Alkenes cyclometallation

A wide variety of five-membered zirconacydes 8 may be formed by the formal co-cycliza-tion of two 7i-components (3 and 6 alkene, alkyne, allene, imine, carbonyl, nitrile) on zir-conocene ( Cp2Zr ) (Scheme 3.2) [2,3,8]. The co-cydization takes place via the r 2-complex 5 of one of the components, which is usually formed by complexation of 3 with a zircono-cene equivalent (path a) ( Cp2Zr itself is probably too unstable to be a true intermediate) or by oxidation on the metal (cyclometallation/p-hydrogen elimination) (path b). Two additional routes to zirconocene r 2-complexes are by the reverse of the co-cyclization reaction (i. e. 8 reverting to 5 or 9 via 7), and by rearrangement of iminoacyl complexes (see Section... 

The reaction of an alkene (in excess) with a polyhydride can strip hydrogen from the metal. The open sites formed in this process can now react, whether by cyclometalation or by coordinating the alkenes. If the alkene has accessible C—H bonds (e.g. propene) these may be broken by the metal. Indeed dehydrogenation may proceed until a stable 16-electron compound has been formed. In most cases this hydrogen stripped from the ligand is passed to the excess of alkene. Examples of these types of reaction have been described by Wilkinson (Scheme 5).131... 

Alkene cyclozirconation is a really useful means by which 1,2-trans difunctional-ized cyclopentane rings can be produced. The initially formed cyclometallic species is decomposed in the presence of oxygen and aqueous acid to give the desired product (equation 110)429 -433. 

Cyclohexyldienyl complexes, with Ti(IV), 4, 327 Cyclohexylisocyanides, with gold(I) halides, 2, 281 Cyclohexylphosphine, for semiconductor growth, 12, 9 Cyclohexyl selenides, preparation, 9, 480 Cyclohydrocarbonylation alkenes, 11, 515 alkynes, 11, 522 dienes, 11, 522 overview, 11, 511-555 for ring expansion, 11, 527 Cycloisomerizations, via silver catalysts, 9, 558 Cyclomanganation, product types, 5, 777-778 Cyclometallated azobenzenes, liquid crystals, 12, 251 Cyclometallated complexes for OLEDs... 

Cyclometallation (also called oxidative coupling) is a rather special case of oxidative addition. In this reaction, two unsaturated molecules, X=Y and X =Y, add to the same metal atom M. One of the X—Y bonds and one of the X —Y bonds are broken, and new M X and M -Y bonds form. However, a new Y—Y bond also forms, and the overall result is a cyclometaUated compound (Figure 3.7a). As in oxidative addition, the oxidation state of the metal center increases by 2. Cyclometallation is common with alkynes (Figure 3.7b), as well as with alkenes activated by electron-withdrawing groups [21]. 

In the presence of Ni(dppe)Br2 the oxabicyclic alkenes 693 react with propynoic esters to yield 2H-benzo[ ]coumarins 696 (Scheme 172) <2001AGE1286>. The reaction mechanism involves cyclometallation of the propynoic ester and oxabicyclic alkene 693 to form the nickelacyclopentene intermediate 694. (3-Oxy elimination then forms the intermediate 695, which undergoes protonation and isomerization of the double bond followed by intramolecular lactonization to afford the desired 2/7-benzo[ ]coumarins 696 (Scheme 172) <2001AGE1286>. 

Attempts to prepare titanaindanes and titanaindenes from thermolysis of 28a or 28b in the presence of alkenes and alkynes failed only the cyclometalated complexes 30a and 30b from intramolecular C-H activation in 29 were observed [Eq. (7)].42 A similar reaction has been observed for substituted Cp2Zr-benzyne complexes.2... 

A range of P—C ligand complexes are known and these are fully discussed under the headings cyclometallated (Sections 45.5.4.5.V and 45.10.3.2.iii) and alkene complexes (Section 45.5.4.5.vi). 

Five-membered cyclometallated complexes similar to (191) are produced from the reaction between alkenic imines and iridium complexes. The reaction of alkenic imines with [Ir(Cl)(C8H,4)2]2 in the presence of cyclohexylphosphine yields complex (193),442 which, when treated with Cl2, undergoes substitution without Ir—C bond rupture to give (194 reaction 114).441 Azobenzene or its derivatives react with [Ir(Cl)(C8Hl4)2]2 in the presence of PR3 (R=Ph, cyclohexyl) according to reaction (112) to yield the cyclometallated complex (195).441,442... 

