Page 90 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

GROUP 13 ELEMENTS
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Borate minerals may have come to the rescue at this point. Boric acid, B(OH) ,
3
readily forms borate esters, B(OR) , with alcohols. Indeed, this reaction was the basis
3
of the old-fashioned flame test for borates, because trimethyl borate burns with an
intense green flame. The reaction is even more facile with 1,2 diols, which form cyclic
borate esters; this is shown in Figure 3.2 for glyceraldehyde. The borate-complexed
glyceraldehyde is then reacts with glycolaldehyde enolate to yield a borate-complexed
pentose. The borate mineral, on whose surface the reaction is thought to occur, may
afford a cleaner tar-free version of the formose reaction, which is much more promising
as a source of ribose. Experiments provide support for this picture. In the presence of
Ca(OH) , an aqueous solution of glycolaldehyde and glyceraldehyde rapidly turns brown.
2
In the presence of borate minerals such as ulexite (see Figure 3.2) or colemanite, the same
reaction does not turn brown, even over months, while slowly producing ribose and other
pentoses.
This is where the theory, advanced by the American chemist and origin-of-life researcher
Steven Benner, takes a somewhat otherworldly turn. Borate minerals are scarce in the
earth’s crust and may not have been around in the watery environment of early Earth. Mars,
however, was drier, and evidence from a meteorite suggests that it also had boron. Thus,
according to Benner, “The evidence seems to be building that we are actually all Martians;
that life started on Mars and came to Earth on a rock.” With characteristic humor, he has
added: “It’s lucky that we ended up here, nevertheless—as certainly Earth has been the bet-
ter of the two planets for sustaining life. If our hypothetical Martian ancestors had remained
on Mars, there may not have been a story to tell.”

REVIEW PROBLEM 3.3

An important point that we have glossed over in the above discussion concerns the
exact manner in which boron complexation leads to a cleaner formose reaction. What
is your opinion on the matter?

3.2 HYDROBORATION

Boron hydrides or boranes are an important class of compounds that we won’t quite do jus-
tice to in this book. Many of the polyhedral boranes exhibit inherently multicenter bonding
and, accordingly, their chemistry, though utterly fascinating, is less suited to analysis by
arrow pushing. Their chemistry was first explored in detail by the German chemist Alfred
Stock (1876–1946), whose influential book, as noted above, led the American chemist
H. C. Brown (1912–2004) to ultimately discover hydroboration. Hydroboration is one of
the quirkier reactions we will meet in this chapter. It typically refers to the addition of a B–H
bond to carbon–carbon multiple bonds (although addition to C=N and C=Oalsoinvolve
a similar mechanism). A concerted mechanism with a four-membered transition state is
thought to operate:
R 5
R 5
H B 6
R H B R 6
R 1 C C R 3 C C (3.3)
R 2 R 4
R 1 R 3
R 2 R 4