The interconversion of cyclobutene-1,2-diones (1) and 1,2-bisketenes (RCCO)2 (2) has been surveyed for different combinations of substituents R = H, Me, t-Bu, Ph, Me3Si, CN, Cl, Br, R1O, alkynyl, and PhS. The bisketenes 2 have been generated by flash photolysis, and the kinetics of their conversion to 1 have been studied by time-resolved infrared and ultraviolet spectroscopy. The rate constants of the ring closure of 2 are correlated by the ketene stabilization parameters (SE) and with calculated barriers. The rate constant of ring closure of the di-tert-butyl bisketene 2g to cyclobutenedione 1g is only 40 times smaller than for the dimethyl analogue, showing a rather modest steric barrier. The quinoketene 2s has a fast rate of ring closure, but not as fast as anticipated on the basis of calculated geometric and thermodynamic factors. A lag in the attainment of aromatic stabilization in the transition structure for ring closure is a possible cause of this diminished reactivity.

Cycloaddition reactions of the bisketenes OdCdC(SiMe 3 )CRdCdO (15, R ) SiMe 3 ; 16, R ) Ph) include BF 3 -catalyzed [2+2] cycloaddition of 15 with CH 3 CHO to form an isolable β-lactone 18 adduct which undergoes thermal decarboxylation to the vinylketene 19. Reactions of 15 and 16 with CH 2 N 2 proceed by [4+1] cycloaddition to give mixtures of cyclopentene-1,3-diones 20 and methylenefuranones 21, while Me 3 SiCHN 2 and PhCHN 2 give only 20. The reactions are interpreted in terms of a steric preference for nucleophilic attack by the substituted diazomethanes from the side of the ketene bearing the Me 3 SiCdCdO substituent, leading to formation of 20. With the less bulky CH 2 N 2 , attack occurs from both sides and approach from the side of the R group leads to formation of lactones 21. Reaction of tetramethoxyethylene with 15 yields both a cyclopentene-1,3-dione 24 from net addition of dimethoxycarbene and a spirocyclopropylbutenolide 25. Free dimethoxycarbene generated by heating an oxadiazoline precursor also reacted with 15 to give dione 24. Various electrophilic dienophiles do not react with 15, but nucleophilic alkynes react with 16 in thermal reactions to give spirocyclopropenylfuranones 33-36, and Me 3 SiCtCOEt and 16 react by net [4 + 2] cycloaddition to give the quinone 37 as the major product.The [2+2] cycloaddition reactions of ketenes 1, including dimerization and reactions with alkenes, imines, and aldehydes to form cyclobutenones, β-lactams, and β-lactones 2, respectively (eq 1), are the most distinctive reactions of these fascinating species. 1,2 There have been authoritative arguments for the concerted nature of these reactions 1f-h and also strong arguments for stepwise processes. 3 These reactions are also the subject of intense theoretical study. 2 It is characteristic of monoketenes that they do not ordinarily give observed [4+2] cycloaddition products except in unusual circumstances. 4 Bisketenes have been rare species, and only a few cycloaddition reactions of this family have been studied. The metal complex 3 reacts by a net [4+2] cycloaddition to form quinone complexes 4 (eq 2). 5a Photolyses of cyclobutene-1,2-diones such as 5 in the presence of cyclopentadiene were proposed to form bisketenes 6