tert -Butyl N -Hydroxycarbamate ( N -Boc-Hydroxylamine)

: The X-ray structure of the title compound has been determined for the ﬁrst time. This features ribbons formed by two parallel chains of molecules joined by C=O ··· H–N hydrogen bonding, which are then cross-linked by C=O ··· H–O hydrogen bonds. The structure is compared to those of closely related compounds


Introduction
The simple compound tert-butyl N-hydroxycarbamate or N-tert-butoxycarbonylhydrox ylamine 1 (Figure 1) was first reported in 1959 [1] and has proven to be useful in a number of recent applications, including the generation of t-Boc-N=O for use as a Diels Alder dienophile [2].Such simple multifunctional compounds have the potential to form a wide range of interesting hydrogen bonding patterns in the solid state, and similar compounds such as the corresponding carbazate 2 [3], N-pivaloylhydroxylamine 3 [4] and tert-butyl carbamate 4 [5,6] have all been reported to show such patterns, as have a range of N-acylhydroxylamines such as 5 [7] and 6-9 [8].We describe here the X-ray structure determination of compound 1.

Introduction
The simple compound tert-butyl N-hydroxycarbamate or N-tert-butoxycarbonylhydroxylamine 1 (Figure 1) was first reported in 1959 [1] and has proven to be useful in a number of recent applications, including the generation of t-Boc-N=O for use as a Diels Alder dienophile [2].Such simple multifunctional compounds have the potential to form a wide range of interesting hydrogen bonding patterns in the solid state, and similar compounds such as the corresponding carbazate 2 [3], N-pivaloylhydroxylamine 3 [4] and tertbutyl carbamate 4 [5,6] have all been reported to show such patterns, as have a range of N-acylhydroxylamines such as 5 [7] and 6-9 [8].We describe here the X-ray structure determination of compound 1.

Results
The compound was prepared using a method from the literature [9] involving the reaction of hydroxylamine hydrochloride, potassium carbonate and di-tert-butyl dicarbonate.It had physical and NMR data (see Supplementary Materials) in agreement with published values [9].Crystals suitable for diffraction were obtained by recrystallisation from hexane.The resulting molecular structure (Figure 2) shows fairly conventional bond

Results
The compound was prepared using a method from the literature [9] involving the reaction of hydroxylamine hydrochloride, potassium carbonate and di-tert-butyl dicarbonate.It had physical and NMR data (see Supplementary Materials) in agreement with published values [9].Crystals suitable for diffraction were obtained by recrystallisation from hexane.The resulting molecular structure (Figure 2) shows fairly conventional bond lengths and angles.The N-hydroxycarbamate functionality is quite accurately planar with a torsion angle O(1)-C(1)-N(1)-(O3) of +2.1(2) • .In fact, this is characteristic of the compounds of Figure 1 with none having a corresponding torsion angle above 12 • (Table 1).
However, as expected, it is the pattern of interactions formed by hydrogen bonding that is more interesting.Each carbonyl oxygen is hydrogen bonded to the N-H and O-H of two separate adjacent molecules, resulting in a ribbon structure featuring a series of ten-membered rings (Figure 3, Table 2).The molecules are linked in two parallel chains by C=O•••H-N bonding and these are then cross-linked by C=O•••H-O hydrogen bonds, giving a graph set description of C(4)R 2 3(10) [10].

D-H•••
Molecular structure of 1 with anisotropic displacement ellipsoids drawn at 50% probability level and the numbering system used.
However, as expected, it is the pattern of interactions formed by hydrogen bonding that is more interesting.Each carbonyl oxygen is hydrogen bonded to the N-H and O-H of two separate adjacent molecules, resulting in a ribbon structure featuring a series of ten-membered rings (Figure 3, Table 2).The molecules are linked in two parallel chains by C=O•••H-N bonding and these are then cross-linked by C=O•••H-O hydrogen bonds, giving a graph set description of C(4)R 2 3 (10) [10].
However, as expected, it is the pattern of interactions formed by hydrogen bonding that is more interesting.Each carbonyl oxygen is hydrogen bonded to the N-H and O-H of two separate adjacent molecules, resulting in a ribbon structure featuring a series of ten-membered rings (Figure 3, Table 2).The molecules are linked in two parallel chains by C=O•••H-N bonding and these are then cross-linked by C=O•••H-O hydrogen bonds, giving a graph set description of C(4)R 2 3(10) [10].

