T-Butylcyanoketenes

The ketene (2) also undergoes cycloaddition to acetylenes to give 2-cyclobutcnc-1 -one derivatives in 40-80% yield. Thus t-butylcyanoketene reacts with phenylacctylene in benzene at room temperature to give the cyclobutenone (10). Only one of the two possible products of cycloaddition is formed thus the reaction is rcgiospecific. The structure... 

The basic reagem is used for dehydrochlorination of a quinone dichloride in ether in two steps of a synthesis of t-butylcyanoketene. ... 

Cychadditions. Moore and Weyler1 noted that t-butylcyanoketene (2) is unusually reactive in cycloaddition reactions. Thus it reacts with cyclohexene (3) to form the cyclobutanone (4) in 63 % isolated yield. Note that phenylcyanoketene does not react with cyclohexene.3... 

Thiazole adds t-butylcyanoketene and dichloroketene to give a zwitterion wherein A-thiazolium and enolate moieties are linked by carbon-nitrogen bond. This intermediate leads to the corresponding 2-acylthiazole (35). The reaction with r-butyldicyanoketene leads also to the 2 1 cyclic adduct (36) (Scheme 4) <84JOC590>. The same reactions have been observed with 4- and 5-methyl-thiazoles. 

Thiazolo[4,5-d]pyrans [CjNS-CsO]. 2-Dimethylamino-thiazoles will undergo cycloaddition with two moles of t-butylcyanoketen to give a mixture of two thiazolo[4,5-f/]tetrahydropyran-2-ones (153 = H or Me, = CN, R =... 

Bampfield and Brook have examined the steric course of the cycloadditions of t-butylcyanoketen to allenes. Unlike diphenylketen, this keten adds preferentially to the more substituted double bond of unsymmetrical allenes. The stereochemical results were very similar to the findings reported by Moore in 1973 and are accounted for by the intermediacy of free-rotating zwitterions. ... 

Recent interest has been focused on the reactions of t-butylcyanoketen, which resembles bis-trifluoromethyl-keten in a number of respects. Thus, addition to nor-bornadiene gives, as well as the expected cyclobutanone, a formal [2 -I- 2 -I- 2] addition product (310), and with 1,1-dimethylethylene an ene product (311) is obtained in addition to the normal cyclobutanone. 1,2- and 1,4-addition compete in the reactions of diphenylketen with conjugated dienes, the product ratios depending on the substitution of the diene. " ... 

The zwitterion (588) has been implicated in the thermal rearrangements of the t-butylcyanoketen-l,l-dimethylallene adducts (587 R = H or CDj) and the same intermediate appears to be involved in the conversion of (587 R = Me) into (589) by a prototropic shift from the allenic methyl to the enolate system. By contrast, the four adducts formed by addition of t-butylcyanoketen to 1,3-dimethylallene isomerize by a different pathway, possibly via the dienol (590). ... 

The reaction of t-butylcyanoketen with racemic and optically active diphenylallene gives exclusively 2- -benzylidenecyclobutanones (112) and (113). The structure of the major product (112) has been established by X-ray crystallography. With partially resolved diphenylallene, the adduct was also optically active, demonstrating that the transition state for the addition is also, to some extent, chiral. The absence of the Z isomers, the chiral transition state, and the formation of a torsionally strained major product are all consistent with the view that the reaction is a concerted [tc2 + 7i2J cycloaddition. 

The full paper on the stereoselectivity and stereospecificity in electron-deficient diene cycloadditions, the preliminary results on which were reported extensively last year, has now appeared. As compared with other ketenes, t-butylcyanoketene is extraordinarily reactive towards norbornene, and only one cyclobutanone, the exoadduct (193), is obtained. Reaction of 4,4-dimethylcyclobutenone with cyclo-pentadiene under boron trifluoride etherate catalysis produces the exo-adduct (194). ... 

Partially resolved cyclonona-1,2-diene has also been treated with t-butylcyanoketen to give optically active products. The major product (329) has a cw-relationship between the t-butyl group and the adjacent CH2, which again suggests a synchronous reaction via a 2s (allene) and 2a (keten) addition (330).- ... 

In the reaction of the same ketene with optically active 1,3-dimethylallene, optically active and racemic [2+2] cycloadducts are obtained, indicating that the reaction is a stepwise process. Optically active 1,3-diphenylallene reacts with t-butylcyanoketene to give cyclobutanones with the -configuration. In contrast, reaction of the same ketene with optically active 1,2-cyclononadiene gives a 2 3 mixture of the E Z stereoisomeric cycloadducts. ... 

The [2+2] cycloaddition reaction of ketenes with vinyl ethers and thioethers also occurs very readily. Even allyl ethers undergo this reaction. The reaction of diphenylketene with vinyl ethers is stereospecific, indicating a concerted one-step process . Also, dimethylketene and -MeOCH=CHMe affords a cycloadduct in which the alkene stereochemistry is maintained In contrast, the [2+2] cycloadduct obtained fl om t-butylcyanoketene and CH2=CHOEt or CH2=CHOAc did not give a 100% stereoselectivity and linear products are often also obtained, indicating the formation of a switter ionic intermediate. The latter are detected in the reaction of bis(trifluoromethyl)ketene with ethyl vinyl ether (see the General Introduction ). The initial reaction occurs across the C=0 bond of the ketene, which rearranges via switter ionic intermediates to form the cyclobutanone reaction product . 

The additions of t-butylcyanoketen to cis- and trans-cyclo-octene have been shown to be stereospecific, and the stereochemistry of the addition of dichloroketen to norbomadiene has been confirmed, by chemical degradation of the adduct, to be entirely consistent with a [ 2 -I- 2j] concerted mechanism. The use of lanthanide shift reagents makes the configurational assignment of such adducts by nm.r. spectroscopy less ambiguous. ... 

Addition of t-butylcyanoketen to cis-cyclo-octene is also discussed, see ref. 159. 

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