Flash vacuum pyrolysis of -acyl- o -methoxybenzylidene- triphenylphosphoranes to give 2-substituted benzofurans

The title stabilised ylides, readily prepared in one step from acid chlorides, are converted upon FVP at 850 °C into 2-substituted benzofurans. When the acyl group is aromatic it appears unchanged as the 2-substituent in the product while for aliphatic examples degradation processes may lead to 2-alkenyl products


Introduction
Some time ago we reported that, while flash vacuum pyrolysis (FVP) of the 2methoxybenzoyl substituted stabilised ylides 1 at 700 °C gave the expected alkynes 2 by extrusion of Ph 3 PO, simply increasing the pyrolysis temperature to 850 °C resulted in loss of the OMe group and cyclisation of the resulting phenoxyl radicals to give 2substituted benzofurans 3. 1 Depending on the nature of R, the products were formed with R 1 = Me and Et (R = Me), R 1 = vinyl (R = Et or Pr i ), R 1 = Me, Et and vinyl (R = Pr n , Bu n or Pent n ) and R 1 = Ph (R = Ph).and these patterns could be accounted for by interaction of the initially formed benzofuran-3-yl radicals with the 2-alkyl group.As shown access to 1 was gained by reaction of 2-methoxybenzoyl chloride with an appropriate non-stabilised ylide.We have subsequently extended this chemistry to construction of more complex systems by using groups R which can interact with the benzofuran-3-yl radical intermediate in a more constructive way leading to tri-and tetracyclic heterocyclic systems. 2 In the course of this work it has become apparent that it is sometimes advantageous to approach the key intermediates 2 by starting from an ylide bearing the 2-methoxyphenyl group and reacting it with a suitable acid chloride.In this paper we describe the preparation and pyrolytic behaviour of the isomeric ylides 5-10 which has remained unreported until now.
As shown these were readily prepared by treatment of 2methoxybenzyltriphenylphosphonium bromide 4 3 with butyllithium followed by 0.5 equiv. of an acid chloride.The reaction takes place with transylidation to give the required ylides 5-10 and regenerate 0.5 equiv. of the quaternary phosphonium chloride.The products were stable pale yellow solids which showed the expected 31 P NMR signals in the range δ P +10.9-14.4 and also had highly informative 13 C NMR spectra with phosphorus coupling extending throughout the P-phenyl groups and to the first one or two carbons of both the acyl and 2-methoxyphenyl groups (Table 1).
When the ylides were subjected to FVP at 850 °C and 10 -2 -10 -3 Torr using the previously described apparatus, 5 Ph 3 PO was obtained at the furnace exit and in the cold trap colourless liquids which proved to be the desired 2-substituted benzofurans.
As shown in Table 2, the nature and yield of the products closely paralleled those obtained from the isomeric ylides 1.Thus, the acetyl ylide 5 gave a mixture of 2-methyl and 2-ethyl products 11 and 12, the propionyl and isobutyryl ylides gave only 2vinylbenzofuran 13, the n-butyryl ylide gave a mixture of 11, 12 and 13, while the benzoyl ylide gave the phenyl product 15.The mechanism for formation of these was fully described in our previous paper and the similar product distribution is of course to be expected from the common intermediacy of 2. One R group not previously investigated was t-butyl and the result from this was in accord with those for R = Et and Pr i where if only β (and no γ) hydrogens are present on the alkyl group, β -hydrogen abstraction occurs with loss of H (R = Et) or Me (R = Pr i ) to give 13.In the case of 9 a similar process occurred with loss of Me to give 2-isopropenylbenzofuran 14 which showed excellent agreement with published spectroscopic data.Table 1. 13 C NMR Spectra of ylides 5-10, δ C (J P-C )

Table 2 .
Results of pyrolysis of ylides 5-10 Melting points were determined using a Reichert hot-stage microscope and are uncorrected.Infra red spectra were recorded as nujol mulls on a Perkin Elmer 1420 instrument.NMR spectra were obtained for 1 H at 80 MHz using a Bruker WP80 instrument and for 13 C and 31 P at 75 MHz and 121 MHz respectively using a Varian Gemini 2000 instrument.All spectra were run on solutions in CDCl 3 with internal Me 4 Si as reference for 1 H and 13 C and external 85% H 3 PO 4 as reference for 31 P. Chemical shifts are reported in ppm to high frequency of the reference and coupling constants J are in Hz.Mass spectra were obtained on an A. E. I. MS-902 spectrometer using electron impact at 70 eV.