Dimethylbarbiturate, allyl transfer

Cleavage of allylamines is rather difficult. However, unexpectedly the allylamine 340 is deprotected by allyl transfer to dimethylbarbiturate (341) [158] or to 2-thiobenzoic acid using DPPB as a ligand [159]. The 2-thiobenzoic acid method has been applied to indole synthesis [160], Allylamines are also cleaved with Pd/C in EtOH in the presence of methanesulfonic acid [161] or with Ni(dppp)Cl2 [162], Thus diallylamine (339) can be regarded as protected ammonia. 

The CH-acid dimedone (pKa = 5.2) was chosen as the allyl transfer reagent in order to facilitate separation from the Liberated amine. When used in 7- to 8-fold excess, it also protonates the free amino group (pKa = 8) to such an extent that the Latter cannot function as an allyl acceptor. However, if the reaction times are long, an enamine may form with the dimedone in which case dimethylbarbituric acid (pKfl = 4.7) can be used instead.184185 It is also noteworthy that the thioether function of methionine is not detrimental to the activity of the Pd catalyst. A mechanism for the Pd 0)-cata lysed deprotection of Aloe derivatives using dimedone or NtN --dimethylbarbituric acid as scavengers is given in Scheme 8.75. 

Allyl-based N"-protection, stable to base and moderate acid, can be achieved with the allyl group itself,or with an allyloxymethyl (Alom) group (Scheme 7).P4,34,5S] githej- gagg cleavage by Pd(0)-catalyzed allyl transfer to A,A -dimethylbarbituric acid l (which probably also scavenges the formaldehyde released from the latter) proceeds well, provided care is taken over the integrity of the catalytic complex. 

Another method for deallylation is the Pd-catalyzed transfer of the allyl group to reactive nucleophiles. Sodium 2-ethylhexanoate [141], morpholine [142], dimedone [143] and AA-dimethylbarbituric acid [144] are used as allyl scavengers. It is claimed that benzenesulfinic acid 331 or sodium toluenesulfinate are the best allyl scavengers [145]. 

The enolate formed in this process then condenses irreversibly with the r -allylpalladium species. At the outset, dimedone and A,A-dimethylbarbituric acid had been more specifically devisedP for the deprotection of allyl carbamates in solution or in solid-phaseP chemistry in which case the enolate species also forms by proton transfer from the (3-dicarbonyl allyl scavenger to the liberated amine but since then it has also been used for the deprotection of allyl carboxylic esters (see Section 2.2.1.1.3).P 1... 

---