Cyclohexadiene complex

Alkenes in (alkene)dicarbonyl(T -cyclopentadienyl)iron(l+) cations react with carbon nucleophiles to form new C —C bonds (M. Rosenblum, 1974 A.J. Pearson, 1987). Tricarbon-yi(ri -cycIohexadienyI)iron(l-h) cations, prepared from the T] -l,3-cyclohexadiene complexes by hydride abstraction with tritylium cations, react similarly to give 5-substituted 1,3-cyclo-hexadienes, and neutral tricarbonyl(n -l,3-cyciohexadiene)iron complexes can be coupled with olefins by hydrogen transfer at > 140°C. These reactions proceed regio- and stereospecifically in the successive cyanide addition and spirocyclization at an optically pure N-allyl-N-phenyl-1,3-cyclohexadiene-l-carboxamide iron complex (A.J. Pearson, 1989). 

The analogue [Fe(C6H6)(C6Me6)]+ + (PFg )2 [23] was made by double hybride abstraction from the known cyclohexadiene complex [Fe(r 4-C6H8)(r 6-C6Me6)] [29], It behaved in the same way, although the reactions were smoother. 

Such double C-C bond formation had also been attempted by Vollhardt et al. by nucleophilic attack on [CoCp(C6H6)]++. Formation of disubstituted cyclohexadiene complexes was possible only when the nucleophile was CH30-or C5H5-. 

Ring-member substitution, a very characteristic reaction of some 18-e borabenzene complexes (see Section VII,B), can also occur with 1,4-dibora-2,5-cyclohexadiene complexes. The cobalt complex 53 cleanly reacts with MeCOCl/ A1C13 to give the cation 54 (Scheme 7) (75). The Rh complex (C5Me5)Rh[MeB(CHCH)2BMe] reacts analogously (75). 

Scheme 16.2 illustrates the catalytic mechanism proposed by Muetterties and coworkers [13]. Salient features of this mechanism are the coordination of benzene in the -fashion, to give a transient Col I( 4-C, iH, i)(PR3)2 complex, and the intramolecular hydride transfer to form the allylic intermediate Co(//3-Ctl l7) (PR3)2. Hydrogen addition would give an 4-1,3-cyclohexadiene complex that ultimately releases cyclohexane via H2 addition/hydride migration steps. Complete cis stereoselectivity of hydrogen addition was demonstrated by replacing H2 with D2. 

The 1,3-cyclohexadiene complex 64 may be prepared by addition of two equivalents of hydride to the (C6H6)Mn(CO)3+ cation 65 (R = H, Scheme 16)90. The first equivalent of hydride generates the neutral ( j5-cyclohexadienyl)Mn(CO)3 complex... 

More recently, an environmentally benign method using air as oxidant has been developed for the oxidative cyclization of arylamine-substituted tricarbonyl-iron-cyclohexadiene complexes to carbazoles (Scheme 19). Reaction of methyl 4-aminosalicylate 45 with the complex salt 6a affords the iron complex 46, which on oxidation in acidic medium by air provides the tricarbonyliron-complexed 4a,9a-dihydrocarbazole 47. Aromatization with concomitant demetalation by treatment of the crude product with p-chloranil leads to mukonidine 48 [88]. The spectral data of this compound are in agreement with those reported by Wu[22j. 

Cycloheptene complexes with gold, 12 348 Cyclohexadiene complexes with cobalt, 12 286 with group VIB metals, 12 225-227 with group VIIB metals, 12 240, 241 with iron, 12 264 with palladium, 12 313 with ruthenium, 12 278 with silver, 12 340... 

Although tricarbonylbutadieneiron (1) was prepared by Reihlen et a/.1 in 1930, some considerable time passed before the corresponding cyclohexadiene complex (2 equation 1) was reported.2 Fischer and Fischer described the conversion of (2) to the cationic cyclohexadienyliron complex (3 equation 1) by reaction with triphenylmethyl tetrafluoroborate in dichloromethane.3 This particular complex is extremely easy to prepare and isolate as the hydride abstraction reaction proceeds the product (3) crystallizes out. Precipitation is completed by pouring the reaction mixture into wet diethyl ether, the small amount of water present serving to destroy any excess triphenylmethyl tetrafluoroborate by conversion to triphe-nylmethanol. Filtration, followed by washing the residue with ether, gives pure dienyl complex. 

