Alkyne insertion products, formation

The superior donor properties of amino groups over alkoxy substituents causes a higher electron density at the metal centre resulting in an increased M-CO bond strength in aminocarbene complexes. Therefore, the primary decarbo-nylation step requires harsher conditions moreover, the CO insertion generating the ketene intermediate cannot compete successfully with a direct electro-cyclisation of the alkyne insertion product, as shown in Scheme 9 for the formation of indenes. Due to that experience amino(aryl)carbene complexes are prone to undergo cyclopentannulation. If, however, the donor capacity of the aminocarbene ligand is reduced by N-acylation, benzannulation becomes feasible [22]. 

Insertion of the alkyne into the chromium carbene bond in intermediate B affords vinyl carbene complex D, in which the C=C double bond may be either (Z) or (E). A putative chromacydobutene intermediate resulting from a [2+2] cydoaddition of the alkyne across the metal-carbene bond on the way to chromium vinylcarbene D, as was sometimes suggested in early mechanistic discussions, has been characterized as a high energy spedes on the basis of theoretical calculations [9c]. Its formation and ring-opening cannot compete with the direct insertion path of the alkyne into the chromium-carbene bond. An example of an (E)-D alkyne insertion product has been isolated as the decarbonylation product of a tetracarbonyl chromahexatriene (4, Scheme 4) [14], and has been characterized by NMR spectroscopy and X-ray analysis. 

Addition of hexafluorobutyne to CpW(CO)3SR (R = Me, Et, Pr ) also yields 172-vinyl products (189). Isolation of Cp(CO)2W[Tj2-CF3CC(CF3)-C(O)SMe) reflects CO insertion in addition to i72-vinyl formation (190). Other alkyne insertion products have been identified as well Cp-... 

Although numerous alkyne insertion products have been reported for Mo(II) and W(II), simple dimerization to form cyclobutadiene or tri-merization to form an arene ligand is rare. One brief report of cyclobutadiene formation from a bisalkyne complex (206) has been followed by a full paper which suggests that an rj2-vinyl complex may be the precursor to CpM(S2CNR2) [ 174-C4(CF3)4] (207). 

The formation of the tricarbonylchromium-complexed fulvene 81 from the 3-dimethylamino-3-(2 -trimethylsilyloxy-2 -propyl)propenylidene complex 80 and 1-pentyne also constitutes a formal [3+2] cycloaddition, although the mechanism is still obscure (Scheme 17) [76]. The rf-complex 81 must arise after an initial alkyne insertion, followed by cyclization, 1,2-shift of the dimethylamino group, and subsequent elimination of the trimethylsilyloxy moiety. Particularly conspicuous here are the alkyne insertion with opposite regioselectivity as compared to that in the Dotz reaction, and the migration of the dimethylamino functionality, which must occur by an intra- or intermo-lecular process. The mode of formation of the cyclopenta[Z ]pyran by-product 82 will be discussed in the next section. 

P-H oxidative addition followed by alkyne insertion into a Pd-P bond gives the re-gio-isomeric alkenyl hydrides 15 and 16. Protonolysis with diaUcyl phosphite regenerates hydride 17 and gives alkenylphosphonate products 18 and 19. Insertion of alkene 18 into the Pd-H bond of 17 followed by reductive eUmination gives the bis-products, but alkene 19 does not react, presumably for steric reasons. P-Hydride elimination from 16 was invoked to explain formation of trace product 20. 

Depending on the nature of the substrates, selectivity could be completely reversed between the two isomeric products. For example, switching R1 group between Buc and Ph gave high yields of the first and second product structures, respectively. The authors noted that the reaction did not proceed if the imine contained an ortho-MeO group at R2 or if the imine was replaced with an aldehyde, oxime, or hydrazone. The catalytic cycle is initiated by C-H activation of the imine, that is, the formation of a five-membered metallocycle alkyne insertion affords the intermediate drawn in Scheme 69. It is noteworthy that this is the first report of catalytic synthesis of indene derivatives via a C-H insertion mechanism (C-H activation, insertion, intramolecular addition). 

Malacria and co-workers76 were the first to report the transition metal-catalyzed intramolecular cycloisomerization of allenynes in 1996. The cobalt-mediated process was presumed to proceed via a 7r-allyl intermediate (111, Scheme 22) following C-H activation. Alkyne insertion and reductive elimination give cross-conjugated triene 112 cobalt-catalyzed olefin isomerization of the Alder-ene product is presumed to be the mechanism by which 113 is formed. While exploring the cobalt(i)-catalyzed synthesis of steroidal skeletons, Malacria and co-workers77 observed the formation of Alder-ene product 115 from cis-114 (Equation (74)) in contrast, trans-114 underwent [2 + 2 + 2]-cyclization under identical conditions to form 116 (Equation (75)). 

