Patai S., Rappoport Z. 2000 The chemistry of functional groups. The chemistry of dienes and polyenes. V. 2 / 0885 989

(cyclodienyl)MLnC cations (170, Scheme 42)116,117,124a,125,136a,163b,173. The regioselectivity of hydride abstraction from a variety of substituted (cyclohexadiene)Fe(CO)3 complexes has been examined116a – c. There are only a few examples of hydride abstraction for the preparation of acyclic (pentadienyl)MLnC cations (171), since the success of this reaction requires the presence of a cis-alkyl substituent on the diene138,174. In comparison, reaction of Ph3CC with (C8H8 Fe(CO)3 generates the ( 5-cyclooctatrienyl)Fe(CO)3C cation 172 via C C bond formation rather than via hydride abstraction175.

H

Fe(CO)3

(172)

SCHEME 42

b. Acylium ions. Reaction of acylium ions with (cycloheptadiene)- or (cyclooctatetraene)Fe(CO)3 occurs at an uncomplexed double bond to afford the acyl substituted (dienyl) Fe(CO)3C cations 173 and 174 respectively (Scheme 43)176. While cation 174 is observable by NMR spectroscopy at low temperature, warming the solution results in cyclization to generate the final product 175. In general, attempted acylation of uncomplexed 1,3-dienes results in polymerization. Coordination of the Fe(CO)3 group moderates the electrophilic acylation of diene complexes to generate the corresponding cis-dienone complexes 176 (Scheme 43)177. Electrophilic attack occurs on the same face of the ligand as that bound to the metal to initially generate the cationic ( 3-allyl) complex 177. Deprotonation gives 176. The initially formed 176 may be subsequently isomerized to the more stable trans-dienone complex under the influence of additional acyl halide or base. Substitution is always observed to occur at the diene termini. In contrast to former assertions in the literature, Franck-Neumann and coworkers have reported that complexes bearing electron-withdrawing substituents slowly undergo acylation in the presence of two or more equivalents of AlCl177a3 . Electrophilic substitution of (diene)Fe(CO)3 complexes

with alkoxychloromethane or with orthochloroformates has been reported178. Acylation of the (cyclohexadiene)RhCp complex occurs at the cyclopentadienyl ligand136a.

D. Deprotonation

A number of cationic (diene)metal complexes undergo ˛-deprotonation. Treatment of (cyclodiene)Mo(CO)2CpC cations (e.g. 44) with NEt3 or other non-nucleophilic bases

yields the neutral ( 3-cyclodienyl)Mo(CO)2Cp complexes (178, equation 28)81b,179. Deprotonation of (1-hydroxy-1,3-butadiene) cations 179 [MLn D Mo(CO)2CpŁ , Ru(CO)Cp] with NEt3 affords the anti-1-formyl- -allyl products 18030,32 while the (1,3-pentadiene)Mo (CO 2CpŁC cation (181) requires a stronger base for deprotonation to give 182 (Scheme 44)87.

(181) R = Me

R = Me

MLn = Mo(CO)2Cp* LiHMDS

Cp*(CO)2Mo

Me

(182)

SCHEME 44

In contrast, deprotonation of neutral (diene) metal complexes results in the formation of carbanions. Deprotonation of (isoprene)Fe(CO)3 with LDA ( 78 °C) generates the anion 183 (Scheme 45)180. The anion reacts directly with alkyl, benzyl or allyl halides or, in the presence of ZnBr2, with aldehydes. The insect pheromone, ipsdienol, has been prepared by this method. In a similar fashion, deprotonation of cyclic and acyclic (diene)Fe(CO)3 complexes bearing an electron-withdrawing group occurs ˛ to this group and on the exo face of the complex. Alkylation of the resultant cyclic anions occurs in a diastereospecific fashion, also on the exo face of the ligand due to the steric bulk of the metal – ligand array181. This has been extended to acyclic dienes; alkylation of (methyl 3,5-hexadienoate)Fe(CO)3 (184) occurs in a highly diastereoselective fashion (Scheme 46)182. This is proposed to occur via approach of the electrophile to the s-trans rotamer of the ester enolate anion on the face opposite to Fe(CO)3. This methodology, along with electrophilic acylation (Section V.C.2.b), was utilized in the preparation of the C8 C15 segment of protomycinolide IV. Attempts to generate and alkylate a dithianyl anion adjacent to (butadiene)Fe(CO)3 were unsuccessful183.

