Alkenes into aldehydes

Acyl chromates are used for epoxidation [679] and for the conversion of alkenes into aldehydes [650]. 

Copper compounds catalyze an exceedingly varied array of reactions, hetereogeneously, homogeneously, in the vapor phase, in organic solvents and in aqueous solutions. Many of these reactions, particularly if in aqueous solutions, involve oxidation-reduction systems and a Cu -Cu11 redox cycle. Molecular oxygen can often be utilized as oxidant, e.g., in copper-catalyzed oxidations of ascorbic acid and in the Wacker process (page 798) for conversion of alkenes into aldehydes. 

Palladium catalysts are widely used in liquid phase aerobic oxidations, and numerous examples have been employed for large-scale chemical production (Scheme 8.1). Several industrially important examples are the focus ofdedicated chapters in this book Wacker and Wacker-type oxidation of alkenes into aldehydes, ketones, and acetals (Scheme 8.1a Chapters 9 and 11), 1,4-diacetoxylation of 1,3-butadiene (Scheme 8.1b Chapter 10), and oxidative esterification of methacrolein to methyl methacrylate (Scheme 8.1c Chapter 13). In this introductory chapter, we survey a number of other Pd-catalyzed oxidation reactions that have industrial significance, including acetoxylation of ethylene to vinyl acetate (Scheme 8. Id), oxidative carbonylation of alcohols to dialkyl oxalates and carbonates (Scheme 8.1e), and oxidative coupling of dimethyl phthalate to 3,3, 4,4 -tetramethyl biphenylcarboxy-late (Scheme 8.1f). 

The hydroformylation, or 0x0, process was introduced in 1938 and is the oldest homogeneous catalytic process in commercial use. It is used to convert terminal alkenes into aldehydes and other organic products, especially those having their carbon chain increased by one. Approximately 10 million tons of hydroformylation products are produced annually. The conversion of an alkene of formula R2C=CH2 into an aldehyde R2CH—CH2—CHO is outlined in Figure 14.18. ... 

The selectivity of a catalyst can be measured in terms of regioselectivity and chemoselectivity. Let s look at the typical transformation of alkenes into aldehydes in the presence of CO and H. This reaction is called hydroformylation and is presented in Scheme 6.1. 

Allylic carbonates are most reactive. Their carbonylation proceeds under mild conditions, namely at 50 C under 1-20 atm of CO. Facile exchange of CO2 with CO takes place[239]. The carbonylation of 2,7-octadienyl methyl carbonate (379) in MeOH affords the 3,8-nonadienoate 380 as expected, but carbonylation in AcOH produces the cyclized acid 381 and the bicyclic ketones 382 and 383 by the insertion of the internal alkene into Tr-allylpalladium before CO insertion[240] (see Section 2.11). The alkylidenesuccinate 385 is prepared in good yields by the carbonylation of the allylic carbonate 384 obtained by DABCO-mediated addition of aldehydes to acrylate. The E Z ratios are different depending on the substrates[241]. 

Usually, organoboranes are sensitive to oxygen. Simple trialkylboranes are spontaneously flammable in contact with air. Nevertheless, under carefully controlled conditions the reaction of organoboranes with oxygen can be used for the preparation of alcohols or alkyl hydroperoxides (228,229). Aldehydes are produced by oxidation of primary alkylboranes with pyridinium chi orochrom ate (188). Chromic acid at pH < 3 transforms secondary alkyl and cycloalkylboranes into ketones pyridinium chi orochrom ate can also be used (230,231). A convenient procedure for the direct conversion of terminal alkenes into carboxyUc acids employs hydroboration with dibromoborane—dimethyl sulfide and oxidation of the intermediate alkyldibromoborane with chromium trioxide in 90% aqueous acetic acid (232,233). 

The effect of introducing -hybridized atoms into open-chain molecules was discussed earlier, and it was noted that torsional barriers in 1-alkenes and aldehydes are somewhat smaller than in alkanes. Similar effects are noted when sp centers are incorporated into six-membered rings. Whereas the fiee-energy barrier for ring inversion in cyclohexane is 10.3 kcal/mol, it is reduced to 7.7 kcal/mol in methylenecyclohexane and to 4.9 kcal/mol in cyclohexanone. ... 

Another interesting biooxygenation reaction with alkenes, recently identified, represents an enzymatic equivalent to an ozonolysis. While only studied on nonchiral molecules, so far, this cleavage of an alkene into two aldehydes under scores the diversity of functional group interconversions possible by enzymatic processes [121,122]. 

