Carbenes alkoxy amino

The 1,3,4-oxadiazole 113 is formed from the azo compound 112 by the action of triphenylphosphine <96SL652>. A general synthesis of 1,3.4-oxadiazolines consists in boiling an acylhydrazone with an acid anhydride (e.g., Scheme 18) <95JHC1647>. 2-Alkoxy-2-amino-l,3,4-oxadiazolines are sources of alkoxy(amino)carbenes the spiro compound 114, for instance, decomposes in boiling benzene to nitrogen, acetone and the carbene 115, which was trapped as the phenyl ether 116 in the presence of phenol <96JA4214>. 

BC NMR Shifts (S Values, CDC13) and IR Frequencies (cm-1) of Selected Alkoxy(2-aminoalkenyl)carbene (= 4-Amino-1-chroma-1,3-dienes) and Alkoxy(2-PHOSPHINO)CARBENE COMPLEXES (= 4-PHOSPHINO-l-CHROMA-1,3-DIENEs)... 

H-Pyran, 2-alkoxy-4-methyl-2,3-dihydro-conformation, 3, 630 4H-Pyran, 2-amino-IR spectra, 3, 593 synthesis, 3, 758 4H-Pyran, 4-benzylidene-synthesis, 3, 762 4H-Pyran, 2,3-dihydro-halogenation, 3, 723 hydroboration, 3, 723 oxepines from, 3, 725 oxidation, 3, 724 reactions, with acids, 3, 723 with carbenes, 3, 725 4H-Pyran, 5,6-dihydro-synthesis, 2, 91 4H-Pyran, 2,6-diphenyl-hydrogenation, 3, 777 4H-Pyran, 6-ethyl-3-vinyl-2,3-dihydro-reactions, with acids, 3, 723 4H-Pyran, 2-methoxy-synthesis, 3, 762 4H-Pyran, 2,4,4,6-tetramethyl-IR spectra, 3, 593 4H-Pyran, 2,4,6-triphenyl-IR spectra, 3, 593... 

The superior donor properties of amino groups over alkoxy substituents causes a higher electron density at the metal centre resulting in an increased M-CO bond strength in aminocarbene complexes. Therefore, the primary decarbo-nylation step requires harsher conditions moreover, the CO insertion generating the ketene intermediate cannot compete successfully with a direct electro-cyclisation of the alkyne insertion product, as shown in Scheme 9 for the formation of indenes. Due to that experience amino(aryl)carbene complexes are prone to undergo cyclopentannulation. If, however, the donor capacity of the aminocarbene ligand is reduced by N-acylation, benzannulation becomes feasible [22]. 

The electrophilic carbene carbon atom of Fischer carbene complexes is usually stabilised through 7i-donation of an alkoxy or amino substituent. This type of electronic stabilisation renders carbene complexes thermostable nevertheless, they have to be stored and handled under inert gas in order to avoid oxidative decomposition. In a typical benzannulation protocol, the carbene complex is reacted with a 10% excess of the alkyne at a temperature between 45 and 60 °C in an ethereal solvent. On the other hand, the non-stabilised and highly electrophilic diphenylcarbene pentacarbonylchromium complex needs to be stored and handled at temperatures below -20 °C, which allows one to carry out benzannulation reactions at room temperature [34]. Recently, the first syntheses of tricyclic carbene complexes derived from diazo precursors have been performed and applied to benzannulation [35a,b]. The reaction of the non-planar dibenzocycloheptenylidene complex 28 with 1-hexyne afforded the Cr(CO)3-coordinated tetracyclic benzannulation product 29 in a completely regio- and diastereoselective way [35c] (Scheme 18). 

Alkoxy(carbene)iron(0) and amino(carbene)iron(0) complexes usually react with alkynes to give rj4-pyrone iron complexes and furans, respectively [54]. Nevertheless the chemoselective formation of naphthols was reported for alkoxy(carbene)iron(0) complexes with the electron-poor alkyne dimethyl... 

A combination of a Diels-Alder and a Fisher carbene-cyclopentannulation is described as the last example in this subgroup. Thus, Barluenga and coworkers used a [4+2] cycloaddition of 2-amino-l,3-butadienes 4-115 with a Fischer alkoxy-arylalky-nylcarbene complex 4-116 this is followed by a cyclopenta-annulation reaction with the aromatic ring in 4-116 to give 4-117 (Scheme 4.25) [36]. An extension of this domino process is the reaction of 4-118 with 2equiv. of the alkynyl carbene 4-119 containing an additional C-C-double bond (Table 4.2) [37]. The final product 4-120, which was obtained in high yield, is formed by a second [4+2] cycloaddition of the primarily obtained cyclopenta-annulated intermediate. 

