Rhodium cyclobutenones

It should be noted that upon reaction with an electron-rich cyclobutenone (R1 = R3 = H, R2 = OEt), the major product formed was a cobaltacyclopen-tenone, which may also be considered to be an 772-vinylketene complex. A similar restructure was isolated after heating 114.a with a large excess of triphenylphosphine, which replaces the ligand site vacated by the central C2 unit. Interestingly, such 772-vinylketene complexes are the expected products from the analogous insertion of rhodium into cyclobutenones (e.g., 7). 

An intramolecular allenylidene-alkynyl coupling was also observed in the reaction of the mixed alkynyl-allenylidene rhodium(I) complex 73 with carbon monoxide (Scheme 25). In this case, the initially formed thermally unstable allenyl derivative 74 evolved into the metallated cyclobutenone 75 when an excess of CO was present [276]. 

A rhodium-catalyzed ring opening of the cyclobutenones 637 forms the r 4-vinylketene intermediates 638, which can dimerize to afford (/. )-6-vinyl-2//-pyran-2-oncs in excellent yield (Scheme 146) <2004AGE5369>. A ruthenium-mediated ring opening and dimerization of cyclobutenones forms a mixture of (E)- and (Z)-6-vinyl-2//-pyran-2-ones in high yield (Equation 257) <2004AGE5369>. 

Lawlor, M. D., Lee, T. W., Danheiser, R. L. Rhodium-Catalyzed Rearrangement of a-Diazo Thiol Esters to Thio-Substituted Ketenes. Application in the Synthesis of Cyclobutanones, Cyclobutenones, and 3-Lactams. J. Org. Chem. 2000, 65, 4375-4384. 

Cyclobutenone also oxidatively adds onto rhodium(I) or cobalt(I) to afford a metallacycle [40]. 

Huffman MA, Liebeskind LS, Pennington WT (1990) Synthesis of metallacyclopentenones by insertion of rhodium into cyclobutenones. Organometallics 9(8) 2194-2196. doi 10.1021/ om00158a009... 

Kondo T, Niimi M, Nomura M, Wada K, Mitsudo TA (2007) Rhodium-catalyzed rapid synthesis of substituted phenols from cyclobutenones and alkynes or alkenes via C-C bond cleavage. Tetrahedron Lett 48(16) 2837-2839, http //dx.doi.Org/10.1016/j.tetlet.2007.02.091... 

In the reaction of the rhodium alkynyl complexes 7 with excess carbon monoxide at room temperature, the cyclobutenone 8 is isolated in 90 % yield... 

The application of a rhodium catalyst to decarbonylative and direct coupling reactions of cyclobutenones with 2-norbornene provides stereoselective access to fused cyclopentenes (Scheme 6.23). Argon atmosphere is crucial for this reaction to give fused cyclopentenes, while in the presence of carbon monoxide the reaction affords cyclohexenones by direct coupling [27]. 

Cyclobutenones undergo oxidative addition to rhodium(I) and cobalt(I) (Scheme 1.14) [22]. The metals inserted site-selectively into the C(carbonyl)-C(sp ) bond to form the corresponding metallacyclopentenones. 

The rhodium-catalyzed reaction of 2,3-disubstituted cyclobutenone 89 with electron-deficient alkenes dehvered 2,3,6-substituted phenols 90 (Scheme 3.51) [60]. 

When the cyclobutenone 33 was treated with a rhodium catalyst, the C(arene) -C(carbonyl) bond was site-selectively cleaved and the alkene moiety was intramolecularly inserted therein to construct the tetracyclic skeleton of 34. The isopropyl group was introduced by electrophilic aromatic bromination. 

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