Ylides heteroatom

Intermediate heterocyclic ylides may be stabilized by [1,2]- or [2,3]- rearrangements (Scheme 5). The [l,2]-shift may result in expelling an ylide heteroatom from the initially formed ylide (Scheme 6). The 1,3-dipolar cycloaddition of ylides generated by carbene cyclization onto the heteroatom of the C=Z double-bond allows an additional ring to be formed and provides a route to bridged and bridged-fused systems (Scheme 7). 

Nonplanar TS geometries in the Ph3P=CHR reactions are favored by a-branching in the aldehyde substituent R because the 1,3-interactions become increasingly important. However, the steric constraints decrease carbonyl reactivity and work against an early TS. The result is a consistent but relatively small trend toward the cis-selective pathway for the reaction of tertiary aldehydes with a variety of ylides. Heteroatom branching at the a-carbon has a similar effect on selectivity. In this case, the steric constraints may be smaller, but there are new constraints due to the need to minimize lone-pair or dipole-dipole interactions. There are fewer conformational options for the carbonyl component in the TS, and the result is an increase in the importance of 1,3-interactions. Since a-heteroatom substituents in the carbonyl reactant will also increase carbonyl reactivity, there will be a trend toward an earlier, more puckered TS. The combination of conformational constraints and increased reactivity results in higher selectivity for the cis-disubstituted oxaphosphetane. 

Although widely applied in functional group modification in a variety of heterocyclic systems, phosphorus ylides have only been employed sparingly in heterocyclic ring construction with two or more heteroatoms in the nucleus. Their potential is shown in the applications illustrated below. 

Electron deficient species can attack the unshared electron pairs of heteroatoms, to form ylides, such as in the reaction of thietane with bis(methoxycarbonyl)carbene. The S —C ylide rearranges to 2,2-bis(methoxycarbonyl)thiolane (Section 5.14.3.10.1). A"-Ethoxycar-bonylazepine, however, is attacked by dichlorocarbene at the C=C double bonds, with formation of the trans tris-homo compound (Section 5.16.3.7). 

An ylide can be defined as a compound in which a carb-anion is attached directly to a heteroatom carrying a high degree of positive charge ... 

Similar to phosphur ylides, sulfur ylides 1 and 2 possess the nucleophilic site at the carbon atom and the pendant leaving group at the heteroatom (sulfur). Different from the Wittig reaction, the Corey-Chaykovsky reaction does not lead to olefins. 

The ylides may be defined as dipolar compounds in which a carbanion is covalently bonded to a positively charged heteroatom. They are represented by the following general formula ... 

A comparative study on ylide stability as a function of the heteroatom type was carried out by Doering et al. [3,4]. They concluded that the phosphorus and sulfur ylides are the most stable ones. The participation of three-dimensional orbitals in the covalency determines the resonance stabilization of the phosphorus and sulfur ylides [5-8]. The nitrogen ylides are less stable from this point of view. The only stabilization factor involves electrostatic interactions between the two charges localized on adjacent nitrogen and carbon atoms [9]. 

This review concerns in the first part the works published during the last three years on the synthesis and reactivity of stabilized ylides C-substituted by electron-withdrawing groups (COR, CO2R, CN, etc.). The second part deals with the works published in the same period on the chemistry of phosphorus ylides mainly C-substituted by heteroatoms of groups 1-16 (metals, metalloids and nonmetal elements Li, Ba, Ca, Ti, Zr, Nb, Mo, Re, Fe, Ru, Rh, Pd, Pt, Au, Zn, Hg, B, Si, Sn, N, P, As, Sb, O, S, Te). 

The second chapter (M. Taillefer and H. J. Cristau) is dedicated to new trends in ylide chemistry. The preparation and the reactivity of phosphorus ylides, C-substituted by heteroatoms is presented, ylides being substituted by groups 1 and 2 elements, by transition metals or by elements of groups 13 to 16. A rich and versatile chemistry is thus reported. 

Muller et al. have also examined the enantioselectivity and the stereochemical course of copper-catalyzed intramolecular CH insertions of phenyl-iodonium ylides [34]. The decomposition of diazo compounds in the presence of transition metals leads to typical reactions for metal-carbenoid intermediates, such as cyclopropanations, insertions into X - H bonds, and formation of ylides with heteroatoms that have available lone pairs. Since diazo compounds are potentially explosive, toxic, and carcinogenic, the number of industrial applications is limited. Phenyliodonium ylides are potential substitutes for diazo compounds in metal-carbenoid reactions. Their photochemical, thermal, and transition-metal-catalyzed decompositions exhibit some similarities to those of diazo compounds. 

Transition metal-catalyzed carbenoid transfer reactions, such as alkene cyclopro-panation, C-H insertion, X-H insertion (X = heteroatom), ylide formation, and cycloaddition, are powerful methods for the construction of C-C and C-heteroatom bonds [1-6]. In contrast to a free carbene, metallocarbene-mediated reactions often proceed stereo- and regioselectively under mild conditions with tolerance to a wide range of functionalities. The reactivity and selectivity of metallocarbenes can be... 

There are a few isolated cases of the addition of amines, thiols, carboxylic acids, and a phosphorus ylide to doubly bonded germanium compounds. Again, the reactions are regioselective, with the nucleophilic portion of the weak acid adding to the germanium and the proton adding to the heteroatom. 