As mentioned earlier, steric effects can be important in determining the outcome of the hydride abstraction reaction. This is particularly vexing in cases where an alkyl substituent is present at the sp carbon of the cyclohexadiene complex. For example, complexes such as (47 equation 19) are untouched by trityl cation, provided traces of acid are not present (these are formed by hydrolysis of the trityl tetra-fluoroborate due to atmospheric moisture, and will cause rearrangement of the diene complex). This is due to the fact that only the hydride trans to the Fe(CO)3 group can be removed, and the methyl substituent prevents close approach to this hydrogen. 

As mentioned earlier, direct hydride abstraction from 5-exo-substituted cyclohexadiene complexes is in general difficult, except for the 2-trimethylsilyl-substituted derivatives such as (48) and (50). Oxidative cyclization techniques have been developed to overcome this problem, exemplified by the conversion of (52) to (53) and thence to (54 Scheme 7). Stereocontrolled addition of a second nucleophile has already been illustrated by the conversion of (54) to (126) or (127), and the limitations imposed by a sterically demanding 6-exo substituent have been mentioned. 

Complex 49 was converted to the oxime complex 52. Reaction of 52 with organocuprate, followed by treatment with AC2O and CO, afforded the interesting but rather unstable [(1,2,3,4- /)-1 -0V-acctoxy-A-rnethoxyarnino)-5-c < /o-acyl-1.3-cyclohexadiene] complex 55 via 53 and 54 [15]. 

Nucleophilic attack occurs at C(2) of the diene. The 1,3-cyclohexadiene complex 66 is converted to the homoallyl anionic complex 67 by nucleophilic attack, and the 3-alkyl-1-cyclohexene 68 is obtained by protonation. Insertion of CO to 67 generates the acyl complex 69, and its protonation and reductive elimination afford the aldehyde 70 [20]. Reaction of the butadiene complex 56 with an anion derived from ester 71 under CO atmosphere generates the homoallyl complex 72 and then the acyl complex 73 by CO insertion. The cyclopentanone complex 74 is formed by intramolecular insertion of alkene, and the 3-substituted cyclopentanone 75 is obtained by reductive elimination. The intramolecular version, when applied to the 1,3-cyclohexadiene complex 76 bearing an ester chain at C(5), offers a good synthetic route to the bicyclo[3.3.1]nonane system 78 via intermediate 77 [21]. 

The 1,3-cyclohexadiene complex 66 was expanded to cycloheptadienone (80) by an interesting reaction of CO mediated by AICI3 via 79. The bicyclo[3.2.1]octenedione 81 was prepared by the twofold carbonylation of 66 under high pressure of CO via the intermediate 79 [22,23], This interesting transformation has been applied to the stereoselective construction of the dicyclopenta[a, JJcyclooctene core 83 of ceroplastin terpene from 82 under 5 atm of CO [24],... 

The usefulness of 1,3-cyclohexadiene complexes is enhanced by their conversion to stable cationic complexes. The if -cationic complex 102 is prepared as a stable salt by the hydride abstraction from the neutral complex 66 via 101, and its highly regio- and stereoselective reaction with nucleophiles is used for synthetic purposes. Complex 102 reacts with nucleophiles such as amines, active methylenes, alkyl copper or alkoxides at C(l) or C(5) from the uncomplexed exo side. In other words, the nucleophilic attack occurs regioselectively at a dienyl terminus, and stereoselectively anti to Fe(CO)3 to give 103. Hydride abstraction from 103 affords 104, which reacts with a nucleophile to form 105. Decomplexation of 105 produces the 5,6-disubstituted-l,3-cyclohexadiene 106. 

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)]. 

Through a significant rc-backbonding interaction, the coordination of benzene to [Os] (1) serves both to activate the arene toward the electrophilic addition of dimethoxymethane (Table 1, entries 1-4) or 3-penten-2-one (entry 5) and to stabilize the resulting benzenium intermediate 8. If manipulated at low temperature (—40 °C), 8 can be trapped with either a silyl ketene acetal (entries 1, 4, and 5), 2-trimethylsiloxypropene (entry 2), or phenyllithium (entry 3) to yield the substituted 1,4-cyclohexadiene complex 9 [15]. This species can be oxi-... 

