Bis olefins

Nickel and palladium react with a number of olefins other than ethylene, to afford a wide range of binary complexes. With styrene (11), Ni atoms react at 77 K to form tris(styrene)Ni(0), a red-brown solid that decomposes at -20 °C. The ability of nickel atoms to coordinate three olefins with a bulky phenyl substituent illustrates that the steric and electronic effects (54,141) responsible for the stability of a tris (planar) coordination are not sufficiently great to preclude formation of a tris complex rather than a bis (olefin) species as the highest-stoichiometry complex. In contrast to the nickel-atom reaction, chromium atoms react (11) with styrene, to form both polystyrene and an intractable material in which chromium is bonded to polystyrene. It would be interesting to ascertain whether such a polymeric material might have any catal3dic activity, in view of the current interest in polymer-sup-ported catalysts (51). 

Diels-Alder cycloaddition of 5-bromo-2-pyrone with the electron-rich tert-butyldi-methylsilyl (TBS) enol ether of acetaldehyde, using superheated dichloromethane as solvent, has been investigated by Joullie and coworkers (Scheme 6.90) [188]. While the reaction in a sealed tube at 95 °C required 5 days to reach completion, the anticipated oxabicyclo[2.2.2]octenone core was obtained within 6 h by microwave irradiation at 100 °C. The endo adduct was obtained as the main product. Similar results and selectivities were also obtained with a more elaborate bis-olefin, although the desired product was obtained in diminished yield. Related cydoaddition reactions involving 2-pyrones have been discussed in Section 2.5.3 (see Scheme 2.4) [189]. 

TABLE 1. Enthalpies of formation of bis-allenes, the related bis-olefins and bis-acetylenes and their acyclic analogs... 

The 0/7/fo-alkylation of aromatic ketones with olefins can also be achieved by using the rhodium bis-olefin complex [C5Me5Rh(C2H3SiMe3)2] 2, as shown in Equation (9).7 This reaction is applied to a series of olefins (allyltrimethyl-silane, 1-pentene, norbornene, 2,2 -dimethyl-3-butene, cyclopentene, and vinyl ethyl ether) and aromatic ketones (benzophenone, 4,4 -dimethoxybenzophenone, 3,3 -bis(trifluoromethyl)benzophenone, dibenzosuberone, acetophenone, />-chloroacetophenone, and />-(trifluoromethyl)acetophenone). 

Kouwenhoven (6) sjmthesised two bis-olefinic ligands related to the simple pent-4-enyl compounds mentioned above. These were of the general formula PhD[(CH2)aCH =CH2]2 (D = P, As) and formed platinum(II) and mercury(II) complexes. The platinum complexes LPtCla (L = PhD[(CH2)3 CH=CH2]2) were found to be monomeric species containing, as shown by their i. r. spectra, one coordinated and one free double bond. The two mercury complexes [LHgClaJs did not contain coordinated olefinic groups. 

The calculated transition state for -hydrogen transfer (Fig. 3) has a non-planar 6-membered ring structure. There is no direct interaction between the aluminium atom and the hydrogen being transferred. Thus, the situation differs sharply from that for transition metal polymerization, where the transition state has some resemblance to a hydride-bis(olefin) complex, as illustrated for a typical metallocene case in Fig. 3. 

Another example is the cyclization of the racemic allylic alcohol 232 at -80°C which furnished the racemic tetracyclic bis-olefin 233 in 70% yield (89, 90). Ozonolysis of 233 gave the bicyclic triketone aldehyde 234 which underwent under acidic conditions a double intramolecular aldol cyclodehydration to produce racemic 16,17-dehydroprogesterone 235. This represents the first synthesis of a steroid via the now so-called "biomimetic" polyene cyclization method. 

Reaction of the bis-olefin 301 with Pd(PPly )4 afforded the isoindoline 302 together with the 1/7-2-benzazepine 303 the former was shown to rearrange to 303 in the presence of the palladium complex (Scheme 39) <1996CC2257>. 

More interestingly, as an oxidative condition with Mn(OAc)3, various macrolide compounds (230) are obtained in high yields by the treatment of bis(acetylacetate) (228) and bis(olefin) (229) with Mn(OAc)3 in acetic acid under highly diluted conditions (2 mM of substrate) [237,238]. This reaction proceeds as follows. The initial formation of a (3-keto ester radical via single electron oxidation by Mn(OAc)3 and its addition to olefin occur the formed benzylic radical is oxidized by Mn(OAc)3 to form a benzylic cation, then cyclization by oxygen of the enol form of the (3-keto ester occurs, and again this reaction sequence is repeated in another (3-keto ester group to ultimately form the macrolide compounds. 

