Alkenes stepwise radical

A -Nitroso dialkylamines, when irradiated under nitrogen with light < 350 nm at low temperature in methanol in the presence of an alkene and mineral acid, undergo addition to a C-C double bond by a stepwise radical mechanism involving the aminium radical R,NH 28 29. 

An unusual example of copper-catalyzed epoxidation using substoichiometric amounts of soluble Cu(ll) compounds in methylene chloride uses MCPBA as a terminal oxidant. As a mixture of cis- and /ra j -epoxides can be obtained from a single stereoisomeric alkene, a stepwise radical mechanism is suggested <2000TL1013>. 

The oxidative properties of iodosylbenzene in the presence of Mn(TPP)Cl towards hydroxylation of alkanesand epoxidation of alkenes" were simultaneously reported by Hill and Groves. These oxidations have the characteristics of a stepwise radical reaction. 

For the analogous synthesis of ethyl 1-cyanocyclopropane-l-carboxylates from ethyl di-bromocyanoacetate and alkenes, see Section 1.2.1.2.4.2.2. These transformations are likely to occur by a stepwise, radical mechanism. 

Resctions with Alkenes and Aromatics. Unsymmetrically substituted olefins form head-to-head dimers selectively. We mention the classic vinylcarba-zole," vinyl ethers,indenes, and p-methoxystyrene. The regio-chemistry of the addition is compatible with a stepwise mechanism proceeding via a singly linked 1,4-bifunctional radical cation, in which spin and charge are separated (see above). Several dimerizations have quantum yields greater than... 

Stepwise reactions by way of diradical intermediates are also possible they often require rather high temperatures, but they are probably involved in the formation of cyclobutanes by the thermal coupling of alkenes like the halogenated alkene 2.161 with themselves or with dienes like butadiene giving the cyclobutane 2.163. The radical centres in the intermediate 2.162... 

An ab initio method has been employed to study the mechanism of the thermal isomerization of buta-1,2-diene to buta-1,3-diene. The results of the study have indicated619 that the transformation proceeds in a stepwise manner via a radical intermediate. Experimental free energies of activation for the bond shift in halocyclooctatetraenes have been reported and analyzed by using ab initio MO calculations.620 The isomerization of hexene using a dihydridorhodium complex in dimethyl sulfoxide has been reported,621 and it has been suggested622 that the Pd(II)-catalysed homogeneous isomerization of hexenes proceeds by way of zr-allylic intermediates. A study has been made623 of alkene isomerization catalysed by the rhodium /-phosphine-tin dichloride dimeric complex, and the double-bond isomerization of olefinic amines over potassium amide loaded on alumina has been described.624... 

The formation of diazetidines by [2+2] cycloaddition can be achieved by a thermal or photoinduced reaction. The reaction may proceed stepwise, either by dipolar or radical intermediates. A large number of 1,2-diazetidine derivatives have been prepared by [2+2] cycloaddition of an alkene to an azo compound. A large number of syntheses involving these types of fragments have been complied by Richter and Ulrich <1983HC(42)443> and also by Timberlake and Elder <1984CHEC(7)449>. 

Yates and Wright, 1967). Bimolecular nucleophilic attack on an acetylene (Miller, 1956) (158) or radical attack on an alkene (Readio and Skell, 1966) (159) are illustrative of both stepwise processes and overall stereoselectivity. 

We consider the anti process (144) as the normal, if ideal, course of addition. Closer to reality is the stepwise process. If we begin with the schemes (138) or (142) for substitution at an alkene carbon, and divert the first intermediates as in (160), we acquire a basic scheme for addition. Now, the attacks of nucleophiles, radicals and electrophiles on alkenes and of nucleophiles and radicals on alkynes produce Jrans-oriented intermediates, in which the memory of their parents is impressed. If > lcz, and 3 s and fc4 k5, the path to product should be stereoselective (stereoelectronic axiom 2). Indeed anti attack of the gegen... 

The photochemistry of tethered alkenes is more predictable than the nontethered situation. There is evidence of Ti-stacking for closely held moieties which presumably improves the orbital interactions between the alkenes. Exciplex formation is likely involved when the reacting groups are within 5 angstroms. Cornil et al. have shown that k systems can couple within this distance [11]. This exciplex could lead to a concerted cycloaddition from the excited state which would be consistent with the observed products. Although stepwise addition (see Sch. 3) cannot be ruled out even in these tethered singlet reactions. Ring closure must be very rapid if diradicals are involved, since no radical-trapped species have been found. 

The second problem in the addition of 1,1-dichlorodifluoroethylene to 1,3-butadiene is regiospecificity. Carbon 1 of 1,3-butadiene may become attached either to the carbon holding two chlorines or to the carbon with two fluorines. The cycloaddition of fluorinated alkenes is usually not a concerted four-center reaction in which the bonds are formed simultaneously or nearly so. Instead, the reaction is a stepwise biradical process in which the first step is formation of a free-radical intermediate with a single electron at that end of the double bond that can better accomodate it. That happens at the carbon linked to two chlorine atoms. Thus, a biradical is formed that cyclizes to form 2,2-dichloro-l,l-difluoro-3-vinylcyclobutane M [118, 119, 720]. 

Stepwise reactions by way of diradical intermediates are also possible, as in the coupling of the halogenated alkene 6.102 with butadiene 6.103. As we saw in Chapter 2 (see pages 67-68), any group, C, Z or X, stabilises a radical. Both radical centres in the intermediate 6.104 are stabilised, the one at the top by the o-chlorines and the /3-fluorines, and the one below because it is allylic. These combine rapidly 6.104 (arrow on the upper drawing) to give the cyclobutane 6.105. 

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