Rotation barriers about bonds

Fhe van der Waals and electrostatic interactions between atoms separated by three bonds (i.c. the 1,4 atoms) are often treated differently from other non-bonded interactions. The interaction between such atoms contributes to the rotational barrier about the central bond, in conjunction with the torsional potential. These 1,4 non-bonded interactions are often scaled down by an empirical factor for example, a factor of 2.0 is suggested for both the electrostatic and van der Waals terms in the 1984 AMBER force field (a scale factor of 1/1.2 is used for the electrostatic terms in the 1995 AMBER force field). There are several reasons why one would wish to scale the 1,4 interactions. The error associated wilh the use of an repulsion term (which is too steep compared with the more correct exponential term) would be most significant for 1,4 atoms. In addition, when two 1,4... 

D H- H EXSY NMR spectroscopy (EXSY - exchange spectroscopy) was used for the determination of the rotational barrier about the C(2)-Ph bond in 2-phenyl-l-boraadamantane trimethylamine adduct 16 (Figure 5) Eact= 14.0 0.6kcalmoF1, lnd = 28 0.6, AG = 14.5 0.7 kcal mol-1 <2000IZV497>. 

The values of rotation barriers about the central E14-C bond are 11.6-13.0kcal/mol. 

The rotation barriers about the central E14-C bond in betaines of the third group are 15.3-18.2 kcal/mol. The barrier value increases in the series Si-Ge-Sn and on going from X = O to NMe. 

Reaction of 3 with Ph3C+PF6" resulted in the formation of methylidene complex [(n-C5H5)Re(N0)(PPh3)(CH2)]+ PF6 (8) in 88-100% spectroscopic yields, as shown in Figure 11. Although 8 decomposes in solution slowly at -10 °C and rapidly at 25 °C (She decomposition is second order in 8), it can be isolated as an off-white powder (pure by H NMR) when the reaction is worked up at -23 °C. The methylidene H and 13C NMR chemical shifts are similar to those observed previously for carbene complexes [28]. However, the multiplicity of the H NMR spectrum indicates the two methylidene protons to be non-equivalent (Figure 11). Since no coalescence is.observed below the decomposition point of 8, a lower limit of AG >15 kcal/mol can be set for the rotational barrier about the rhenium-methylidene bond. 

The rotational barrier about the C—O bond in the cyanomethoxymethyl radical, [35]/[36], constitutes a similar case, although the situation is somewhat more complicated (Beckwith and Brumby, 1987). As oxygen carries two lone pairs of electrons, the transition structure for rotation about the C—O bond can still be stabilized by conjugation. Compared to the methoxy-methyl radical, the barrier in the captodative-substituted radical is 1-2 kcal mol higher. 

An intriguing feature is that the previously unknown bisindoles 154 display atropisomerism as a result of the rotation barrier about the bonds to the quaternary carbon center. With the use of A-triflyl phosphoramide (1 )-41 (5 mol%, R = 9-phenanthryl), bisindole 154a could be obtained in 62% ee. Based on their experimental results, the authors invoke a Brpnsted acid-catalyzed enantioselective, nucleophilic substitution following the 1,2-addition to rationalize the formation of the bisindoles 154 (Scheme 65). 

The rotational barrier about the P=C bond is very small. Absar and Van Wazer64 first estimated the barrier as 0.003 kcal mol"1. The barrier is 0.13 kcal mol 1 at HF/DZ + P70. 

Schaefer and coworkers20 have used long-range NMR coupling constants to investigate rotational barriers about the C(sp2)—C(sp3) bonds in benzyl compounds. The barrier for benzylsilane was found to be 1.77 kcalmol-1, compared to 1.2 kcalmol-1 for ethylbenzene. The increased barrier for benzylsilane is attributed to increased stabilization of the stable conformer, in which the C—Si bond lies in a plane perpendicular to the benzene plane, by a hyperconjugative interaction between the C—Si bond and the jr-system. 

Hydroxyborane, H2BOH, has been studied171 with a (9,5,1)->[4,2,1] basis set. The OH bond length and the BOH angle were optimized. Of particular interest in this study was the rotational barrier about the BO bond. The planar form of the molecule is the most stable, and the computed barrier was 68.6 kJ mol-1. A partial 7r-bond is superimposed on the c-bond. An analysis of the energy and population components was carried out. 

Although a diradical intermediate might be expected to give both cis and trans products, a high rotational barrier about the C2—Cg bond created by two t-butyl substituents may inhibit formation of the cis isomer. 

The PES for rotation around the C—C bond of 58, M=Si, Ge (Figure 18) shows two minima the s-trans and the gauche, while the s-cis structure is a saddle point that connects two gauche enantiomers. The gauche rotamer is by ca 4333 and 3 kcal mol-1335 for 58, M = Si, Ge, respectively, less stable than the s-trans rotamer, an energy difference very similar to that in 1,3-butadiene. However, the rotation barrier about the central C—C bond in 58, M = Si and Ge, of ca 10-11 and 9.5 kcalmol-1, respectively, is... 

