Alkenyl allyl carbonates

Enones (cf. 11, 391-392). PdlOAc). is the preferred catalyst for dehydrogenation of alkenyl allyl carbonates to give a.p-enals or -enones. Use of a phosphine ligand can catalyze allylation/protonation in addition to p-elimination. 

Allyl carbonates with a sp -substituent (alkenyl, aryl) are privileged substrates regioselectivities of b/1 >98 2 are obtained, using ligand LI in conjunction with procedures (d) or (e) or ligand L2 in conjunction with procedures (b), (d) or (e). 

Rhodium(ll)-catalyzed etherification of an allylic carbonate with enantiopure alkenyl alcohols 246 followed by RCM proceeds with excellent diastereoselectivity to afford syn-247 or /f-3,6-dihydropyrans 248 in high yield (Scheme 68) <2004AGE4788>. This methodology can be extended to the preparation of chiral 2,3,6-trisubstituted 3,6-dihydropyrans <2004JA8642>. 

For the aziridination of 1,3-dienes, copper catalysis gave better yields of A-tosyl-2-alkenyl aziridines with 1,3-cyclooctadiene, 1,4-addition occurred exclusively (50%) [46]. Good results were also obtained on rhodium catalysed decomposition of PhI=NNs (Ns = p-nitrophenylsulphonyl) with some alkenes the aziridination was stereospecific, whereas with chiral catalysts asymmetric induction (up to 73% ee) was achieved. However, cyclohexene gave predominantly (70%) a product derived from nitrene insertion into an allylic carbon-hydrogen bond [47]. 

Preparation of 1,4-dienes can be affected by the palladium-catalyzed crosscoupling of alkenylsilanes with allyl carbonates or 1,3-butadiene monoxides (Scheme 6) [36,37] or alternatively by the reaction of allylsilanes with alkenyl triflates (Eq. 36) [32]. 

In a recent development, Castano and Echavarren [42] have described couplings of allyl carbonates with alkenyl (and aryl) tins Pd2(dba)3 is used as the catalyst. These reactions are... 

Alkenyl, allyl, and aryl halides undergo oxidative addition to Pd° complexes to form alkenyl-, allyl-, and aryl-palladium a-complexes which then react with carbon monoxide, alkenes and alkynes. 

Sequential Rh-catalysed etherification of the allylic carbonate using the Cu(I) alkoxide derived from the enantiomers of the alkenyl alcohols followed by a RCM occur with excellent regio- and enantiospecificity and lead to cis- and traws-disubstituted dihydropyrans <04AG(E)4788>. 

Reactions between hydrido complexes and unsaturated hydrocarbons generally proceed in two steps. The first step involves coordination of the hydrocarbon to form a 7r-complex followed by hydrido ligand migration and the formation of the product possessing metal-carbon bond. Dependent upon the hydrocarbon alkyl, alkenyl, allyl, or vinyl complexes may be formed ... 

Additionally, a rhodium-catalyzed silicon-assisted activation of a C-CN and C-Cl bond was reported in 2008. Under these catalytic conditions, carbon-cyano bonds in aryl, alkenyl, allyl, and benzyl cyanides bearing a variety of functional groups can be silylated. The observation of an enamine side product in the silylation of benzyl cyanides and related stoichiometric... 

A study has been reported regarding the ruthenium-catalysed reaction of benza-mides with alkynes, which yields ort/io-alkenylated derivatives. Here, the mechanism is likely to involve rate-limiting metalation, followed by alkyne insertion to form intermediates such as (63) which on protonolysis yield the alkenylated products. An allylic carbon-carbon double bond has also been used as a coordination site in palladium-catalysed alkenylation reactions, as shown in Scheme 3. Here measurement of kinetic isotope effects suggests that coordination of the palladium with the allylic double bond occurs before palladation to give (64). Insertion of the alkene into the carbon-palladium bond gives (65) and -hydride elimination " leads to the product... 

Aryl or alkenyl halides attack the central carbon of the allene system in the 2,3-butadien-l-ol 120 to form the 7r-allyl intermediate 121, which undergoes elimination reaction to afford the o,/3-unsaturated ketone 122 or aldehyde. The reaction proceeds smoothly in DMSO using dppe as a ligandflOl]. 

Allenes also react with aryl and alkenyl halides, or triflates, and the 7r-allyl-palladium intermediates are trapped with carbon nucleophiles. The formation of 283 with malonate is an example[186]. The steroid skeleton 287 has been constructed by two-step reactions of allene with the enol trillate 284, followed by trapping with 2-methyl-l,3-cyclopentanedione (285) to give 286[187]. The inter- and intramolecular reactions of dimethyl 2,3-butenylmalonate (288) with iodobenzene afford the 3-cyclopentenedicarboxylate 289 as a main product) 188]. 

Dienes and allylarcncs can be prepared by the Pd-catalyzcd coupling of allylic compounds with hard carbon nucleophiles derived from alkenyl and aryl compounds of main group metals. Allylic compounds with various leaving groups can be used. Some of them are unreactive with soft nucleophiles, but... 

The chiral information of stereogenic centers in the allyl moiety of the precursor is destroyed on deprotonation. While an i/3-bound ion pair with a planar carbon frame is a chiral compound, usually rapid racemization takes place by intra- or intermolecular migration of the cation from one face to the opposite one. The sole exceptions known at present are secondary 2-alkenyl carbamates with X = dialkylaminocarbonyloxy21, in which the cation is tied by the chelating ligand, see Section 1.3.3.3.1.2. 

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