Alkenes vinylic oxidation

Triple bonds can be monohydroborated to give vinylic boranes, which can be reduced with carboxylic acids to cis alkenes or oxidized and hydrolyzed to aldehydes or ketones. Terminal alkynes give aldehydes by this method, in contrast to the mercuric or acid-catalyzed addition of water discussed at 15-4. However, terminal alkynes give vinylic boranes (and hence aldehydes) only when treated with a hindered borane such as 47, 48, or catecholborane (p. 798)," or with BHBr2—SMe2. The reaction between terminal alkynes and BH3 produces 1,1-... 

With the exception of the parent compounds, where the Michael adducts are isolated, acrylic esters [see, e.g. 6,7,31,105,111 ] and nitriles [6,7], and vinyl ketones [26, 113, 115] generally yield the cyclopropanes (Table 7.6) under the standard Makosza conditions with chloroform. Mesityl oxide produces a trichlorocyclopropy-lpropyne in low yield (10%) [7]. When there is no substituent, other than the electron-withdrawing group at the a-position of the alkene, further reaction occurs with the trichloromethyl anion to produce spiro systems (35-48%) (Scheme 7.12) [7, 31]. Under analogous conditions, similar spiro systems are formed with a,p-unsaturated steroidal ketones [39]. Generally, bromoform produces cyclo adducts with all alkenes. Vinyl sulphones are converted into the dichlorocyclopropane derivatives either directly or via the base-catalysed cyclization of intermediate trichloromethyl deriva-... 

Alkenes can be transformed to carbonyl compounds through the oxidation of the vinylic carbon atom. A special case of vinylic oxidation is acetoxylation of alkenes and dienes. 

Palladium-catalyzed allylic oxidations, in contrast, are synthetically useful reactions. Palladium compounds are known to give rise to carbonyl compounds or products of vinylic oxidation via nucleophilic attack on a palladium alkene complex followed by p-hydride elimination (Scheme 9.16, path a see also Section 9.2.4). Allylic oxidation, however, can be expected if C—H bond cleavage precedes nucleophilic attack 694 A poorly coordinating weak base, for instance, may remove a proton, allowing the formation of a palladium rr-allyl complex intermediate (89, path by694-696 Under such conditions, oxidative allylic substitution can compete... 

Vinylic Oxidation. Various alkenes are oxidized to the corresponding ketones using fm-BuOOH in the presence of Pd(II) catalysts bearing perfluorinated ligands in a fluorous biphasic system.1309 The catalyst can be reused, but progressively longer reaction times are required. 

Vinyl sulfones.1 The photoadducts of 1 to alkenes on oxidation (H202) undergo selenoxide elimination to provide vinyl sulfones in 60-95% yield. Example ... 

Most of the reactions listed in Table 6 involve prior activation of the substrate by coordination to palladium in the form of a v-, a 77-ally lie, a 77-benzylic, or an alkyl or aryl complex. Once coordinated to the metal, the substrate becomes an electron acceptor and can react with a variety of different nucleophiles. The addition of nucleophiles (Nu) to the coordinated substrate may occur in two different ways, as shown by Scheme 9 for 7r-alkene complexes 397"399 (a) external attack leading to trans addition of palladium and nucleophile across the 77-system (path A) or (b) internal addition of the coordinated nucleophile to the complexed alkene resulting in cis addition of palladium and nucleophile to the double bond. The cis and trans adducts (120) and (121) may then undergo /3-hydride elimination (/3-H), producing the vinylic oxidation product... 

Some terminal alkenes are oxidized to aldehydes depending on their structure. As described before, acrylonitrile and acrylate are oxidized to acetals of aldehydes in alcohols or ethylene glycol.Selective oxidation of terminal carbons in 4-hydroxy-1-alkenes (18) gave the five-membered hemiacetals (19), which can be converted to y-butyrolactones by PCC oxidation (Scheme 4). Formation of a tricyclic six-membered hemiacetal (62%) from a 5-hydroxy-1-alkene system was used for the synthesis of rosa-ramicin. Formation of aldehydes as a major product from terminal alkenes using (MeCN)2Pd(Cl)(N02) and CuCU in r-butyl alcohol under selected conditions was reported. The vinyl group in the -lactam was oxidized mainly to the aldehyde as shown below (equation 12). ... 

See [6]. The following reaction types have been listed (a) Geometric isomerization of alkenes (b) Allylic [1,3] hydrogen shift (c) Cycloaddition of alkenes. Dimerization, Tri-merization. Polymerization (d) Skeletal rearrangments of alkenes and methathesis (e) Hydrogenation of alkenes (f) Additions to alkenes (g) Additions to C = X (h) Aliphatic substitutions (i) Aromatic substitution (j) Vinyl substitution (k) Oxidation of alkenes (1) Oxidation of alcohols (m) Oxidation of arenes (n) Oxidative decarboxylation (o) Oxidation of amines (p) Oxidation of vinylsilanes and sulfides (q) Oxidation of benzal-dehyde (r) Dehydrogenations. 

A typical example of an application of ozonolysis in synthesis is the cleavage of the alkene unit in 351 to give aldehyde 352, with loss of formaldehyde, a part of Smith s synthesis of (+)-thiazinotrienomycin In this case, the reductive workup used triphenylphosphine. Ozonolysis of 353 gave aldehyde 354, an intermediate in Corey s synthesis of the Cecropia Moth Juvenile hormone, via a reductive workup with DMS. Note that the electron rich vinyl ether moiety reacted in preference to the simple alkene moiety. The ozonolysis product of the methyl vinyl ether was a methyl ester. This method is particularly useful for the preparation of protected acids. In general, electron rich alkenes are oxidized faster than electron-poor alkenes. 

