Allyl complexes synthetic applications

As noted in the introduction, in contrast to attack by nucleophiles, attack of electrophiles on saturated alkene-, polyene- or polyenyl-metal complexes creates special problems in that normally unstable 16-electron, unsaturated species are formed. To be isolated, these species must be stabilized by intramolecular coordination or via intermolecular addition of a ligand. Nevertheless, as illustrated in this chapter, reactions of significant synthetic utility can be developed with attention to these points. It is likely that this area will see considerable development in the future. In addition to refinement of electrophilic reactions of metal-diene complexes, synthetic applications may evolve from the coupling of carbon electrophiles with electron-rich transition metal complexes of alkenes, alkynes and polyenes, as well as allyl- and dienyl-metal complexes. Sequential addition of electrophiles followed by nucleophiles is also viable to rapidly assemble complex structures. 

The nonacarbonyldiiron-induced transformation of oxazabicyclo[2.2.2] octenes (12) into condensed azetidinones is intriguing mechanistically but will obviously have limited synthetic application (Scheme 15).37 The /(-lactam (14a), among other products, is isolated directly from the iron carbonyl reaction but the dimethyl analog (14b) is obtained by pyrolysis of an isolable intermediate <7-7r-allyl complex (13b). 

Another approach in the study of the mechanism and synthetic applications of bromination of alkenes and alkynes involves the use of crystalline bromine-amine complexes such as pyridine hydrobromide perbromide (PyHBts), pyridine dibromide (PyBn), and tetrabutylammonium tribromide (BiMNBn) which show stereochemical differences and improved selectivities for addition to alkenes and alkynes compared to Bn itself.81 The improved selectivity of bromination by PyHBn forms the basis for a synthetically useful procedure for selective monoprotection of the higher alkylated double bond in dienes by bromination (Scheme 42).80 The less-alkylated double bonds in dienes can be selectively monoprotected by tetrabromination followed by monodeprotection at the higher alkylated double bond by controlled-potential electrolysis (the reduction potential of vicinal dibromides is shifted to more anodic values with increasing alkylation Scheme 42).80 The question of which diastereotopic face in chiral allylic alcohols reacts with bromine has been probed by Midland and Halterman as part of a stereoselective synthesis of bromo epoxides (Scheme 43).82... 

A similar palladium-catalyzed cyclization procedure has recently been developed which involves enol ethers capable of (3-H elimination.373 Significant evidence has been accumulated suggesting that an oxa-ir-allyl complex is not an intermediate in these reactions, but that it is better characterized as an enolate addition to a Pdn-alkene complex.376 377 Synthetic applications of this reaction have also appeared.376-379... 

Porphyrin complexes, however, are prone to oxidative decomposition and therefore synthetic applications are hampered by rapid catalyst deactivation. This problem can be overcome by attaching electron-withdrawing groups to the periphery of the porphyrin system. Another problem is the poor chemoselectivity. In many cases, addition to the C=C double bond and formation of the epoxide are much faster than the corresponding hydrogen abstraction, which leads to the allylic alcohols. This is... 

Among 7i-allyl complexes of several transition metals, the chemistry of n-allylpalladium has been studied most extensively. From the standpoint of organic synthesis, reactions involving 7i-allylpalladium complexes are by far the most important therefore, their synthetic applications are mainly treated in this chapter. 

Among the reactions catalyzed by titanium complexes, the asymmetric epoxidation of allylic alcohols developed by Sharpless and coworkers [752, 807-810] has found numerous synthetic applications. Epoxidation of allylic alcohols 3.16 by ferf-BuOOH under anhydrous conditions takes place with an excellent enantioselectivity (ee > 95%) when promoted by titanium complexes generated in situ from Ti(0/ -Pr)4 and a slight excess of diethyl or diisopropyl (R,R)- or (iS, 5)-tartrates 2.69. The chiral complex formed in this way can be used in stoichiometric or in catalytic amounts. For catalytic use, molecular sieves must be added. Because both (RJ )- and (5,5)-tartrates are available, it is posable to obtain either enantiomeric epoxide from a single allylic alcohol. Cumene hydroperoxide (PhCMe20OH) can also be used in place of ferf-BuOOH. This method has been applied to industrial synthesis of enantiomeric glycidols [811, 812]. 

