Cisoid intermediate

Reactions a and b in Scheme 8 represent different ways of coordination of butadiene on the nickel atom to form the transoid complex 27a or the cisoid complex 27b. The hydride addition reaction resulted in the formation of either the syn-7r-crotyl intermediate (28a), which eventually forms the trans isomer, or the anti-7r-crotyl intermediate (28b), which will lead to the formation of the cis isomer. Because 28a is thermodynamically more favorable than 28b according to Tolman (40) (equilibrium anti/syn ratio = 1 19), isomerization of the latter to the former can take place (reaction c). Thus, the trans/cis ratio of 1,4-hexadiene formed is determined by (i) the ratio of 28a to 28b and (ii) the extent of isomerization c before addition of ethylene to 28b, i.e., reaction d. The isomerization reaction can affect the trans/cis ratio only when the insertion reaction d is slower than the isomerization reaction c. 

Figure 16 (a) Structures of adenylation domain intermediates and inhibitors aminoacyl-sulfamoyl adenosine (AMS) and cisoid -like macrocyclic inhibitor, (b) Alkyne-functionalized chemical probe for NRPS A and PCP domains, (c) Structure of aminoacyl PCP, SNAC substrate analogue, and hydrolytically stable phosphopantetheinyl analogue, (d) Structure of vinylsulfonamide probe. R represents a peptide component and R an amino acid side chain. 

Synthetic cA-l,4-polyisoprene (structure 5.42) is produced at an annual rate of about 100,000 t by the anionic polymerization of isoprene when a low dielectric solvent, such as hexane, and K-butyllithium are used. But, when a stronger dielectric solvent, such as diethy-lether, is used along with w-butyllithium, equal molar amount of tra i -l,4-polyisoprene and cA-3,4-polyisoprene units is produced. It is believed that an intermediate cisoid conformation assures the formation of a cis product. An outline describing the formation of cA-1,4-polyisoprene is given in structure 5.42. 

Notably, the proposed stereoselectivity of a-insertion seems to run opposite to that observed for some related processes (vide supra). Such systems differ from the present system, though, because they involve an octahedral Rh(III) center, whereas intermediate 88 is a Rh(I)-square-planar complex. In Werner s original stoichiometric studies, the same mechanistic dichotomy was evident (Scheme 9.14) - square planar Rh(I)-complexes underwent cisoid insertion in the absence of other factors and transoid addition in the presence of an oxidant (HX). In intermediate 88, coordination of the alkene moiety to the rhodium center may also play a role in directing insertion. 

Dienes, 11 addition to, 194-198 cisoid conformation, 197, 350 conjugated, 11 Cope rearrangement, 354 cycUsation, 346 cycloaddition to, 348 Diels-Alder reaction, 197, 349 excited state, 13 heat of hydrogenation, 16,194 isolated, 11 m.o.s of, 12 polymerisation, 323 Dienone intermediates, 356 Dienone/phenol rearrangement, 115 Dienophiles, 198, 350 Digonal hybridisation, 5 Dimedone, 202 Dimroth s Et parameter, 391 solvatochromic shifts, 391 solvent polarity, 391 Y and,392 Dinitrofluorobenzene proteins and, 172... 

Combining substrate-induced diastereoselection and mutual diastereoselectivity, as illustrated for the crotylzincation of the alkenyllithium derived from 209, led to excellent results as the gewt-dimetallic species 217 was obtained in a highly stereoselective fashion. The stereochemical outcome was explained by the addition of the kinetically reactive cisoid metallotropic form of the crotylzinc reagent anti to the propyl group in the chelated allyl alkenylzinc intermediate. After hydrolysis, compound 218 was obtained as a single diastereomer (equation 106)148,149. 

This step, yielding a cisoid P-hydroxyethylpalladium intermediate (71), is preceded by a trans-cis isomerization (69 to 70).498 A series of hydride shifts (66 to 67 and 67 to 68) eventually leads to the product carbonyl compound. 

