For this lithiation experiment, tert-butyl lithium amide (LiNHtBu) is synthesized via lithiation of tert-butyl amine (tBuNH 2 ) using tert-butyl lithium (tBuLi), forming isobutane as a side product. The described protocol is a modification of a previously-reported protocol31 and proceeds according to the following reaction:

tBuNH 2 + tBuLi → tBuH + 1/8 [LiNHtBu] 8 . (2)

The original report for the synthesis of LiNHtBu differs from this protocol in that it employed the use of less reactive n-butyl lithium as the organolithium reagent. In general, one should always choose the less reactive organolithium reagent whenever possible. However, for the purpose of this paper, the authors have elected to demonstrate the safe usage of the more reactive tert-butyl lithium solution so that viewers can observe the proper handling of the most challenging reagent. This protocol may be easily applied to the use of the less reactive organolithium reagents.

Critical Steps

Due to the highly pyrophoric nature of organolithium reagents, all operations must be carried out under inert atmosphere conditions, necessitating the use of a Schlenk or inert gas line, or an inert atmosphere glovebox. While operation in a glovebox is a far simpler approach, it is associated with its own risks, different from those of performing lithiations on an inert gas line. Either of these approaches therefore requires great care and adherence to protocol. Described here are two protocols for lithiation: one on an inert gas (Schlenk) line, and one within a glovebox. When performing a lithiation on an inert gas line, a familiarity with the operation of air-free glassware and protocols is invaluable. However, since different laboratories may adopt slightly different practices, a step-by-step protocol for each method is thoroughly described. The chemical vendor offers its own recommended glassware apparatus and protocol for proper use of air-sensitive reagents32. The Protocol section outlines a procedure similar to the vendor's, but which has been modified to maximize safety and ease, specifically for alkyllithium protocols. The detailed procedure is available in the Protocol section, but here, some important points are highlighted to maximize safety and success.

NOTE: Never work in the laboratory alone.

PPE

An extremely important consideration is the use of proper personal protective equipment (PPE), which for lithiation includes a proper-fitting lab coat, safety glasses, long pants (preferably made of non-flammable material), closed-toed shoes, and a hair tie (if applicable). While best practices can ensure that no fires occur in most cases, tert-butyl lithium is extremely pyrophoric, and accidents can happen. When they do, the safety of the researcher is better secured if they are shielded by the proper PPE. The UCLA alumna's most significant mistakes were that she performed a lithiation with no laboratory coat and that she was wearing clothing made of flammable material20.

Ventilation

Lithiations outside the glovebox should always be performed in a hood. If a clear hood is not available, do not perform a lithiation until a clear, uncluttered hood space free of other flammable chemicals is secured. The sash should be lowered as much and as often as possible. An additional mistake of the UCLA alumna was that there were other flammables in the hood (hexanes), which spilled and caught fire, igniting her clothes20.

Inert Gas

A lithiation requires the use of inert gas. A Schlenk line (double manifold switchable between inert gas and vacuum) is ideal, though any inert gas source with good flow control will work.

Syringe

Glass syringes are preferable to plastic syringes due to their chemical inertness and smoother plunger motion. A long (1-2 ft)32, flexible needle must always be attached securely to the delivery syringe. Another of the UCLA alumna's mistakes was the use of a too-short (1.5 inches)20 needle, which may have necessitated inverting the reagent bottle to draw the reagent into the syringe, which can lead to spills and fire. Thus, a long needle should always be used so that the bottle does not need to be inverted. The needle should be attached securely so that it does not pop off during reagent delivery. Luer-lock style syringes (Figure 2) are best. If using a push-on "slip-tip" syringe needle system, ensure that the needle is extremely well attached before proceeding. A syringe should always be selected that is at least twice the volume of the desired quantity of organolithium reagent32. This is due to the fact that head space always occupies some volume of the syringe while drawing a reagent. Another of the UCLA alumna's mistakes was the use of a syringe that was too small. When the syringe reached capacity, it likely popped open, splashing tBuLi onto her unprotected arm20.

Quenching Agents

A small beaker containing toluene (volume approximately equal to the volume of organolithium reagent to be delivered) should be located in the hood within reach of – but not right next to – the reaction vessel. A watch glass appropriately sized to cover this beaker in case of fire should also be placed over the beaker. This beaker will be used to dilute the residual reagent contaminating the syringe after the reagent addition (Figure 1).

A second beaker containing isopropanol (volume approximately five times the volume of organolithium reagent to be delivered) should also be located in the hood within reach of – but not right next to – the reaction vessel. A second watch glass appropriately sized to cover this beaker in case of fire should also be placed on top of the beaker. This vessel is used to quench the residue left in the syringe after the addition (Figure 1).

