Cyclopentadiene with Mo and

Cyclopentadiene reacts with transition metals by oxidative addition of one H—C bond at the saturated carbon. Metal-atom vapors of Mo and W codeposit with cyclopentadiene to form (h -C5Hj)2MH2. Irradiation of Mo(CO)j with cyclopentadiene in isooctane yields (h -C5H5)Mo(CO)2C5H2-h ) that involves overall H transfer between ligands. 

The neutral complexes (Tr-CsHs ReH (XIX), (tt-CsHs M (XX) where M = Mo and W, and (jr-CiHs TaHs (XXI) may all be prepared by treating solutions of sodium borohydride and sodium cyclopentadiene in tetrahydrofuran with anhydrous metal chlorides, e.g., 

An ab initio MO calculation by Jorgensen revealed enhanced hydrogen bonding of a water molecule to the transition states for the Diels-Alder reactions of cyclopentadiene with methyl vinyl ketone and acrylonitrile, which indicates that the observed rate accelerations for Diels-Alder reactions in aqueous solution arise from the hydrogenbonding effect in addition to a relatively constant hydrophobic term.7,76 Ab initio calculation using a self-consistent reaction field continuum model shows that electronic and nuclear polarization effects in solution are crucial to explain the stereoselectivity of nonsymmetrical 

Heck et al. reported an Ru-group 6 metal bimetallic system that was stabilized with bis(cyclopentadienyl) ligands.The two metal centers were first coordinated by the cyclopentadiene, and intramolecular metathesis was carried out to join the two metal centers. In contrast to the common 77 -coordination, a unique coordination of the cyclopentadiene in the Ru-Mo and Ru-W systems (complexes 31 and 32) was observed by Bitterwolf et al  

An ab initio MO computational study of the Diels-Alder addition of ethylene to butadiene reveals a symmetrical transition state and a concerted reaction profile. Extended Htickel MO calculations on the endo.exo product distribution in [4 + 2] reactions of cyclopentadiene indicate, within the range of intermolecular distances investigated, that the ewdo-orientation is energetically favoured. A study of the relationship between isomer distribution and solvent polarity in the reactions of cyclopentadiene with acrylonitrile and with methyl acrylate reveals that the endo-adduct. dicyclopentadiene and exo-adduct rdicyclopentadiene ratios are increased linearly with the empirical solvent polarity factor The favoured endo- 

Methylcyclopentadiene readily undergoes reaction with metal carbonyls 47) and even 1,3-diphenylcyclopentadiene reacts with iron carbonyl 4S). A novel method for obtaining a ferrocene with one substituted ring, is the reaction of 1,3-diphenylcyclopentadiene with the dimer of cyclopentadienyl-iron dicarbonyl at 170° C to produce 1,3-diphenylferrocene in 15% yield. Other substituted cyclopentadienes which have been used are tetraphenyl-cyclopentadienone in the preparation of tetraphenyl(hydroxy)cyclopenta-dienylmanganese tricarbonyl 49) and indene in the preparation of the dimer of indenylmolybdenum tricarbonyl (50). A variety of substituted fulvenes have been used in the preparation of substituted cyclopentadienyl metal tricarbonyl compounds of Cr, Mo, and W (57, 52). This latter reaction proceeds best in the presence of a solvent, such as 1,2-dimethoxyethane, to permit easy abstraction of hydrogen. 

A large number of metal-coordinated chalcogenacycles were prepared from the heteroaldehyde and -ketone complexes [M(CO)5 E=C(Ph)R ] (M = Cr, Mo, W E = S, Se, Te R = H, Aryl) and 1,3-dienes. The cycloadditions proceeded rapidly, even at low temperatures.179180,228-233 As dienes 2,3-dimethyl-l,3-butadiene [for an example see Eq. (27)], isoprene, transA, 3-pentadiene, cyclopentadiene, pentamethylcyclopentadiene, and 1,3-cyclohexadiene were used. Decomplexation with pyridine/THF yielded the free heterocycles. Although the tellurobenzaldehyde complex [W(CO)5 Te = C(Ph)H ] (99c) proved too unstable for isolation, it could be generated and employed in subsequent cycloadditions with dienes.180 

There are probably several factors which contribute to determining the endo exo ratio in any specific case. These include steric effects, dipole-dipole interactions, and London dispersion forces. MO interpretations emphasize secondary orbital interactions between the It orbitals on the dienophile substituent(s) and the developing 7t bond between C-2 and C-3 of the diene. There are quite a few exceptions to the Alder rule, and in most cases the preference for the endo isomer is relatively modest. For example, whereas cyclopentadiene reacts with methyl acrylate in decalin solution to give mainly the endo adduct (75%), the ratio is solvent-sensitive and ranges up to 90% endo in methanol. When a methyl substituent is added to the dienophile (methyl methacrylate), the exo product predominates.  

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