1,3-isomerization, alkyne complexes

Terminal RCH—CH2 1-Hexene C4H9CH=CH2 is isomerized by complex 1 in accordance with the factors influencing the thermodynamic stability of cis- and trans-2 -hexene [15], At the end of the reaction, the alkyne complex 1 was recovered almost quantitatively. No alkene complexes or coupling products were obtained. The corresponding zirconocene complex 2a did not show any isomerization activity. Propene CH3CH=CH2 reacts with complex 6 with substitution of the alkyne and the formation of zirconacydopentanes as coupling products, the structures of which are non-uniform [16]. 

Complex condensation products are obtained by reaction of the alkyne complex with excess of 3,3-dimethyl-l-butyne, which yields two isomeric products of formulas Ru3(CO)6[HC2C(Me)3-COCH2CMe3][HC2CMe3]2 (125). The X-ray structure of one of those adducts (Fig. 19) shows that both dimerization of two alkyne molecules and the insertion of carbon monoxide into the alkyne metal bonds have occurred. The Ru-Ru distances of 2.820,2.828, and 2.686 A in the ring are of interest. The value of 2.686 A is one of the shortest found in a... 

The isomerization, itself, originates from the a complex (B in Figure 3). However the total activation energy depends critically on the relative energy of A and B (Figure 3). An alkyne C=C triple bond binds more efficiently to a transition metal complex than a o C-H bond since the % C-C orbital is a better electron-donor and the 71 C-C orbital a better electron acceptor than the a and a C-H orbitals, respectively. However, the difference in energy between the two isomers is relatively low for a d6 metal center because four-electron repulsion between an occupied metal d orbital and the other n C-C orbital destabilizes the alkyne complex. This contributes to facilitate the transformation for the Ru11 system studied by Wakatsuki et al. 

For other systems the transformation is still energetically accessible but die barrier may be higher if the alkyne complex is less destabilized. In d2 and d4 complexes, the four-electron repulsion with the alkyne can be avoided (the alkyne behaves like a 4-electron donor) and this stabilizes the alkyne complex. The intraligand 1,2 shift is then associated to a higher activation barrier. This has been illustrated in a study by Fledos et al. on [Cp2Nb(HC=CH)(L)]+ and [Cp2Nb(C=CH2)(L)]+ (L = CO, PH3) where DFT calculations show that the barrier to isomerization is +29.2 kcal.mof1 (resp. 

Alkynes react readily with a variety of transition metal complexes under thermal or photochemical conditions to form the corresponding 7t-complexes. With terminal alkynes the corresponding 7t-complexes can undergo thermal or chemically-induced isomerization to vinylidene complexes [128,130,132,133,547,556-569]. With mononuclear rj -alkyne complexes two possible mechanisms for the isomerization to carbene complexes have been considered, namely (a) oxidative insertion of the metal into the terminal C-Fl bond to yield a hydrido alkynyl eomplex, followed by 1,3-hydrogen shift from the metal to Cn [570,571], or (b) eoneerted formation of the M-C bond and 1,2-shift of H to Cp [572]. 

Alkynes coordinated to platinum(O) are susceptible to electrophilic attack. The reaction which has been most fully studied is the protonation of complexes Pt(alkyne)(PPh3)2 to give vinyl platinum(Il) complexes then alkenes. The reaction has been discussed in Section 52. The vinyl complexes formed undergo isomerization in the final step, since the cis vinyl complex yields some tracts-alkene. Carbene intermediates have been proposed in the pathway for this isomerization.848 Platinum(II) alkyne complexes can be converted into carbene complexes, and this reaction has been discussed in Section 52.4,6. This pattern of differential reactivity is apparent in the IR spectra of the two sets of complexes. For alkyne complexes of platinum(O) the C==C stretching frequency is lowered by some 450 cm-1 upon coordination, but with the platinum(II) analogs the difference is only in the region of 200 cm-1. 

Zirconocene 1,9-anthracenediyl complex 69 presumably undergoes rearrangement to an isomeric benzyne complex prior to the insertion of external alkyne (Equation 26). The isomerization can be understood as a /3-hydrogen elimination/reductive elimination process, resulting in a formal reduction to Zr(ll), followed by a typical alkyne/ alkyne oxidative cyclization to the observed zirconacyclopentadiene product 70. The coordinated benzyne intermediate can be observed spectroscopically as a trimethylphosphine adduct <2000JA9880>. 