Notably, it is not just heteroatom-functionalized alkenes that can behave as dienophiles with metal-bound phospholes. It has been demonstrated that reaction of 2equiv of 3,4-dimethyl-l-phenylphosphole with a cationic platinum(n) complex of an enantiomerically pure cyclometallated A, A -dimethyl-l-(l-naphthyl)ethylamine ligand affords, following decomplexation with cyanide, the novel optically pure diphosphine (-f)-267 quantitatively as an air-sensitive oil (Scheme 93) <2000CC167>. The high-frequency chemical shift of one of the phosphoms centers (5 4.9 and 104.2 ppm (7pp = 43.9 Hz)) is indicative of rwt>-jy -stereochemistry. Similar reactivity has been... 

A limited number of cyclometalation reactions at alkene and related C-H bonds have been carried out. In part, this may have arisen because such ligands can form Pd-aUcene TT-complexes rather than give C-H insertion products. Nevertheless, examples can be found, as in the cyclometalation of vinylic oximes (equation 74). 

The Murai reaction (Scheme 4), the replacement of an ortho-CH on an aromatic ketone by an alkyl group derived from a substrate olefin, is catalyzed by a variety of Ru complexes. This C bond formation occurs via chelate directed C-H bond activation (cyclometalation) in the first step, followed by alkene insertion into RuH and reductive elimination of the alkylated ketone. In a recent example of the use of a related cyclometalation in complex organic synthesis, Samos reports catalytic arylation (Suzuki reaction) and alkenylation (Heck reaction) of alkyl segments of a synthetic intermediate mediated by Pd(II). 

Metallacyclic complexes play an important role as reactive intermediates in catalytic cycles initiated by homogeneous transition-metal complexes. Thus, metallacyclobutanes are discussed as intermediates in alkene metathesis, isomerization of strained cyclopropane compounds and many other reactions. On the other hand, numerous examples of isolable me-tallacyclobutane complexes have been reported. These can be formed by different routes such as carbon-carbon bond cleavage of cyclopropane compounds (A), cyclometallation via C — H bond cleavage (B), nucleophilic addition to allyl complexes (C), rearrangement of metallacyc-lopentanes (D) or transmetalation of 1,3-dimetallalated carbon chains (E). ... 

We discovered the thermal reversibility of the diene cyclometallation when we attempted cyclization of the diene 9 (Scheme 3) (8,9). The reaction proceeded to completion after two hours at room temperature, but rather than the expected tricyclic alcohol 13, the product was the dimer 11, from the reaction of two monosubstituted alkenes. We repeated the cyclozirconation, but let it proceed for a longer time. After 18 hours at room temperature, a new product had appeared (TLC analysis). After 1.5 hours at 75°C, the reaction was complete. The tricyclic alcohol 13 was isolated in 63% yield from 9. 

Shortly after the discovery of enyne metathesis, Trost began developing cycloisomerization reactions of enynes using Pd(ll) and Pt(ll) metallacyclic catalysts (429-433), which are mechanistically divergent from the metal-carbene reactions. The first of these metal catalyzed cycloisomerization reactions of 1,6-enynes appeared in 1985 (434). The reaction mechanism is proposed to involve initial enyne n complexation of the metal catalyst, which in this case is a cyclometalated Pd(II) cyclopentadiene, followed by oxidative cyclometala-tion of the enyne to form a tetradentate, putative Pd(IV) intermediate [Scheme 42(a)]. Subsequent reductive elimination of the cyclometalated catalyst releases a cyclobutene that rings opens to the 1,3-diene product. Although this scheme represents the fundamental mechanism for enyne metathesis and is useful in the synthesis of complex 1,3-cyclic dienes [Scheme 42(fe)], variations in the reaction pathway due to selective n complexation or alternative cyclobutene reactivity (e.g., isomerization, p-hydride elimination, path 2, Scheme 40) leads to variability in the reaction products. Strong evidence for intermediacy of cyclobutene species derives from the stereospecificity of the reaction. Alkene... 

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