Reactions on the NMR Time Scale

Normally, rotation around single bonds has a barrier below 5 kcal mol and occurs faster than the NMR time scale. Rotation around the double bond of alkenes, on the other hand, has a barrier that is normally above 50 kcal mol and is slow on the NMR time scale. There are numerous examples of intermediate bond orders, whose rotation occurs within the NMR time scale. Hindered rotation about the C —N bond in amides such as A, A-dimethylformamide (5-4) provides a classic example of site exchange. At room 

Hindered rotation occurs on the NMR time scale for numerous other systems with partial double bonds, including carbamates, thioamides, enamines, nitrosamines, alkyl nitrites, diazoketones, aminoboranes, and aromatic aldehydes. Formal double bonds can exhibit free rotation when alternative resonance structures suggest partial single bonding. The calicene 5-5, for example, has a barrier to rotation about the central bond of only 20 kcal mol .  

Steric congestion can raise the barrier about single bonds enough to bring it into the NMR range. Rotation about the single bond in the biphenyl 5-6 is raised to a measurable 

13kcalmoF by the presence of the ortho substituents, which also provide diastereotopic methylene protons as the dynamic probe. Hindered rotation about an sp -sp bond can sometimes be observed when at least one of the carbons is quaternary. Thus, at - I50°C, the ren-butyl group in rerr-butylcyclopentane (5-7) gives two resonances in the ratio of 2 1, since two of the methyl groups are different from the third (5-7a). 

Hindered rotation has frequently been observed in halogenated alkanes. The increased barrier probably arises from a combination of steric and electrostatic interactions. 2,2,3,3-Tetrachlorobutane (5-8) exhibits a 2 1 doublet below —40°C from anti and gauche rotamers that are rotating slowly on the NMR time scale. 

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Fig. 43. H NMR evidence that the reversible reaction between [TpBu Me]ZnOH and C02 is rapid on the NMR time scale ( = internal standard). Reprinted with permission from Ref. (151). Copyright 1993 American Chemical Society.

Si-P bonds are formed in the reaction with di- and tri-phosphinomethanides (Eqs.(l), (2)), and the resulting ylides 4 and 5 are fluctional in solution. In the case of monophosphinomethanide 3, both Si-C and Si-P bond formation is observed. The ylide 6 is rigid in solution on the NMR time scale, but it rearranges to the phosphinomethane derivative 7 within several days at 20 °C. 

D.R. McMillin, Purdue University In addition to the charge effects discussed by Professor Sykes, I would like to add that structural effects may help determine electron transfer reactions between biological partners. A case in point is the reaction between cytochrome C551 and azurin where, in order to explain the observed kinetics, reactive and unreactive forms of azurin have been proposed to exist in solution (JL). The two forms differ with respect to the state of protonation of histidine-35 and, it is supposed, with respect to conformation as well. In fact, the lH nmr spectra shown in the Figure provide direct evidence that the nickel(II) derivative of azurin does exist in two different conformations, which interconvert slowly on the nmr time-scale, depending on the state of protonation of the His35 residue (.2) As pointed out by Silvestrini et al., such effects could play a role in coordinating the flow of electrons and protons to the terminal acceptor in vivo. 

Reaction of the primary phosphane Bu3SiPH2 If with MgBu2 furnishes the solvent-free hexameric cluster 17 (Eq. 10) (47). Yellow crystals, have been isolated in 39% yield, which are thermochromic. The NMR spectrum, especially the 31P NMR signal at S = -263.8, suggested that the molecule prefers a high symmetry or dissociates rapidly on the NMR time scale. Since 15 is highly soluble in aromatic hydrocarbons even at low temperature and free of metal oxide, it can thus be regarded as a valuable source of phosphandiide, that is, for nucleophilic RP2 transfer reactions. 

The reaction of [W(CN)814 with H2 at 400 °C was reported to give a green-black product of composition l W CN ]. Recently the same reaction has been shown to give [W(CN)7H]4, and in the presence of base the following equilibrium is established,261 as depicted in equation (14). [W(CN)7]5 can also be prepared directly from [W2Cl9]3- with an excess of cyanide. The sharpness of the 13C NMR lines suggests that on the NMR time scale there is no exchange of coordinated cyanide and that both structures are fluxional in solution. 

Intermolecular secondary-secondary hydride transfer between 2 and propane in SbFs-SOiCIF solution has been observed by Hogeveen and Gaasbeek.111 The reaction was rapid on the NMR time scale, and a single peak was obtained from the two types of methyl groups down to at least — 100°C (AG < 6 kcal mol-1). 

Reaction of the bis-tridentate receptor [L14-2H]2 with R(C104)3 in water produces the highly stable homobimetallic triple-stranded helicates [R2(L14-2H)3] (R = La-Tb except Pm and R = Er-Yb-Lu, log/S[S2(Li4 2H)3] = 51(4), Elhabiri et al. (1999)). NMR spectra point to rigid D3-symmetrical complexes for which exchange between the helical enantiomers is slow on the NMR time scale (i.e., the methylene protons of the ethyl residues are systematically diastereotopic, fig. 55). 

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