Cbz Protecting Group: N-Cbz Protection & Deprotection Mechanism

You can zoom and play around with this 3D model of the Cbz protecting group!
Note the orientation of the phenyl ring: it’s not co-planar with the carbamate system!

What is the Cbz Protecting Group?

Cbz is a benzyloxycarbonyl group (formerly carboxybenzyl) and protects amines as carbamates. More rarely, chemists might use Cbz to protect alcohols as their carbonates.

Leonidas Zervas (no not the one from the movie “300”) introduced the Cbz group, thus also abbreviated as Z [1]. With this discovery, Leonidas and his advisor Bergmann spearheaded the field of controlled peptide chemistry. In the 1930s, Cbz unlocked the synthesis of previously inaccessible oligopeptides. Zervas continued to be a driving force in peptide chemistry, including development of other protecting groups.

Cbz Protection Mechanism

Cbz protection is typically performed with Cbz-Cl either under Schotten-Baumann conditions (carbonate base) or with an organic base. The mechanism is attack of the nucleophilic amine to the highly reactive chloroformate. As chloride is the leaving group, the reaction liberates HCl and requires some base.

Like we’ve seen for Boc, Cbz₂O or other activated agents (e.g., Cbz-OSu) can offer more optionality, depending on the system. If you are into exotic reagents, you might like reagents A or B in the next figure – basically, anything with an activated “Cbz+” synthon works.

As is common the case, we can protect amines selectively given their higher nucleophilicity. However, there have been some reports of challenging selective protection of secondary amines over secondary alcohols. As always, our rules of thumb depend on the specific system at hand.

Cbz DeProtecTION Mechanism With Hydrogenolysis

Hydrogenolysis deprotects Cbz protecting groups, usually in an easy and rapid manner. The mechanism is a reduction with H₂, releasing toluene and the free carbamate. Consequently, decarboxylation to the deprotected amine is very much favoured (particularly at elevated temperatures).

Besides molecular H₂, it is possible to use other “H₂ donors” through transfer hydrogenation reactions (e.g., see procedures below).

Cbz protecting group Orthogonality

Cbz is orthogonal to Boc, Trt, Fmoc and other common protecting groups. As mentioned, it was an essential part of the early days of peptide synthesis.

But beware thinking it’s fully orthogonal! Although Cbz tolerates some acid, harsh conditions can cleave it as well (e.g., excess HCl, HBr)! This mechanism includes protonation of the carbamate and liberation through S_N2 and decarboxylation.

Beyond hydrogenation, Cbz can be susceptible to other transition metal catalysis as well, for instance Ni(0) or Pd(0). This interesting case report demonstrated selective removal of double Cbz-protected histidine [2] . Compared to heteroaromatic nitrogen atoms, originally basic amines did not engage in any reaction.

This is a great question for you: Suggest a mechanism for the reaction (note the other reagent!) and propose an explanation for the selectivity!

Cbz use cases and tricks

As always, it would be boring to just look at the simplest case of nitrogen protection and deprotection. Let’s briefly discuss three additional topics [3].

First, Cbz protects other nucleophilic functional groups like alcohols, phenols, thiols… as well. In these cases, an organic base (not carbonate) in dichloromethane or some ether-based solvent is typically used. To increase reactivity, NaH as base allows for protection of deactivated, tertiary alcohols:

Notably, Cbz-Cl as a reagent can activate pyridines to regioselective nucleophilic attack. The use of electrophiles for these purposes is a key concept in heterocyclic chemistry. There is a nice collection on 1-acylpyridiniums by the Baran Lab.

Third and coolest, the Cbz group can serve as a masked N-methylamine. Treatment with LiAlH₄ exhaustively reduces the carbamate to the alkane.

Cbz Protection experimental procedure [4]

“To the SM (1.70 g, 2.64 mmol) in THF/H2O (2:1, 15 mL) was added NaHCO3 (443 mg, 5.27 mmol) and Cbz-Cl (0.56 mL, 3.96 mmol) at 0 °C and the solution was stirred for 20 h at the same temperature. The reaction mixture was diluted with H2O and extracted with AcOEt. The combined organic layers were washed with brine, dried over Na2SO4, and concentrated in vacuo. The resulting residue was purified by silica gel column chromatography (40% AcOEt/n-hexane) gave 20 (1.85 g, 2.38 mmol) in 90% yield as a white powder.”

Cbz deprotection experimental procedure

[4] Normal hydrogenolysis
To a solution of SM (20.7 mg, 15.0 micromol) in 2 mL of MeOH were added 5% Pd-C (6.4 mg), and the mixture was stirred at 60 °C for 40 h under atmospheric pressure of H2. Then, the catalyst was filtered of on pad of celite. The filtrate was concentrated in vacuo to give a crude material containing 27, which was used without purification for the next step.

[5] Deprotection of 5 Cbz groups in the final step via transfer hydrogenation.
To a solution of protected caprazamycin A (32) (6.5 mg, 3.21 micromol) in EtOH/HCO2H (1.9 ml, v/v = 20:1) was added Pd black (66.5 mg, 625 micromol) at 25 °C, and the resultant mixture was stirred for 1.5 h at 25 °C. After filtration through celite, the filtrate was concentrated under reduced pressure. The resultant residue was purified by C18 reversed phase. Caprazamycin A eluted at 11-18 min as an isolated peak. The eluent was collected and concentrated under reduced pressure to give a pure caprazamycin A (3.6 mg, 98%) as a colorless powder:

Cbz Protecting Group References

• [1] M. Bergmann, L. Zervas, Berichte der deutschen chemischen Gesellschaft. 1932, 65, 1192 | Über ein allgemeines Verfahren der Peptid-Synthese
• [2] B. H. Lipshutz, S. S. Pfeiffer, A. B. Reed, Org. Lett. 2001, 3, 4145 | Selective Cleavage of Cbz-Protected Amines
• [3] J. Pearson, W. R. Roush, Handbook of Reagents for Organic Synthesis: Activating Agents and Protecting Groups (Wiley)
• [4] Org. Lett. 2014, 16, 3364 | Catalytic Asymmetric Total Synthesis of (+)-Caprazol
• [5] Angew. Chem. Int. Ed. 2015, 54, 3136 | Total Synthesis of (−)-Caprazamycin A
• P. Kocienski, Protecting Groups (Thieme Verlag)
• P. Wuts, T. Greene, Greene’s Protective Groups in Organic Synthesis (Wiley)