Cyclohexenyl complexes

In the above-mentioned preparation of the u-cyclohexenyl complex, [PdCliV-CeHglb, benzene and cyclohexane are formed. ThisTr-cyclohexenyl complex is also formed from cyclohexa-1,3-diene, either by treating it with sodium chloropalladite 191) or with palladium chlorocarbonyl, [PdCl2-(CO)]2 81, 88). 

By D-NMR spectroscopy, up to three hindered ligand movements were detected in [Cr(CO)2L(tj4 CH-diene)] complexes, a situation comparable to [Mn(CO)3(t/3 CH-cyclohexenyl)] complexes (46-51). Only small activation barriers are found for exchange processes of the C—H—Cr bridge. Higher barriers are observed for the site exchange of the three monodentate ligands. Similar energies are necessary to activate 1,5-H shifts (Table VI). 

The spectrum of a ir-allyl system where H has been substituted is often v ry simple but when substitution at or occurs considerably more complex spectra are observed. Nonetheless, the spectrum of 77-crotyl-palladium chloride can be interpreted satisfactorily on the basis of an AMM X system 16, 44). In some cyclohexenyl complexes of palladium, the hydrogens are substituted by a (—CH2—group. A comparison of their spectra with those of analogous rr-allyl complexes confirms the assignments of the H hydrogens to the higher doublet in the spectra of the unsubstituted complexes 33, 61, 99). In the case of the platinum complex... 

In the case of cyclohexene the reaction has lead to cyclohexenyl complexes (Me5C5)2-Ln (Ln = La, Nd) [59]. Such processes are considered in Section V.l. 

In fact, the highest anti-Cram selectivity reported to date (96% de) was observed with the MAT-mediated addition of methylmagnesium bromide to 2-(l-cyclohexenyl)propanal3 i 36. The stereochemical outcome of this addition reaction can be explained as follows on treatment of the carbonyl compound with the large aluminum reagent, the sterically least hindered complex 9 is formed. Subsequent addition of the nucleophile from the side opposite to the bulky aluminum reagent produces the anti-Cram diastereomer preferentially. 

X-ray structures of 10c-151 a (prisms) and 10c-151a (needles) were analyzed and compared. In the former complex, the shortest contact between the aromatic carbon and cyclohexenyl carbon which are going to bind is 3.161(4) A. In the latter complex, however, those two carbons are too distant to permit reaction [34]. 

Palladium(O) or readily reduced paUadium(II) complexes were the most efficient catalysts, giving higher yields than analogous Pt catalysts. The Markovnikov product was formed with high regioselectivity. In dialkynes, both C=C bonds could be hy-drophosphorylated, while the C=C double bond in a cyclohexenyl alkyne subshtuent did not react. With trimethylsilylacetylene, unusual anti-Markovnikov selectivity was observed. 

Andrus et al. (109) proposed a stereochemical rationale for the observed selec-tivities in this reaction. The model is based on the Beckwith modification (97) of the Kochi mechanism, suggesting that the stereochemistry-determining event is the ally lie transposition from Cu(III) allyl benzoate intermediates 152 and 153, Fig. 13. Andrus suggests that the key Cu(III) intermediate assumes a distorted square-planar geometry. Steric interactions are decreased between the ligand substituent and the cyclohexenyl group in Complex 152 as opposed to Complex 153 leading to the observed absolute stereochemistry. 

In the example shown in Figure 4.4 either of these mechanisms leads to insertion of the alkyne into the C-Rh double bond of the initially formed acylcarbene rhodium complex. The resulting vinylcarbene complex undergoes intramolecular cyclopropanation of the 1-cyclohexenyl group to yield a highly reactive cyclopropene, which is trapped by diphenylisobenzofuran. 

Reactions of Fischer carbene complexes with diynes have been extensively studied as a synthetic approaches to alkynylarenes and biaryls. In general, Cr =C(0R )R (C0>5 (R = Me, Bu R = Ph, 1-nap, 1-cyclohexenyl) react with... 

