Allylic functions

As with vinyl compounds, the outcome of an allylic hydrogenation is the resultant of two competing reactions. 

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Both conjugated and isolated dienes are usually accessible by extension of the methods suitable for mono-olefins. Allylic functions for ehmination may be produced by double bond introduction a to a functional group or by allylic substitution of an olefin. Both reduction of allylic systems to mono-olefins and elimination to give dienes, may involve rearrangement. 

The order for decreasing hydrogenolysis of allylic functions appears to be Pd > Pt Rh = Ru. Hydrogenolysis of allylic functions over palladium may... 

Dan and Henbesi (ii) demonslraied ihai ihe amount of salts remaining in platinum oxide catalysts had an important bearing on the hydrogenation-hydrogenolysis ratio of allylic functions. Hydrogenolysis is inhibited by salts remaining from the catalyst preparation or by salts such as sodium nitrite, cyanide, or hydroxide added later. 

Steric hindrance around an allylic function will diminish its hydrogenolysis as access of the function to the catalyst surface is impeded. Reduction of 5-methylthebaine (32) proceeds smoothly over Pd-on-C in ethanol at 1 atm to afford 5-methyldihydrothebaine (33), whereas reduction of thebaine itself is less clean and gives dihydrothebainol, dihydrothebainone, and dihydro-thebaine (/b). 

Nanaomycin A 103 and deoxyfrenolicin 108 are members of a group of naphthoquinone antibiotics based on the isochroman skeleton. The therapeutic potential of these natural products has attracted considerable attention, and different approaches towards their synthesis have been reported [65,66]. The key step in the total synthesis of racemic nanaomycin A 103 is the chemo-and regioselective benzannulation reaction of carbene complex 101 and allylacety-lene 100 to give allyl-substituted naphthoquinone 102 after oxidative workup in 52% yield [65] (Scheme 47). The allyl functionality is crucial for a subsequent intramolecular alkoxycarbonylation to build up the isochroman structure. However, modest yields and the long sequence required to introduce the... 

A combination of NMR spectroscopy and MALDI-TOF MS is commonly employed in our laboratory for the characterisation of PPG polymers. Analysis of di-hydroxyl end-capped PPG (18) is initially described. The [H NMR spectrum [54] can be used to confirm the backbone structure of the polymer, as can be seen in Figure 22 (a and b are from the backbone of the polymer, with c from the methyl side chains). Peaks of low intensity, downfield of those from the backbone of the polymer, in the HNMR spectrum may be used to identify and quantify the allyl functionality in the polymer [55]. These resonances (d, e and f) are... 

Other allylic functionalizations have been reported. The catalytic heteroannulation of allylic benzylamines leads to heterocyclic products, whereas a stoichiometric version of this reaction leads to both allylic and aryl functionalization (Equation (189)).1S3 1S3a... 

Geraniol is an interesting substrate that presents a hindered terminal C = C bond and an allylic function. The carbonylation reaction gives, beside the acid, the six-membered lactone and not the butyrolactone that means that initially an isomerization step occurred as depicted in Scheme 13. The most adapted ligand to obtain good selectivity in this lactone is dppb [98]. In the case of perillyl alcohol such an isomerization does not occur and the C5-lactone is produced (Scheme 14) [99]. 

Ligand 50 reacts with the metal carbonyls as it is well known for the heteroscorpionate ligand Hbdmpza. This is also observed with the allyl-functionalized ligand 48 (Scheme 28). 

The concept of establishing an optimized platform to construct the organo-silica monolith has been demonstrated using allyl functionality [95], although any other... 

In contrast to reactions with vinyl epoxides and palladium catalysts, the reactions with rhodium retain the stereochemistry of the alkene fragment during the reaction [20]. This is illustrated by the reactions of trans-37a/h and cis-37a/b, which give only one product possessing the same olefin geometry as the starting epoxides (Eqs. 4 and 5). The retention of olefin stereochemisty has also been documented in allylic functionalizations with iridium catalysts, indicating that similar modes of action may be present [21, 22]. 

The insertion of allenes in the palladium-carbon a bond of cyclopalladated pyridine derivative 295 (cf. 00CRV3067) affords stable, isolable (ry -allyl) palladium complexes (e.g., 296) (03JOM(687)313). The ideally located imine unit when depalladated reacts selectively with the allyl functionality to yield methylene morphanthridizinium salts 297a-c. 

Allylic acetates are usually prepared by esterification from allylic alcohols. However, the corresponding alcohols are often only accessible by the fairly expensive hydride reduction of carbonyl compounds. Consequently, direct allylic functionalization of easily available olefins has been intensively investigated. Most of these reactions involve peroxides or a variety of metal salts.However, serious drawbacks of these reactions, (e.g. toxicity of some metals, stoichiometric reaction conditions, or nongenerality) may be responsible for their infrequent use for the construction of allylic alcohols or acetates. 

Allylic acetoxylation with palladium(II) salts is well known however, no selective and catalytic conditions have been described for the transformation of an unsubstituted olefin. In the present system use is made of the ability of palladium acetate to give allylic functionalization (most probably via a palladium-x-allyl complex) and to be easily regenerated by a co-oxidant (the combination of benzoquinone-manganese dioxide). In contrast... 

The third intra-pair reaction to be discussed involves bond formation between radical anion and cation without intervening transfer both singlet and triplet radical ion pairs can couple. For example, the bifunctional radical cation 24 generates two chloranil adducts, most likely via zwitterions (e.g., 74 and 75 ), initiated by forming a C O bond. The CIDNP results indicate that 74 and 75 are formed from a singlet radical ion pair. Adduct 75 is a minor product, as the major spin density of 24 + is located in the allyl function which, therefore, is expected to be the principal site of coupling. 

Synthesis and Characterization of Allyl-Functionalized Octa-Arm PIB Stars... 

The synthesis of allyl-functionalized octa-arm stars was achieved by a one-pot two-step procedure [64]. Scheme 6 shows the steps involved in the synthe-... 

The stars were characterized by GPC (LLS) and NMR spectroscopy. The RI traces (see Fig. 5) showed the formation of monomodal narrow dispersity stars. Molecular characteristics of two representative samples are summarized in Table 2. According to NMR evidence (Fig. 6), the arms were quantitatively functionalized with allyl groups. The end-functionality of the stars was calculated by comparing the integrated peak area of the core protons [aromatic (5= 6.82 ppm), -CH2- (6=4.0 ppm)] and chain end allyl protons (-CH2- (5=2.0 ppm), -CH= (5=5.8 ppm), =CH2 (5=5.1 ppm), and was found to be 8.1, after correcting for the presence of -10% linear contaminant. Quantitative allyl-functionalization is direct proof for the formation of the octa-arm stars. 

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