The cyclohexadiene complex 29 has been further elaborated to afford either the cydo-hexenone 34 or the cyclohexene 36 in moderate yields (Scheme 1) [21]. The addition of HOTf to 29 generates the oxonium species 33, which can be hydrolyzed and treated with cerium(IV) ammonium nitrate (CAN) to release the cyclohexanone 34 in 43 % yield from 29. Alternatively, hydride reduction of 33 followed by treatment with acid eliminates methanol to generate the r 3-allyl complex 35. This species can be trapped by the conjugate base of dimethyl malonate to afford a cyclohexene complex. Oxidative decomplexation of this species using silver trifluoromethanesulfonate liberates the cyclohexene 36 in 57 % overall yield (based on 29). 

The chiral anisole derivative 37 has been used in the synthesis of several asymmetric functionalized cyclohexenes (Table 9) [22]. In a reaction sequence similar to that employed with racemic anisole complexes, 37 adds an electrophile and a nucleophile across C4 and C3, respectively, to form the cyclohexadiene complex 38. The vinyl ether group of 38 can then be reduced by the tandem addition of a proton and hydride to C2 and Cl, respectively, affording the alkene complex 39. Direct oxidation of 39 liberates cydohexenes 40 and 41, in which the initial asymmetric auxiliary is still intact. Alternatively, the auxiliary may be cleaved under acidic conditions to afford /y3 -allyl complexes, which can be regioselectively attacked by another nucleophile at Cl. Oxidative decomplexation liberates the cyclohexenes 42-44. HPLC analysis revealed high ee values for the organic products isolated both with and without the initial asymmetric group. 

A carbon-carbon double bond of this intermediate then reacts with two alkyne units via a cobalt-catalyzed [2 + 2 + 2] process leading to a cobalt-1,3-cyclohexadiene complex which undergoes subsequent reductive elimination (Scheme 14). The regioselective outcome of this reaction was explained in terms of steric congestion. 

Selective reduction of the methyl ester to the corresponding aldehyde using DIBAL at low temperature and subsequent reductive amination with iodopiperonyl-ammonium chloride affords the tricarbonyliron-cyclohexadiene complex with the secondary alkylamine in the side chain. Iron(O)-mediated oxidative cydization... 

Electrophilic aromatic substitution of 2,3-dimethyl-4-methoxyani]ine by reaction with the tricarbonyliron-coordinated cyclohexadienylium salt generates the aryl-substituted tricarbonyliron-cyclohexadiene complex. Treatment of this complex with very active manganese dioxide results in oxidative cyclization and aromatization with concomitant demetallation to afford directly 4-deoxycarbazomycin B, a degradation product of the antibiotic carbazomycin B [32]. Using ferricenium hexafluorophos-... 

Electrophilic aromatic substitution of 5-hydroxy-2,4-dimethoxy-3-methylaniline by reaction with the iron complex salts affords the corresponding aryl-substituted tricarbonyliron-cyclohexadiene complexes. O-Acetylation followed by iron-mediated arylamine cydization with concomitant aromatization provides the substituted carbazole derivatives. Oxidation using cerium(IV) ammonium nitrate (CAN) leads to the carbazole-l,4-quinones. Addition of methyllithium at low temperature occurs preferentially at C-1, representing the more reactive carbonyl group, and thus provides in only five steps carbazomycin G (46 % overall yield) and carbazomycin H (7 % overall yield). 

The 77 -bound benzo[/ ]thiophene and dibenzothiophene complexes of iridium undergo reduction to the corresponding 77-complexes by hydride reaction with 2 mol of Red-Al results in addition of two H to give the cyclohexadiene complexes (Scheme 78) <2000CCR63>. 

The /4-cyclohexadiene complex [Ru(//4-C6H8)(PF3)3], which is readily obtained by displacement of benzene (method E) from [Ru(//4-C6H8)(//6-C6H6)], is a fiuxional molecule (1). The 31P H) NMR spectrum at room temperature (Fig. 17) exhibits a basic 1-3-3-1 quartet pattern (coupling to F) and shows the further complicated fine structure expected for an [AX3]3 spin system (X = F, A = P) indicative of... 

Arenes bound to iridium are susceptible to nucleophilic attack to form first cyclohexadienyl and then cyclohexadiene complexes. For example, Maitlis showed that [Cp Ir( -toluene)]+ reacts with NaBH4 yielded the cyclohexadiene... 

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