SCHEME 47

50 °C, complex 185 isomerizes to the (anti-allyl)Fe(CO)3 anion (186), which may be trapped by reaction with Me3SnCl to give the corresponding (anti-crotyl)Fe(CO)3SnMe3 complex (187). Isomerization of 187 to the thermodynamically more stable (syn-crotyl) isomer (188) occurs only at a higher temperature (55 °C).

In contrast, reaction of (cyclohexadiene)Fe(CO)3 (189) with strong carbon nucleophiles (conjugate acid pKa > ca 28) in THF/HMPA/ 78 °C, followed by protic workup, gives cyclohexene products 190a and 190b (Scheme 48)185. If the reaction is run under an atmosphere of carbon monoxide, products incorporating CO (e.g. 191) may be obtained. This reaction is proposed to occur via nucleophilic attack at an internal diene carbon on the face opposite to iron, to afford a (1,3,4- 3-butenyl)Fe(CO)3 anion 192 which has been partially characterized by 1H NMR spectroscopy at low temperature ( 60 °C 186. Upon warming the solution to 0 °C, the signals attributed to 192 disappear and are replaced by signals corresponding to the (allyl)Fe(CO)3 species 193. Protonation of either 192 or 193 gives the olefinic products 190a and 190b. Under a positive pressure of carbon monoxide, CO insertion into 192 gives the anionic acyl species 194 which has been characterized by IR and 1H NMR spectroscopy186. Protonation of 194 yields 191.

Examination of the reactivity of acyclic (diene)Fe(CO)3 complexes indicates that this nucleophilic addition is reversible. The reaction of (C4H6 Fe(CO)3 with strong carbon nucleophiles, followed by protonation, gives olefinic products 195 and 196 (Scheme 49)187. The ratio of 195 and 196 depends upon the reaction temperature and time. Thus, for short reaction time and low temperature (0.5 h, 78 °C) the product from attack at C2 (i.e. 195) predominates while at higher temperature and longer reaction time (2 h, 0 °C) the product from attack at C1 (i.e. 196) predominates . This selectivity is rationalized by kinetically controlled attack at the more electron-poor carbon (C2) at low temperature. Nucleophilic attack is reversible and, under conditions where an equilibrium is established, the thermodynamically more stable (allyl)Fe(CO)3 is favored. The regioselectivity for nucleophilic attack on substituted (diene)Fe(CO)3 complexes has been reported187. The

78 ! 0 °C) gives cyclopentanone products (197, Scheme 49)188. Intramolecular variants of this reaction have been reported for the preparation of bicyclo[n.3.0]alkanones (equation 29)189.

SCHEME 50

2. Cationic (diene)cobalt complexes

The reaction of (diene)Co(CO)3C cations with a range of carbon and heteroatom nucleophiles has been examined. As might be expected, since these are positively charged species the range of nucleophiles which are reactive is more extensive than for neutral (diene)Fe(CO)3 complexes (see Section V.E.1) and includes such nucleophiles as pyridine and phosphines. In contrast to the (diene)Fe(CO)3 complexes, nucleophilic attack on the (diene)Co(CO)3C cations occurs exclusively at the diene terminus to give neutral (anti- 1-substituted-allyl)Co(CO)3 complexes 198 in ‘moderate to good yield’ (Scheme 50)28. The regioselectivity for nucleophilic attack on substituted (diene)Co(CO)3C cations has been examined134b. In general, for hydride, phenyl magnesium bromide or pyridine as nucleophile, attack at the less hindered diene terminus is preferred; 1-substituted (diene)Co (CO)3C cations 199 give predominantly 200, while 2-substituted (diene)Co(CO)3C cations 201 give predominantly 202 (equation 30).

The neutral (allyl)Co(CO)3 products are themselves susceptible to nucleophilic attack. Thus reaction of (C4H6)Co(CO)3C with two equivalents of sodium dimethyl malonate anion gives the tetraester 203, presumably via initial attack at C1 to generate the intermediate -allyl complex 204 followed by regiospecific attack by the second equivalent at C4 (Scheme 50)28. The intramolecular variant of this reaction using a single equivalent of a dinucleophile, such as a ˇ-dicarbonyl dianion or the corresponding 1,3-bis(silyloxy)diene, leads to the formation of vinyldihydrofuran products 205190. For these reactions, it would