Allenes Cope rearrangement

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

Thermodynamic and kinetic data for Cope rearrangements leading to allenes have been measured [511]. For preparatively useful yields the equilibrium can be shifted to the allene, for example by the classical use of allylic alcohols leading to carbonyl compounds [512],... 

Much experimental and theoretical work has been performed with the two allenes 1,2,6-heptatriene (32) and 1,2,6,7-octatetraene (34). Thermal isomerization of 32 leads to 3-methylene-l,5-hexadiene (346), a process that at first sight looks like a typical Cope rearrangement. However, trapping experiments with either oxygen or sulfur dioxide have shown that at least half of the rearrangement passes through the diradical 345 (Scheme 5.52) [144],... 

In addition to a-allenic a-amino acids, the corresponding allenic derivatives of y-aminobutyric acid (GABA) have also been synthesized as potential inhibitors of the pyridoxal phosphate-dependent enzyme GABA-aminotransferase (Scheme 18.49) [131,138-142]. The synthesis of y-allenyl-GABA (152) and its methylated derivatives was accomplished through Crabbe reaction [131], aza-Cope rearrangement [138] and lactam allenylation [139], whereas the fluoroallene 153 was prepared by SN2 -reduc-tion of a propargylic chloride [141]. 

Furthermore, the oxy-Cope rearrangement of allenic cycloheptane alcohol 47 (NaOEt, THF, 20 °C, 12 h, 80%) gave rise only to ring-enlarged product 48 without transannular cyclization (equation 15)29. The above transformations can be rationalized either by the fragmentation-recombination mechanism or by a concerted oxy-Cope mechanism29. 

CASSCF calculations.7 Treatment of substituted 5-triethylsilyloxyhexa-l,2,5-triene derivatives with catalytic amounts of W(CO)6 was found to give the products of formal Cope rearrangement 2-triethylsilyloxyhex-l-en-5-ynes. The mechanism is believed to involve 6-endo attack by the silyl enol ether on the tungsten-activated allene, followed by ring opening with simultaneous loss of W(CO)s.8... 

The preparation of other y-allenic amino acids by the aza-Cope rearrangement is described (1). 

Castelhano, A. L., Krantz, A. Allenic amino acids. 1. Synthesis of y-aiienic GABA by a novei aza-Cope rearrangement. J. Am. Chem. Soc. 1984, 106, 1877-1879. 

A carbanion -accelerated hetero-Cope rearrangement was reported by Blechert 170). Reaction of the sodium salt of the hydroxamic acid (202) with the allene sulfone 203 led at 0 °C within 10 min to 206. Undoubtedly, the carbanions 204 and 205 are the important intermediates in this transformation. A carbanion -accelerated... 

Anionic oxy-Cope rearrangement of allene alcohol 40 under standard conditions (KH, 18-crown-6, I2, THF, 2 h at 20 °C, in 40% yield) gave rise to the carboxylic acid 41 (equation Treatment of 40 with a catalytic amount of NaOEt (THF, 20 °C, 12 h)... 

---