Cyclopentenyl complexes

Similarly, reduction of chromocene with hydrogen in the presence of carbon monoxide also aifords a w-cyclopentenyl complex (27, 2S). 

The cyclopentenyl complex may also be prepared from nickel tetra-carbonyl and cyclopentadiene [7, 8]. 

Cyclopentadiene iron tricarbonyl has been prepared and decomposes thermally to the binuclear carbonyl [a -C5H5Fe(CO)2]2 [26o]. The binuclear iron complex may further react with cyclopentadiene or thermally decompose ( 200°) [27, 28] to give ferrocene. Monosubstituted ferrocenes may be prepared by the former reaction [27]. Chromium hexacarbonyl and cyclopentadiene at 280-350° react to give chromocene [29] the reaction is reversible since treatment of chromocene with carbon monoxide under pressure affords chromium hexacarbonyl, together with intermediate products such as [jr-CpCr(CO)3]2, [jr-Cp2Cr][3r-CpCr(CO)3] and, when hydrogen is also present, the cyclopentenyl complex jr-CsH5CrC5H7(CO)2, 4.1, is formed [30, 31, 32]. 

It is well known that, when treated with complex substrates, cyclopenta-diene can form complexes with cyclopentadiene, cyclopentadienyl, and even cyclopentenyl ligands. The same possibilities are found for bora-2,5-cyclohexadienes, but with the additional complexity of ligand isomerism. 

The reaction of Co2(CO)8 with 25 is complicated at room temperature, but above 60°C Co(CO)2(C5H5BPh) (11) is obtained as the only complex product (29). Ni(CO)4 reacts with 25 in boiling hexane to give the (cyclopentadienyl)(cyclopentenyl)nickel analog 49 [5(nB) = 24.4 ppm, one broad signal] this complex contains a novel boron-carbon ligand with... 

Tris(oxazoline) complexes have also been investigated as ligands in the allylic oxidation reaction. Katsuki and co-workers (116) observed that Cu(OTf)2 com-plexed to the tris(oxazoline) 160 is a more selective catalyst than one derived from CuOTf, Eq. 99, in direct contrast to results observed with bis(oxazohnes) or pyridyl bis(oxazohnes) as ligands (cf. Section III.A.3). When the reaction is conducted at -20°C, the cyclopentenyl benzoate is delivered in 88% ee albeit in only 11% yield after 111 h. Larger cycloalkenes are less selective (cyclohexene 56% ee, cyclohep-tene 14% ee, cyclooctene 54% ee). 

Acylation with cyclopentenyl- or cyclohexenylacetyl chloride gave three-membered ring systems.113 114 An interesting example is the reaction of ethylene with acid chlorides complexed with excess A1C13 to give the 17 p.y-unsaturated ketones 115... 

Allyloxypyrimidine largely resists rearrangement even at 200°C. However, ally lie cyclopentenyl 2-pyrimidinyl ethers (852) can be rearranged on catalysis by tetrakis(tri-isopropyl phosphite)palla-dium, a Pd(0)-complex (Scheme 68). 

Scheme 1.2 Preparation of ( y5-cyclopentadienyl)(jy3-cyclopentenyl)nickel(II) from nickel tetracarbonyl via the nickel(0) complex Ni(C5H6)2 as the supposed intermediate...

The topic of 7r-allyl-transition metal chemistry is considerably broader than complexes involving the tt-CsHs ligand itself. A variety of complexes is now known in which the 7r-allyl radical is part of a carbocyclic ring system. Some examples of 7r-cyclopropenyl-, 7r-cyclobutenyl-, and tt-cyclopentenyl- transition metal complexes are presented below. 

Olefin-coupling reactions of Tj -allyliron complexes with a variety of cationic iron-olefin complexes (ethylene, propene, styrene, etc.) were utilized to give cationic bimetallic complexes with cr,7r-hydrocarbon bridges (80,81). The condensation of simple [FpColefin)]" substrates with Fp(allyl) precursors was extended to the reaction with Fp(l,3-butadi-ene)+. Initial attack at C-1 or C-4 leads to the formation of dinuclear complexes with cr-coordinated and 7r-coordinated Fp fragments, which by subsequent intramolecular condensation could give either cyclohexenyl or cyclopentenyl intermediates. Attack at C-2 yields a dinuclear complex incapable of further intramolecular reaction [Eqs. (6-8)]. 

They have presented a further library synthesis of phosphines which show very different properties in catalysis to those above. A library of phosphines 98 was prepared based on the well-known /3-turn forming motif -Pro-D-Yyy-(where Yyy is a D-amino acid) with the phosphine-containing amino acids flanking this element.The structural features that were varied in the initial library prepared were the amino acids at the N- and C-termini and substitutions of D-amino acids next to the proline of the -Pro-D-Yyy- fragment. Palladium complexes of the library members were formed on the pins from [PdCl(77 -C3Hs)]2 and these screened for catalytic activity in the asymmetric addition of dimethyl malonate to cyclopentenyl acetate (Scheme 49). The selectivities obtained ranged from 34% ee for the... 

Similarly, treatment of (l,2-dioxobenzocyclobutene)chromium complex 52 with excess of 2-propenyllithium, 1-lithio-l-phenylethene and cyclopentenyl-lithium at -78 °C followed by hydrolysis with diluted aqueous hydrochloric acid afforded similar type benzocyclooctenediones as diastereomeric mixture along with formation of cyclopentaanellated indanones (Eq. 39) [37]. 

In a related process, 4-amino-1 -metala-1,3,5-hexatrienes such as 63 - accessible from the addition of secondary amines to 2-(l-cyclopentenyl)ethynylcarbene complexes 62 - undergo a spontaneous cyclization to an rj cyclopentadiene species 64 (Scheme 32). [82] The uncoordinated cyclopentadiene could not be isolated on a preparative scale instead cyclopentenone 65 was obtained upon subsequent treatment with HBF4 and aqueous KOH. [82]... 

The further reactions of the Y(C6 0)D0 radical are more complex and result in several important products through several paths. One of these paths include the ring opening and the unzipping of the non cyclic radical to the formation of two acetylenes, CO and HC =0. The other paths result in the formation of a cyclopentenyl ketone formaldehyde radical, and to the cyclopentadienyl radical + CO2 channel. Kinetic parameters for important product channels are reported and a reduced mechanism is proposed. 

Electrocyclic reactions are not limited to neutral polyenes. The cyclization of a pentadienyl cation to a cyclopentenyl cation offers a useful entry to five-membered carbocycUc compounds. One such reaction is the Nazarov cyclization of divinyl ketones. Protonation or Lewis acid complexation of the oxygen atom of the carbonyl group of a divinyl ketone generates a pentadienyl cation. This cation undergoes electrocyclization to give an allyl cation within a cyclopentane ring. The allyl cation can lose a proton or be trapped, for example by a nucleophile. Proton loss occurs to give the thermodynamically more stable alkene and subsequent keto-enol tautomerism leads to the typical Nazarov product, a cyclopentenone (3.220). 

Commercially available, air-stable Pd phosphinous acid complex is an active catalyst for the thioether formation by the reaction of 1-cyclopentenyl chloride (49) with thiophenol (50) and hexylmercaptan (52) to give the thioethers 51 and 53 [15]. 1-Cyclopentenyl phenyl thioether (55) was obtained by the reaction of 1-cyclopentenyl triflate (54) with lithium phenyl sulfide [16]. 

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