In Figure 2.2 the most important synthetic approaches to alkoxy- or (acy-loxy)carbene complexes from non-carbene precursors are sketched. Some of these strategies can also be used to prepare amino- and thiocarbene complexes. These procedures will be discussed in detail in the following sections. In addition to the methods sketched in Figure 2.2, many complexes of this type have been prepared by chemical transformation of other heteroatom-substituted carbene complexes. Because of the high stability of most of these compounds, many different reactions can be used to modify the substituents at C without degrading the carbon-metal double bond. The generation of heteroatom-substituted carbene complexes from other carbene complexes will be discussed in Section 2.2. 

SiMe3) in the presence of ethanol, i-propanol, or diphenylamine to account for the formation of alkoxy- and amino (alkenyl) allenylidene complexes [25] or of a buta-trienyl(methoxy)carbene complex in the presence of methanol [26]. Two representative examples are depicted in Scheme 3.12. 

If we treat alkoxycarbcne complexes not with phosphines but with primary or secondary amines, we observe a new kind of reaction, reminiscent of the reactions of esters. This observation led us into peptide chemistry along a path that proved to be quite surprising to a coordination chemist. We could show that the alkoxy group of alkoxy(organo) carbene complexes can be substituted not only by mono- or dialkylamino residues but also by free amino groups of amino acid and peptide esters (63, 64). The principle of this reaction is shown in Scheme 2. 

An open question was also how pentacarbony 1 [[ethoxy (diethylamino) -carbene[]tungsten(0) would react with boron trihalides, since, as we have learned previously, in principle both the alkoxy and the amino group are removable. The answer was given by the exclusive formation of trans-(bromo)tetracarbonyl(diethylaminocarbyne)tungsten(0) (95), a com-... 

Replacing the alkoxy carbene substituent by a better electron-donating amino group stabilizes the metal carbonyl bond. As a result, CO insertion in vinyl carbene D is hampered instead, cyclopentannulation via the chromacyclohexadiene I leads to aminoindenes K, which are readily hydrolyzed to indanones L (Scheme 6) [20]. 

A complementary access to alkoxy- and aminocarbene complexes ( Semmelhack-Hegedus route ) involves the addition of the pentacarbonylchromate dianion 18 (obtained from the reduction of hexacarbonylchromium with C8K) to carboxylic acid chlorides and amides [27] (Scheme 10). While alkylation of acyl chromate 19 leads to alkoxycarbene complexes 12, addition of chromate dianion 18 to carboxylic amides generates the tetrahedral intermediates 20, which are deoxygenated by trimethylsilyl chloride to give amino carbene complexes 14. 

Scheme 10. Semmelhack-Hegedus route to alkoxy and amino carbene complexes.

Typical modes for the preparation of (l-alkynyl)carbene complexes, as well as spectroscopic and structural data, are briefly summarized. Alkoxy-and amino(l-alkynyl)carbene complexes are available from metal carbonyls and metal isocyanides, respectively. 

Aminomethylenation of [2-(AT/-amino)ethenyI]carbene complexes affords alkoxy(l-alkynyl)carbene complexes (22-30% yield) by /3-elimination, together with [2-amino-l-(iminoacyl)ethenyl]carbene complexes (61-83% yield),37 e.g., (CO)5M=C(OEt)-CH = CPh(NHPh) + HCONR2 + POCl3/Et3N - (CO)5M = C(OEt) — C=CPh (M = Cr, W R = pyrrolidine, morpholine). 

Conjugated 2-alkoxy-6-amino-l-metalla-l,3,5-hexatrienes of type M= C(OR) — C=C—C=C(NR2) are most easily accessible by addition of enamines to alkoxy(l-alkynyl)carbene complexes in a broad array of different substituents. Due to the highly unsaturated character of such compounds a variety of transformations into organic products can be anticipated. Since... 

Thermolysis of the pentacarbonyl chromium compounds 151 affords tet-racarbonyl chelate complexes by loss of a cis CO group,20 in the same fashion that has been found for ortho-alkoxy phenylcarbene complexes181 and similar products.182 The C=C triple bond of alkoxy(l-alkynyl)carbene complexes 1 is more reactive than that of amino(l-alkynyl)carbene complexes. (CO)5Cr = C(NHMe)—C=CPh was shown to add ethanol very slowly to give (CO)sCr = C(NHMe)-CH=C(OEt)Ph in 79% yield.21... 

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