Alkene complexes reactivity

Zirconocene-catalyzed kinetic resolution of dihydrofurans is also possible, as illustrated in Scheme 6.8 [18]. Unlike their six-membered ring counterparts, both of the heterocycle enantiomers react readily, albeit through distinctly different reaction pathways, to afford — with high diastereomeric and enantiomeric purities — constitutional isomers that are readily separable (the first example of parallel kinetic resolution involving an organome-tallic agent). A plausible reason for the difference in the reactivity pattern of pyrans and furans is that, in the latter class of compounds, both olefmic carbons are adjacent to a C—O bond C—Zr bond formation can take place at either end of the C—C 7T-system. The furan substrate and the (ebthi)Zr-alkene complex (R)-3 interact such that unfavorable... 

However, styrene and cyclohexene gave complex product mixtures, and 1-octene did not react under the same reaction conditions. Thus, the activity of this catalyst is intrinsically low. Jacobs and co-workers [159,160] applied Veturello s catalyst [PO WCKOj ]3- (tethered on a commercial nitrate-form resin with alkylammonium cations) to the epoxidation of allylic alcohols and terpenes. The regio- and diastereoselectivity of the parent homogeneous catalysts were preserved in the supported catalyst. For bulky alkenes, the reactivity of the POM catalyst was superior to that of Ti-based catalysts with large pore sizes such as Ti-p and Ti-MCM-48. The catalytic activity of the recycled catalyst was completely maintained after several cycles and the filtrate was catalytically inactive, indicating that the observed catalysis is truly heterogeneous in nature. 

The results outlined above highlight numerous similarities between dioxygen-and BQ-mediated oxidation of palladium(O). The key steps in the respective pathways appear virtually identical (Scheme 10). Recent studies of the reactivity of dioxygen and electron-deficient alkenes with Pd°-alkene complexes reinforce these similarities. 

Note that despite the death of the carbene-alkene complex in the study of benzylchlorocarbene (53) (see above), benzene is able to modulate the intramolecular reactivity of ferf-butylcarbene. ° Some sort of complex must be involved here. Benzene complexes with carbenes have been proposed before. Kahn and Goodman found a transient species on photolysis of diazomethane in benzene, and attributed it to a complex. Moss et al. found that benzene modulated the ratio of intramolecular rearrangement to intermolecular addition for three different carbenes (53), chloropropylcarbene, and chlorocyclopropylcarbene, and proposed that a carbene-benzene complex 70 favored the intramolecular rearrangement (Scheme 7.31). Their proposal was bolstered by ab initio calculations that found such stable complexes for CCI2 and CH3CCI. 

The Os(II) alkene complexes [OsCl(NO)(R2C=CR2)(PPh3)2] are quite reactive toward addition of 0SNS02C6H4Me-4 (Section II,C,2,f) (163,164). 

Alkene complexes can be prepared with platinum in a divalent or a zerovalent oxidation state. The electron density at the platinum center exerts significant changes in bonding between the alkene and platinum. These effects exhibit themselves in both structural features and chemical reactivities. 

Coordination of an alkene to platinum(O) differs from complexation to platmum(Il) Zerovalent platinum is an electron-rich metal center, whereas platinum(II) is electron poor. A a consequence alkenes coordinated to platinum(0) became more electron rich than in their fret state, and therefore susceptible to electrophilic attack. For alkenes complexed to platinum(II) their primary mode of reactivity is by attack from an external nucleophile. 

Although t/3-allyl complexes of platinum(II) are not rare, their occurrence is not as frequent as for -alkene complexes. This situation is reversed for palladium(II) where r 3-allyl complexes are very common, and much of modern organopalladium chemistry is becoming dominated by the reactivity of j)3-allyl complexes. 

A study of the scope of the reaction has shown that mono- and disubstituted alkenes and, particularly, aryl-substituted alkenes are the best substrates. Various limitations have been noted, some not unexpected (sensitivity to steric effects), others quite surprising (complex reactivity of cyclohexenes). Nevertheless, the exceptionally high stability of the reagent should make it available from the shelf, and in appropriate cases its use is to be considered as an alternative to the Simmons-Smith reaction. 

The reactivity of alkenes increases with their nucleophilic nature in the order tetra-substituted>trisubstituted>disubstituted>monosubstituted. Further, the epoxidation rate V = /c2X[alkene][complex]/(l + J [alkene]) shows that decomposition of the alkene-metal complex represents the rate determining step in this reaction. 

The cyclic peroxymetalation process (inner half-circle of Scheme 3) involves the peroxo species (70a) as the reactive intermediate. The cyclic adduct (72a) can result (a) from the reaction of an Rhin-peroxo complex with an alkene or (b) from the reaction of an Rh -ir-alkenic complex with 02. Indeed, both of these reactions (a) and (b) have been shown to occur. 

Nitrogen nucleophiles such as amines (and in intramolecular cases, amides and tosamides) readily add to alkenes complexed to palladium(ll) and iron(ll) with reactivity and regiochemical features parallel to those observed for oxygen nucleophiles. However, these metal-assisted animation reactions are subject... 

Cationic cyclopentadienyliron dicarbonyl (Fp) alkene complexes are generally reactive towards a wide variety of nucleophiles, including nitrogen nucleophiles, but they too generate stable chemical step (usually oxidation). This makes catalysis impossible and severely limits application of this methodology to organic synthesis (equation 21 ).36 However, in contrast to palladium, iron is relatively inexpensive and stoichiometric procedures... 

Iodine isocyanate, preformed or made in situ from AgNCO and h, adds to alkeites - with the regio- and stneo-chemistry expected of reactions proceeding via cyclic iodonium ions. When the INCO is made in situ, a competing mechanism also occurs (except with the most reactive alkenes) in which the alkene complexes with the iodine, and the complex then reacts with the isocyanate ion to generate the same P-iodoisocyanate as obtained firom INCO direct The reaction can be carried out at -35 to +20 C in Et20, CHzCh, THF, pentane or excess alkene as solvent. Dichloromethane or ether are... 

Comparison of equation (52) with Schemes 10 and 11 suggests diat substitution at C-4 confers substantial benefit in terms of lx>th yield and reaction time. This is most likely a result of a gem-dialkyl effect in which conformations placing the alkene and complexed-alkyne ends of the system in close proximity are rendered endialpically more favorable by increasing substitution on the intervening atoms. The reduced contribution of to AG associated with alkene complexation allows it to better compete with intermolecular side reactions (i.e. trimerization of the otherwise mote reactive alkyne moieties). Hua has made similar observations in simpler systems (equation 53). No stereocontrol at C-4 is available, however, the position being too remote from the reaction centers (equation 54). ... 

J. F. Biellmann, H. Hemmer, and J. Levisalles Alkene complexes of transition metals as reactive intermediates, pp. 215-265 (258). 

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