The hydrosi(ly)lations of alkenes and alkynes are very important catalytic processes for the synthesis of alkyl- and alkenyl-silanes, respectively, which can be further transformed into aldehydes, ketones or alcohols by estabhshed stoichiometric organic transformations, or used as nucleophiles in cross-coupling reactions. Hydrosilylation is also used for the derivatisation of Si containing polymers. The drawbacks of the most widespread hydrosilylation catalysts [the Speier s system, H PtCl/PrOH, and Karstedt s complex [Pt2(divinyl-disiloxane)3] include the formation of side-products, in addition to the desired anh-Markovnikov Si-H addition product. In the hydrosilylation of alkynes, formation of di-silanes (by competing further reaction of the product alkenyl-silane) and of geometrical isomers (a-isomer from the Markovnikov addition and Z-p and -P from the anh-Markovnikov addition. Scheme 2.6) are also possible. 

The reactions of allylmetal reagents with carbonyl compounds and imines have been extensively investigated during the last two decades [1], These carbon—carbon bondforming reactions possess an important potential for controlling the stereochemistry in acyclic systems. Allylmetal reagents react with aldehydes and ketones to afford homo-allylic alcohols (Scheme 13.1), which are valuable synthetic intermediates. In particular, the reaction offers a complementary approach to the stereocontrolled aldol process, since the newly formed alkenes may be readily transformed into aldehydes and the operation repeated. 

The introduction of umpoled synthons 177 into aldehydes or prochiral ketones leads to the formation of a new stereogenic center. In contrast to the pendant of a-bromo-a-lithio alkenes, an efficient chiral a-lithiated vinyl ether has not been developed so far. Nevertheless, substantial diastereoselectivity is observed in the addition of lithiated vinyl ethers to several chiral carbonyl compounds, in particular cyclic ketones. In these cases, stereocontrol is exhibited by the chirality of the aldehyde or ketone in the sense of substrate-induced stereoselectivity. This is illustrated by the reaction of 1-methoxy-l-lithio ethene 56 with estrone methyl ether, which is attacked by the nucleophilic carbenoid exclusively from the a-face —the typical stereochemical outcome of the nucleophilic addition to H-ketosteroids . Representative examples of various acyclic and cyclic a-lithiated vinyl ethers, generated by deprotonation, and their reactions with electrophiles are given in Table 6. 

Substrates suitable for oxidative conversion into carbonyl compounds are alkenes, primary or secondary alcohols, and benzyl halides. Polystyrene-bound alkenes have been converted into aldehydes (with the loss of one carbon atom) by ozonolysis followed by reductive cleavage of the intermediate ozonide (Entry 1, Table 12.3). 

The direction of insertion of the alkene into the metal-hydride bond can take place to yield the normal or branched metal alkyl, which ultimately determines the nib ratio of aldehydes (equation 4). 

Nevertheless, in those cases in which the proportion of hydrate in equilibrium with the aldehyde is low, it is possible to obtain a useful yield of aldehyde.60,61 Electron donating groups,68,69 conjugation with alkenes and aromatic rings5 and steric hindrance69 decrease the proportion of hydrates in equilibrium with aldehydes. This explains the fact that alcohols successfully transformed into aldehydes by Jones oxidation, normally belong to the allyl,70 benzyl71 or neopentyl kind.72... 

Collins reagent is used for the introduction of carbonyl groups at allylic positions." This transformation of alkenes into enones is much slower than the oxidation of alcohols, requiring a great excess of Cr03 2Py and prolonged reaction times. Consequently, alcohols can be oxidized to aldehydes and ketones by Collins reagent without interference from alkenes. 

Most functional groups resist Collins oxidation, including the oxidation-sensitive sulfides106 and thioacetals.103 Although Collins reagent can transform alkenes into enones" and alkynes into inones,107 these reactions are slower than the oxidation of alcohols into aldehydes or ketones. Therefore, alcohols can be usually oxidized with no interference from alkenes108 or alkynes.109... 

Under oxidation with PCC, migration of alkenes into conjugation with aldehydes or ketones can be avoided by the addition of calcium carbonate (see page 47). 

Migration of alkenes into conjugation with the aldehydes or ketones, produced during the oxidation,... 

Because of the action of Et3N on the activated alcohol, some side reactions—beginning with a deprotonation—can happen in sensitive substrates. For example, a-epimerization of sensitive aldehydes and ketones,260 and migration of alkenes into conjugation with carbonyl groups261 are occasionally found. 

