Cyclopropylimines

An intramolecular version of an azide cycloaddition of 221 and 222 provided cyclopropylimines 224 and 225 via formation of triazoline 223 followed by extrusion of nitrogen with concomitant 1,2-hydrogen shift (Scheme 36) [58], The cyclization was found to be solvent dependent polar solvents such as DMF gave the best yields, whereas benzene gave several side products. 

Several cyclopropylimines have been synthesized and their reactions with a range of nucleophiles have been investigated. Mild hydrolysis of diimine (16) produces, amongst other products, the /3-ketoimine (17), stabilized by intramolecular hydrogen bonding. 

The first compound selected for this study was the 2-aza-1, 4-diene 68 (Scheme 11). Triplet-sensitized irradiation of azadiene 68, using acetophenone, for 25 min affords two new products, identified as the cyclopropylimine 69 (11%)... 

The aza-di-TT-methane (ADPM) rearrangement of aza-1,4-dienes via radical-cat-ions suggests the possibility that other radical-ion intermediates (e.g., radical-anions) could also be responsible for this rearrangement reaction. In order to test this proposal, the azadiene 101 was irradiated for 20 min in acetonitrile using A,iV-dimethylaniline (DMA) as an electron-donor sensitizer. The reaction leads to formation of the cyclopropylimine 102. Separation of product mixture by column chromatography on silica gel affords the aldehyde 34 (21%) resulting from hydrolysis of the imine 102, (Scheme 18) [70]. 

Intramolecular imine formation between a coordinated aminate ion and a 2-oxo acid has been utilized to synthesize the two racemic amino acids 2-cyclopropylglycine and proline.463 Thus anation of [Co(NH3)5OH2]3+ by Br(CH2)3C0C02H at pH 5 gives two major products (145) and (146). Both are converted to tetraammineiminocarboxylato chelates by attack of an adjacent deprotonated ammonia. The cyclopropylimine complex can, for example, be reduced by alkaline BH4 to give the (RS)-2-cyclopropylglycine complex. 

Dihydroazepines have been synthesized by the first rhodium-catalyzed hetero-[5+2] cycloaddition of cyclopropylimines and alkynes (Scheme 8.62) [138]. The reaction proceeds via formation of metallacycle 147 which undergoes migratory insertion of dimethyl acetylenedicarboxylate (DMAD) to form 148. Finally, dihydroa-zepine 149 is obtained via reductive elimination. 

An experimental and computational investigation into the photorearrangement of N-cyclopropylimines to yield pyrrolines (Scheme 122) has shown that the, regiochem-istry and stereochemistry can be understood in terms of a mechanism involving barrierless evolution in three different (S2, Si, and So) singlet states and sequential decay through two different (S2/S1 and S1/S0) conical intersection funnels.183... 

P.y-Unsaturatod imines and their derivatives undergo ADPM rearrangement upon triplet-sensitized irradiation in the presence of acetone and acetophenone (Scheme 4.31) [45-47]. The alkyl-substituted cyclopropylimines readily underwent hydroxylation on Si02 to afford the corresponding cyclopropanecarbaldehydes... 

Campos, P.J., Soldevilla, A., Sampedro, D., and Rodriguez, M.A. (2001) N-Cyclopropylimine-1 -pyrroline rearrangement. A novel photochemical reaction. Organic Letters, 3, 4087 -089. 

The first 1-ADPM rearrangement of an acyclic derivative, reported five years later, was observed in studies of the sensitized irradiation of the (3,y-unsaturated imine 8 that yielded the corresponding cyclopropylimine 9 exclusively (Sch. 5) [11]. This photoproduct undergoes hydrolysis during isolation to generate the corresponding cyclopropane carbaldehyde 10. 

For more than 30 years, di-7i-methane processes were limited to the three different types of (3,y-unsaturated systems mentioned above. However, recent studies have shown that the rearrangement is applicable to 2-aza-1,4-dienes [13]. Thus, triplet-sensitized irradiation of compound 11 affords the cyclopropylimine 12 and the TV-vinylaziridine 13 (Sch. 6). The photoreaction of 11 represents the first example of a 2-aza-di-7i-methane rearrangement (2-ADPM) that brings about the formation of a heterocyclic product. The reaction has been extended to other 2-azadienes that yield the corresponding cyclopropylimines regioselectively [13b]. 

The 1-ADPM rearrangement of 1-aza- 1,4-dienes affords the corresponding cyclopropylimines regioselectively. However, in order to predict the outcome of the reaction in pyrazinobarrelenes, quinoxalinobarrelenes, and other polycyclic systems, two different factors should be taken into account. One is the possible competition between the DPM and the 1-ADPM rearrangements, and the other is the influence of substitution on the stability of the reaction intermediates. However, in most of the cases studied, the 1-ADPM process takes place preferentially over the DPM reaction [3e]. An example of competition between these two reactions is shown in Sch. 4 for compound 5a [10a]. 

Several related carbene complexes 28 with M = Cr, Mo, or W, have been shown to undergo photocycloadditions with alkenes, in particular acrylates, to furnish a series of 1 -pyrrolines 29 in moderate yields. The mechanism is believed to involve an initial cyclopropanation of the alkene, followed by a light-induced [l,3]-sigmatropic rearrangement of an intermediate N-cyclopropylimine <020M4076>. 

For synthesis of certain alkaloids, the requisite cyclopropylimines were prepared by bisalkylation of the appropriately substituted benzyl cyanide. Cyanides could be selectively reduced in high yield to the corresponding aldehydes by employing diisobutylaluminium hydride. Treatment of these aldehydes with a primary amine completes the synthesis of the imines. Rearrangement191,432-435 with acids afforded the 2-pyrrolines in high yield (equation 26). 

Cyclopropylimines are usually generated by reduction of nitriles or condensation of carbonyl compounds with amines. An alternative to this process involves the generation of cyclopropylinunoni-um salts such as (209) at room temperature from cyanohydrin equivalent (208 Scheme 49). ... 

The cyclopropylimine ynoline rearrangement (equation 68) has been exploited by Stevens in alkaloid syntheses. - Wenkert s cyclopropylcarbinyl rearrangement (equation 69) served extremely well in the design of 1,4-dicarbonyl synthons or 3,-y-unsaturated carbonyl compounds which then were expressed in numerous syntheses of terpenoid and alkaloid natural products. Donor-acceptor concepts continue to be express in the applicability of these rearrangements to organic synthesis. 

Af-Cyclopropylimines undergo rearrangement to cyclic imines (pyrrolines) under... 

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