Transoid couplings

Three-bond carbon-phosphorus couplings JCP depend on configuration, as expected, transoid couplings being larger than cisoid ones, as demonstrated for cyclohexyl- and 2-norbornylphosphonates in Table 4.52. The Karplus relation (4.13) can be derived from the data for 2-norbornyl derivatives [376]. 

In inter alia the t-butyl substituted compound 118, the cisoid allylic couplings constants to H, 2.3 Hz and that to H", 1.9 Hz together with the transoid counterparts to H, 2.6 Hz and to H", 2.4 Hz indicated a slight variation of the coupling constant with substituent electronegativity In 119 the cisoid and transoid coupling constants were 2.4 and 2.8 Hz... 

Following this pattern, 3JCC couplings of transoid carbon atoms in aromatic compounds are larger than those in a cisoid coupling path [129]. 

Three-bond carbon-mercury couplings are larger for transoid in comparison to cisoid coupling routes, as shown for the endo-exo pair of 2-norbornylmercury acetate, and met a... 

Reaction of benzynes with acyclic dienes generally gives Diels-Alder products in poor yields because of competitive [2 + 2] cycloadditions and ene-type processes (Scheme 113). The product distribution, obtained from 2,3-dimediyl-l,3-butadiene (484)," is more characteristic of a two-step mechanism than of a concerted addition. Widiout pursuing this argument, it tqrpears diat the extremely poor yield of [4 + 2] cycloadduct (485) results from die very fast reaction of benzyne with the transoid diene (484). Bond formation at C-1 coupled with a hydrogen transfer (- 487) or cyclobutane ring closure (-> 486) seems to occur rapidly before diene (484) can adopt the s-cis conformation. 

However, Hamman et al.136 analyzed the rotameric populations of 2-fluoro-l,2-disubstituted ethanes and found that transoid 3/hcch and Vhccf couplings could be described by linear fits... 

Figure 12.16. Dependence of allylic coupling constants on stereochemistry. Solid lines Transoid (Jbx) dashed lines Cisoid Ja - 0 is the... 

Seshadri, V., et al. 2003. Gonjugated polythiophene consisting of coupling through locked transoid conformation. Poly Mater Sci and Eng 88 292. 

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