A Novel Route to 2‐Ethoxybutadienes; Diels‐Alder Reaction of 1,2‐λ⁵‐Oxaphosphorines

Abstract

Dedicated to Professor Siegfried Hiinig on the occasion of his 60th birthday 2-Ethoxybutadienes are, as enol ethers of a$-unsaturated carbonyl compounds, versatile and useful synthons. We describe here, a variable synthetic method for this class of compounds starting from primary alkyl halides, carboxylic acids and aldehydes.Alkylidenetriphenylphosphoranes ( I ) , whose ylide substructure is formed from a primary alkyl halide o r alcohol, can be transformed by treatment with the carboxylic acid derivatives (2) to acyl ylides (3)I'l. The ambidentateL2] phosphoranes (3) are O-alkylated[31, by treatment with ethyl bromide, to the phosphonium salts (4). from which a yproton can be abstracted by sodium amidel"]. The 2-cupies the apical position[81. In the 3'P-NMR spectra, particularly that of (9) (R2 =! = H), two broad signals appear at 6= + 10 to + 16 and -20 to -50 (H3P04 as external standard); these indicate an equilibrium between (9) and (12). In polar solvents such as CDCI,, this lies more on the side of (9), while in nonpolar solvents, such as benzene and toluene, (12) is more predominant. If solutions of (9)/(12) R' = H in toluene are treated with N-methylmaleimide (13), the substituted N-methylphthalimides ( I 7 ) [ ' I and triphenylphosphane oxide (15) can be isolated whose formation we account for by the following mechanism: (12). in a Diels-Alder reaction with (13), yields the bicycle (14). After Iigand rearrangement to (14'), the bridgehead C-atom bonded to the phosphorus occupies a n apical position in the trigona! bipyramid, permitting cleavage of the C-P bondfg1, which is cleaved in a cycloreversion to (IS). The dihydrophthalimide derivative (16) is formed, which is dehydrated in the work-up to (17). In principle, a cycloaddi6Et ethoxyallylidenetriphenylphosphoranes, which are described in the mesomeric limiting structures (5) and (5') are thus formed, and formally react as (5') with aldehydes (8) in a Wittig reaction to the 2-ethoxybutadienes (10) ( Table 1); as en01 ethers they are readily converted into a,b-unsaturated ketones by known methods. On the other hand, the ylides (5) react with acid chlorides (6) at the y-position to the P atom[51. Thus, apart from 1 mole of the phosphonium salt (C1' instead of Br'), the acylphosphoranes (9) are formed from (5) and (6) (2 : 1 molar ratio). lsocyanates ( 7 ) also attack the ally1 ylides (5) at the y-position to form, after proton rearrangement, the carbamoyl-substituted compounds (9)16]. Wittig reaction of (9) with (8) produces the 1-acyl-2-ethoxybutadienes (11) (Table l), from which, inter alia, y,&unsaturated 1,3-dicarbonyl compounds and derivatives of the 2,3-dihydro-4-pyrones can be obtained"'.(9). which is describable via mesomeric limiting structures, should exist in several different conformations. It therefore follows, in particular when R2 =/= H, that reaction of (8) with (9) should lead to formation of compound (11) having a mixture of Z. E-isomers at the I-position. In contrast, carbonyl olefination forms a new double bond having E-geometry whi...

Substituent effect on the transition from ionic to covalent bonding in triphenylphosphonium ylide derivatives: reactivity of 3-methyl-2,2,2-triphenyl-2H-cyclohepta[d][1,2λ5]oxaphosphole with heterocumulenes

“…This feature is completely different from that of 1,2λ 5 -oxaphosphines 12a-f, which exhibit two broad signals at δ P Ϫ20 to Ϫ50 and ϩ10 to ϩ16 in CDCl 3 or benzene-d 6 possibly due to an equilibrium between 12a-f and 13a-f, respectively (Scheme 4). 10 Thus, these features suggest 1a-d exist as resonance hybrids of A, B, and C, and are not in equilibrium between structures A, B, and C. The chemical shifts (δ C ) of C8a and C3 and their coupling constants between the carbon and phosphorus atoms in the 13 C NMR spectra of 1a-d are also summarized in Table 1. The values for C8a of 1a-d become lower in the order 1a > 1b > 1c > 1d, and the values for C3 become higher in the order 1a < 1b < 1c < 1d.…”

One-Pot Cyclizations of (2,4-Dioxobutylidene)phosphoranes and (2-Alkoxy-4-oxobut-2-enylidene)phosphoranes

“…Bestmann et al reported the reaction of methylidenephosphorane 97 with carbonic acid derivatives 98 to give the acylated ylides 99 (Table 16). 80 Alkylation of the latter afforded phosphonium salts 100. Their treatment with sodium amide gave ethoxyphosphoranes 101 which were subsequently acylated with acyl chlorides 102 to yield the (2-ethoxy-4-oxobut-2-enylidene)phosphoranes 88.…”

“…In contrast, aldehydes and ketones 93 react at the a-position of (2-alkoxy-4-oxobut-2-enylidene)phosphoranes 88 to give normal Wittig products 94. [76][77][78][79][80]…”