Jt-Allyl complexes

A cobalt-mediated formal Alder ene reaction of an allenyne takes place to give a mixture of adducts and ( 4-cycloliexadiene)cobalt complexes (Scheme 16.77) [88], The reaction may proceed via coordination and ensuing jt-allyl complex formation. 

Palladium(II) is one of the most important transition metals in catalytic oxidations of allenes [1], Scheme 17.1 shows the most common reactions. Transformations involving oxidative addition of palladium(O) to aryl and vinyl halides do not afford an oxidized product and are discussed in previous chapters. The mechanistically very similar reactions, initiated by nucleophilic attack by bromide ion on a (jt-allene)pal-ladium(II) complex, do afford products with higher oxidation state and are discussed below. These reactions proceed via a fairly stable (jt-allyl)palladium intermediate. Mechanistically, the reaction involves three discrete steps (1) generation of the jt-allyl complex from allene, halide ion and palladium(II) [2] (2) occasional isomeriza-... 

Scheme 17.1 Selected palladium-catalyzed reactions of allenes proceeding via Jt-allyl complexes.

The intermediate Jt-allyl complex is formally the palladium(II) complex of an allylic anion that can be represented by the two mesomeric forms shown in Scheme 17.2. It is important to note that this is not a fast equilibrium between two cr-allyl complexes but a stable species where palladium is simultaneously bound to both carbon-1 and carbon-3. All eight atoms of the Jt-allyl moiety are almost in the same plane. All three carbon atoms have sp2 character and the rotation between the Cl-C2 and C2-C3 bonds is blocked. As a consequence of the hindered rotation, four dia-stereomeric Jt-allyl complexes are possible. For example, in Scheme 17.2 both R and R are syn to the hydrogen on carbon-2, therefore this complex is called the syn,syn diastereomer. 

In Scheme 17.2 palladium is coordinated from below, but it is also possible that it coordinates from above and forms the other enantiomer of the chiral syn,syn Jt-allyl complex. If palladium has another chiral ligand then these Jt-allyl complexes become diastereomers. Thus, from an unsymmetrically substituted allene (R R ), eight diastereomeric Jt-allyl complexes can be formed. If one of the diastereomers is preferred then further reaction of the Jt-allyl moiety leads to an enantiomerically enriched product. 

In 1997, Backvall and Jonasson published a procedure for the 1,2-oxidation of terminal allenes 7 [5]. In this case the reaction conditions were chosen so that the (vinyl)palladium complex equilibrates back to the allene complex. Using bromide instead of chloride as a nucleophile, the 2-bromo-jt-allyl complex 9 is the major intermediate present in the reaction mixture. A catalytic reaction was developed with the use of 5 mol% palladium acetate and p-benzoquinone (BQ) as terminal oxidant (Scheme 17.5). 

Good diastereoselectivity was obtained with BQ as the oxidant in acidic media but the reaction times were relatively long (1-2 days at 40 °C). Using the copper(II)-oxy-gen system in slightly basic media permits a much faster reaction (0.5-1 h at 20 °C) with better isolated yields but with poor or even reversed diastereoselectivity. The slower reaction with BQ as oxidant is due to the fact that this oxidant requires an acidic medium, which lowers the nucleophilicity of the acid moiety. It is also likely that BQ or copper(II) has to coordinate to palladium(II) before the second nucleophile can attack to make the Jt-allyl complex more electrophilic. Coordination of cop-per(II) would make a more electrophilic intermediate than coordination of BQ. The relation between reaction time and diastereoselectivity supports a mechanism analogous to that in Scheme 17.7. 

The reaction of an allene with an aryl- or vinylpalladium(II) species is a widely used way of forming a Jt-allyl complex. Subsequent nucleophilic attack on this intermediate gives the product and palladium(O) (Scheme 17.1). Oxidative addition of palladium ) to an aryl or vinyl halide closes the catalytic cycle that does not involve an overall oxidation. a-Allenyl acids 27, however, react with palladium(II) instead of with palladium(O) to afford cr-vinylpalladium(II) intermediates 28 (Scheme 17.12). These cr-complexes than react with either an allenyl ketone [11] or with another alle-nyl acid [12] to form 4-(3 -furanyl)butenolides 30 or -dibutenolides 32, respectively. 

