Allene reaction + alkyl radicals

Brown proposed a mechanism where the enolate radical resulting from the radical addition reacts with the trialkylborane to give a boron enolate and a new alkyl radical that can propagate the chain (Scheme 24) [61]. The formation of the intermediate boron enolate was confirmed by H NMR spectroscopy [66,67]. The role of water present in the system is to hydrolyze the boron enolate and to prevent its degradation by undesired free-radical processes. This hydrolysis step is essential when alkynones [68] and acrylonitrile [58] are used as radical traps since the resulting allenes or keteneimines respectively, react readily with radical species. Maillard and Walton have shown by nB NMR, ll NMR und IR spectroscopy, that tri-ethylborane does complex methyl vinyl ketone, acrolein and 3-methylbut-3-en-2-one. They proposed that the reaction of triethylborane with these traps involves complexation of the trap by the Lewis acidic borane prior to conjugate addition [69]. 

A complicating feature proved to be the fact that some alkyl radicals also induced the blue color, although it appeared possible to allow for this by varying the distance of the molybdenum oxide layer from the front face of the reaction cell. The silica surfaces were at first thought to be perfect reflectors for H atoms, but corrections were applied subsequently by Allen et al. (2) when this was found not to be strictly correct. 

At 750 K, k 2x 10 cm molecule s for these reactions, compared with ksA 2 X for the analogous reaction of alkyl radicals, but is a factor of about 10" greater than for allene formation from CH2CHCH2. 

Reaction Wave function Alkene, photocycloaddition, 366. 420-23 addition to benzene. 420 substituted, 432 Alkyl amines, tertiary. 466 Alkyl aryl ketones, 399, 402 Alkylethylenes, 420 Alkylidenecyclopropene. 57-58 N-Alkylimine, 375 Alkyl iodide, 471 Alkyl methyl ketones. 383-84 Alkyl radical, 380 Allene. 416 Ally radical. 102, 460 Allyl resonance, 461... 

Aryl- and alkylsulfonyl radicals have been generated from the corresponding iodides and added to, e.g., propadiene (la), enantiomerically enriched (P)-(+)-propa-2,3-diene [(P)-(lc)] and (P)-(-)-cyclonona-l,2-diene [(P)-(lk)] [47]. Diaddition of sulfo-nyl radicals may compete considerably with the monoaddition [48,49]. Also, products of diiodination have been purified from likewise obtained reaction mixtures, which points to a more complex reactivity pattern of these substrates towards cumulated Jt-bonds. An analysis of regioselectivities of arylsulfonyl radical addition to allenes is in agreement with the familiar trend that a-addition occurs in propadiene (la), whereas alkyl-substitution at the cumulated Jt-bond is associated with a marked increase in formation of /3-addition products (Scheme 11.7). 

Dick turned up some interesting chemistry of caprolactam and its O-alkyl imino ethers. He and collaborators went on to explore the chemistry of allene, for example, its reactions with acetylene, carbon monoxide, and tetrafluo-roethylene. He did extensive work on the chemistry of cyclooctatetraene and of ferrocene. In the cyanocarbon area he collaborated on studies of the anion radical of tetracyanoethylene, that is, tetracyanoethylene bearing an extra electron. He was author or coauthor of 45 papers and 16 U.S. Patents that came out of the Central Research Department. 

Terminal allenes. In the presence of a radical initiator, alkyl bromides or iodides react with 1 to provide allenes in moderate to good yield. The synthesis of a natural amino acid (3) from 2 by this reaction proceeds without racemization. 

The reaction of triethyl phosphite with maleic anhydride has been re-investi-gated. The product is not the dimeric phosphorane (11), but the ylid (12). Reactions of alkyl or aryl diphenylphosphinites with 1-nitro-l-alkenes have been studied in detail. Radical ion-pairs are involved as shown by e.s.r. and P CIDNP experiments. Allene may be hydrophosphorylated in a radical-initiated process to give allyl- and isopropenyl-phosphonates. A phosphorane (14) has been observed during the reaction of ethyl diethylthiophosphinite (13) with methacrylic acid. ... 

Tin alkynes rearrange to yield allene products in much the same way as do lithium alkynes, except that the reaction involves a radical mechanism. It is very similar to the reaction of allyl stannanes with alkyl halides, which substitutes the allyl group. Similar reactions are reported for allyl derivatives of cobalt, rhodium and iridium, but this work has not been extended to alkyne derivatives. 

Radical reactions ofallenes are largely unknown [57]. Recently, Ma and coworkers [58] developed the first example of a radical addition/cyclization reaction of allene-enes in an alkene-to-allene manner (Scheme 5.32). The reaction of the allene 146 and polyfluoro alkyl iodide 147 with zinc powder as a cheap, readily available, efficient, and mild initiator led to 148 in moderate to good yields (53-86%), which on treatment with TBAF (tetra-n-butylammonium fluoride) gave the allene 149. 

The kinetic competition between C -C and C -C cyclization of enyne-allene 3.535 is determined by the nature of substituents in the acetylene branch. Hydrogen or n-alkyl substituents lead to the domination of the C -C reaction to form naphthalene derivative 3.538, while substituents stabilizing the radical and bulky substituents at QJ favor the C -C process and the formation of cyclised compounds 3.539 and 3.540 (the formal product of the Diels-Alder reaction with = Ph) (Scheme 3.35) [269aj. 

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