D-H•••A D-H H•••A D•••A D-H•••A
Ref.Among the related compounds shown in Figure 1, the structure is most similar to that of N-pivaloylhydroxylamine 3 [4].This also features a ribbon with each carbonyl oxygen hydrogen-bonded to the N-H and O-H of two separate molecules (Figure 4, Table 2).However, the important distinction is that, while 1 has the two chains of molecules aligned in parallel, in 3, they are antiparallel, and the structure can be viewed as a series of R 2 2 (10) dimers formed by C=O•••H-O bonding, which are then joined in the chain by the C=O•••H-N bonding, resulting in a first-order network of C(4)R 2 2 (10) and a second-order network of R 2 4 (10) [10].
Molbank 2023, 2023, x FOR PEER REVIEW 3 of 5 Among the related compounds shown in Figure 1, the structure is most similar to that of N-pivaloylhydroxylamine 3 [4].This also features a ribbon with each carbonyl oxygen hydrogen-bonded to the N-H and O-H of two separate molecules (Figure 4, Table 2).However, the important distinction is that, while 1 has the two chains of molecules aligned in parallel, in 3, they are antiparallel, and the structure can be viewed as a series of R 2 2(10) dimers formed by C=O•••H-O bonding, which are then joined in the chain by the C=O•••H-N bonding, resulting in a first-order network of C(4)R 2 2(10) and a second-order network of R 2 4(10) [10].Although it is not obvious from Figure 3, the ribbons of hydrogen-bonded molecules of 1 are not planar, but distinctly V-shaped.The angle between the mean planes defined by O(2), C(1), O(1), N(1) and O(3) of molecules on the two sides of the ribbon is 62.62(4)°.A view of the unit cell along the a-axis shows how the hydrogen-bonded chains of molecules are aligned in a zig-zag pattern (Figure 5).By comparison, the remaining compounds identified in Figure 1 show an interesting variety of hydrogen bonding patterns.For tert-butyl carbazate 2, there are four independent molecules joined in a rectangular array by N•••H-N (NH2) interactions, and these units are then joined in infinite chains by the C=O•••H-N (NH) hydrogen bonding [3].For tertbutyl carbamate 4, we have simple R 2 2(8) dimers [5,6].N-Propionylhydroxylamine 5 has three independent molecules and six separate hydrogen bonding interactions [7], and Nacetylhydroxylamine 6 has two independent molecules with five separate hydrogen bonding interactions [8], resulting, in each case, in highly complex three-dimensional networks.In contrast, the longer chain analogues N-hexanoyl-, N-decanoyl-and N-dodecanoylhydroxylamine 7-9 all form two-dimensional layers with a central region formed by only two hydrogen bonding interactions and an outer hydrophobic region accommodating the hydrocarbon chains [8].Although it is not obvious from Figure 3, the ribbons of hydrogen-bonded molecules of 1 are not planar, but distinctly V-shaped.The angle between the mean planes defined by O(2), C(1), O(1), N(1) and O(3) of molecules on the two sides of the ribbon is 62.62(4) • .A view of the unit cell along the a-axis shows how the hydrogen-bonded chains of molecules are aligned in a zig-zag pattern (Figure 5).
Molbank 2023, 2023, x FOR PEER REVIEW 3 of 5 Among the related compounds shown in Figure 1, the structure is most similar to that of N-pivaloylhydroxylamine 3 [4].This also features a ribbon with each carbonyl oxygen hydrogen-bonded to the N-H and O-H of two separate molecules (Figure 4, Table 2).However, the important distinction is that, while 1 has the two chains of molecules aligned in parallel, in 3, they are antiparallel, and the structure can be viewed as a series of R 2 2(10) dimers formed by C=O•••H-O bonding, which are then joined in the chain by the C=O•••H-N bonding, resulting in a first-order network of C(4)R 2 2(10) and a second-order network of R 2 4(10) [10].Although it is not obvious from Figure 3, the ribbons of hydrogen-bonded molecules of 1 are not planar, but distinctly V-shaped.The angle between the mean planes defined by O(2), C(1), O(1), N(1) and O(3) of molecules on the two sides of the ribbon is 62.62(4)°.A view of the unit cell along the a-axis shows how the hydrogen-bonded chains of molecules are aligned in a zig-zag pattern (Figure 5).By comparison, the remaining compounds identified in Figure 1 show an interesting variety of hydrogen bonding patterns.For tert-butyl carbazate 2, there are four independent molecules joined in a rectangular array by N•••H-N (NH2) interactions, and these units are then joined in infinite chains by the C=O•••H-N (NH) hydrogen bonding [3].For tertbutyl carbamate 4, we have simple R 2 2(8) dimers [5,6].N-Propionylhydroxylamine 5 has three independent molecules and six separate hydrogen bonding interactions [7], and Nacetylhydroxylamine 6 has two independent molecules with five separate hydrogen bonding interactions [8], resulting, in each case, in highly complex three-dimensional networks.In contrast, the longer chain analogues N-hexanoyl-, N-decanoyl-and N-dodecanoylhydroxylamine 7-9 all form two-dimensional layers with a central region formed by only two hydrogen bonding interactions and an outer hydrophobic region accommodating the hydrocarbon chains [8].By comparison, the remaining compounds identified in Figure 1 show an interesting variety of hydrogen bonding patterns.For tert-butyl carbazate 2, there are four independent molecules joined in a rectangular array by N•••H-N (NH 2 ) interactions, and these units are then joined in infinite chains by the C=O•••H-N (NH) hydrogen bonding [3].For tert-butyl carbamate 4, we have simple R 2 2 (8) dimers [5,6].N-Propionylhydroxylamine 5 has three independent molecules and six separate hydrogen bonding interactions [7], and N-acetylhydroxylamine 6 has two independent molecules with five separate hydrogen bonding interactions [8], resulting, in each case, in highly complex three-dimensional networks.In contrast, the longer chain analogues N-hexanoyl-, N-decanoyland Ndodecanoylhydroxylamine 7-9 all form two-dimensional layers with a central region formed by only two hydrogen bonding interactions and an outer hydrophobic region accommodating the hydrocarbon chains [8].
In summary, we obtained the X-ray crystal structure of tert-butyl N-hydroxycarbamate for the first time and found the molecules to exist, when viewed along the b-axis, in crosslinked parallel chains with each carbonyl oxygen hydrogen bonded to both NH and OH of adjacent molecules.These then pack in a zig-zag pattern when viewed along the a-axis.

