Acetals of alkenes

Efficient acetalization of alkenes bearing various EWG with an optically active 1.3-diol 72 proceeds smoothly utilizing PdCN, CuCI. and O2 in DME to give the 1,3-dioxane 73[113], Methacrylamide bearing 4-t-butyloxazolidin-2-one 74 as a chiral auxiliary reacts with MeOH in the presence of PdCE catalyst... 

Acetalization of alkenes can be achieved in good yields when the oxidation is carried out in the presence of alcohols or diols. Acetalization of terminal alkenes such as 1-butene occurs preferentially at the 2-position in the presence of PdCl2-CuCl2 (equation 158),414 whereas terminal alkenes bearing electron-withdrawing substituents are acetalized at the terminal position in the presence of PdCl2-CuCl in 1,2-dimethoxyethane (equation 159).415... 

Our studies on the acetalization of alkenes, first reported in 1983, revealed that terminal alkenes bearing electron-withdrawing substituents such as those shown in Scheme 21 are regioselectively acetalized at the terminal olefinic carbon with ROH or diols by PdCl2-CuCl catalyst under Homochiral acetals, which serve as the chi-... 

Diborane or aUcylboranes are used for reduaion of alkenes and alkynes via hydrobora-tion (see pp. 37f., 47f., 130f.) followed by hydrolysis of the borane with acetic acid (H.C. Brown, 1975). 

In contrast to oxidation in water, it has been found that 1-alkenes are directly oxidized with molecular oxygen in anhydrous, aprotic solvents, when a catalyst system of PdCl2(MeCN)2 and CuCl is used together with HMPA. In the absence of HMPA, no reaction takes place(100]. In the oxidation of 1-decene, the Oj uptake correlates with the amount of 2-decanone formed, and up to 0.5 mol of O2 is consumed for the production of 1 mol of the ketone. This result shows that both O atoms of molecular oxygen are incorporated into the product, and a bimetallic Pd(II) hydroperoxide coupled with a Cu salt is involved in oxidation of this type, and that the well known redox catalysis of PdXi and CuX is not always operalive[10 ]. The oxidation under anhydrous conditions is unique in terms of the regioselective formation of aldehyde 59 from X-allyl-A -methylbenzamide (58), whereas the use of aqueous DME results in the predominant formation of the methyl ketone 60. Similar results are obtained with allylic acetates and allylic carbonates[102]. The complete reversal of the regioselectivity in PdCli-catalyzed oxidation of alkenes is remarkable. 

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 oxidation of terminal alkenes with an EWG in alcohols or ethylene glycol affords acetals of aldehydes chemoselectively. Acrylonitrile is converted into l,3-dioxolan-2-ylacetonitrile (69) in ethylene glycol and to 3,3-dimetho.xy-propionitrile (70) in methanol[28j. 3,3-Dimethoxypropionitrile (70) is produced commercially in MeOH from acrylonitrile by use of methyl nitrite (71) as a unique leoxidant of Pd(0). Methyl nitrite (71) is regenerated by the oxidation of NO with oxygen in MeOH. Methyl nitrite is a gas, which can be separated easily from water formed in the oxidation[3]. 

The carbonylation of alkene in AcOH-acetic anhydride in the presence of NaCl affords the /9-acetoxycarboxylic anhydride 242 in good yields and the method offers a good synthetic method for / -hydroxycarboxylic acid 243[222],... 

The oxidative coupling of alkenes which have two substituents at the 2 posi-tion, such as isobutylene, styrene, 2-phenylpropene, 1,1-diphenylethylene, and methyl methacrylate, takes place to give the 1,1,4.4-tetrasubstituted butadienes 285 by the action of Pd(OAc)2 or PdCF in the presence of sodium acetate[255-257]. Oxidation of styrene with Pd(OAc)2 produces 1.4-diphenylbutadiene (285, R = H) as a main product and a- and /3-acetoxystyrenes as minor pro-ducts[258]. Prolonged oxidation of the primary coupling product 285 (R = Me) of 2-phenylpropene with an excess of Pd(OAc)2 leads slowly to p-... 

