Bis(trimethylstannyl) Benzopinacolate

[39157-60-3]  · C32H38O2Sn2  · Bis(trimethylstannyl) Benzopinacolate  · (MW 692.03)

(reagent for radical mediated formation of C-C and Sn-X (X = C, O, N, etc.) bonds)

Alternate Name: TINPIN.

Physical Data: mp 122-124 °C.

Solubility: slightly sol petroleum ether; sol benzene, CH2Cl2.

Form Supplied in: not commercially available.

Preparative Method: prepared as a colorless solid by photolysis of a mixture of Hexamethyldistannane and benzophenone.1

Handling, Storage, and Precautions: avoid exposure of the reagent to light and heat; store protected from light in a refrigerator or freezer. Organotin compounds are highly toxic.

Preparation and Thermolysis.

Bis(trimethylstannyl) benzopinacolate (TINPIN) is prepared in the form of colorless crystals by photolysis of a mixture of benzophenone and Me3SnSnMe3 in benzene.1 On thermolysis, TINPIN undergoes homolytic C-C cleavage, resulting in the formation of a (trimethylstannyloxy)diphenylmethyl radical, which can undergo further cleavage to give benzophenone and a trimethylstannyl radical (eq 1).

Radical C-C Bond Formation.

Under typical radical conditions, employing Tri-n-butylstannane and Azobisisobutyronitrile or Me3SnSnMe3, the intermolecular addition of alkyl radicals to O-benzylformaldoxime is inefficient. Hart and Seely overcame this problem by employing TINPIN in place of these conventional tin reagents (eq 2), a particularly interesting example being the synthesis of a C-glycosidic product starting with a glucosyl bromide (eq 3).2

An interesting extension of this work, employing a functionalized oxime derivative, allowed the synthesis of a novel dinucleoside (eq 4).3

The report of Hart and Seely also mentions that TINPIN is a highly effective replacement for Bu3SnH for the intermolecular radical addition of cyclohexyl iodide to ethyl acrylate (eq 5). The reaction is remarkable in that high yields of the desired adduct are obtained without the use of a large excess of acrylate radical acceptor. It is thought that this may be a radical nonchain process, involving trapping of the initial adduct radical by a (trimethylstannyloxy)diphenylmethyl radical, followed by a thermal retroaldol process to give the observed product (eq 6).4

The TINPIN reagent has also been employed in an intramolecular [3 + 2] radical-mediated addition reaction, enabling the synthesis of the cyclopenta[b]benzofuran ring system of the natural product rocaglamide (eq 7).5 This stereoselective transformation involves initial attack of a trimethylstannyl radical on the thioamide group, followed by cyclopropane opening, double intramolecular radical cyclization, and final expulsion of the tin radical.

Formation of Tin-Heteroatom Bonds.

TINPIN is a highly effective reagent for the formation of stannylated derivatives from a range of heteroatom containing systems, including benzoquinone and NBS (eqs 8 and 9).1,6

Examples of other systems which are effectively bis-stannylated include benzil and azobenzene.

1. Hillgärtner, H.; Neumann, W. P.; Schroeder, B. LA 1975, 586.
2. Hart, D. J.; Seely, F. L. JACS 1988, 110, 1631.
3. Debart, F.; Vasseur, J-J.; Sanghvi, Y. S.; Cook, P. D. TL 1992, 33, 2645.
4. Curran, D. P. COS 1991, 4, 715.
5. Feldman, K. S.; Burns, C. J. JOC 1991, 56, 4601.
6. Neumann, W. P.; Hillgärtner, H.; Baines, K. M.; Dicke, R.; Vorspohl, K.; Kobs, U.; Nussbeutel, U. T 1989, 45, 951.

Nigel S. Simpkins

University of Nottingham, UK

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