Ylide compounds structural studies

Alkaloids are another family of compounds that are easily accessible from synthetic routes utilizing carbonyl ylides. The complex structure of naturally occurring alkaloids has been the driving force for the generation of new carbonyl ylide methodology. These studies have resulted in the discovery of several new reaction manifolds as well as the total synthesis of several natural products. 

A theoretical study on 5V-methylselenabenzene 192 and its oxygen and sulfur analogues has been carried out using ab initio calculations with the DFT method <2006M1791> (cf. Section 7.11.2, structure 3). The X-H compounds were also evaluated. These structures have 6ji electrons and are homoaromatic with ylide character. Structural parameters were also determined for these structures. The substituent on the chalcogen atom is calculated to be more stable in a pyramidal rather than a planar conformation. 

The structures of a number of stabilized ylides have been studied by l C, iH, and 3ip n.m.r.H and by X-ray crystallography. 12 The X-ray crystal structures of (24) and (27) were compared to those previously determined for (25) and (26) and this data, together with that obtained from Raman and infrared studies on these and related compounds, was used to determine the extent and nature of hydrogen bonding in these systems. 

NMR spectroscopic studies f111,13C, and 31P) are consistent with the dipolar ylide structure and suggest only a minor contribution from the ylene structure.234 Theoretical calculations support this view.235 The phosphonium ylides react with carbonyl compounds to give olefins and the phosphine oxide. 

Structure and bonding in cyclic ylide complexes have been studied extensively in earlier experimental and theoretical work.2 There is general agreement that transannular aurophilic bonding has a significant influence on the stability and reactivity of this family of compounds. There is also evidence for inter -molecular aurophilic bonding between these auracycles, but the number of examples is still limited (Scheme 43) 27455-i57,i8i... 

The dipolar structure 1 describes the chemical behavior of thiocarbonyl ylides best, although other mesomeric forms have been used for the representation of the electronic structure of these dipoles. The parent compound, thioformaldehyde (5)-methylide (1), was studied by means of spectroscopic and theoretical methods (2-5), which showed that the molecule possesses a bent allyl-type structure (6). According to theoretical calculations, structures lA and IB have the largest contribution (31.5% each) in the representation of the electronic structure, whereas 1C, which reflects the 1,3-dipolar character, has only a 4.2% contribution (5). 

In two recent works the phosphorus analogues of pyridones 109-111 (Scheme 58) have been studied computationally [317, 318], 110 is planar, while for 109 and 111 the planar structures are transition states with 11.0 and 19.8 kcal moH above the nonplanar minimum, respectively. The low barriers to inversion indicate that the planar structures have considerable aromaticity, which is shown by the NICS values exhibiting 67%, 51%, and 33% of the benzene value for 109,110, and 111, respectively [317], ELF analysis also indicates a certain aromaticity in these compounds [317], The bonding situation in 109 is similar to the cyclic phosphinocarbene isomer of phosphinine 112 [319] (Scheme 59), which was shown to be planar and aromatic. Stabilization of this carbene can be achieved by incorporating it to an anne-lated ring system 113, and also by the electron donor effect of an ylide formed with an additional o4,l5-P (114) [319],... 

While most of the initial studies have involved the transition metal-catalyzed decomposition of a-carbonyl diazo compounds and have been reviewed [3-51], it appears appropriate to highlight again some milestones of these transformations, since polycyclic structures could be nicely assembled from acyclic precursors in a single step. Two main reactivities of metalo carbenoids derived from a-carbonyl diazo precursors, namely addition to a C - C insaturation (olefin or alkyne) and formation of a ylid (carbonyl or onium), have been the source of fruitful cascades. Both of these are illustrated in Scheme 27 [52]. The two diazo ketone functions present in the same substrate 57 and under the action of the same catalyst react in two distinct ways. The initially formed carbenoid adds to a pending olefin to form a bi-cyclop. 1.0] intermediate 58 that subsequently cyclizes to produce a carbonyl ylide 59, that is further trapped intramolecularly in a [3 + 2] cycloaddition. The overall process gives birth to a highly complex pentacyclic structure 60. 

At the same time, the synthesis of mononuclear silver ylide complexes were achieved, Schmidbaur and his colleagues discovered that the ylide anions produced dinuclear species, Ag2[(CH2)2PMe2]. The remarkably stable methyl compound melts at 153-155°C and can be sublimed at 150°C and 0.1 atm. Mass spectral and H and P H NMR studies demonstrated the ring structure of the compound, although no structural analysis could be performed due to the photosensitivity of the compound. The synthesis of the dinuclear silver(I) ylide complex (6) is described in Inorganic Syntheses, and although colorless crystals are obtained, no X-ray stracture was possible due to sample decomposition in the X-ray beam. 

---