Radical reactions allenylations

Allenylcobaloximes, e.g. 26, react with bromotrichloromethane, carbon tetrachloride, trichloroacetonitrile, methyl trichloroacetate and bromoform to afford functionalized terminal alkynes in synthetically useful yields (Scheme 11.10). The nature of the products formed in this transformation points to a y-specific attack of polyhaloethyl radicals to the allenyl group, with either a concerted or a stepwise formation of coba-loxime(II) 27 and the substituted alkyne [62, 63]. Cobalt(II) radical 27 abstracts a bromine atom (from BrCCl3) or a chlorine atom (e.g. from C13CCN), which leads to a regeneration of the chain-carrying radical. It is worth mentioning that the reverse reaction, i.e. the addition of alkyl radicals to stannylmethyl-substituted alkynes, has been applied in the synthesis of, e.g., allenyl-substituted thymidine derivatives [64],... 

Additional routes to a-allenic-a-amino acids were described more recently and utilize radical [136] or transition metal-catalyzed [137] allenylations, in addition to copper-promoted Michael additions [15b]. Thus, sterically demanding amino acid derivatives (e.g. 151) are accessible via a 1,6-addition reaction of lithium di-tert-butyl-cyanocuprate with acceptor-substituted enynes of type 150 (Scheme 18.48). 

Usanov and Yamamoto recently found that catalytic amounts of Co(TPP) 367 led to a dramatic rate acceleration of Nozaki-Kishi-Hiyama reactions catalyzed by chromium complex 368 (Fig. 101) [460]. The authors attributed the rate enhancement to initial reduction of 367 to a Co(I) complex. The latter is able to undergo an Sn2 substitution at propargyl bromide 366 giving an allenylCo(ffl) species. It was proposed that its homolysis leads to allenyl radical 366A, which couples to Cr(II) complex 368. The resulting allenyl Cr(III) complex adds in an SN2 process... 

The reaction mechanism of photocyclization of aryl vinyl ethers was derived from results obtained by means of flash photolysis. The ground state intermediate rearranges by mono-or bi-molecular 1,4-hydrogen shifts to yield the products (Scheme 62) (81JOC978). The photocyclization of 2-aryloxyenones was used in the total synthesis of ( )-lycoramine (77JA8065). The formation of dihydrobenzo[6 ]furans by radical cyclization from o-allenyl-oxyarenediazonium salts with tri-n-butyltin(IV) hydride was successful (81CC136). 

The other reactions which are observed in this system are due to the CsHs and CHs- radicals. The duality in the structure of the CsHs-radical was pointed out by Collin and Lossing. They concluded that the radical behaved more like CH=C—CH2 (propargyl) than CH2=C=CH (allenyl) at low pressure since it reacted with methyl to give 1-butyne rather than 1,2-butadiene. 

In the presence of the radical substitution promoter [Fe2(CO)4()u,-SR)i(PPh3)2], tertiary amines react with transition metal carbonyl clusters under exceptionally mild conditions. Modified allenyl and allylic clusters similar to those described earlier have been isolated from such reactions. Two distinct types of products have been isolated (i) those involving the elimination of an alkyl group and (ii) those involving C-C coupling reactions. The product formation described in Scheme 31 was preceded by amine coordination, C-H activation, C-N cleavage, carbene-amine complex formation, transamination, and C-C coupling (6Ia.h)- Such processes are of interest in the area of hydrodenitrification (6/c). 

Although propargyl recombination is not an elementary reaction and most probably passes through the linear C6H6 compound (Miller, 2001), it can be conveniently lumped into a single equivalent reaction (Appel et al., 2000). Similarly, C3H3 can also react with methyl-allenyl (k-C4H5) to form toluene or H and benzyl radical ... 

In 1986 Inanaga et al. found that allylic or propargylic acetates could be reduced in THF by diiodosamarium in the presence of a catalytic amount of a Pd(0) complex and 1 equiv of 2-propanol [160,161]. Some examples are indicated in Scheme 60. The mechanism of the reaction likely involves jt-allyl (or a-allenyl) palladium intermediates which are reduced first to radicals and then to carban-ions (see Scheme 60). A final protonation by the alcohol generates the products. 

Also, as shown in this example and similar to the Diels-Alder reaction, the normal or traditional mode of reaction is for the lowest unoccupied molecular orbital (LUMO) of the electron-deficient enophile to react with the highest occupied molecular orbital (HOMO) of the electron-rich ene. However, there are some carbon and hydrogen containing systems that have been proposed to proceed via radical or stepwise pathways. For example, allenyl alkynes 7 and 8 were thermally cyclized using PhMe under relatively mild conditions to provide 10 and 11 in good yield. The authors suggested a radical mechanism for this transformation via the fulvene biradical intermediate 9. ... 

The propargyl-allenyl free radical (236) has been studied by e.s.r. by reaction of methylacetylene and allene with photochemically generated t-butoxyl radicals. 1,1-Dimethylallene reacts by abstraction of either an allenyl... 

The reaction of lithium diisopropylamide with propargyl acetate 3.559 followed by treatment of the enolate formed with trimethylchlorosilane at —78°C affords ketene acetal 3.561. After heating the reaction mixture to 40°C, the intermediate vinylpropargyl ether 3.560 undergoes the Claisen rearrangement to produce silyl ester 3.561, which after hydrolysis forms allenyl-acetic acid 3.562 with a 50% overall yield (Scheme 3.42) [281]. This reaction is of interest as a synthetic strategy for obtaining the benzoannelated enyne-allene that by radical cyclization forms polycyclic aromatic compounds with an embedded fluorene section [281]. (Scheme 3.43)... 

---