Allyl carbonates specificity

The mechanism of this interconversion is known as apparent allyl rotation and results in a switching of the allylic termini in addition to the movement of the central allyl carbon from one side of the coordination plane to the other although, for situations with symmetrically substituted allyl units, only this latter change is noticeable As the name suggests, the process is non-trivial and, in reality, the mechanism is dependent on the specific conditions.1161 Nevertheless, in most cases, the interchange is rapid compared with the rate of nucleophilic attack and, therefore, the product outcome does not necessarily depend on the intermediate isomer ratio (Curtin-Hammett principle). 

The Tsuji-Trost reaction, more commonly applied to carbocyclic nucleoside synthesis, has been used in the synthesis of famciclovir 21 (Scheme 5) <2000T4589>. Reaction of 2-amino-6-chloropurine with allylic carbonate 22 in the presence of Pd2DBA3 and bis(diphenylphosphino)ethane (DPPE) gave a 1 1 ratio of 23 to 24 after 1 h (DBA = dibenzylideneacetone). However, a ratio of >95 5 in favor of the thermodynamically favored N-9 isomer 23 was subsequently obtained upon further stirring, highlighting the reversibility of the reaction in this specific instance. 

The termination reactions appear to be quantitative and specific for the reaction of saturated polymer molecules attached to aluminium and titanium [116], but applied to diene polymerizations the method is less satisfactory mainly because of the greater stability of allylic carbon-transition metal bonds. Polybutadiene has been labelled by terminating with tritiated methanol with the Cr(acac)3/AlEt3 catalyst [55], and similarly polyisoprene prepared with VCl3/AlEt3 [107]. Polybutadiene prepared with Til4/Al(i-Bu)3 has been labelled using C02 [115]. 

A feature specific to allyl protection is the possibility of introducing the allyl group indirectly via the allyl carbonate by Pd-mediated decarboxylation. This has... 

Rh-catalyzed domino reactions have been published during the past decade, such as rhodium-catalyzed hydroallylation of activated alkenes (Scheme 12.93). This was achieved by a one-pot operation of neutral components, an allylic carbonate 188, an a,P-unsaturated ketone or ester 187, and a hydrosilane, which are activated in a specific order by [Rh(cod)(P(OPh)3)2]OTf under almost neutral conditions. The products 189 are obtained in high yields, but the regioselectivity is strongly dependent on the nature of the allylic substrate [197]. 

When the cationic i7 -allylpalladiuin(II) complex is formed in the absence of nucleophiles, elimination of proton and conjugate diene formation ensues (see Sect V.2.5.1 for a specific coverage of eliminations). Formally this is a dehydration of allylic alcohols in neutral medium by prior conversion into allylic carbonate however, in some cases a tertiary base is added to accelerate proton elimination. Tsuji s group has reported the elimination in steroidal and related compounds. Different configurations at C-3 (A ring) afford different diene systems (Scheme 50). The combination of Pd(0Ac)2 and tributylphosphine is the precatalytic combination preferred by this group, which uses NMR to control the quality of the catalytic mixture. ... 

More specific cycfizations are shown in Scheme 56. Carbonate of hydroxyacetone is not an allyl carbonate, but it can give rise to an 17 -l-oxaallylpalladium complex, which reacts with rigid olefins to afford cyclopropylketones.f ° Monocarbonate of cis-butene-l,4"di i forms a zwitterionic r/ -allylpalladium complex, which reacts as a dipole with Ceo to afford a vinyltetrahydrofuran derivative of fullerene. A... 

When reacted with the allylacetoxy-compound (118) in the presence of a palladium catalyst, the alkenylalane (119) gives specifically (86%) the trans-product (120), in what is described as the first demonstration of nearly complete inversion at the participating allylic carbon centre in a Pd-catalysed allylation of a carbon nucleophile. ... 

The anion formed from a substituted t-butyldimethylsilyl cyanohydrin [Li+ RC(OTBS)CN ] is the nucleophile in the regio- and stereo-specific 5 2 reaction of tertiary allylic carbonates. A [Rh(COD)Cl]2-P(C-2,4-di-f-BuC6H3)3 ligand catalyst... 

In the Huckel theory of simple hydrocarbons, one assumes that the election density on a carbon atom and the order of bonds connected to it (which is an election density between atoms) are uninfluenced by election densities and bond orders elsewhere in the molecule. In PPP-SCF theory, exchange and electrostatic repulsion among electrons are specifically built into the method by including exchange and electrostatic terms in the elements of the F matrix. A simple example is the 1,3 element of the matrix for the allyl anion, which is zero in the Huckel method but is 1.44 eV due to election repulsion between the 1 and 3 carbon atoms in one implementation of the PPP-SCF method. 

Uses. There are about forty to fifty organic peroxides commercially available in more than seventy formulations designed for specific applications which include (1) initiators for vinyl monomer polymerizations, and copolymerizations of monomers such as vinyl chloride, ethylene, styrene, vinyl acetate, acrylics, fluoroolefms and buta-dienestyrene (2) curing agents for thermoset polyesters, styrenated alkyds and oils, silicone rubbers and poly allyl diglycol carbonates ... 

The allyl-resonance stabilized E- and Z-pent-l,3-dienyl-2-cations (22 and 23) are the smallest member of vinyl cations observed as persistent species in superacid solution 49 These are difficult to generate experimentally50 but structures with only five heavy atoms are suitable candidates for coupled cluster model calculations. A challenging task of quantum chemistry was to assign the 13C NMR spectrum of the mixture of isomers (Fig. 3), which exhibits pairs of signals of 22 and 23 which differ only by a few ppm, to the chemical shifts for the specific carbon atoms of the E- and Z-isomers, respectively. 

Also among the oxidants that add oxygen at carbon-carbon double bonds is singlet oxygen.121 For most alkenes, this reaction proceeds with the specific removal of an allylic hydrogen and shift of the double bond to provide an allylic hydroperoxide as the initial product. 

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