Cyclopentadiene with chromium hexacarbonyl

Upon UV irradiation in hydrocarbon solution, the hexacarbonyls of chromium, molybdenum, and tungsten react differently with conjugated dienes like 1,3-butadiene (la), ( )-l,3-pentadiene (lb), 2-methyl-1,3-butadiene (lc), ( , )-2,4-hexadiene (Id), ( )-2-methyl-l,3-pentadiene (le), 2-ethyl-1,3-butadiene (If), or 1,3-cyclohexadiene (Ig). Chromium hexacarbonyl (2) yields, with the acyclic dienes la-lf, tetracarbonyl-r/2-dienechromium(0) complexes (3a-3f) in a smooth reaction (8-10). With 1,3-cyclohexadiene, in addition to 3g, dicarbonylbis(>/4-l,3-cyclohexadiene)chromium(0) (4g) is obtained [Eqs. (7) and (8)j. During chromatography on silica gel, the 1,3-cyclohexadiene complex 3g dismutates readily to [Cr(CO)6] and 4g [Eq. (9)]. Under the same conditions with 2 1,3-cyclopentadiene (lh) yields, in a hydrogen-transfer reaction, the stable dicarbonyl- / 5-cyclopentadienyl-r/ 3-cyclopent-enylchromium (5) (11-13) [Eq. (10)]. 

The sodium sand is treated with excess of freshly cracked cyclopentadiene (see preparation of cyclopentadienylvana-dium tetracarbonyl for details) to prepare a red solution of cyclopentadienylsodium. To the resulting solution is added 8.8 g. (0.04 mol) of solid chromium hexacarbonyl, and the mixture is then refluxed with stirring until no unreacted chromium hexacarbonyl, or very little, remains in the reflux condenser (3 to 6 hours). The resulting yellow solution of... 

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]. 

Acylmetallates lp,q generated by addition of lithio prop-2-ynylic ethers to hexacarbonyl chromium or tungsten afford oxacyclopenten-2-ylidene complexes 173 on contact with wet silica gel (Scheme 73).194 The chromium complex 173 (M = Cr) undergoes a condensation with tolane to give a 1,4-dioxy aromatic compound 174. Addition of cyclopentadiene to 173 (M = W) affords a Diels-Alder adduct 176 in a 1 3 exo/endo ratio,195 while addition of dimethylamine quite unexpectedly does not lead to production of an aminocarbene complex but to formation of 5,5 -diphenyl-2,2 -bifuran 175 (Scheme 73).196... 

Chromium forms a cyclopentadienyl compound similar to ferrocene (p. 499). Cyclopentadienyl sodium reacts with anhydrous chromium (I I) chloride in tetrahydrofuran to give red (C5H5)2Cr. By oxidising this, compounds containing the (C5H5)2Cr+ ion are easily obtained (Cotton and Wilkinson, 1954). When cyclopentadiene mixed with molybdenum or tungsten carbonyl is passed through a tube heated to 300 the dicyclopentadienyl hexacarbonyl is formed (Wilkinson, 1954) ... 

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