Agouri and co-workers prepared crystalline-amorphous copolymers of the alkenic-vinylic type. They first initiated the crystalline sequence in the presence of Et2Zn or Et2Cd which are good transfer agents and then switched to a radical initiation by activating the C-metal bond through oxidation as shown in Scheme 9. 

Oxidation of ethylene in alcohol with PdCl2 in the presence of a base gives an acetal and vinyl ether[106,107], The reaction of alkenes with alcohols mediated by PdCl2 affords acetals 64 as major products and vinyl ethers 65 as minor products. No deuterium incorporation was observed in the acetal formed from ethylene and MeOD, indicating that hydride shift takes place and the acetal is not formed by the addition of methanol to methyl vinyl etherjlOS], The reaction can be carried out catalytically using CuClj under oxygen[28]. 

The 7, i5-unsaturated alcohol 99 is cyclized to 2-vinyl-5-phenyltetrahydro-furan (100) by exo cyclization in aqueous alcohol[124]. On the other hand, the dihydropyran 101 is formed by endo cyclization from a 7, (5-unsaturated alcohol substituted by two methyl groups at the i5-position. The direction of elimination of /3-hydrogen to give either enol ethers or allylic ethers can be controlled by using DMSO as a solvent and utilized in the synthesis of the tetronomycin precursor 102[125], The oxidation of the optically active 3-alkene-l,2-diol 103 affords the 2,5-dihydrofuran 104 in high ee. It should be noted that /3-OH is eliminated rather than /3-H at the end of the reac-tion[126]. 

The best procedures for 3-vinylation or 3-arylation of the indole ring involve palladium intermediates. Vinylations can be done by Heck reactions starting with 3-halo or 3-sulfonyloxyindoles. Under the standard conditions the active catalyst is a Pd(0) species which reacts with the indole by oxidative addition. A major con.sideration is the stability of the 3-halo or 3-sulfonyloxyindoles and usually an EW substituent is required on nitrogen. The range of alkenes which have been used successfully is quite broad and includes examples with both ER and EW substituents. Examples are given in Table 11.3. An alkene which has received special attention is methyl a-acetamidoacrylate which is useful for introduction of the tryptophan side-chain. This reaction will be discussed further in Chapter 13. 

Uses. Magnesium alkyls are used as polymerization catalysts for alpha-alkenes and dienes, such as the polymerization of ethylene (qv), and in combination with aluminum alkyls and the transition-metal haUdes (16—18). Magnesium alkyls have been used in conjunction with other compounds in the polymerization of alkene oxides, alkene sulfides, acrylonitrile (qv), and polar vinyl monomers (19—22). Magnesium alkyls can be used as a Hquid detergents (23). Also, magnesium alkyls have been used as fuel additives and for the suppression of soot in combustion of residual furnace oil (24). 

Compounds considered carcinogenic that may be present in air emissions include benzene, butadiene, 1,2-dichloroethane, and vinyl chloride. A typical naphtha cracker at a petrochemical complex may release annually about 2,500 metric tons of alkenes, such as propylenes and ethylene, in producing 500,000 metric tons of ethylene. Boilers, process heaters, flares, and other process equipment (which in some cases may include catalyst regenerators) are responsible for the emission of PM (particulate matter), carbon monoxide, nitrogen oxides (200 tpy), based on 500,000 tpy of ethylene capacity, and sulfur oxides (600 tpy). 

To control the stereochemistry of 1,3-dipolar cycloaddidon reacdons, chiral auxiliaries are introduced into either the dipole-part or dipolarophile A recent monograph covers this topic extensively ° therefore, only typical examples are presented here. Alkenes employed in asymmetric 1,3-cycloaddidon can be divided into three main groups (1) chiral allyhc alcohols, f2 chiral amines, and Hi chiral vinyl sulfoxides or vinylphosphine oxides. 

The chemistry of alkynes is dominated by electrophilic addition reactions, similar to those of alkenes. Alkynes react with HBr and HC1 to yield vinylic halides and with Br2 and Cl2 to yield 1,2-dihalides (vicinal dihalides). Alkynes can be hydrated by reaction with aqueous sulfuric acid in the presence of mercury(ll) catalyst. The reaction leads to an intermediate enol that immediately isomerizes to yield a ketone tautomer. Since the addition reaction occurs with Markovnikov regiochemistry, a methyl ketone is produced from a terminal alkyne. Alternatively, hydroboration/oxidation of a terminal alkyne yields an aldehyde. 

Oxidative cleavage of an alkene with Klvln04 gives a carboxylic acid if the alkene has at least one vinylic hydrogen (Section 7.9). 

In an extension of this work, the Shibasaki group developed the novel transformation 48—>51 shown in Scheme 10.25c To rationalize this interesting structural change, it was proposed that oxidative addition of the vinyl triflate moiety in 48 to an asymmetric palladium ) catalyst generated under the indicated conditions affords the 16-electron Pd+ complex 49. Since the weakly bound triflate ligand can easily dissociate from the metal center, a silver salt is not needed. Insertion of the coordinated alkene into the vinyl C-Pd bond then affords a transitory 7t-allylpalladium complex 50 which is captured in a regio- and stereocontrolled fashion by acetate ion to give the optically active bicyclic diene 51 in 80% ee (89% yield). This catalytic asymmetric synthesis by a Heck cyclization/ anion capture process is the first of its kind. 

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