Synthesis and reactions of CARBENE., Slides of Organic Chemistry

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CARBENE

Reaction Intermediate

CONTENTS:

1.INTRODUCTION 2.HOW DO WE KNOW THAT CARBENES EXIST? 3.TYPES OF CARBENES 4.SINGLET AND TRIPLET CARBENES 5.WAYS TO MAKE CARBENES 6.CARBENE FORMATION 7.How Carbenes react

How do we know that carbenes exist? The best evidence for the existence of carbenes comes from a group of structures which contain a carbene but are stable compounds. The most important of these are known as the ‘N-heterocyclic carbenes’—the carbene is incorporated into a five-membered ring and stabilized by the presence of two adjacent electron- donating nitrogen atoms and the bulky N-substituents. The example below was first made in 1991: it is crystalline, and its X-ray crystal shows the bond angle at the carbene carbon to be 102°, and 13C NMR confirms that the carbene C atom is electron deficient.

These stable carbenes are very much the exception: most carbenes

are too reactive to be isolated. Reactive carbenes can, however, be

observed by irradiating precursors (often diazo compounds like

diazomethane) trapped in frozen argon at very low temperatures

(less than 77 K). IR and ESR spectroscopy can then be used to

determine their structure.

Carbenes can be divided into two types Spectroscopic investigations of a number of carbenes of differing structures have shown that they fall broadly into two groups:

All these observations can be accounted for by considering the electronic

structure of a carbene. Carbenes have two-coordinate carbon atoms: you might therefore expect them to have a linear (diagonal) structure—like that of an alkyne—with an sp hybridized carbon atom. Such a linear carbene would have six electrons to distribute amongst two σ orbitals and two (higher-energy) p orbitals. The two electrons in the degenerate p orbitals would remain unpaired because of electron repulsion in the same way as in molecular oxygen •O–O•.

Yet few carbenes are linear: most are bent, with bond angles between 100° and 150°, suggesting a trigonal (sp2) hybridization state. An sp2 hybridized carbene would have three (lower-energy) sp2 orbitals and one (high-energy) p orbital in which to distribute its six electrons.

There are two ways of doing this:

1.Either all of the electrons can be paired, with each pair

occupying one of the sp2 orbitals, or

2.two of the electrons can remain unpaired, with one elec-

tron in each of the p orbitals and one of the sp2 orbitals.

These two possibilities explain our two observed classes of carbene, and the two

possible arrangements of electrons (spin states) are termed triplet and singlet. The

orbitals are the same in both cases but in triplet carbenes we have one electron in

each of two molecular orbitals and in singlet carbenes both electrons go into the sp

orbital.

Singlet and triplet carbenes Triplet carbenes have two unpaired electrons, one in each of an sp2 and a p orbital, while singlet carbenes have a pair of electrons in a non-bonding sp2 orbital and have an empty p orbital.

The existence of the two spin states explains the different behaviour of triplet and singlet carbenes towards ESR spectroscopy; the orbital occupancy also explains the smaller bond angle in singlet carbenes, which have an electron-repelling lone pair in an sp2 orbital.

IT is much more common in modern chemistry to use a transition metal, such as copper or rhodium, to promote formation of the carbene.

● Carbenes from tosylhydrazones: Good starting materials for these

reactions are tosylhydrazones, which produce transient diazo compound is not normally isolated, and decomposes to the carbene on heating.

Carbene formation by α elimination α Eliminations follow a mechanism akin to an E1cB β elimination—a strong base removes an acidic proton adjacent to an electron-withdrawing group to give a carbanion. Loss of a leaving group from the carbanion creates a carbene.

When geminal dibromoalkanes are treated with BuLi, a halogen–metal exchange reaction produces a lithium carbenoid, with a metal atom and a halogen attached to the same carbon atom. Lithium carbenoids are stable at very low temperatures—they can be observed by NMR, but they decompose to carbenes at about – 100 °C.

The essence of this type of carbenoid is that it should have a leaving group, such as a halo-gen, that can accept a pair of electrons and another, usually a metal, that can donate a pair of electrons. If the metal leaves first, a carbanion is created that can lose the halogen to make a carbene. They might also leave together. Both mechanisms are α eliminations.