Acetamide, rotation

Next, examine the equilibrium structure of acetamide (see also Chapter 16, Problem 8). Are the two NH protons in different chemical environments If so, would you expect interconversion to be easy or difficult Calculate the barrier to interconversion (via acetamide rotation transition state). Rationalize your result. Hint Examine the highest-occupied molecular orbital (HOMO) for both acetamide and its rotation transition state. Does the molecule incorporate a n bond. If so, is it disrupted upon rotation ... 

Restricted rotation in a series of ditert-buty1 phosphines (30 R = H, Aik, Ph etc) has been studied using molecular mechanics calculations.10 Rotation about the P-N bond in phosphonyl acetamides has a barrier of dG 16.3 heal mol 1. 66... 

Fig. 7.20 Illustration of the additivity of conformational geometry functions for the 41-rotation in (CH3NH)(0=)C-C(CH3)(H)(NHCOCH3) (ALA). During the torsional motion about the C(a)-C bond of ALA, the interactions within the system are the same as those encountered during the C(=0)-C torsion in N-methyl propanamide (NMPA), plus those encountered during the C(ct)-C torsion in N-acetyl N -methyl glycine amide offset by 120° as shown (GLY), minus those encountered during the C-C torsion in N-methyl acetamide (NMA).

Table 5.27. Methyl rotation barriers Ai+b for various H-bonded andprotonated acetamide X complexes (cf. Fig. 5.64), with comparison NRT bond orders bco and bcs and bond lengths Rco and Rq n of the amide moiety in each complex...

Figure 5.65 The dependence of the acetamide methyl-rotation barrier (AT ) on NRT bond-order differences in the amide group (Ab = bco - cn) for various H-bonded complexes of the pseudo-cA (occH(in) = 0°) rotamer (see Table 5.27).

The effects of the substituents on nitrogen on rotational barriers were discussed by Yoder and Gardner (34) for formamides and acetamides. The pertinent data, given in Table 5, suggest that the barriers to rotation of formamides are not affected by the bulkiness of the alkyl group on nitrogen, but such a conclusion... 

Effect of Substituents on Nitrogen on the Barrier to Rotation of Formamides and Acetamides (RCONR2) ... 

Kessler and Rieker (50) studied the barriers to rotation and equilibrium constants of a series of /V-(2,4,6-trialkylphenyl)acetamides (13). The barriers, which are generally low relative to those in the corresponding /V-methyl compounds, and equilibrium constants are summarized in Table 8. The barrier is large when... 

The thermal reaction of 6-amino-4-oxopyrano[3,4-d][l,2,3]thiadiazoles (32) leads to 6-hydroxy-4-oxo-[l,2,3]thiadiazolo[4,5-c]pyridines (33) and 2-cyano-2-(l,2,3-thiadia-zol-5-yl)acetamide (34).58 Formation of (33), a Dimroth-type rearrangement, proceeds by thermal opening of the pyrane ring, followed by the simultaneous rotational isomerization of the ketene intermediate and its recyclization on to the amido group to form the pyridin-2-one cycle. 

Nagashima and colleagues showed subsequently that 5-10 mol% of Pd(PPh3)4 catalyzed atom transfer radical cyclizations (ATRC) of /V-allyl-difluoroiodo-acetamides 158b in fluorescent lab light at ambient temperature [194], Under these conditions, 34—98% yield of cyclized compounds 159b were obtained. No reaction occurred in the dark, while it was considerably slower in the absence of the catalyst. The low yield of 34% in one example (R=Bn) is due to the unfavorable rotational barrier in the substrate, which cannot be influenced by the presence of the catalyst. 

THF205. More recently, the acetamide AcNH2 bare enolate has been re-inspected by DFT methods218. In particular, a 5-7 kcal mol 1 rotation barrier of the pyramidalized NH2 group has been computed (in agreement with experiment), for the monomer as well as for the mixed aggregate with LiOH. 

Ozonolysis and reduction of the product gives 2,2,2-trichloro-7V-[(27 )-l-hydroxyhex-2-yl]acetamide with Me,0 + 23.7" (c = 1.0, CHC1,), which is in good agreement with the optical rotation of the t.-Norleucin derived enantiomer, Mq° —24.3r (c = 1.0, CHCf,). 

A similar conformational analysis has been done with formamide derivatives, with secondary amides, and for hydroxamide acids. It is known that thioformamide has a larger rotational barrier than formamide, which can be explained by a traditional picture of amide resonance that is more appropriate for the thioformamide than formamide itself. Torsional barriers in a-keto amides have been reported, and the C—N bond of acetamides, thioa-mides, enamides carbamates (R2N—C02R), and enolate anions derived... 

The dielectric relaxation data for dimethylformamide (DMF) and dimethyl-acetamide (DMA) can be described by two Debye processes [9]. The low-frequency, high-amplitude process is attributed to rotational diffusion. For... 

A highly selective ring opening of 6-azabicyclo[3.2.0]hept-3-en-7-one (45) was achieved by using a whole cell preparation of ENZA-1 (Rhodococcus equi) [20, 86, 87]. The hydrolysis selectively gave the amino acid 46, which was converted directly to the ester acetamide 47. The recovered lactam 48 was reduced to 49. This was hydrolysed to 5, which displayed a similar optical rotation to that of natural cispentacin (Scheme 7). It is worthy of mention that ENZA-1 exhibited only poor hydrolytic activity towards ( )-49. 

Stereochemistry — The conformational analysis of various deuteriated ethyl-phosphine-borane adducts and dimethyl methylphosphonates were based on vibrational spectral data. The stereochemistry of diethylphosphonyl acetamide,the unusual vinyl compounds (60 X = L.E.P., O, and a number of l,3,2-dioxaphosphorinanes have been studied. It was found that the Raman-active ring vibration is related to the orientation of the phosphoryl group. Conformational data for dioxaphosphepane was compared with calculated parameters. A low-temperature study of several cyclopropyl-phosphonates (61) revealed rotational isomerization about the P-O bonds but not about the P-C bond. The variable temperature i.r. and Raman spectra of the silyl phosphates (62) also revealed rotational isomerism. ... 

Fig. 10 Potential-energy profile (MM2) for the rotation of the neopentyl groups in. V,Ai-dineopentyl-acetamide (lower trace) and -thioacetamide (upper trace). The reaction coordinate involves rotation of both neopentyl groups to varying degrees (Berg et al., 1985).

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