Phenyl azide amines

The thermal, and more importantly, the photolytic decomposition of aryl azides in the presence of nucleophiles, generally amines or alcohols, is the commonest method for preparing 3H-azepines. In fact, jV-phenyl-3//-azepin-2-amine (32, R = Ph), the first example of a 3//-azepine, was prepared by thermal decomposition of phenyl azide in aniline.32... 

Pioneering studies have shown that the yield of iV-phenyl-3//-azepin-2-amine (32, R = Ph) from the thermolysis of phenyl azide in aniline increases as the ratio of azide to aniline decreases, and in dilute solution with an azide to aniline ratio of 1 200 a 54% yield of the 3//-azepine can be achieved.34 The reaction is successful with other arylamines, but the procedure is of limited preparative value as large volumes of amine are required and only moderate yields of 3H-azepines are obtained. 

Interestingly, photolysis of phenyl azide in liquid ammonia yields 3//-azepin-2-amine (39)35 (see experimental procedure in Houben-Weyl, Vol.4/5b, pi268). 

An attempt to improve the yield of the azepin-2-amine by the addition of sodium amide to the photolysate failed, as a rapid nonphotolytic reduction of the phenyl azide to aniline took place. 

Photolysis of aryl azides in amine solution, with a tertiary amine as cosolvent to promote stabilization of the singlet nitrene, has met with some success. For example, the yield of 2-piperidino-3 W-azepme. obtained by the photolysis of phenyl azide in piperidine, is increased from 35 to 58% in the presence of A A /V. /V -tetramethylethylenediamine (TMLDA).180 Also, an improved yield (36 to 60 %) of A,(V-diethyl-3W-azepin-2-amine (38, R = Et) can be obtained by irradiating phenyl azide in triethylamine, rather than in dicthylaminc, solution.181 Photolysis (or thermolysis) of phenyl azide in TMEDA produces, in each case, 38 (R = Et) in 40% yield.181 In contrast, irradiation of phenyl azide in aniline with trimethylamine as cosolvent furnishes jV-phenyl-377-azepin-2-amine (32, R = Ph) in only low yield (2%).35... 

Photolysis of 2-(cu-phenylalkyl)phenyl azides 42 (n = 1-4) in diethylamine, followed by heating with methanol, furnishes 7V,Ar-diethyl-3-(a>-phenylalkyl)-3//-azepin-2-amines 43, albeit in low yields.295... 

Base-catalyzed loss of hydrogen fluoride from the initially formed //../V-diethylG-ltrifluoro-methyl)-3//-azepin-2-amine (66) to give iV,ALdiethyl-3-(difluoromethylene)-3//-azepin-2-amine (67) occurs on photolysis of 2-(trifluoromethyl)phenyl azide (65) in diethylamine.10... 

Figure 4.21 BASED can react with molecules after photoactivation to form crosslinks with nucleophilic groups, primarily amines. Exposure of its phenyl azide groups to UV light causes nitrene formation and ring expansion to the dehydroazepine intermediate. This group is highly reactive with amines. The cross-bridge of BASED is cleavable using a disulfide reducing agent.

Figure 5.16 Photoactivation of a phenyl azide group with UV light results in the formation of a short-lived nitrene. Nitrenes may undergo a number of reactions, including insertion into active carbon-hydrogen or nitrogen-hydrogen bonds and addition to points of unsaturation in carbon chains. The most likely route of reaction, however, is to ring-expand to a dehydroazepine intermediate. This group is highly reactive toward nucleophiles, especially amines.

HSAB (N-hydroxysuccinimidyl-4-azidobenzoate) is a heterobifunctional reagent containing an amine-reactive NHS ester on one end and a photoreactive phenyl azide group on the other end... 

Figure 5.20 Sulfo-HSAB is a short photoreactive crosslinker that can be used to modify amine-containing molecules through its NHS ester end to form amide linkages. After photoactivation, the phenyl azide group can react with amines to create a covalent bond.

SANPAH (N-succinimidyl-6-(4 -azido-2 -nitrophenylamino)hexanoate) is a heterobifunctional crosslinking agent containing an NHS ester and a photoreactive phenyl azide group (Thermo Fisher). The NHS ester end can react with amine groups in proteins and other molecules, forming... 