Hydride abstraction from 225 gives the dibenzannuleted tropyne complex 226 (Scheme 27).85 As in the case of 223, 226 reacts rapidly with HBr to give 227, which, in solution, isomerizes slowly to 228. KBEt3H and bis(dicyclohexylphosphine)ethane attack the 4-position of the tropyne to give alkyne complexes 225 and 229, respectively. 

Cross-coupling ofalkynes. A route to isomerically pure 1,3-dienes involves hydrozirconation of an alkyne followed by conversion to the alkyne complex 3, which couples with a second alkyne to give a metallacyclopentadiene 4. Treatment of 4 with dilute acid provides the diene 5, usually as a single isomer (>97%). Overall yields are in the range 53-80%. 

Reaction of Fe3(CO)12 with 3-pentyn-l-ol gives the hydrido cluster 4, formed by coupling of an allenylidene unit with a coordinated carbonyl ligand and methoxy group. In contrast, reaction with the isomeric alkyne 2-methyl-3-butyn-2-ol affords a binuclear iron complex.18 Treatment of Fe3(p3-S)2(CO)9 with Me3NO affords the vinylferrocene complex 5, whereas the similar reaction with Fe3(p3-Te)2(CO)9 gives no cluster products.19... 

Scheme 6.12 Redox-induced isomerization of a ruthenium vinylidene and alkyne complex.

The use of chiral ligands has also been explored as a method of asymmetric induction in the Pauson-Khand reaction. To date, all of these reports have involved the addition of (P)-(-i-)-Glyphos to a preformed Co-alkyne complex and subsequent separation of the resulting diastereomers by HPLC. Under reaction conditions which prevented isomerization of the alkyne complex, the subsequent Pauson-Khand reaction proceeded with high levels of diastereoselectivity (Scheme 8)... 

Stoichiometric studies of M-H additions to alkynes also show mixed stereochemical results. The more common cw-addition is typified by the CO-promoted transformation of Cp2Nb(H)(RCECR) to ds-Cp2Nb(CO)(Ti -CR=CHR) [90]. In mononuclear systems where frans-additions have been found, radical-type mechanisms have been implicated [91] or cis/trans isomerization of the intermediate vinyl species [92] has been found. Although the intermediacy of alkyne complexes has not been established, Schwartz s hydrozirconation of alkynes [93] by Cp2ZrHCl represents a general entry to vinyl-metal species which can be transformed stereoselectively to alkenes, vinyl halides, and/or carboxylic acids. 

In contrast to the behavior of free alkynes, 1,3-isomerization of alkyne complexes to produce complexed allenes [140,141) is rather rare. A synthetically useful example [140] is provided by the CpMn(CO)2 complexes of electrophilic alkynes 89 which undergo alumina-promoted isomerization to the corresponding allene derivatives 90 these, in turn, can be oxidatively demetalated to the free allenes 91 (Scheme 4-45). 

The combined action of zinc and tantalum(V) chloride on alkynes generates low-valent tantalum-alkyne complexes, which react with carbonyl compounds to yield ( )-allylic alcohols stereoselectively. The tantalum-6-dodecyne complex, for instance, and j -phenyl-propionaldehyde afford the alcohol 370 in 96% yield. Mixtures of structurally isomeric a,/ -unsaturated ( )-amides 371 and 372 (R, = alkyl or Mc3Si) are obtained from the... 

Catalysis by alkali metal ions has recently been reported as an alternative route. In an argon matrix, acetylene forms a n complex with the metal. On irradiation, it isomerizes to the vinylidene form, M C=CH2. When complexed with metals, vinylidene is much more stable, in the same way that metal carbenoids are generally much more stable than carbenes, and rearrangement of a tungsten alkyne complex to a tungsten vinylidene complex has been reported. ... 

Fe(CO)5] to form a Fe(alkyne)+ complex on CAD, the latter produces structure-indicative fragments, based on which the position of the acetylenic bond can be determined ". This is illustrated for isomeric octynes in Figure 16 ". ... 

Cyclopropanation proceeds through the formation of an electrophilic (2-furyl)carbene-chromium complex which in turn forms through 5-exo dig cyclization. As shown in Scheme 10.13, cyclization of ene-yne-ketone 15 begins with the nucleophilic attack of carbonyl oxygen to an internal carbon of an alkyne in q -alkyne complex A. A might be the most plausible pathway for generation of (2-furyl)carbene-chromium complex 18. A slipped, polarized q -complex B would be an alternatively possible intermediate. It is known that A is prone to isomerize to B, which has been... 

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