A mechanistic picture which reconciles the experimental results is given in Scheme 24. It is assumed that both the heteroatom and the double bond of the allyl halide compete for an electrophilic metal carbene. Heteroatom attack yields a metalated ylide 129, which may go on to ylide 131 by demetalation and/or to allylmetal complex 130. Symmetry-allowed [2,3] rearrangement of 131 accounts for product 132, and metal elimination from 130 gives rise to products 132 and 133, corresponding to [2,3] and [1,2] rearrangement, respectively, as well as haloacetate (if R3 = CHc ). 

Electrocyclization of 1,4-dienes is an efficient process when a heteroatom with a lone pair of electrons is placed in the 3-position, as in 77 (Scheme 20)38. Photoexcitation of these systems generally results in efficient formation of a C—C bond via 6e conrotatory cyclization to afford the ylide 78. These reactive intermediates can undergo a variety of processes, including H-transfer (via a suprafacial 1,4-H transfer) to 79 or oxidation to 80. In a spectacular example of reaction, and the potential it holds for complex molecule synthesis, Dittami and coworkers found that the zwitterion formed by photolysis of divinyl ether 81 could be efficiently trapped in an intramolecular [3 + 2] cycloaddition by the... 

Cyclodditions to Carbon-Heteroatom Triple Bonds. Transient electrophilic carbenes are known to react with nitriles to give transient46 or even stable nitrile ylides 30.47 No reaction of transient nucleophilic carbenes with nitriles has been reported. 

Diaminocarbene complexes were reported as early as 1968 [152], Preparation and applications of such complexes have been reviewed [153], Because of 7t-electron donation by both nitrogen atoms, diaminocarbenes are very weak tt-acceptors and have binding properties towards low-valent transition metals similar to those of phosphines or pyridines [18,153]. For this reason diaminocarbenes form complexes with a broad range of different metals, including those of the titanium group. Titanium does not usually form stable donor-substituted carbene complexes, but rather ylide-like, nucleophilic carbene complexes with non-heteroatom-substituted carbenes (Chapter 3). 

The chemical behavior of heteroatom-substituted vinylcarbene complexes is similar to that of a,(3-unsaturated carbonyl compounds (Figure 2.17) [206]. It is possible to perform Michael additions [217,230], 1,4-addition of cuprates [151], additions of nucleophilic radicals [231], 1,3-dipolar cycloadditions [232,233], inter-[234-241] or intramolecular [220,242] Diels-Alder reactions, as well as Simmons-Smith- [243], sulfur ylide- [244] or diazomethane-mediated [151] cyclopropanati-ons of the vinylcarbene C-C double bond. The treatment of arylcarbene complexes with organolithium reagents ean lead via conjugate addition to substituted 1,4-cyclohexadien-6-ylidene complexes [245]. 

Several reaction sequences have been reported in which Fischer-type carbene complexes are converted in situ into non-heteroatom-substituted carbene complexes, which then cyclopropanate simple olefins [306,307] (Figure 2.22). This can, for instance, be achieved by treating the carbene complexes with dihydropyridines, forming (isolable) pyridinium ylides. These decompose thermally to yield pyridine and highly electrophilic, non-heteroatom-substituted carbene complexes (Figure 2.22) [46]. 

Photolysis or thermolysis of heteroatom-substituted chromium carbene complexes can lead to the formation of ketene-like intermediates (cf. Sections 2.2.3 and 2.2.5). The reaction of these intermediates with tertiary amines can yield ammonium ylides, which can undergo Stevens rearrangement [294,365,366] (see also Entry 6, Table 2.14 and Experimental Procedure 2.2.1). This reaction sequence has been used to prepare pyrrolidones and other nitrogen-containing heterocycles. Examples of such reactions are given in Figure 2.31 and Table 2.21. 

Non-heteroatom-substituted carbene complexes can also be generated by treatment of electrophilic transition metal complexes with ylides (e.g. diazoalkanes, phosphorus ylides, nucleophilic carbene complexes, etc. Section 3.1.3). Alkyl complexes with a leaving group in the a-position are formed as intermediates. These alkyl complexes can undergo spontaneous release of the leaving group to yield a carbene complex (Figure 3.2). 

Fig. 3.21. Preparations of non-heteroatom-substituted alkylidene complexes from phosphorus ylides [516,517].

Ylides are species in which a positively charged heteroatom X (such as P, S, N, or As) is connected to a negatively charged atom possessing an unshared electron... 

Ylides, by definition, are nucleophiles. Probably the most complete definition has been given by AW Johnson [2], who stated that an ylide is a carbanion directly bonded to a heteroatom with a high degree of formal positive charge, this charge... 

Ylides in which the heteroatom is N, P, As, S, or Se are well known. Other ylides containing Sb, Bi, O, Te, I, or Br are also known, but they are rarely used as ligands since they are very unstable, and they will not be treated here. The synthesis of the ylides is achieved through several preparative methods, most of which have been comprehensively reviewed [2-11]. The most relevant of these requires two steps, and involves the reaction of a halide with an EZ nucleophile (NR3, PR3, ASR3, SR2, etc.) and subsequent dehydrohalogenation of the onium salt (method a) as represented in Scheme 2 [2-6]. This process has been reported in a wide variety of experimental conditions, using virtually all kinds of solvents and bases (provided that they are compatible). The desilylation of some a-SiMe3 onium salts (method b)... 

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