The Ene Reaction in the Chemistry of Low-Coordinate Phosphorus

Abstract

The ene reaction belongs to one of the simplest and most versatile reaction categories in organic chemistry. Recently ene reactions involving compounds of low-coordinate phosphorus, phospha-ene reactions, have found wide application in the synthesis of different classes of organophosphorus compounds. For phospha-ene reactions four main types are currently known which involve the employment of phosphaalkenes, iminophosphines and phosphaalkynes. Also included are retro-phospha-ene reactions, which reveal access to short-lived and highly reactive phosphaalkenes. The experimentally observed regiochemistry is in agreement with MO calculations, carried out on the simplest parent reactions of type I-IV. Neilson has recently shown that 5 avoids the also feasible [2+2] cycloaddition in reactions with 1,1-dimethylallene, Table 1. Products and reaction conditions for type I phospha-ene reactions of 5 with all-carbon enes (entries 1-5, 21 6-7 19 and 8 20)

Organophosphorus Compounds; 129. Mesitylphosphaacetylene: Synthesis and Reactivity Studies of a New Phosphaalkyne

“…The same effect has been observed in the heterocyclic compounds 6, 8, 10, 12, 14, and 22 (Table). In contrast to C3, 13 C NMR data of the remaining skeletal carbon atom are in good agreement with those of the tert-butyl derivative.…”

“…Again the mesityl substitution leads to a marked low-field shift (δ = 335.9) in the 31 P NMR spectrum when compared with the tert-butyl derivative (δ = 314.0). In the 13 C NMR spectrum of 20 the former sp-carbon of 1b (now C3) is observed at δ = 224.0, i.e., a dramatic upfield shift (∆δ = -15.8 vs. tert-butyl substitution). The same effect has been observed in the heterocyclic compounds 6, 8, 10, 12, 14, and 22 (Table).…”

“…As previously mentioned, tert -butylphosphaacetylene (1a) has been used mainly as the model substrate for studies on the reactivity of phosphaalkynes. 8 Compound 1a shows a very pronounced tendency to undergo cycloaddition, [2][3][4][5] ene, 13 and cyclooligomerization 5,14 reactions. In this paper we concentrate mainly on novel 1,3-dipolar and [4+2] cycloaddition reactions of mesitylphosphaacetylene (1b) .…”

“…13 C NMR (CDCl 3 ): δ = 21.0 (s, p-CH 3 ), 21.5 (s, o-CH 3 ), 37.9 (s, NCH 3 ), 127.6 (s, aryl-C), 127.7 (s, aryl-C), 127.8 (s, aryl-C), 128.1 (s, aryl-C), 128.3 (s, aryl-C), 129.5 [d, 2 J(C,P) = 8.0 Hz, aryl-C], 129.7 (s, aryl-C), 133.8 [d, 2 J(C,P) = 15.3 Hz, aryl-C], 134.0 (s, aryl-C), 135.6 (s, aryl-C), 135.7 (s, aryl-C), 136.9 [d, 2 J(C,P) = 3.3 Hz, aryl-C], 141.3 [d, 1 J(C,P) = 43.8 Hz, C4], 142.1 [d, 2 J(C,P) = 4.6 Hz, C5], 170.8 [d, 1 J(C,P) = 47.1 Hz, C2]. MS (EI, 70 eV): m/z (%) = 369 (40.3) [M + ], 251 (43.0) [M + -Mes + H], 118 (100.0) [Mes + -H], 91 (7.4) [C 6 The reaction and workup of phosphaalkyne 1b (100 mg, 0.62 mmol) in toluene (2 mL) and the mesoionic compound 13 (157 mg, 0.62 mmol) in toluene (1 mL) were analogous to the preceding procedure and furnished 14 as a yellow oil; yield: 135 mg (59%).…”

Diastereoselective Intramolecular Alder−Ene Reaction on Chiral Perhydro-1,3-benzoxazines. A Rapid Entry to Enantiopure cis-3,4-Disubstituted Pyrrolidines

“…The ene reaction represents one of the most important and versatile bond-forming reactions in organic chemistry owing to a great variety of suitable starting substrates. Ene reactions involving alkenes (olefin−ene or Alder−ene reaction), carbonyl functionalities (carbonyl−ene reaction), azo compounds, singlet oxygen, nitroso groups, or phosphorus derivatives as enophiles have been used in carbon−carbon and carbon−heteroatom transformations with olefins. Reactions involving hetero−ene components such as enoles and enamines are also known.…”