Bonding inversion

Special cases of these involving transition states for rotation about single bonds, inversion of pyramidal nitrogen and phosphorus centers and ring inversion in cyclohexane, have been discussed in the previous chapter. The only difference is that these conformational processes are typically well described in terms of a simple motion, e.g., rotation about a single bond, whereas the motion involved in a chemical reaction is likely to be more complex. 

SCHEME 1. The dynamic behavior of organozinc compounds involves C—Zn exchange and C—Zn bond inversion as main processes. Aggregation and ligand exchange dynamics complete the overall picmre describing the dynamics of these compounds... 

TABLE 4. Experimental activation parameters for the primary carbon-metal bond inversion as measured in Et20 by VT-NMR ... 

TABLE 6. Attempts to measure secondary alkyl carbon-metal bond inversion by NMR"... 

EXAMPLES OF ONE-BOND INVERSE CORRELATION (HMQC AND HSQC) WITHOUT 13C DECOUPLING... 

Fig. 6 Sketch of the Si/So conical intersection seam in fulvene along the methylene torsion and bond inversion coordinates (a) and energy profile along the seam (b) (adapted with permission from [44])...

Inverse detected heteronuclear experiment. Cross peaks show correlations between carbons and protons usually 2-4 bonds away. A critical experiment in structure elucidation. Inverse detected heteronuclear experiment. Cross peaks show correlations between carbons and their attached protons (1-bond). Inverse detected heteronuclear experiment. Cross peaks show correlations between carbons and their attached protons (1-bond). Cross peaks are sharper (and therefore easier to detect) than in HMQC because H- H couplings are removed. Usually performed as sensitivity enhanced and with editing. Establishes correlations between like spins of low natural abundance (e.g. C- C). In the case of C, the experiment only observes 1 in every 10 000 molecules, so large sample amounts are needed. 

Sulphur. In acyclic compounds containing sulphur-sulphur bonds inversion at the sulphur is relatively easy, but in the cyclic compound (62) there is no evidence for inversion at sulphur. The first n.m.r. kinetic study of ring inversion for a 1,4-dithian has been of the perfluoro-compound... 

A problem inherent to one-bond, inverse-detected experiments was the suppression of the unwanted 99% of the signal arising from protons directly bound to and thus of no interest spectroscopically. A number of ap-... 

Full details are now available of another versatile and highly stereoselective synthesis of vinyl bromides and chlorides from alkynes (Scheme 44). Conversion into either the cis- or /rans-vinyl silanes is followed by trnns-addition of halogen and then base-mediated tran5-elimination of halogenotrimethylsilane the overall result from vinyl silane is replacement of trimethylsilyl by halide with double-bond inversion. 

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