The intramolecular insertion of a conjugated diene into 7r-allylpalladium, initially formed in 789, generates another rr-allyl complex 790, which is trapped with acetate anion to give a new allylic acetate 791. No further reaction of the allylic acetate with alkene takes place[489]. 

Addition of nucleophiles to both activated and unactivated alkenes is catalyzed by Pd(II). Addition of alcohols or AcOH to alkenes bearing EWGs is catalyzed by PdCl2(PhCN)2 to give the corresponding ethers and esters. The addition of an alcohol to the cyclic acetal of acrolein 82 to give the ether 83 is also possible with the same catalyst[64]. Amines add to the vinylic ether 84 to give 85, but not to simple alkenes[65]. 

In 1959 Carboni and Lindsay first reported the cycloaddition reaction between 1,2,4,5-tetrazines and alkynes or alkenes (59JA4342) and this reaction type has become a useful synthetic approach to pyridazines. In general, the reaction proceeds between 1,2,4,5-tetrazines with strongly electrophilic substituents at positions 3 and 6 (alkoxycarbonyl, carboxamido, trifluoromethyl, aryl, heteroaryl, etc.) and a variety of alkenes and alkynes, enol ethers, ketene acetals, enol esters, enamines (78HC(33)1073) or even with aldehydes and ketones (79JOC629). With alkenes 1,4-dihydropyridazines (172) are first formed, which in most cases are not isolated but are oxidized further to pyridazines (173). These are obtained directly from alkynes which are, however, less reactive in these cycloaddition reactions. In general, the overall reaction which is presented in Scheme 96 is strongly... 

Chlorohydnns and 1,2-dichloro denvatives are obtamed by oxidation of alkenes with fert-butyl hypochlorite when the reaction is performed in acetic acid instead of water, chlorohydrm acetate is formed [Ji] (equation 25)... 

BH3 SMe2, CH2CI2, 0°, 1 h, then BF3, 5 min. Simple benzylidene acetals are cleaved efficiently without hydroboration of alkenes that may be present, and acetonides are converted to the hydroxy isopropyl ethers. 

Replacement of silver nitrite by inexpensive sodiiunor potassium nitrite enhances the imlity of this process Treatment of alkenes v/ith sodiiun nitrite and iodine in ethyl acetate and water in the presence of ethylene glycol gives conjngatednitroalkenesin49-82% yield The method for generation of nitryl iodide is improved by the treatment of iodme v/ith potassium nitrite complexed v/ith 18-crovm-6 in THF under sonicadon, as shovmin Eq 2 32 ... 

A suitable catalyst for carboxy-de-diazoniations was found by Matsuda s group in their work on arylations of alkenes. As in the case of alkene arylations (Sec. 10.9), they used Pd11 acetate (2 mole %) and carbon monoxide (9 atm) for reactions with benzenediazonium tetrafluoroborate and sodium acetate in acetonitrile as solvent at room temperature (Nagira et al., 1980 82-85% yield). Similar results were obtained... 

We have investigated the bromo-addition of alkenes and their related compounds with BTMA Br3. Thus, we found that the reaction of alkenes with BTMA Br3 in aprotic solvents such as dichloromethane and chloroform gave 1,2-dibromo adducts in a manner of stereospecific anti-addition, and, in such protic solvents as methanol and acetic acid, gave the corresponding dibromo adducts along with considerable amounts of solvent-incorporated products in regioselective manner (Fig. 18) (ref. 29). 

Formylation of alkenes can be accomplished with N-disubstituted formamides and POCl3. ° This is an aliphatic Vilsmeier reaction (see 11-15). Vilsmeier formylation can also be performed on the ot position of acetals and ketals, so that hydrolysis of the products gives keto aldehydes or dialdehydes ... 

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