A complexity beyond mere memory

One of the most prominent features of chemistry in recent decades has been the advances in the methods of organic synthesis. There seems to be no limit to the complexity of an organic structure that organic chemists can prepare by a sequence of molecular transformations. This has been the basis of the success of the pharmaceutical chemical industry in recent years.

The catalogue of these basic steps is continually increasing, and the chemist has the problem of selecting the method which will give the best chemo-, regio-, and stereo- selectivity in the best yield. Mere memory has long ceased to be sufficient, and a number of databases have been developed from which the available methods, and their relative merits, for achieving a particular transformation can be retrieved.

This encyclopedia (acronym EROS) is a monumental attempt to help organic chemists to chose the best synthetic method. A team of around 1,000 practicing chemists from 40 countries has been assembled to write, to a standard format, on more than 3,000 reagent catalysts presently available "which with some generality effect the transformation of a substrate into a product". It is a great success, and should be not only a source of reference for anyone starting an organic synthesis, but also a choice for browsing on any organic chemist's long train journey.

Entries are around 1.8 pages long, and arranged alphabetically under the IUPAC name of the reagent, each with the structural and molecular formulae, the molecular weight, the CAS Registry number, and a summary of its field of use. For example: Borane-Dimethyl Sulfide, BH3.SMe2, [13292-87-0], C2H9BS, (MW 75.96) (hydroborating and reducing agent). There follow alternate names, physical data, form supplied, analysis for purity, preparative methods, and handling, storage, and precautions.

The various applications of the reagent are then given, with critical comments on its advantages and disadvantages, concluding with a list of related reagents. A substantial part of each entry consists of equations which makes browsing easier. References are thorough and up to date, many dated 1994, and the first reference for each entry is to reviews, when available.

For example, the discussion of the use of tributylstannane covers five pages, with an introductory section on mechanism, then sections on reactions initiated by Bu3Sn radical (C-X homolysis followed by H abstraction, radical rearrangements, intramolecular additions to unsaturated centres, intermolecular additions, fragmentation and homolytic substitution reactions, and hydrostannylation of alkenes, alkynes, and carbonyl compounds), applications in organometallic methodology, carbonylation and oxygenation of radicals, and TBTH as a source of Bu3Sn anion; there are eight references to reviews, and about 120 to the primary literature.

2,6-Di-t-butylpyridine is covered in one page under the headings of proton scavenger, catalysis of enolization and addition reactions, and inhibition of acid-catalyzed reactions, with a total of 21 references. 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone is covered in five pages under the headings of dehydrogenation of hydrocarbons, dehydrogenation of carbonyl compounds, oxidation of alcohols, benzylic oxidation, phenolic cyclization and coupling reactions, and miscellaneous reactions, with a total of about 110 references. Some of the longer entries, such as that on lipases (eight pages, 112 references), are introduced by an essay on general aspects of the reagent.

In the final volume, 640 pages are taken up by four indexes which greatly enhance the value of the entries. First a formula index of reagents, then an index of structural classes of reagents, a reagent function index, and finally (475 pages) a subject index.

I could not find any significant omissions of reagents, and the errors of commission are few, unimportant, and usually obvious. For example, di-t-butyl peroxide is consistently referred to as 1,1-di-t-butyl peroxide which it is not; the text refers to the reaction of ethylene glycol with dibutyltin oxide, but the equation shows propylene glycol as the reactant, and an oxastannolane as the product, whereas ethylene glycol would give a dioxastannolane, and propylene glycol would give a dioxastanninane; and the statement that the product from ethylene glycol is an infinite 6-coordinate chain polymer is true only of the crystalline state and not of the solutions which are used in synthesis. Some of the entries in the indexes show evidence of being generated by computer. For example, tris(trimethylsilyl)aluminium is indexed under nitrating reagents, but this surprising attribute is not supported by the text.

I found the encyclopedia easy to use, and absorbing reading. I would have appreciated a little more attention to mechanistic aspects, since it is only through the mechanism that the behaviour of the reagent can be understood and improved.

The inevitable comparison is with Fieser and Fieser's Reagents for Organic Synthesis. The information is broadly similar, but EROS, written as a coherent whole, is more critical, less staccato in style, and a lot more user-friendly.

The chemical database ISIS, accessible from the Chemical Database Service at Daresbury over the Internet, lists 535,000 reactions which can be searched by computer. The user has to make his own critical assessment of the data. For example, putting in Br2 as reagent produces 4615 hits, and some experience is needed to narrow this down and make the best use of the system. But one big advantage is that the database is regularly updated.

There is no mention of plans to make EROS available on compact disc, but surely that is the way it should develop. In a few years' time, the listing will begin to appear rather dated, but once the basic material is on CD-Rom, it will be simple and inexpensive to update it annually. In early Greek mythology, Eros was the son of Chaos, and was characterised by order and harmony. EROS should do much to achieve the same for the synthetic organic chemist.

Alwyn Davies is emeritus professor of chemistry, University College London.