Synthetic applications of electrochemical reduction of allyl ethers in the presence of nickel complexes with bipyridines and tetraazamacrocycles as ligands 00CCC844. 

Electrophilic substitution of r/ -allyl complexes, especially those of Si and Sn, has found extensive synthetic applications, but the overall transformation is stoichiometric with regard to the amount of the metal atom. A catalytically useful reaction of r/ -allyl intermediate was involved in telomerization of 1,3-dienes in the presence of Pd catalyst shown in Scheme 8.41. r/ -Allylmetal complexes were reactive with not only H+ but also electrophilic alkenes (e.g. Scheme 8.72) [62,133]. Recent development in Pd-catalyzed amphiphilic allylation of alkenes and imines (e.g. Scheme 8.73) relied on the high susceptibility of Pd-bound r -allyl ligand to the attack of unsaturated carbon electrophile [134]. 

This year has again emphasized the growing importance of organo-transition metal complexes in organic synthesis. In catalysed reactions the major advances have been in asymmetric catalysis with the first reports of chiral induction in catalytic epoxidation and further reports on improved catalysts for asymmetric hydrogenation and allylic alkylation. The formation of carbon-carbon bonds continues to attract attention, and several novel and potentially useful synthetic applications of organometallic complexes have been reported. 

The first enantioselective intermolecular C—H bond insertion which could be of practical synthetic application was reported in 1997 by Davies and Hansen (Scheme 1.2). In the presence of a variety of relatively unreactive cycloalkane solvents 2 (compared with C—H bonds at the allylic and ben lic positions, as well as C—H bonds a to a heteroatom), the Rh complex Rh2(S-DOSP)4, a privileged catalyst derived from L-proline, was found capable to catalyze the decomposition of aryldiazoacetates 1, inducing the functionalization of cycloalkanes 3 (Scheme 1.2, eqn (1)). Circumventing chemoselectivity and... 

Asymmetric Induction on the Nucleophile The use of the tBu-PHOX ligand led to the first catalytic enantioselective Tsuji allylations of simple alkanone enol derivatives 62. These mild, operationally straightforward and stereoselective reactions described by Stoltz et al. [52] produce chiral cycloalkanones 63 with quaternary stereocenters at the a-position with high enantiopurities and in excellent chemical yields (Scheme 12.31). Mechanistic studies showed the incorporation of an O-bound enolate in the intermediate Pd-allyl complex [53]. Further investigations on the substrate scope led to several applications in the synthesis of natural products [54]. Recently, a similar approach was used to afford enantiopure quaternary lactams 65 that intercept synthetic intermediates previously used in the synthesis of the Aspidospcrma alkaloids quebrachamine and rhazinilam, but that were previously only available by chiral-auxiliary-assisted approaches or as racemic mixtures (Scheme 12.32) [55],... 

Although the first Ptotalyzed allylic substitutions have been known for several decades, not many of them have been used in synthetic applications [165b]. Noteworthy is the fact that, compared to Pd complexes, the Pt congeners give an inverse regioselectivity with the preferential formation of branched products when soft nucleophiles (sNus) such as 1,3-diketones or p-keloesters are used (Scheme 12.85) [186]. 

The catalytic version of allylation of nucleophiles via 7r-allylpaUadium intermediates was discovered in 1970 using allylic esters and aUyl phenyl ethers as substrates (Scheme Formation of 7r-allylpaUadium complexes by oxidative addition of various allylic compounds to Pd(0) and subsequent reaction of electrophilic rr-allylpalladium complexes with soft carbon nucleophiles are the basis of the catalytic allylation. After the reaction, Pd(0) is regenerated, which undergoes oxidative addition to the allylic compounds again, making the whole reaction catalytic. The efficient catalytic cycle is ascribed to the characteristic feature that Pd(0) is more stable than Pd(II). Allylation of carbon nucleophiles with allyhc compounds via TT-allylpalladium complexes is called the Tsuji-Trost reaction. The reaction has wide synthetic applications, particularly for cyclization. " ... 

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