Generally phenol formation is the major reaction path however, relatively minor modifications to the structure of the carbene complex, the alkyne, or the reaction conditions can dramatically alter the outcome of the reaction [7]. Depending on reaction conditions and starting reactants roughly a dozen different products have been so far isolated, in addition to phenol derivatives [7-12], In particular, there is an important difference between the products of alkyne insertion into amino or alkoxycarbene complexes. The electron richer aminocarbene complexes give indanones 8 as the major product due to failure to incorporate a carbon monoxide ligand from the metal, while the latter tend to favor phenol products 7 (see Figure 2). 

Coupling of 1-alkyne 410 with 1-alkene 411, catalysed by CpRu(cod)Cl in aqueous DMF, affords the diene 414 as an ene-type product in good yield. One explanation of the reaction is the formation of the (71-al ly I )(> 2-al kync)i n termediate from the 1 -alkene and insertion of the alkyne [161]. However, formation of the ruthenacyclopentene 412, subsequent /1-elimination to form 413, and reductive elimination offer a more easily understandable mechanism. A formal synthesis of altemaric acid (415) was achieved by this reaction [162],... 

Whereas the catalytic hydrosilylation of alkynes was one of the first methods of controlled reduction and functionalization of alkynes, the ruthenium-catalyzed hydroamination of alkynes has emerged only recently, but represents a potential for the selective access to amines and nitrogen-containing heterocydes. It is also noteworthy that, in parallel, the ruthenium activation of inert C-H bonds allowing alkyne insertion and C-C bond formation also represents innovative aspects that warrant future development. Among catalytic additions to alkynes for the production of useful products, the next decade will clearly witness an increasing role for ruthenium-vinylidenes in activation processes, and also for the development of ruthenium-catalyzed hydroamination and C-H bond activation. 

C(OMe)C6H4-o-C=CPh (CO)j leads directly to the formation of a chrysene derivative via the formal dimerization of the carbene ligand. A plausible explanation for the formation of the final product involves a doubly alkyne-bridged dinuclear complex, alkyne insertions into metal-carbene bonds, and coupling of the carbene carbons. 

Insertion stereochemistries for the alkyne insertion process were found to be variable. Sometimes cis addition of the Ni-CHs bond occurred (e.g., for PhCsCCHs), sometimes trans addition occurred (e.g., for PhC=CPh), and sometimes a mixture of isomers was obtained. Allowing the complexes to stand led to a thermodynamically controlled mixture of cis and trans insertion products. Equilibration of cis and trans vinylaluminum compounds is known. Initial cis insertion of the alkyne yields a coor-dinately unsaturated vinylnickel intermediate that accounts for the different stereochemistry of products formed under kinetic control. Isomerization of the double bond occurs for this intermediate in competition with product formation. Thus the stereochemistry of the kinetic product does not necessarily give the stereochemistry of a preceding insertion step. 

The silicon-carbon bonds of silacyclobutanes are readily activated by Pd and Pt complexes [691]. This notable characteristic has been used for carbon-carbon bond formation using silacyclobutanes [692-695]. The Pd-catalyzed reaction of 191a with alkynes affords silacyclohexenes and allylvinylsilanes (Scheme 10.262) [693]. A plausible mechanism for formation of the cyclic product involves three steps - oxidative addition of 191 a to a Pd(0) species, alkyne insertion into the Si-Pd bond, and reduc-... 

The introduction of an o-methoxy group on the benzene ring of the alkynyl substituent increased the yields of the cyclization products. The reaction proceeds through the nucleophilic attack of the nitrogen atom on electron-deficient alkyne 317, the formation of the alkenylpalladium intermediate 318, the insertion of the alkenes 315 into the C—Pd bond 319, and -hydride elimination. They reported many examples of this type of reactions, but those are summarized in their own review in this issue of Chemical Reviews.168... 

Scheme 25) was observed when cyclic alkenes (e.g., 214) were treated with ruthenium carbene complex 18 in the presence of terminal alkynes (e.g., 215). A mechanism involving initial ROM, followed by alkyne insertion of the intermediate carbene complex, followed by ROM from intermediate 217, was proposed. In order to account for the unexpectedly high yield (the yield is higher than the anticipated E Z selectivity in the formation of 217) of the process, a second source of the observed product involving metathesis of an additional mole of cyclopentene from intermediate 217 was suggested. 

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