The extraordinary reactions of phenyldimethylsilyllithium with N,N-disubstituted amides

Abstract

Phenyldimethylsilyllithium reacts with N,N-dimethylamides in a variety of ways, depending upon the stoichiometry, the temperature and, most subtly, on the structure of the amide, with quite small-seeming changes in structure leading to profound changes in the nature of the products. When equimolar amounts of the silyllithium reagent and N,N-dimethylamides 6 are combined in THF at -78 degrees C, and the mixture quenched at -78 degrees C, the product is the corresponding acylsilane . If the same mixture is warmed to -20 degrees C before quenching, the product is a cis enediamine 11. The enediamines are easily isomerised from cis to trans, easily oxidised to dienediamines , and, with more difficulty, hydrolysed to alpha-aminoketones 13. If two equivalents of the silyllithium reagent are used, the product is an alpha-silylamine 20. The mechanism of formation of the enediamines appears to be by way of a Brook rearrangement of the tetrahedral intermediate 17 followed by loss of a silanoxide ion to give a carbene or carbene-like species. The 'carbene' combines with the Brook-rearranging nucleophile to give an intermediate 28, which loses another silanoxide ion to give the enediamine. The same carbene can be attacked by a second equivalent of the silyllithium reagent to give the alpha-silylamine 20. Other nucleophiles, like alkyllithiums, phenyllithium, and tributylstannyllithium also trap the carbene to give products 48-52. The intermediate anions in these reactions, when benzylic, can be further trapped with alkylating agents to give the products 33, 34 and 53-55. In special cases, the anion formed by attack on the carbene can be trapped by intramolecular reactions displacing internal leaving groups, as in the formation of the enamine 37 and the cyclopentane 41, or attacking a carbonyl group, as in the formation of the indanone 61, or attacking a double or triple bond, as in the formation of the cyclopentanes 71 and 75. In another special case, the carbene reacts with vinyllithium to give an allyllithium intermediate 56, which selectively attacks another molecule of carbene to give eventually the gamma-aminoketone 58. Small changes in the structure of the amide lead to a variety of other pathways each of which is discussed in the text. Notably, each member of the homologous series of amides Ph(CH2)nCONMe2 gives rise to a substantially different product: when n= 0, the reaction is normal, and the yield of the alph]-silylamine 20e is high; when n=1, proton transfer in the intermediate anion 64 and displacement of the phenyl group leads to the silaindane 66; when n=2, fragmentation of the intermediate anion 80, and capture of the carbene by benzyllithium leads to the 1,4-diphenylbut-2-ylamine 83; and when n=3, proton transfer in the intermediate anion 67 and displacement of the phenyl group leads to the silacyclopentane 69.

Versatility in the Brook Rearrangement for the Selective Ring‐Opening of Three‐Membered Rings

“…Knowing the importance of skipped dienes, structures found in a number of natural products, and the challenges associated with their preparation, particularly when a quaternary carbon stereocenter is present, we decided to extend our strategy to their synthesis. When one equivalent of commercially available Bu 2 Mg was added to 6 at room temperature in THF, a mixture of cyclobutene 5 and skipped diene 9 was obtained.…”

Polycyclic N‐Heterocyclic Compounds. Part 86: Synthesis and Evaluation of Antiplatelet Aggregation Activity of 2,4‐Disubstituted 9‐Chloro‐5,6‐dihydropyrimido[5,4‐d]benzazepine and Related Compounds

“…To the Vilsmeier reagent prepared from N,N ‐dimethylcyclohexanecarboxamide and phosphoryl chloride was added compound 7 , and the reaction was stirred at 90 °C for 1 h. The product was recrystallized from ethanol to give 8d (77%) as colorless plates, mp 171–174 °C; IR (potassium bromide) cm −1 : 1347, 1153 (SO); 1 H NMR (deuterochloroform): δ 1.03–2.09 (m, 10H, cyclohexyl‐H), 2.32 (s, 3H, Me), 2.42–3.01 (m, 3H, H5 and 1′), 4.31 (t, 2H, J = 6.0 Hz, H6), 6.98–7.89 (m, 7H, Ar‐H); FAB‐ms m/z 502 (MH + ), 504 (MH + + 2). Anal.…”

Polycyclic N‐Heterocyclic Compounds. Part 75: Synthesis of 2,4‐Disubstituted 5,6‐dihydro[1]benzoxepino[5,4‐d]pyrimidines and 12‐Substituted 1,2,4,5‐Tetrahydro[1]benzoxepino[4,5‐e]imidazo[1,2‐c]pyrimidines as Potential Antiplatelet Aggregators

“…By using the same procedure, reaction of 5 (1.00 g, 5.37 mmol) with N ‐(chloro(cyclohexyl)methylene)‐ N ‐methylmethanaminium chloride solution (prepared from a stirred mixture of N , N ‐dimethylcyclohexanecarboxamide (2.40 g, 15.5 mmol), phosphorus oxychloride (1.68 mL, 18.0 mmol), and dry chloroform (20 mL) at 0°C for 30 min) at reflux for 5 h gave 6c (1.20 g, 71%), purified by silica gel column chromatography (benzene) and recrystallized from ethyl acetate as colorless needles, mp 81–83°C; 1 H NMR (deuterochloroform): δ 1.19–2.20 (m, 10H, cyclohexyl‐H), 2.60–3.05 (m, 1H, H1′), 3.07 (t, 2H, J = 6.0 Hz, H5), 4.55 (t, 2H, J = 6.0 Hz, H6), 7.02–7.27 (m, 2H, H9 and 10), 7.46 (dd, 1H, J = 5.4, 2.7 Hz, H8), 8.04 (dd, 1H, J = 6.3, 2.7 Hz, H11); FABMS m / z 315 (MH + ), 317 (MH + + 2). Anal .…”