Mo containing Y zeolites were also tested for cyclohexene oxidation with oxygen as oxidant and t-butyl hydroperoxide as initiator [86]. In this case the selectivity for cyclohexene oxide was maximum 50%, 2-cyclohexene-l-ol and 2-cyclohexene-l-one being the main side products. The proposed reaction scheme involves a free radical chain mechanism with intermediate formation of cyclohexenyl hydroperoxide. Coordination of the hydroperoxide to Mo + in the zeolite and oxygen transfer from the resulting complex to cyclohexene is believed to be the major step for formation of cyclohexene oxide under these conditions. 

The proposed mechanism for allyhc acetoxylation of cyclohexene is illustrated in Scheme 15. Pd -mediated activation of the allyhc C - H bond generates a Jt-allyl Pd intermediate. Coordination of BQ to the Pd center promotes nucleophilic attack by acetate on the coordinated allyl ligand, which yields cyclohexenyl acetate and a Pd -BQ complex. The latter species reacts with two equivalents of acetic acid to complete the cycle, forming Pd(OAc)2 and hydroquinone. The HQ product can be recycled to BQ if a suitable CO catalyst and/or stoichiometric oxidant are present in the reaction. This mechanism reveals that BQ is more than a reoxidant for the Pd catalyst. Mechanistic studies reveal that BQ is required to promote nucleophilic attack on the Jt-allyl fragment [25,204-206]. 

Similarly, treatment of the ir-cyclohexenylpalladium complex [PdCl-(ir-CeHg) with cyclopentadienylsodium gives the red, crystalline complex ir-cyclohexenyl-ir-cyclopentadienylpalladium(II) (LI), the structure of... 

Catalytic hydrogenation of l,2,3,4-tetrahydro-ll//-pyrido[2,l-6]quina-zolin-ll-one and its 6-, 7-, 8-, and 9-methyl derivatives over Pd/C catalyst in ethanol gave 1,2,3,4,6,7,8,9-octahydro-ll //- derivatives (87JMC1543). A solution of a 1 1 mixture of 9-methyl-l,2,3,4-tetrahydro-ll/f-pyrido-[2,l-6]quinazolin-11 -one (73) and 2-benzyl-9-methyl-l,2,3,4-tetrahydro-ll//-dipyrido[l,2-a 4,3-catalyst yielded a complex reaction mixture containing 15% of 9-methyl-l,2,3,4,6,7,8,9-octahydro-ll//-pyrido[2,l-/)]quinanolizin-Tl-one and other products (87T1157). The double bonds of the pyridine of compound 73 was also saturated by intermolecular catalytic hydrogen transfer from 2-(4-cyclohexenyl)-l,2,3,4,5,6,7,8-octahydroquinazolin-4-one in the presence of Pd/C catalyst [85H(23)3095]. 

We studied the oxidation of cyclohexene at 70°C in the presence of cyclopentadienylcarbonyl complexes of several transition metals. As with the acetylacetonates, the metal center was the determining factor in the product distribution. The decomposition of cyclohexenyl hydroperoxide by the metal complexes in cyclohexene gave insight into the nature of the reaction. With iron and molybdenum complexes the product profile from hydroperoxide decomposition paralleled that observed in olefin oxidation. When vanadium complexes were used, this was not the case. Variance in product distribution between the cyclopentadienylcarbonyl metal-promoted oxidations as a function of the metal center were more pronounced than with the acetylacetonates. Results are summarized in Table V. 

Thus, depending on the metal complex used, cyclohexene oxidation can occur via one or more of at least three major pathways, as shown in Reaction 20 path A, radical initiated decomposition of cyclohexenyl hydroperoxide path B, metal catalyzed epoxidation of the olefin and path C, metal catalyzed epoxidation of an allylic alcohol. Ugo found that path B becomes more pronounced when molybdenum complexes are used to modify the oxidation of cyclohexene in the presence of group... 

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