiajia Dong of Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, and Barry Sharpless published in Nature the use of fluorosulfuryl azide as a safer and more efficient reagent for the preparation of various azides and their use in the Click reaction (aka Double Click) to construct a triazole library with more than a thousand compounds (Figure 3).
Fluorosulfuryl azide, FSO2N3, introduced in this paper shows superior efficiency in diazo transfer to primary amines. Compared with the traditional diazo transfer reagents, it presents many advantages: the reactions are carried out at room temperature, 1:1 stoichiometric, fast and efficient conversion, suitable for all primary amines, including tertiary alkyl amines (e.g. t-BuNH2), near quantitative conversion. In this chapter, we’ll discuss QM analysis of the reaction mechanism of this useful diazo transfer reagent.
Discussion on Reaction Mechanism
Fluorosulfuryl azide is highly electrophilic. Among the three azido nitrogen, we need to determine which nitrogen atom is more electrophilic to interact with the incoming amine (Fig. 4).
The LUMO and LUMO map of fluorosulfuryl azide were calculated first (see Chapter 1: Application of LUMO Calculation in Nucleophilic Reactions ). Results are shown on Figure 5, with LUMO lobe on N-3 > N-2 > N-1, and LUMO map clearly shows that N-3 is more accessible than N-2, suggesting that nucleophilic attack is more likely to occur on N-3. This is consistent with results from a 15N isotope labeling experiment reported by Samuelson et al. in 2014.
Modeling of the First Step
The Nature paper pointed out that water is crucial for fluorosulfuryl azide to transfer a diazo group to amines efficiently. Our preliminary calculations indicated that nucleophilic attack of amine groups is a stepwise mechanism, not a one-step concerted mechanism (Figure 6), and shall include a water molecule in the reaction modeling, i.e. the amine attacks N-3 of fluorosulfuryl azide first, followed by transfer of a hydrogen atom from the amino group via a water molecule to N-1 of the reagent.
Shown on Figure 7-1 is the reaction energy profile calculated for the addition step, with the distance of the amine N—N-3 bond changing from 2.4 Å to 1.5 Å over 10 steps, and an energy barrier less than 5 kcal/mol. Next, we used the final structure in the addition step as the starting point to calculate for its reaction energy profile of the hydrogen atom transfer step. The results show that the activation energy of this step is about 15 kcal/mol, an energy barrier that could readily be overcome under room temperature conditions (Figure 7-2) . The hydrogen transfer is relayed via a water molecule, with a seven-membered ring transition state.
We selected the structures at the highest energy point from the two reaction energy profiles for more accurate Transition State and imaginary frequency calculations. Resulting transition state structures are shown on Figure 8, with their respective imaginary frequency of 136 and 1280 cm-1 that are corresponding to the bonds being made and broken, satisfying the criteria for them to be considered as transition states.
Modeling of the Second Step
Next, we calculated for the reaction energy profile of the N-1—N-2 bond cleavage of the amine-FSO2N3 addition complex and the transfer of the second hydrogen atom from amine to N-1 step, via a water molecule. Result is supportive of a concerted mechanism with an activation energy of about 20 kcal/mol (Figure 9). This step has the highest energy barrier for the whole diazo transfer reaction, indicating that this is the Rate Limiting Step of the reaction. Calculated transition state has a single ifrequency at 239 cm-1, offering further support to our water catalysis model.
Shown on Figure 10 is an overall reaction energy profile of the reaction. We set the relative energy of the starting species as zero, and included the key reaction intermediates and transition states in the energy plot. The energy barrier of each step of the reaction is relatively low, consistent with room temperature reaction conditions. The reaction is exothermic, ~22 kcal/mol. Reversing the reaction from the products side will encounter an energy barrier of >30 kcal/mol, consistent with the observation that this diazotransfer reaction is not reversible at room temperature.
In summary, to establish our current QM model of this FSO2N3 diazotransfer reaction, we calculated reaction energy profiles for various possible mechanisms that could account for the key experimental observations. We conducted further transition state and imaginary frequency calculations to ensure that they meet the essential criteria. A useful protocol for qualitative QM study of reaction mechanism of interest.
Building on What We Just Learned
Reaction of carbon dioxide with water to form carbonic acid is a process of important geological and biological significances. Naturally, we are curious to find out how many water molecules are involved in the formation of a molecule of carbonic acid  . One, two, three, or four?
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 Spartan’20 Tutorial and User’s Guide (2021). Irvine, CA, USA: Wavefunction, Inc. pages 160, 538, & 667. In computational chemistry, molecular vibrations near the transition state can be described approximately with a harmonic oscillator model. As shown in the equation below, the vibration frequency is proportional to the square root of the ratio of force constant k and reduced mass μ (a combination of the masses of atoms involved in motion along that coordinate). The force constant k is the curvature of the potential energy surface along a specific coordinate (defined by the second derivative of the energy relative to the geometric coordinate). Transition state is located at the saddle point on the potential energy surface. The curvature of the transition state on the reaction path is negative and the reduced mass is positive, so the frequency calculated is an imaginary number. Having one and only one imaginary frequency that corresponds to vibration of the bonds being made/broken is a crucial criteria to satisfy for the calculated structure to be considered as a transition state.
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This article is written and edited by Zhong Zheng, Qiuyue Wang, Jian Wang, Dong Pan, Yongsheng Chen, and John S. Wai.