Figure 1 .
Figure 1.Structure of compound 1 and similar crystallographically characterised compounds with CSD RefCodes.

Figure 1 .
Figure 1.Structure of compound 1 and similar crystallographically characterised compounds with CSD RefCodes.

Figure 2 .
Figure 2. Molecular structure of 1 with anisotropic displacement ellipsoids drawn at 50% probability level and the numbering system used.

Figure 3 .
Figure 3. Hydrogen bonding pattern in 1 viewed along the b-axis and schematic representation.

Figure 2 .
Figure 2. Molecular structure of 1 with anisotropic displacement ellipsoids drawn at 50% probability level and the numbering system used.

Figure 3 .
Figure 3. Hydrogen bonding pattern in 1 viewed along the b-axis and schematic representation.

Figure 3 .
Figure 3. Hydrogen bonding pattern in 1 viewed along the b-axis and schematic representation.

Figure 4 .
Figure 4. Hydrogen bonding pattern in 3 viewed along the b-axis and schematic representation.

Figure 5 .
Figure 5. V-shaped end view and unit cell of 1 viewed along the a-axis, showing the packing of hydrogen-bonded ribbons.

Figure 4 .
Figure 4. Hydrogen bonding pattern in 3 viewed along the b-axis and schematic representation.

Figure 4 .
Figure 4. Hydrogen bonding pattern in 3 viewed along the b-axis and schematic representation.

Figure 5 .
Figure 5. V-shaped end view and unit cell of 1 viewed along the a-axis, showing the packing of hydrogen-bonded ribbons.

Figure 5 .
Figure 5. V-shaped end view and unit cell of 1 viewed along the a-axis, showing the packing of hydrogen-bonded ribbons.

Table 1 .
Torsion angles in 1 and similar compounds.

Table 1 .
Torsion angles in 1 and similar compounds.

Table 1 .
Torsion angles in 1 and similar compounds.