In a similar manner, Jt-allyl complexes of manganese, iron, and molybdenum carbonyls have been obtained from the corresponding metal carbonyl halides [5], In the case of the reaction of dicarbonyl(r 5-cyclopentadienyl)molybdenum bromide with allyl bromide, the c-allyl derivative is obtained in 75% yield in dichloromethane, but the Jt-allyl complex is the sole product (95%), when the reaction is conducted in a watenbenzene two-phase system. Similar solvent effects are observed in the corresponding reaction of the iron compound. As with the cobalt tetracarbonyl anion, it is... 

Pd(H) complexes with strongly electron-withdrawing ligands can insert into the allylic C—H bond (path c) to form directly the Jt-allyl complex via oxidative addi-tion.502,694,697 Pd(OOCCF3)2 in acetic acid, for example, ensures high yields of allylic acetoxylated products.698 The delicate balance between allylic and vinylic acetoxylation was observed to depend on substrate structure, too. For simple terminal alkenes the latter process seems to be the predominant pathway.571... 

The rearrangement exhibits some stereochemical preference for c/s-vinyl carbene complex (with respect to the metal) compared to the //ww-isomer. Thus, 2-methyl-2-m-vinyl cyclopropyl (methoxy) carbene chromium pentacarbonyl rearranges to 5-methyl-5-vinyl-2-methoxycyclopentenone approximately 4 times faster (THF, 52 °Q than the trans-isomer, which in turn rearranges faster than phenyl derivatives. This suggests that vinyl complexes undergo initial Cope-type rearrangement to form metallacycloheptadienes, which then rearrange to jt-allyl complexes. Subsequent CO insertion and reductive elimination leads to the vinylcyclopentenones (equation 89)150. 

The cationic Jt-allyl complex is often isolable and has been the subject of considerable study. X-ray structures of the 7t-allyl complexes with chiral ligands have been a primary source of structural information from which the design and predictive model of chiral catalysts derive. Chiral-metal-olefin complexes, which constitute another important class of intermediates have also been isolated, albeit few in number [31]. These static studies have been complemented by a growing number of NMR studies taking advantage of modem heteronuclear correlation and NOE techniques, which offer opportunities to monitor solution structures of the catalytic species [32-34],... 

Complementary to the conjugate substitution reaction in which the nucleophile is transferred directly from the tetraalkyl ferrate to the allylic ligand, preformed low-valent Fe complexes can form reactive allyl-iron complexes via an SN2 -type mechanism (path C, Equations (7.8) and (7.9), Scheme 7.16], These complexes react with incoming nucleophiles and electrophiles in a substitution reaction. Depending on the nature ofthe iron complex employed in the reaction, either o- or Jt-allyl complexes are generated. 

For Pd-catalyzed cross-coupling reactions the organopalladium complex is generated from an organic electrophile RX and a Pd(0) complex in the presence of a carbon nucleophile. Not only organic halides but also sulfonium salts [38], iodonium salts [39], diazonium salts [40], or thiol esters (to yield acylpalladium complexes) [41] can be used as electrophiles. With allylic electrophiles (allyl halides, esters, or carbonates, or strained allylic ethers and related compounds) Pd-i73-jt-allyl complexes are formed these react as soft, electrophilic allylating reagents. 

Platinacyclobutane complex 118 undergoes equilibrium heterolytic scission of the exocyclic carbon-carbon bond to form a cationic allyl complex and the organic enolate ion (Equation 35) <1993OM3019>. Similar dissociative ionization was previously reported for rearrangements of iridium and rhodium metallacyclobutane complexes formed by nucleophilic alkylation < 1990JA6420>. This carbon-carbon bond activation is generally associated with reversible central carbon alkylation of Jt-allyl complexes (Section 2.12.9.3.3), but the homolytic equivalent has recently been... 

Considering the mechanistic rationales of the transition metal-catalyzed enyne cycloisomerization, different catalytic pathways have been proposed, depending on the reaction conditions and the choice of metal catalyst [3-5, 45], Complexation of the transition metal to alkene or alkyne moieties can activate one or both of them. Depending on the manner of formation of the intermediates, three major mechanisms have been proposed. The simultaneous coordination of both unsaturated bonds to the transition metal led to the formation of metallacydes, which is the most common pathway in transition metal-catalyzed cycloisomerization reactions. Hydrometalation of the alkyne led to the corresponding vinylmetal species, which reacts in turn with olefins via carbometalation. The last possible pathway involves the formation of a Jt-allyl complex which could further react with the alkyne moiety. The Jt-allyl complex could be formed either with a functional group at the allylic position or via direct C-H activation. Here the three major pathways will be discussed in a generalized form to illustrate the mechanisms (Scheme 8). 