Figure 5.22 The NHS ester of ANB-NOS reacts with amines to form amide bonds. Subsequent photoactivation of the complex with UV light causes phenyl azide ring expansion and reaction with neighboring amines.

SADP, N-succinimidyl-(4-azidophenyl)l,3 -dithiopropionate, is a photoreactive heterobifunctional crosslinker that is cleavable by treatment with a disulfide reducing agent (Thermo Fisher). The crosslinker contains an amine-reactive NHS ester and a photoactivatable phenyl azide group, providing specific, directed coupling at one end and nonselective insertion capability at the other end. 

Figure 5.25 The reaction of sulfo-SAPB with an amine group is done first to form an amide bond derivative through its NHS ester end. Subsequent exposure to UV light causes the phenyl azide group to ring-expand to a highly reactive dehydroazepine, which can couple to nucleophiles, such as amines.

Sulfo-SAMCA, sulfosuccinimidyl-7-azido-4-methylcoumarin-3-acetate, is a heterobifunctional reagent similar in design to SAED (Section 3.9, this chapter) (Thermo Fisher). One end of the crosslinker contains an amine-reactive sulfo-NHS ester, while the other end is an AMCA derivative (Chapter 9, Section 3) that contains a photosensitive phenyl azide group. Unlike... 

Figure 5.35 ABH reacts with aldehyde-containing compounds through its hydrazide end to form hydrazone linkages. Glycoconjugates may be labeled by this reaction after oxidation with sodium periodate to form aldehyde groups. Subsequent photoactivation with UV light causes transformation of the phenyl azide to a nitrene. The nitrene undergoes rapid ring expansion to a dehydroazepine that can couple to nucleophiles, such as amines.

Figure 5.37 APG can be used to label specifically arginine residues in proteins, producing stable, cyclic Schiff base-like bonds with the side-chain guanidino groups. Photoactivation with UV light then causes ring expansion of the phenyl azide group, initiating covalent bond formation with amines.

Since the active ester end of the molecule is subject to hydrolysis (half-life of about 20 minutes in phosphate buffer at room temperature conditions), it should be coupled to an amine-containing protein or other molecule before the photolysis reaction is done. During the initial coupling procedure, the solutions should be protected from light to avoid decomposition of the phenyl azide group. The degree of derivatization should be limited to no more than a 5- to 20-fold molar excess of sulfo-SBED over the quantity of protein present to prevent possible precipitation of the modified molecules. For a particular protein, studies may have to be done to determine the optimal level of modification. 

Figure 6.1 The Wedekind trifunctional crosslinker can react with amine groups via its p-nitrophenyl ester to form amide bond linkages. The phenyl azide group then can be photoactivated with UV light to generate covalent bond formation with a second molecule. The biotin side chain provides binding capability with avidin or streptavidin probes.

Figure 11.14 Photobiotin can be made to couple spontaneously with nucleophiles by exposure to UV light. The phenyl azide ring undergoes ring expansion to a highly reactive dehydroazepine intermediate, which can react with amines.

Figure 28.12 Sulfo-SBED first is used to label a bait protein through reaction of the sulfo-NHS ester with available amine groups on the protein, yielding an amide bond linkage. This labeled bait protein then is added to a sample containing proteins that potentially could interact with the bait. After an incubation period, the sample is exposed to UV light to photoactivate the phenyl azide group. This reaction causes any interacting prey proteins to be crosslinked with the bait protein, forming a complex containing a biotin affinity tag.

However, in 1978, Chapman and LeRoux discovered that photolysis of phenyl azide, matrix isolated in argon at 10 K, produces a persistent species with a strong vibrational band at 1880 10 cm . The carrier of this species was most reasonably assigned to ketenimine 30 rather than benzazitine 29 or triplet phenylnitrene. This result imphes that it is the ketenimine 30 and not benzazirine 29 that is trapped with amines to form the 37/-azepines (27) that had been isolated earher. It does, however, raise the question as to why two groups observed triplet phenylnitrene by low temperature spectroscopy while a third observed ketenimine 30. 

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