Introduction to General Carbene Chemistry

As shown in the preceding chapter, a characteristic feature of aliphatic diazo compounds is that a diversity of reaction intermediates is formed by dediazoniation. 

Alkanediazonium ions and ion pairs containing carbocations are generated mainly in protic media (Chapt. 7). 

The diazo reactions in this chapter are characterized by processes run either in the gas phase, in relatively inert matrices, or in — typically, but not exclusively — aprotic and comparatively apolar solvents, either thermally or photolytically or with transition metal catalysis of various types. The metastable intermediates are carbenes (RR C ), i. e., neutral, apparently divalent, carbon compounds, or their transition metal complexes (coined carbenoids, see later in this section). It is interesting to recall that the synthesis of a compound that we now call a carbene, namely methylene (H2C ), was already attempted in the early 19th century, i.e., before the tetravalency of carbon was established. Dumas (1835) and Regnault (1839) thought then that it should be possible to obtain a compound consisting of one carbon and two hydrogen atoms by dehydration of methanol (a compound of which only the atomic ratio 1C 4H lO was then known).  

Analogously to the ion pairs involving carbocations mentioned above, diazo compounds are not the only source of carbenes, as they can also be obtained as metastable intermediates by other methods than by dediazoniation of diazoalkanes (see below). 

Diazo Chemistry II Aliphatic, Inorganic and Organometallic Con pounds. By Heinrich Zollinger Copyright 1995 VCH Verlagsgesellschaft mbH ISBN 3-527-29222-5 

---

This article is an attempt at evaluating new important features of tin(II) chemistry the central point is the interrelationship between molecular structure and reactivity of molecular tin(II) compounds. To define these compounds more closely, only those are discussed which are stable, monomeric in solvents and which may be classified as carbene analogs21. Thus, not a complete survey of tin(II) chemistry is given but stress is laid on the structures and reactions of selected compounds. A general introduction to the subject precedes the main chapters. For comparison, also solid-state tin(II) chemistry is included to demonstrate the great resemblance with molecular tin(II) chemistry. Tin(II) compounds, which are either generated as intermediates or only under definite conditions such as temperature or pressure, are not described in detail. 

---