When catalysts of the type 10/GaBr3 are used to initiate the ROMP of norbornene derivatives at low temperature (—50 °C) the intermediate transoid metallacyclobutane complexes (the precursors to the formation of trans double bonds) may be observed. The corresponding cisoid complexes are not stable enough to be detected. No metallacyclobutane complexes are observed in the absence of GaBr3100 112-114. 

In 1989, the irradiation of (E,E)-2,4-hexadiene S3 sensitized by meso-porphyrin IX dimethyl ester led to the formation of cis-3,6-dimethyl-l,2-dioxene (62), which was the major product detected at — 78 °C in Freon 11 [69]. Endoperoxide 62 was purified under vacuum at 0.75 mmHg, and collected in a trap (98% isolated yield). Dienes that can adopt a cisoid conformation, such as 53 or ( , )-l,4-di phenyl butadiene, were photooxidized by the corresponding endoperoxides in high or quantitative yield in a suprafacial Diels-Alder reaction [60, 70], Dienes that cannot readily adopt cisoid conformations, such as (fc, Z)-2,4-hexadienes and (Z, Z)-2,4-hexadienes, lose their stereochemistry in the singlet oxygen [2 + 4]-cyclo-addition [60], (E,Z)- and (Z,Z)-dienes give a complex mixture of hydroperoxides and aldehydes, which suggests the intervention of intermediate zwitterions or 1,4-diradicals [71]. 

As discussed earlier (Section 6.3.4), acyclic (pentadi-enyl)iron cations are intermediates in reactions involving protonation or Lewis acid activation of iron dienol or dienol acetate complexes. With appropriate functionalization, these cations may also be trapped intramolecularly (Scheme 81). In the case of the formation of tetrahydrofinans (278) and (279), equilibration of the transoid and cisoid cations is observed. However, this equilibration is not observed in the stereoselective cyclization to provide (racemic) oxocene (280). Analogous intramolecular cyclizations of sullur and nitrogen... 

A possible mode of formation of 6 could be via the condensation of two cyclic intermediates with cisoid arrangements of Si-X groups (X = Cl or OH), e.g. 9. 

First, the enzyme has at least two and probably three active-site basic groups involved in proton transfers to and from substrates, intermediates, and nascent products and all three bases are located on the si face of the substrate-PLP aldimine system as are the protons to be shuffled about, so all the proton transfers are likely to be economically suprafacial. Several pieces of stereochemical evidence suggest that the j5,y-olefinic PLP-p-quinoidal-a-anion (141) can rotate around its C(P)-C(ol) bond and also implicate that the cisoid isomer of this n complex and then the Z-isomer of the nascent aminocrotonate carry 80 % of the reaction flux. Furthermore, a 15% internal retention of the from the Pro-R methylene of ACPC (9) on B2H (85 % exchange with solvent, 15 % internal return) in the active site and the overall 22/78 H /H5 distribution at C(3) of the mono- and dideutero 2-ketobutyrate (138) products at C(3) are also noted. 

The observed solvent effects on the type of product formed " could also be associated with the structure of the intermediate. Formation of alkenes or epoxides necessitates, respectively, cisoid and transoid arrangements of the arsenic and oxygen atoms, and a transoid structure is likely to be much more favoured in a polar protic solvent such as ethanol than in benzene. Similarly, as observed the presence of lithium ions should favour epoxide formation compared to the effect of sodium or potassium ions, since the transoid intermediate is likely to be stabilized by association of the oxygen with lithium but less so with sodium or potassium. 

From what has been so far reported, it would seem that the intermediacy of the cis-cisoid ring-opened nonplanar intermediate X is much better documented for nitrospiropyrans than for spiroazines, for which the matter is still controversial. No studies have so far been reported on the importance of the X isomer in the photodegradation mechanistic schemes. 

---