Cyclobutenes 1-aryl

The series of wide-bite-angle, bulky ligands derived from a cyclobutene scaffold gave Pd complexes (117) showing appreciable activity in the cross-coupling of reactive aryl bromides with trimethylsilylacetylene. A considerable shift of electron density to the phosphorus atoms, probably arising from alternative aromatic canonical structures, may account for the ligand properties.422... 

Tributylstannyl)-3-cyclobutene-1,2-diones and 4-methyl-3-(tributylstan-nyl)-3-cyclobutene-l,2-dione 2-ethylene acetals undergo the palladium/copper-catalyzed cross coupling with acyl halides, and palladium-catalyzed carbon-ylative cross coupling with aryl/heteroaryl iodides [45]. The coupling reaction of alkenyl (phenyl )iodonium triflates is also performed by a palladium/copper catalyst [46],... 

The initially generated o-diallenylarene 273 electrocydizes to the annelated o-xyly-lene 274, which can either ring close to a naphtho[fo]cyclobutene or be trapped with a dienophile to yield the Diels-Alder adduct 275. Instead of the terminal hydrogen atom, the ethynyl functions can also carry alkyl and aryl substituents. 

Thermolysis of 3-(ort o-anisoyl)-l-(l-piperidinyl)-3-cyclobutenes 807 in the presence of mesitylene affords angular-fused xanthones 809 via formation and ring closure of the intermediate 808 (Scheme 226) <1997TL3663>. Linear-fused xanthones 810 are prepared by nucleophilic addition of aryl and heteroaryl lithiates to dithiane protected benzopyrone-fused cyclobutenediones 811 followed by hydroysis of the dithiane protecting group (Scheme 227) <1996JA12473>. 

Shim and coworkers have published several examples of the 2+2 between aryl-substituted alkynes and alkenes. Their data suggests that the singlet and triplet pathways give rise to the observed cyclobutenes. Exciplex emission was detected for the alkyne-alkene pairs. 

Similarly, enynes substituted at the alkyne with an aryl group led to products resulting from a formal intramolecular [4 + 2] cycloaddition occurring at an unusual low temperature (equation 64). " On the other hand, substrates with R = = H or R = Me, R = H gave cyclobutenes with... 

A final example illustrates the use of squaric acid derivatives in enzyme inhibition. In an effort to discover potent inhibitors (of Yersinia PTPs and PTPIB) with reduced charge and thus improved bioavailabihty, Xie et al. (53) also studied the squaric acid motif as a nonhydrolyzable phosphotyrosine mimic. 3-aryl-4-hydroxy-3-cyclobutene-l,2-diones based on the... 

Liebeskind and co-workers have published a series of papers dealing with the chemistry of cyclobutenones and cyclobutene-diones. Thus an unprotected stannyl derivative bearing an amino function was reacted with an appropriate aryl iodide, leading in excellent yield to a pyrido-annelation product [64] (Scheme 4-23). The reaction of a monoprotected derivative with chlorocyclobutenones led to benzannelated cyclobutenediones [65], and stannyl derivatives of p-benzoquinone also underwent benzannelation when treated with these substrates [66]. Acylation of cyclobutenediones via stannyl derivatives has also been carried out [67], as has functionalization of cyclobutenones [68]. 

Benzocyclobutene is also metallated by n-BuLi-TMED in EtjO, both in the aryl ring and in the cyclobutene ring, i.e., at the benzylic sites L... 

Another group of polymers with good heat resistance includes certain aryl-alicyclic compounds. They are generated from the polycondensation of 4,4 -diaminodiphenyl oxide with various anhydrides such as tricyclo[4,2,2,0 ]dec-9-ene-3,4,7,8-tetracarboxylic acid anhydride, 9-oxatricyclo[4.2,2,0 ]nonane-3.4,7,8-tetracarboxylic acid anhydride, cyclobutene-1,2,3,4-tetracarboxylic acid anhydride, and bicyclo[2,2,2]octane-23,5,6-tetracarboxylic acid anhydride. Thermal stability of these polymers is affected by the ring stability (strain) regardless the true chemical structure. Pyrolysis at 650° C generates some compounds that are the same in nature and some others that are characteristic for each polymer. The main decomposition paths, as indicated by the main pyrolysis products are shown below for each polymer (Ar = 4,4 -diphenylene oxide) ... 

Aryl substituted cyclobutenes undergo cycloreversion to arylalkynes but can also give addition products in hydroxylic solvents. These reactions are singlet state processes. The aryl derivatives of cyclobutene do not open to 1,3-butadienes, and based on substituent effects, the excited states appear to have zwitterionic character. These results suggest that the aryl substituent favors the formation of a zwitterionic excited state. 

The reactions observed with 31b and 31c forming tetra(aryl)ethylene (32) and benzocyclobutene (33) were analogous to those observed with 31a, but the product distributions were significantly different. While 31a underwent dimerization almost exclusively, the formation of cyclobutene increased as more methyl groups were introduced. Carbene 31c produced 33c as major product at the expense of the dimer (32c). 

A cydization/Barbier-type reaction was reported by Curran and coworkers [21] in 2004 in the total synthesis of penitrem D 46. In this work, the aryl radical generated from the iodoarene 41 and Smij proceeded to attack the tethered cyclobutene to from a cyclobutyl radical 42. Subsequently, reaction with Smij led the organo-samarium species 43, which underwent a Barbier-typie reaction with acetone to give the tertiary alcohol 44 in 40% yield. The product contains the BCD ring system of penitrem D 46 (Scheme 5.12). 

---