Third, a beaker of dry ice (approximately ten times the volume of organolithium reagent to be delivered) should be located in reach of the reaction vessel. In the event of the syringe needle coming loose, or something else going wrong, this dry ice may be used to quench the remaining organolithium reagent in the syringe (Figure 1).

Finally, a fire extinguisher should be located nearby in case of emergency, and the location and proper operation of the safety shower should be noted.

The Reagent Bottle

Outside the glovebox, use only organolithium reagent bottles with septum-sealed bottle caps (Figure 3). The purchase of small bottles is recommended since 1) organolithium reagents degrade over time, and long-term storage is not recommended, 2) septa can degrade over time, exposing the reagent to air, and 3) small volumes of pyrophorics are less dangerous than large volumes. The organolithium reagent bottle should be set on the bench and clamped to a ring stand before use (Figure 1).

The Reaction Vessel

The reaction vessel should be oven- or flame-dried and cooled to room temperature under an inert atmosphere to ensure that no traces of water exist on the sides of the glass. The vessel containing the reagent to which the organolithium solution will be added should be clamped above a stir plate and degassed to remove air. This may be done either by purging the vessel with inert gas or by performing several evacuation-inert gas fill cycles on a Schlenk line. Alternatively, the flask can be charged with reagents and solvent in an inert atmosphere glovebox and sealed before removal from the glovebox. The degassed flask should be fitted with a septum and protected by an inert gas blanket (see Protocol and Figure 1). If the synthetic protocol permits, the flask should also be immersed into a cold bath such as dry ice/acetone to control the exotherm that will result when the organolithium reagent is added.

Notes on Lithiation in an Inert-atmosphere Glovebox

The use of air-free gloveboxes makes the handling of air-sensitive reagents vastly simpler, but it comes with its own risks. Since organolithium reagents are shielded from air in the glovebox, it is easier to become complacent and careless. While handling the reagents is simpler, a spill within the glovebox creates a dilemma: the spilled reagent must be wiped up with paper towels, but then the pyrophoric reagent and flammable cloth must be removed from the box and placed back into air, at which point, they will immediately catch fire. To avoid these hazards, reagents and reaction flasks should always be clamped securely within the glovebox, and open bottles and flasks should never be moved or handled by hand. Any materials containing residual reagent should be removed from the glovebox in a sealed desiccator (or similar container) and moved to a hood before being opened and exposed to air.

Know the Location and Operation of Emergency Equipment

Know the location and operation of the lab's fire extinguisher, so that in the event of a fire that cannot be put out by smothering with a watch glass, one can react quickly and decisively. Know also the location and operation of the laboratory's safety shower. In the unlikely event that a piece of clothing catches fire, immediately use the safety shower. If someone else's clothes catch fire, immediately direct them to the safety shower. If the laboratory does not have both a safety shower and a fire extinguisher, do not attempt a lithiation reaction. What may have been the final opportunity to save the life of the UCLA alumna was missed when neither she nor the postdoc working with her used the safety shower or an extinguisher to extinguish the flames. Rather, her postdoctoral coworker attempted to pat out the flames with a lab coat, which also caught fire. Ultimately, she sat on the floor while her postdoctoral coworker attempted to put out the flames by pouring beakers of water, filled from the sink, on the flames20.

Organolithium reagents are excellent for the deprotonation of weakly-acidic hydrogens or for acting as a source of alkyl groups, and they are more aggressive and reactive than the more standard Grignard reagents. Limitations of this technique can include kinetically sluggish reactions, in which case modification of the protocol can aid the chemical transformation19. Additionally, the high reactivity of organolithiums can interfere with desired chemistry. For instance, carbanions are generally excellent nucleophiles. Attempted deprotonation of an electrophilic substrate (such as a carboxylic acid) is likely to lead to nucleophilic attack instead of deprotonation. Thus, chemical knowledge and intuition is required when selecting reagents of this (or any) sort. Lithiation reactions will continue to play a role in synthetic organic and inorganic chemistry for the foreseeable future, and thus, an understanding of safe use is essential. Lithiation reactions are accomplished safely every day, and there is no cause to fear performing this reaction chemistry. However, the reagents deserve a measure of respect and care. It is essential that the multiple required fail-safes be followed to avoid the possibility of injury. In this protocol, a step-by-step procedure for a safe lithiation reaction is demonstrated and published as an open access article so that any researcher in the world can use it as training, free of charge. As such, the authors hope that this report may make the lithiation protocol accessible to a wide array of groups and prevent future tragedies.