Jt-allyl complex can be generated after cyclization, as suggested by Takacs in a Fe(0)-catalyzed cyclization of polyenes. It also can be preformed if an active functional group is present in the allylic position. The palladium-catalyzed intramolecular cycloisomerization reaction of allylic acetates is an efficient method for constructing five- or six-membered rings [56, 57]. An asymmetric approach to this transformation has been studied and so far only poor enantioselectivity has been achieved (0-20% ee) [58]. Very recently, Zhang et al. also reported a Rh-catalyzed cycloisomerization involving a Jt-allylrhodium intermediate formed from an allylic halide [59]. 

Keywords Allylic substitution, Allylation, Allylic alkylation, Jt-Allyl complexes, Palladium, Molybdenum, Ruthenium, Iridium... 

A special case where a racemic mixture can be converted to a single enantiomer was reported by Trost [137]. In this instance, the interconversion of the Jt-allyl complexes 44 and 45 is possible through a furan intermediate and the configuration of the product is controlled by the chiral ligand (Scheme 4). This... 

The stereochemistry of Pd -catalyzed allylic alkylation is net retention (equation 62). This arises from sequential inversion steps. Initially, the Pd approaches from the face of the C3 unit opposite the leaving group, to form the jt-allyl complex. Subsequently, the nucleophile adds to the face of the TT-allyl opposite palladium. If a bulky or umeactive nucleophile is used with allylic acetates, the acetoxy group can add again to the complex. Ultimately, this results in the production of a mixture of stereoisomers upon nucleophihc addition. As an example of the range of allylic substrates that react, nitrogen nucleophiles, in particular primary and secondary amines, undergo palladium-catalyzed substimtion with aUyhc alcohols, acetates, and ethers (equation 63). 

This reaction works (55%) for Co. Furthermore, it is a prototype synthesis for numerous silicon-substituted Jt-allyl complexes of Mo, W and Fe. The yields are moderate to low, however, and isolation of the intermediate -allyl complex and subsequent UV irradiation are required. ... 

These diene dimerizations might start with the formation of cationic Ru(IV) bis(jt-allyl) complexes rather than ruthenacyclopentanes such as 137 (Scheme 4.49). This was confirmed by the stoichiometric reaction of the 1,3-pentadiene complex... 

R = Me) with 1,3-pentadiene and AgOTf, giving rise to the bis(jt-allyl) complex... 

The reaction of this allyHc acetate with the sodium salt of Meldrum s acid (structure in margin) demonstrates the retention of configuration in the palladium(0)-catalysed process. The tetraacetate and the intermediate Jt-allyl complex are symmetrical, thus removing any ambiguity in the formation or reaction of the Jt-allyl complex and hence in the regiochemistry of the overall reaction. 

The presence of five-membered rings such as cyclopentanes, cyclopentenes, and dihydrofurans in a wide range of target molecules has led to a variety of methods for their preparation. One of the most successful of these is the use of trimethylenemethane [3-1-2] cycloaddition, catalysed by pal-ladium(O) complexes. The trimethylenemethane unit in these reactions is derived from 2-[(trimethylsilyl)methyl]-2-propen-l-yl acetate which is at the same time an allyl silane and an allylic acetate. This makes it a weak nucleophile and an electrophile in the presence of palladium(O). Formation of the palladium jt-allyl complex is followed by removal of the trimethylsilyl group by nucleophilic attack of the resulting acetate ion, thus producing a zwitterionic palladium complex that can undergo cycloaddition reactions. 

Palladium coordinates to one face of the diene promoting intramolecular attack by the alcohol on the opposite face. The resulting o-alkyl palladium can form a jt-allyl complex with the palladium on the lower face simply by sliding along to interact with the double bond. Nucleophilic attack of chloride from the lithium salt then proceeds in the usual way on the face opposite palladium. The overall addition to the diene is therefore cis. 

Similarly, thermolysis of ruthenacyclobutane 61 produces Jt-allyl complex 62 [77]. The reaction involves (3-methyl transfer from the central carbon of the ligand to the metal via a formal 16-electron unsaturated intermediate. A kinetic investigation in the presence of excess phosphine revealed that the process is reversible. 

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