Question

The BOC-protecting group may be added by treatment of an amino acid with di- tertbutyl dicarbonate as shown in the following reaction sequence. Propose a mechanism to account for formation of these products.

Short answer

Answer: The proposed mechanism involves a nucleophilic attack of the amino group on the carbonyl carbon of di-tert-butyl dicarbonate, followed by the formation of an intermediate O- ion. The O- ion captures a hydrogen from the amino group, resulting in the BOC-protected amino acid and a water molecule as the products.

Step-by-step solution

Identify the Reactants and Products

: Given the exercise, the reactants are an amino acid and di-tert-butyl dicarbonate, and the products will include the amino acid with the BOC-protecting group attached. First, we will start by understanding the structure of the reactants and then propose the mechanism.

Understand the Structure of Reactants

: For a generic amino acid, the structure will have an amino group (-NH2), a carboxylic acid group (-COOH), and a side chain 'R'. Di-tert-butyl dicarbonate has the following structure: (C(=O)(N(OC(CH3)3)2)OC(CH3)3). Notice that it has two tert-butyl groups (OC(CH3)3) and a carbonyl group (C=O) connected to a nitrogen atom in the middle.

Nucleophilic Attack on Di-tert-butyl Dicarbonate

: The amino group of the amino acid will act as a nucleophile and attack the carbonyl carbon of di-tert-butyl dicarbonate, forming a new bond between the nitrogen of the amino group and the carbon of the carbonyl group. This results in the formation of an intermediate O- ion, which comes from the carbonyl oxygen.

Formation of the BOC-Protected Amino Acid

: The O- ion will attract a hydrogen from the amino group's hydrogen (NH2 to NH), forming a BOC-protected amino acid with the following structure: R-NH-C(=O)ON(OC(CH3)3)2 and a water molecule.

Summary of the Mechanism

: To summarize, the proposed mechanism for the BOC-protection of an amino acid using di-tert-butyl dicarbonate involves a nucleophilic attack of the amino group on the carbonyl carbon, followed by the formation of an intermediate O- ion. The O- ion captures a hydrogen from the amino group, resulting in the BOC-protected amino acid and a water molecule as the products.