Substituted 1,2-Cyclohexadienes

Scheme 6.22 Substituted 1,2-cyclohexadienes that have been successfully generated and trapped.

Scheme 6.23 3-Methylenecyclobutanol derivatives from the interception of the methyl-substituted 1,2-cyclohexadienes 74 and 82 by the enolates of cyclohexanone and diisopropyl ketone, respectively.

The cycloadditions of 1-substituted 1,2-cyclohexadienes and among them their dimerization are of interest because of the position selectivity. Does the reaction occur at the substituted or the unsubstituted ethylene subunit For that question to be answered, 1-methyl- (74), 1-phenyl- (75), 1-cyclopropyl- (76), l-(3-phenylpropyl)-(77) and l-trimethylsilyl-l,2-cyclohexadiene (79) were generated from the corresponding 1-substituted 6,6-dibromobicyclo[3.1.0]hexanes with methyllithium. Several of these dibromides are thermolabile, which particularly applies to the phenyl (93) [76] and the cydopropyl derivative [70], In those cases, it is advisable or necessary to prepare the dibromide in situ, that is, the dibromocarbene is liberated from tetrabro-momethane with methyllithium at -60 °C in the presence of the respective cyclopen-tene. Without workup, from the thus formed 6,6-dibromobicyclo[3.1.0]hexane, the 1,2-cyclohexadiene is then generated by addition of methyllithium at -30°C. 

Scheme 6.26 Cycloadducts offurans and styrenes to 1-substituted 1,2-cyclohexadienes.

Ring closure of more highly substituted cyclohexadienes also follows the Woodward-Hoffmarm rules and, indeed, provided the initial examples of the dichotomy between thermal and photochemical processes that led to development of the concepts underlying the Woodward-Hofimann rules. ... 

The use of chiral bis(oxazoline) copper catalysts has also been often reported as an efficient and economic way to perform asymmetric hetero-Diels-Alder reactions of carbonyl compounds and imines with conjugated dienes [81], with the main focus on the application of this methodology towards the preparation of biologically valuable synthons [82]. Only some representative examples are listed below. For example, the copper complex 54 (Scheme 26) has been successfully involved in the catalytic hetero Diels-Alder reaction of a substituted cyclohexadiene with ethyl glyoxylate [83], a key step in the total synthesis of (i )-dihydroactinidiolide (Scheme 30). 

Photocycloadditions of maleic anhydride and dimethylmaleic anhydride to various other substituted cyclohexadienes and to cycloheptatriene have also been reported/58,97 98 ... 

Utilization of the electroreduction of aryltrimethylsilanes and aryltrimethylgermanes for the preparation of silyl- and germyl-substituted cyclohexadienes was extensively studied203, for example ... 

A further useful reaction sequence, reported by Aumann [219], is based on the Diels-Alder reaction of 2-pyranylidene complexes with enamines (Figure 2.36). Retro-Diels-AIder reaction of the initially formed 3-oxabicycIo[2.2.2]octan-2-ylidene complex leads to elimination of metal hexacarbonyl and formation of a substituted cyclohexadiene. Although this sequence can also be performed with the corresponding carbonyl compounds (2//-2-pyranones), these normally... 

The aprotic double Michael addition was discovered by R. A. Lee and to synthesize functionalized bicyclo[2.2.2]octanes which may serve as starting materials in natural products syntheses (Table I). These ljicyclo[2,2.2]octanes can also be obtained by a Diels-Alder cycloaddition of 2-trimethyl5iloxy-substituted cyclohexadienes and dienophiles H... 

With methyl-substituted cyclohexadienes, very little selectivity is observed during the preparation of the complexes and the hydride abstraction reaction. Dihydrotoluene, on heating with pentacaibonyliron, gives a mixture of complexes (37) and (38 Scheme 4). These cannot be easily separated using standard chromatographic procedures, and little is known about hydride abstraction from the individual complexes. Treatment of the equimolar mixture with trityl tetrafluoroborate gives a mixture of all three possible products (39-41 Scheme 4). 

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. 

Siegel carried out detailed studies with di-fert-butyl-substituted cyclohexadienes with the aim of clarifying their role as intermediates in the hydrogenation of di-tert-butylbenzenes on a 5% Rh-alumina catalyst291. The three isomeric dienes (34, 35, 36)... 

Using cationic tricarbonyl(q5-cyclohexadienyl)iron complexes as starting materials, different synthetic routes to a large number of carbazole alkaloids have been developed [51, 58, 67]. The first step is an electrophilic substitution of a substituted arylamine using the cyclohexadienyliron complex and provides the corresponding 5-aryl-substituted cyclohexadiene-iron complexes (Scheme 1.29). 

Irradiation of the substituted cyclohexadiene resulted in the formation of the Dewar benzene skeleton by a disrotatory ring closure. Reaction with lead tetraacetate (a reaction that is not covered in this book) was used to remove the anhydride group and introduce the final double bond of Dewar benzene. Again, because of the forbidden nature of the conrotatory opening to benzene, Dewar benzene has an appreciable lifetime. At 25°C the half-life for its conversion to benzene is 2 days, and at 90°C its half-life is 30 min. 

The reaction of the silicon-carbon double-bonded intermediate generated from phenylpentamethyldisilane with methyllithium and with methylmagnesium bromide leads to the formation of wholly unexpected products (92). Thus, irradiation of 34 in the presence of methyllithium with a low-pressure mercury lamp in diethyl ether followed by hydrolysis produces l,3-bis(trimethylsilyl)benzene (42) and 1,2-bis(trimethylsilyl)-benzene (43) in 60 and 20% yield, respectively, in addition to an 8% yield of phenyltrimethylsilane (Scheme 13). Surprisingly, no disilyl-substituted cyclohexadienes that might be expected from addition of methyllithium to the intermediate followed by hydrolysis are observed. In this photolysis,... 

The basic reaction of the tricarbonyl(cyclohexadienyl)iron cation [(C6Hy)Fe(CO)3]+ consists of an addition of a nucleophile to the cyclohexadienyl ligand yielding a substituted cyclohexadiene ligand. Assuming the cation contains an iron(O) center and a coordinated cyclohexadienyl cation, the complex was expected to have low-lying MLCT (Fe(0) C6H7+) transitions. Photolysis of the complex in... 

Iron complexes favor the codimerization of BD with alkynes in a 1 1 ratio to (substituted) cyclohexadienes [39]. Two BD molecules and one alkyne give cyclodecatrienes with zerovalent nickel catalysts and good electron-donating ligands such as Ph3P [7, 40]. Ten-membered rings are in fact the principal products of such a reaction the variety of dienes seems to be limited to BD, iso-prene and 1,3-pentadiene, whereas numerous alkynes - simple alkyl-substituted alkynes, alkynes with aprotic functional groups, dialkynes, and cyclic alkynes... 

Taking into account that 158 is much more reactive than 154, we confirm what we said before concerning the poor tendency to the formation of methyl substituted cyclohexadiene. 

The formation of additional products (substituted cyclohexadienes and cyclopentadienes) in some of the reactions, specifically when cyclopropenes with one or two phenyl groups in the 3-position are employed, mechanistically can be rationalized by a reaction pathway involving a nickelacyclobutene 7 with at least one 7t-coordinated alkyne that inserts into the four-mem-bered metallacycle forming a metallacyclohexadiene 8. Reductive elimination may then proceed resulting in cycloalkadienes. 

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