REACTION OF FISCHER TYPE CHROMIUM CARBENE COMPLEXES WITH N,N-DIMETHYLFORMAMIDE DIALKYL ACETALS (DMF-DAA) - AN UNEXPECTED CARBON-CARBON BONDBREAKING

A series of pentacarbonyl(2-oxacyclopentylidene)chromium (5a-f) were r eacted with NN-dimethylformamide dimethyl acetal (DMF-DMA), resulting in the imethylamino)methylidene]cyclopentylidene)chromium (6a-f) and t he unexpected -(cyclopropylmethylidene)cyclopentylidene)chromium (7a-d ). Open chain Fischer type carbene complexes (CO)5Cr=C(OCH3)CH2R, (1a- d) also react easily with DMF-DAA (2a-d). When R1 = H, the carbene com plex (CO)5Cr=C(OCH3)CH=CHN(CH3)2 is formed in high yield; however, whe n R1 = alkyl, the reaction products are dialkoxy complexes (CO)5Cr=C(O CH3)OR2 (4a-d), in which the OR2 group comes from the DMF-DAA. The mec hanisms of the above reactions are elucidated. The cyclopropyl ring pr esent on compounds 7a-d arises from a contraction of the five membered ring of complexes 5a-d due to the breaking of the bond between the ca rbene carbon atom and the a-carbon atom. The same mechanism explains t he formation of dialkoxy complexes 4a-d from complexes 1b-d. In the ca se of the reaction of the complex (CO)5Cr=C(OCH3)CH2CH2CH2CH3 (1d) wit h N,N-dimethylformamide dineopentyl acetal (2d), the intermediary form ation of an enamine derivative has been demonstrated. Moreover the rea ction of rbonyl(2-oxa-4,4-dimethylcyclopentylidene)chromium (5g) with DMF-DMA (2a) allowed the isolation of 2,2-dimethylcyclopropanecarbalde hyde (characterized as the (2,4-dinitrophenyl)hydrazone).