The reagent system TMS-azide/triflic acid performs efficient animation57 of arenes, while the combination of TMS-azide and A-bromosuccinimide with Nafion-H transforms alkenes into /J-bromoalkyl azides58. On the other hand, the combination of TMS-azide and chromium trioxide converts alkenes into a-azidoketones59 and aldehydes into acyl azides60. 

Hydroformylation of alkenes (the oxo reaction) is their conversion into aldehydes by the action of hydrogen and carbon monoxide at 90°-200° C temperature and 100-400 atm pressure in the presence of Co2(CO)g. 

The traditional oxidation of benzylic and other halides to the aldehyde using DMSO (Kornblum oxidation) has been known for about 35 vears246,362,363. Modified oxidations of this type using selenoxides364,365 and 2-nitropropane366,367 have proved more versatile since other functional groups that are sensitive to the traditional oxidation procedure may be tolerated. In the reaction with 2-nitropropane, allylic halides are readily converted into aldehydes with retention of the alkene geometry. 

The oxidation of primary and secondary trialkylboranes with pyridinium chlorochromate (PGG) provided aldehydes or ketones.504-507 An oxidative conversion of alkenes into a carbonyl compound was conducted by tandem hydroboration and oxidation with excess A-methylmorpholine-A-oxide (NMO) in the presence of Pr4NRu04 (TPAP) (Equation (105)).508... 

Carbonylation (the addition of carbon monoxide to organic molecules) is an important industr process as carbon monoxide is a convenient one-carbon feedstock and the resulting metal-acyl cor plexes can be converted into aldehydes, acids, and their derivatives. The 0X0 process is the hydr formylation of alkenes such as propene and uses two migratory insertions to make higher val aldehydes. Though a mixture is formed this is acceptable from very cheap starting materials. 

A convenient catalyst precursor is RhH(CO)(PPh3)3. Under ambient conditions this will slowly convert 1-alkenes into the expected aldehydes, while internal alkenes hardly react. At higher temperatures pressures of 10 bar or more are required. Unless a large excess of ligand is present the catalyst will also have some isomerization activity for 1-alkenes. The internal alkenes thus formed, however, will not be hydroformylated. Accordingly, the 2-alkene concentration will increase while the 1-alkene concentration will decrease this will slow down the rate of hydroformylation. This makes the rhodium triphenylphosphine catalyst... 

The transformation of an alkene into an aldehyde by addition of CO and H2 (syngas) across an olefinic double bond represents one of the world s most important, homogeneously catalysed processes.11"51 Unwanted side-reactions include isomerisation and hydrogenation and, in the case of internal olefins, numerous products may arise as shown in Scheme 4.1 for methyl-3 -pentenoate. 

The third and final general protocol for the hydrolysis of 5 5-acetals exploits the very easy reaction of the sulfur atom of an S-acetal with alkylating agents such as iodomethane, trimethyl- or triethyl-oxonium tetrafluoroborate, and methyl triHuoromethanesuLfonate to form the corresponding trialkylsulfonium salts. Ley s approach to the potent insect antifeedant Azadirachtin [Scheme 2 81]135,171,172 benefited from an easy S-alkylation-hydrolysis sequence. In a synthesis of Epiantillatoxin, a more difficult liberation of an aldehyde from its dithiane derivative was accomplished without rearrangement of a p,y-alkene into conjugation [Scheme 2,82].173... 

Hydroboration. This borane is recommended for hydroboration of alkynes, particularly for regioselective hydroboration of unsymmetrical alkynes (equation I). 1 -Alkynes are converted into aldehydes in high yield. Since alkenes react only slowly with this borane, selective hydroboration of alkynes in the presence of alkenes is possible. 

Oxidations by oxygen and catalysts are used for the conversion of alkanes into alcohols, ketones, or acids [54]-, for the epoxidation of alkenes [43, for the formation of alkenyl hydroperoxides [22] for the conversion of terminal alkenes into methyl ketones [60, 65] for the coupling of terminal acetylenes [2, 59, 66] for the oxidation of aromatic compounds to quinones [3] or carboxylic acids [65] for the dehydrogenation of alcohols to aldehydes [4, 55, 56] or ketones [56, 57, 62, 70] for the conversion of alcohols [56, 69], aldehydes [5, 6, 63], and ketones [52, 67] into carboxylic acids and for the oxidation of primary amines to nitriles [64], of thiols to disulfides [9] or sulfonic acids [53], of sulfoxides to sulfones [70], and of alkyl dichloroboranes to alkyl hydroperoxides [57]. 

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