An understanding of the internal rotation about the amide bond is important because of its relevance to protein structure. Formamide is the simplest amide. The coplanarity and the remarkable rotational barrier about the C-N bond in formamide can be rationalized by resonance between the n electrons of the carbonyl group and the lone pair of the nitrogen atom [1, 50]. According to VB theory, the Jt electronic structure of formamide may be described by six resonance structures. 

Contribution from resonance structure 3, which contains a formal double bond between carbon and nitrogen, is considered to be primarily responsible for the coplanarity and the high rotational barrier about the amide bond [58], The introduction of resonance structure 3 also implies that there is significant charge-delocalization from the nitrogen lone pair to the carbonyl oxygen. 

The rotation barriers about sp3 carbon-heteroatom bonds are usually low in the absence of strong nonbonded interactions between substituents. Factors governing the values reported in Table 7 include the carbon-heteroatom bond length, the... 

One of the difficulties encountered when investigating or discussing rotation barriers about C-0 and especially C-N bonds is that this process may be overshadowed by a competitive process, namely inversion. Inversion barriers for simple amines are in the region of 5 kcal/mol [86], thus somewhat above the rotation barrier reported in Table 7 for methylamine. However, bulky substituents may markedly increase the rotation barrier and cause inversion to become the preferred process of conformational interconversion. This is precisely what happens with ferf-butylamines, where inversion (LXXVa-LXXVb interconversion) but not C-N bond rotation (LXXVa-LXXVc interconversion) is observed [87],... 

The rotational barrier about the C—N bond in vinylamine has been determined theoretically57 and experimentally58-60. The barrier seems to be of the order of 18-25 kJ mol-1. 

Although chiral nonracemic enolates of type 21 and 22 are expected to exist under particular conditions, their half-lives to racemization are usually too short to effect actual asymmetric reactions. To realize an asymmetric transformation via a chiral enolate of type 21, chiral ketone 23 was designed that would generate a chiral enolate with a significantly long half-life to racemization (Scheme 3.8). Steric interactions of C(2)-OMe with C(2,)-OEt and C(8 )-H in the enolate would prevent free rotation of the C(l)-C(l ) bond as well as coplanarity of the enolate double bond with the naphthalene ring. Thus the enolate was expected not to exist as an achiral planar enolate, but it could be a chiral enolate with a chiral C(1)-C(T) axis. The rotational barrier about the C(1)-C(T) bond may be estimated by analogy with 2,2/-disubstituted biphenys because of similarity of the local steric environment around the chiral axes. The half-life to racemization was assumed to be about a few years at —78°C from the reported rotational barrier of various 2,2/-disubstituted biphenys ( 80 kJ/mol).16... 

These compounds (e.g., 142) seemed particularly well suited for the investigation of the carbon-carbon rotational barrier about the Ar-CHS bond as well as the carbon-nitrogen rotational barrier about the adjacent Ar-NH2 bond, but these have not been reported as yet. 

The rotation barrier about the fi-y bond is not negligible, so that the centres exist in either the cis or trans forms (Z, E) mutual transitions between these two forms only occur under certain conditions and at a certain rate. In diene polymerizations in hydrocarbon medium with Li+ as counter-ion, monomer addition transforms a cis centre into a cis segment of the chain, and a trans centre into a trans polymer (the former case is typical for isoprene polymerization in THF the equilibrium is shifted towards the cis form over the whole of the available temperature range). A cis tram transition is, of course, not excluded (for example with butadienyllithium), and when it is more rapid than addition, chain stereospecificity is reduced. 

To probe the transition state structure for these reactions further, the effect of para substituents on amide rotation rates was measured for a series of N,Af-dimethylbenzamides (Berarek, 1973). When the data are correlated with cTp (Ritchie and Sager, 1964), a p value of —1.14 0.06 is obtained (see Fig. 2). The negative p value indicates that electron-donating substituents accelerate the reaction. This can rationalized in the context of Scheme III, where resonance forms for these substrates are shown. The rotational barrier about the C—N bond is decreased as resonance forms I and III predominate. If R is electron donating, these resonance forms will contribute more to the structure of the amide than will II and C-N rotation will therefore be accelerated. 

The nmr data for 13) in Table 3 indicate the existence of two kinds of alkyl groups whose magnetic environments are slightly different from each other, indicating that the rotational barrier about the C—N bond is enhanced by the increased double-bond character of the C—N linkage. 

If the donor-free syn adduct (48) is generated by adding a Lewis acid, then it can rapidly isomerize to the anti adduct (50) at 25 °C. Available evidence indicates that the rotational barrier about the C bond in (48) and (50) is very small. A possible explanation is that pir-pir bonding with the 3p-orbital on aluminum lowers the C=C double bond character. Furthermore, o--bond hyperconjugation in the transition state for rotation (49) reduces its energy and hence the barrier to rotation. That the facile isomeriza-... 

---