Photostimulated SRN1 reactions

The results obtained in the photostimulated SRN1 reaction between carbanions from 2,4,4-trimethyl-2-oxazoline or 2,4-dimethylthiazole and 2-bromopyridine are also consistent with the incomplete formation of the carbanions in the KNH2-NH3(iiq ) system. In these cases, 2-aminopyridine is formed alongside the corresponding pyridyl-2-methylene oxa-zolinyl or thiazolyl substitution products (Wong et al. 1997). When the SRN1 pathway is impeded by conducting the reaction in the dark or in presence of di(tert-butyl) nitroxide, the ionic animation reaction dominates. 

Iodobenzenesulfonamide (32) undergoes photostimulated SRN1 reactions in liquid ammonia, with the potassium enolates derived from acetone, pinacolone, 3-methyl-2-butanone, cyclopentanone, cyclohexanone and cyclooctanone, to give fair to good yields of 2H-l,2-benzothiazine-l, 1-dioxides (Scheme 10.59) [73],... 

Photostimulated SRN1 reactions of bridgehead halides [119, 120] and of tertiary chlorides [122] has been examined and considered as a possible route to nucleophilic substitution of these normally unreactive substrates. In the case of the reaction of 1-iodoadamantane with ketone enolates, the photosubstitution product yield was improved in the presence of 18-crown-6 [103], This is probably related to the ion pairing effect already discussed in a preceding section. 

Intramolecular examples of photostimulated SRN1 reactions implying excited anions with a demonstrated chain mechanism have been reported as in the case of anilide anions and JV-acyl benzylamines used as precursors of oxindoles and isoquinolines [129]. 

Several reports have appeared concerning photostimulated Srn1 reactions of aryl halides. In these processes substitution occurs via a chain mechanism as follows ... 

Photostimulation increases the number of reagents and substrates that can be involved in SRN1 reactions. However, it complicates their performance see the review by Ivanov (2001). Therefore, the dark reactions are of special interest. 

In the reaction of 1-naphthoxide ions, a mixture of 2- and 4-aryl-, along with 2,4-diaryl-l-naphthol, is formed. However, substitution occurs only at C4 with the 2-Me-substituted anion (50-70% yields) [1[. On the other hand, 2-naphthoxide ions react with ArX to give substitution only at Cj of the naphthalene ring [32, 33]. The reactivity of the 2-naphthoxide ions allows the synthesis of naphthylpyridines, naphthylquinolines, and naphthylisoquinolines via their coupling reactions with the corresponding halo arenes, in good to excellent yields (50-95%) [33], The photostimulated reaction between 2-naphthoxide ions and l-iodo-2-methoxy-naphthalene was explored in liquid ammonia, as a novel approach to the synthesis of [1,1 ] binaphthalenyl-2,2 -diol (BINOL) derivatives (Scheme 10.23). This procedure has also been applied to the synthesis of BINOL in moderate yield (40%), which represents the first report of an SRN1 reaction in water [34]. 

Rossi, R.A. and Penenory, A.B. (2003) The photostimulated SRN1 process reaction of haloarenes with carbanions, in CRC... 

Argiiello, J.E., Penenory, A.B. and Rossi, R.A. (2000) Quantum yields of the initiation step and chain propagation turnovers in SRN1 reactions photostimulated reaction of l-iodo-2-methyl-2-phenyl propane with carbanions in DMSO. Journal of Organic Chemistry, 65, 7175-7182. 

Corsico, E.F. and Rossi, R.A. (2000) Synthesis of mono-, di-, and tri-phenyl arenes by sequential photostimulated SRN1 and Pd(0)-catalyzed cross coupling reactions on aryl halides. Synlett, 230-232. 

One of the first reports of an intramolecular SRN1 reaction is the synthesis of cephalotaxinone (43). The iodoketone 42 cyclizes in liquid ammonia under photostimulation affording product 43 in excellent yields (Sch. 45). 

Beugelmans R. The photostimulated SRN1 process reactions of haloarenes with enolates. In Horspool WM, ed. CRC Handbook of Organic Photochemistry and Photobiology. Boca Raton CRC Press Inc., 1994 1200-1217. 

A typical reaction is the photostimulated substitution of aryl halides by ketone [121-131] (and much less efficiently aldehyde [124]) enolate anions (Scheme 5), both inter- [121-128] and intramolecularly [129-131]. The SRN1 reaction with o-bromoacetophenones is a useful method for the construction of an aromatic ring (Scheme 24) [132-133], with o-halophenylalkyl ketones for macrocycles (Scheme 25) [134], with o-haloanilines for indoles [123], with o-halobenzylamines for isoquinolines [135], and several other heterocyclic syntheses are possible [136]. 

The photostimulated reaction of 1,8-diiodonaphthalene with p-methyl-benzenethio-late ions in DMSO yields the substituted cyclized product 10-methyl-7-thia-benzo[de] anthracene (31) in moderate yield (Scheme 10.58) [54], The mechanism proposed to explain product 31 involves an intramolecular radical cyclization after monosubstitution in the propagation cycle of the SRN1 process. 

Barolo, S.M., Lukach, A.E. and Rossi, R.A. (2003) Syntheses of 2-substituted indoles and fused indoles by photostimulated reactions of o-iodoanilines with carbanions by the SRN1 mechanism. Journal of Organic Chemistry, 68, 2807—2811. 

Dichloropyridine reacts in the dark (88% of disubstitution), whereas 2,6- and 3,5-dichloropyridines require photostimulation to afford ca. 80% yield of disubstitution products [26]. Aryltrialkylstannanes are valuable intermediates in organic synthesis, and the fact that they can be easily synthesized through the SRN1 mechanism, opens up important synthetic routes to different reaction schemes. For over a decade, the Pd(0)-catalyzed cross-coupling of organotin compounds with electrophiles, known as the Stille reaction, has been a very important tool in product design [104]. 

Halothiophenes undergo photostimulated reaction with acetone enolate ion to form substitution products (76H(5j377). This is believed to occur by the radical-chain SRN1 mechanism. The propagation steps are as follows ... 

---