Dienes reaction with heteroatom alkenes

Many chiral diphosphine ligands have been evaluated with regard to inducing enantioselectivity in the course of the hydroformylation reaction [25,26]. However, a real breakthrough occurred in 1993 with the discovery of the BI-NAPHOS ligand by Takaya and Nozaki [65]. This was the first efficient and rather general catalyst for the enantioselective hydroformylation of several classes of alkenes, such as aryl alkenes, 1-heteroatom-functionalized alkenes, and substituted 1,3-dienes, and is still a benchmark in this area [66,67]. But still a major problem in this field is the simultaneous control of enantio-... 

Representative data illustrating the influence of Lewis base functional groups in the ADMET reaction are shown in Table 1. When molybdenum catalysts are used to polymerize ether or thioether dienes, little change in reaction rate is observed as compared with the standard, 1,9-decadiene, which possesses no heteroatoms in its structure. When a sulfur atom is three carbons atoms away from the alkene site, the reaction rate is reduced approximately one order of magnitude otherwise, the kinetics are all essentially unaffected [20a]. 

The same transition metal systems which activate alkenes, alkadienes and alkynes to undergo nucleophilic attack by heteroatom nucleophiles also promote the reaction of carbon nucleophiles with these unsaturated compounds, and most of the chemistry in Scheme 1 in Section 3.1.2 of this volume is also applicable in these systems. However two additional problems which seriously limit the synthetic utility of these reactions are encountered with carbon nucleophiles. Most carbanions arc strong reducing agents, while many electrophilic metals such as palladium(II) are readily reduced. Thus, oxidative coupling of the carbanion, with concomitant reduction of the metal, is often encountered when carbon nucleophiles arc studied. In addition, catalytic cycles invariably require reoxidation of the metal used to activate the alkene [usually palladium(II)]. Since carbanions are more readily oxidized than are the metals used, catalysis of alkene, diene and alkyne alkylation has rarely been achieved. Thus, virtually all of the reactions discussed below require stoichiometric quantities of the transition metal, and are practical only when the ease of the transformation or the value of the product overcomes the inherent cost of using large amounts of often expensive transition metals. 

By far the most important reactions of this type are [4 + 2] heterocyclization analogues of the DielsAlder reaction which present a versatile route to six-membered rings . The heteroatom can originate from the dienophile (e.g., Schemes 55-57) or from the diene (e.g., Schemes 58 and 59). Whereas, -unsaturated carbonyl compounds react best with electron-rich alkenes (Scheme 58), enaminothiones prefer electron-deficient dienophiles (Scheme 59). 

The vast majority of work on asymmetric Diels-Alder reactions deals with additions of 1,3-dienes to a, -alkenic carbonyl derivatives XXI) where the chirophore R is attached to the carbonyl group eiAer directly or via a heteroatom X, permitting subsequent removal of the auxiliary (e.g. by attack of a nucleophile Nu Scheme 75). 

Decomplexation of organic ligands bound to the metal in the odd numbered 7 or fashion (alkyl, allyl or dienyl complexes and others) is usually accompanied by formation of new (T-bonds (C-H, C-C, C — Heteroatom) depending on the reaction conditions applied. The ligand cannot be decomplexed unchanged as possible with even-numbered electron ligands, such as alkenes, dienes or alkynes. 

PdCE-catalyzed addition reaction of allyl chloride to alkynes is explained by chloropalladation of a triple bond, followed by insertion of the double bond of allyl chloride to generate 43. No jr-allyl complex is formed from allyl chloride and PdCl2. The final step is elimination of /3-Cl to afford 1-chloro-1,4-diene 44 with regeneration of Pd(ll) [37]. As another example, the Pd(0)-catalyzed Heck reaction of vinyl acetate affords stilbene in this reaction, the primary product is /3-phenylvinyl acetate (45), which reacts again with iodobenzene, and the last step is elimination of /S-OAc to give stilbene. At the same time, Pd(II) is generated, which is reduced to Pd(0) in situ [38]. However, elimination of /3-heteroatoms is not always faster than that of j8-H. For example, the Heck reaction of allyl alcohol with iodobenzene proceeds by preferential elimination of /3-H from the insertion product 46 to afford aldehyde 47, and no elimination of /3-OH from the same carbon occurs to give the alkene 48 [39,40]. 

Allenes (1,2-dienes) are more reactive than alkenes, and Pd-catalyzed reactions of allenes with carbon and heteroatom nucleophiles proceed mostly via the formation of r-allylpalladium intermediates, and offer useful methods for making carbon-carbon and carbon-heteroatom bonds. The hrst report on Pd-catalyzed reaction of allenes with amines and malonate was given by Coulson in 1973 [15]. A comprehensive review on Pd-catalyzed reactions of allenes is available [16]. 

Lewis structure models incorporating radical character are useful in the exposition of photochemical reactions of carbonyl compounds, alkenes, dienes, and aromatic compounds that are not substituted with polar groups. When heteroatoms are substituted onto an aromatic ring, considerable charge transfer character can be introduced into both the ground state and the excited state, and ionic resonance structures become more suitable as models for reactivity. 

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