In term of diversity-oriented strategies, multicomponent reactions (MCR) represent an attractive and rapid access to libraries of macrocycles inspired by biologically active natural products. Combined with Passerini and Ugi reactions, M-RCM has already shown promising synthetic potential, as illustrated by the pioneering work of Domling and coworkers [46]. Condensation of isocyanide 69 with carboxylic acid 70 in the presence of paraformaldehyde leads to bis-olefin 71, which is subsequently submitted to RCM in the presence of G1 and titanium isopropoxide to give the 22-membered macrocycle 72 (Scheme 2.27). 

In order to fit the initial rate data, it is not required that the bound olefin in Cp2ZrHO+, Cp2R 0+ or Cp2RO+ also inserts directly, in an intramolecular fashion. A reaction mechanism based solely on intramolecular insert ion of this bound olefin, without assuming the existence of either a bis(olefin) intermediate or the conerted, bimolecular insertion reaction of a monoolefin species with a second equivalent of monomer-does not fit the data. 

Hydrocarbon 120 is a very reactive species since it can be reduced to the bis(olefin) 122 with lithium aluminum hydride in diethyl ether at room temperature, and trapped with tetracyclone (tetraphenylcyclopentadienone) to give the bis-adduct 121 in refluxing toluene. In both cases, the yields exceed 90 % [74]. 

It has been speculated (5) that the olefin metathesis reaction mech-nism involves a four-centered quasi-cyclobutane transition state. The three basic steps postulated for the reaction, namely, formation of a bis-olefin-tungsten complex, transalkylidenation and olefin exchange, may account, in general, for the initiation and propagation steps in the ringopening polymerization of cycloolefins. Several modes of termination have been considered, but suitable data to test these are not yet available. 

Mono- and diazamacrocycles can be conveniently prepared by a catalyzed ([Cl2(PCy3)2Ru(=CHPh)]) RCM from bis(olefinic) precursors. The reaction is very general and affords monounsaturated macrocycles in reasonable yields. Furthermore, lactam formations by closure of amino-carboxyl derivatives or by Beckmann rearrangement are also methods of the choice however, in the first case, the yields are in general low since polymers are usually produced, as the main product. 

The first isolated example of a bis(olefin)zirconocene has been prepared by this method. Photolysis of 10 in the presence of 2,3,5,6-tetrakis(methylene)bicyclo[2.2.2]octane yields 11, which at elevated temperature rearranges to the (conjugated-diene)zirconocene complex 12... 

Formation of the relatively unstable complexes (olefin)M(C0)5 and (olefin)2M(C0)4 (M = Mo or W) with propylene and butadiene has been accomplished (559) by UV irradiation of M(CO)o with olefin in w-hexane. From W(CO)e, the complexes (cis-2-butene)W(CO)6, (fmws-2-butene)-W(CO)5, and (cis-2-butene)2W(CO)4 have been produced similarly. As with the corresponding ethylene complexes, the olefin ligands in the bis-olefin complexes are in trans positions. Although, in these complexes, the butadiene molecule is coordinated at only one double bond, upon lengthy irradiation of (butadiene)2Mo(CO)4 (559), the previously reported (268) complex (butadiene)2Mo(CO)2 involving chelated butadiene molecules is produced. 

Both cis, trans, trans- and aii-iraws-l,5,9-cyclododecatriene yield upon reaction with Zeise s dimer in acetone, the compounds bis(olefin)-yellow-orange plates melting at about 130°C (456). Dissolution of the all-trans product in an organic solvent results in loss of olefin to produce the polymeric species (Ci2Hi8)4(PtCl2)6 (456). An attempted preparation of the platinum(II) complex of cis,[Pg.320]

Metallacycloalkanes are proven key intermediates in metal-catalyzed cycloadditions and cycloreversions of alkenes. The relationship of some iron metallacyclopentane derivatives with bis(olefin) complexes has been investigated theoretically. Scheme 1 shows a general route from bisalkene... 

Reaction of Cp T Cl2 with 1,4-dilithiobutane at -78 C leads to the titanacyclopentane (29), which is thermally unstable and releases ethylene together with 1-butene [80]. Metallacyclopentane is in equilibrium with bis(olefin) complex and this equilibrium is discussed based on EHMO calculation by Hoffmann et al [16,81]. 

Typical square-planar rhodium-olefin complexes such as acetylacetonates (48) have a stoichiometry of two coordinated olefins per metal-atom. Since chelating olefins are bidentate in their cationic rhodium biphosphine complexes, it would be surprising if bis-olefin complexes were never found under hydrogenation conditions. It seems clear, in fact, that they can be the major coordinated species under certain conditions. Thus examples of 2 1 rhodium enamide complexes with biz-diphenyl-phosphinopropane have been observed (49), although the majority of cases involve a8-unsaturated acids co-complexed with DIOP. 

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